Exoplanet

An **exoplanet** or **extrasolar**
planet**is a planet that orbits a star other**
than the Sun. Over 2000 exoplanets have been discovered
since 1988 (2047 planets in 1296planetary systems including 506 multiple
planetary systems as of 12 January 2016).3

The *Kepler space telescope* has
also detected a few thousand45 candidate
planets,67 of which about
11% may be false positives.8 There is at
least one planet on average per star.9 About 1 in 5 Sun-like starsa have an "Earth-sized"b planet in the habitable zone,c with the nearest
expected to be within 12 light-years distance from Earth.1011 Assuming 200
billion stars in the Milky Way,d that would be 11
billion potentially habitable Earth-sized planets in the Milky Way, rising to
40 billion if planets orbiting the numerous red dwarfs are included.12

The least massive
planet known is PSR B1257+12 A, which is about twice the mass
of the Moon. The most massive
planet listed on the NASA Exoplanet
Archive isDENIS-P
J082303.1-491201 b,1314 about 29 times
the mass of Jupiter, although according to most
definitions of a planet, it is too massive
to be a planet and may be abrown dwarf instead.
There are planets that are so near to their star that they take only a few
hours to orbit and there are others so far away that they take thousands of
years to orbit. Some are so far out that it is difficult to tell whether they
are gravitationally bound to the star. Almost all of the planets detected so
far are within the Milky Way, but there have also been a few possible
detections of extragalactic planets.

The
discovery of exoplanets has intensified interest in the search for extraterrestrial life.
There is special interest in planets that orbit in a star's habitable zone, where it is possible for
liquid water (and therefore life) to exist on the
surface. The study of planetary
habitability also considers a wide range of other factors in
determining the suitability of a planet for hosting life.15

Besides
exoplanets, there are also rogue planets, which do not orbit any star and
which tend to be considered separately, especially if they are gas giants, in which case they are often
counted, like WISE 0855−0714,
as sub-brown dwarfs.16 The rogue
planets in the Milky Way possibly number in the billions (or more).1718

Definition

**IAU**

The
official definition of
"planet" used by the International
Astronomical Union (IAU) only covers the Solar System and thus does not apply to
exoplanets.1920 As of April
2011, the only defining statement issued by the IAU that pertains to exoplanets
is a working definition issued in 2001 and modified in 2003.21 That definition
contains the following criteria:

·
Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses
for objects of solar metallicity) that orbit stars or stellar remnants are
"planets" (no matter how they formed). The minimum mass/size required
for an extrasolar object to be considered a planet should be the same as that
used in the Solar System.

·
Substellar objects
with true masses above the limiting mass for thermonuclear fusion of deuterium
are "brown dwarfs",
no matter how they formed or where they are located.

·
Free-floating objects
in young star clusters with masses below the limiting mass for thermonuclear
fusion of deuterium are not "planets", but are "sub-brown
dwarfs" (or whatever name is most appropriate).

**Alternatives**

However,
the IAU's
working definition is not universally accepted. One alternate suggestion is
that planets should be distinguished from brown dwarfs on the basis of formation.
It is widely believed that giant planets form through core accretion, and that
process may sometimes produce planets with masses above the deuterium fusion threshold;222324 massive planets
of that sort may have already been observed.25 Brown dwarfs
form like stars from the direct collapse of clouds of gas and this formation
mechanism also produces objects that are below the 13 *M*Jup limit
and can be as low as 1 *M*Jup.26 Objects in this
mass range that orbit their stars with wide separations of hundreds or
thousands of AU and have large star/object mass ratios likely formed as brown
dwarfs; their atmospheres would likely have a composition more similar to their
host star than accretion-formed planets which would contain increased
abundances of heavier elements. Most directly imaged planets as of April 2014
are massive and have wide orbits so probably represent the low-mass end of
brown dwarf formation.27

Also,
the 13-Jupiter-mass cutoff does not have precise physical significance.
Deuterium fusion can occur in some objects with a mass below that cutoff.24 The amount of
deuterium fused depends to some extent on the composition of the object.28 The Extrasolar
Planets Encyclopaedia includes objects up to 25 Jupiter masses,
saying, "The fact that there is no special feature around 13 *M*Jup in the observed
mass spectrum reinforces the choice to forget this mass limit".29 The Exoplanet Data
Explorer includes objects up to 24 Jupiter masses with the
advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is
physically unmotivated for planets with rocky cores, and observationally
problematic due to the sin
i ambiguity."30 The NASA Exoplanet
Archive includes objects with a mass (or minimum mass) equal to
or less than 30 Jupiter masses.31 Another
criterion for separating planets and brown dwarfs, rather than deuterium
fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron
degeneracy pressure with the dividing line at around 5 Jupiter
masses.3233 Another
suggestion, based on mass-density relationships, is that the dividing line
should be at 60 Jupiter masses.34

History of detection

For
centuries philosophers and scientists supposed that extrasolar planets existed,
but there was no way of detecting them or of knowing their frequency or how
similar they might be to the planets of the Solar System. Various detection claims made in
the nineteenth century were rejected by astronomers. The first confirmed
detection came in 1992, with the discovery of several terrestrial-mass planets
orbiting the pulsar PSR B1257+12.35The first confirmation
of an exoplanet orbiting a main-sequence star was made in 1995, when
a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the
vast majority have been detected through indirect methods such as the transit method and the radial-velocity method.

**Early speculations**

This space we declare
to be infinite... In it are an infinity of worlds of the same kind as our own.

*Giordano Bruno (1584)*36

In
the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory
that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the
fixed stars are similar to the Sun and are likewise accompanied by planets.

In
the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his *Principia*.
Making a comparison to the Sun's planets, he wrote "And if the fixed stars
are the centers of similar systems, they will all be constructed according to a
similar design and subject to the dominion of *One*."37

In
1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no
compelling reason why planets could not be much closer to their parent star
than is the case in the Solar System, and proposed that Doppler spectroscopy and
the transit method could
detect super-Jupiters in
short orbits.38

**Discred claims**

Claims
of exoplanet detections have been made since the nineteenth century. Some of
the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at
the East India
Company's Madras Observatory reported
that orbital anomalies made it "highly probable" that there was a
"planetary body" in this system.39 In the 1890s, Thomas J. J.
See of the University of Chicago and
the United
States Naval Observatory stated that the orbital anomalies
proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.40 However, Forest Ray Moulton published
a paper proving that a three-body system with those orbital parameters would be
highly unstable.41 During the 1950s
and 1960s, Peter van de Kamp of Swarthmore College made
another prominent series of detection claims, this time for planets orbiting Barnard's Star.42Astronomers now
generally regard all the early reports of detection as erroneous.43

In
1991 Andrew Lyne, M.
Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.44 The claim
briefly received intense attention, but Lyne and his team soon retracted it.45

**Confirmed discoveries**

*Main article: Discoveries of*
exoplanets

*See also: List of exoplanet*
firsts

The three known
planets of the starHR8799, as imaged by the Hale Telescope. The light from the central
star was blanked out by a vector vortex
coronagraph.

2MASS J044144 is a brown dwarfwith a companion about 5–10 times
the mass of Jupiter. It is not clear whether this companion object is asub-brown dwarf or a planet.

Coronagraphic image of AB Pictorisshowing a companion (bottom left),
which is either a brown dwarf or a massive planet. The data was obtained on 16
March 2003 withNACO on
the VLT, using
a 1.4 arcsec occulting mask on top of AB Pictoris.

As
of 12 January 2016, a total of 2047 confirmed exoplanets are listed in the Extrasolar
Planets Encyclopaedia, including a few that were confirmations of
controversial claims from the late 1980s.3 The first
published discovery to receive subsequent confirmation was made in 1988 by the
Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of
the University of
Victoria and the University
of British Columbia.46 Although they
were cautious about claiming a planetary detection, their radial-velocity
observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations
were at the very limits of instrumental capabilities at the time, astronomers
remained skeptical for several years about this and other similar observations.
It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass
between planets and stars. In 1990 additional observations were published that
supported the existence of the planet orbiting Gamma Cephei,47 but subsequent
work in 1992 again raised serious doubts.48 Finally, in
2003, improved techniques allowed the planet's existence to be confirmed.49

On
9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two
planets orbiting the pulsar PSR 1257+12.35 This discovery
was confirmed, and is generally considered to be the first definitive detection
of exoplanets. Follow-up observations solidified these results, and
confirmation of a third planet in 1994 revived the topic in the popular press.50 These pulsar
planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a
second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the
supernova and then decayed into their current orbits.

On
6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced
the first definitive detection of an exoplanet orbiting a main-sequence star, namely the nearby
G-type star 51 Pegasi.5152 This discovery,
made at the Observatoire
de Haute-Provence, ushered in the modern era of exoplanetary
discovery. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of
many new exoplanets: astronomers could detect exoplanets indirectly by
measuring their gravitational influence
on the motion of their host stars. More extrasolar planets were later detected
by observing the variation in a star's apparent luminosity as an orbiting
planet passed in front of it.

Initially,
most known exoplanets were massive planets that orbited very close to their
parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had
indicated that giant planets should only form at large distances from stars.
But eventually more planets of other sorts were found, and it is now clear that
hot Jupiters are a minority of exoplanets. In 1999, Upsilon Andromedae became
the first main-sequence star known to have multiple planets.53 Kepler-16 contains the first discovered
planet that orbits around a binary main-sequence star system.54

On
26 February 2014, NASA announced the discovery of 715 newly verified exoplanets
around 305 stars by the Kepler Space Telescope.
These exoplanets were checked using a statistical technique called
"verification by multiplicity".555657 Prior to these
results, most confirmed planets were gas giants comparable in size to Jupiter
or larger as they are more easily detected, but the Kepler planets are mostly
between the size of Neptune and the size of Earth.55

On
23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting
the habitable zone of a G2-type star.58

**Candidate discoveries**

As
of March 2014, NASA's Kepler mission had
identified more than 2,900 planetary candidates,
several of them being nearly Earth-sized and located in the habitable zone,
some around Sun-like stars.4559

Kepler mission – new
exoplanet candidates – as of 23 July 2015.60

Detection methods

*Main article: Methods*
of detecting extrasolar planets

**Direct imaging**

*Main article: Direct imaging*

Directly imaged planet, Beta Pictoris b

Planets
are extremely faint compared to their parent stars. At visible wavelengths,
they usually have less than a millionth of their host star's brightness. It is difficult
to detect such a faint light source, and furthermore the parent star causes a
glare that tends to wash it out. It is necessary to block the light from the
parent star in order to reduce the glare while leaving the light from the
planet detectable; doing so is a major technical challenge which requires
extreme optothermal stability.61

All
exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their
parent star. Most of them are also very hot, so that they emit intense infrared radiation; the images have then been
made at infrared where the planet is brighter than it is at visible
wavelengths. During the gas-accretion phase of giant-planet formation the
star–planet contrast may be even better in H alpha than it is in infrared—an H alpha
survey is currently underway.62

Specially
designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will
image dozens of gas giants, however the vast majority of known extrasolar
planets have only been detected through indirect methods. The following are the
indirect methods that have proven useful:

**Indirect methods**

·
Transit method

When the star is
behind a planet, its brightness will seem to dim

If
a planet crosses (or transits)
in front of its parent star's disk, then the observed brightness of the star
drops by a small amount. The amount by which the star dims depends on its size
and on the size of the planet, among other factors. This method suffers from a
substantial rate of false positives and confirmation from another method is
usually considered necessary. The transit method reveals the radius of a
planet, and it has the benefit that it sometimes allows a planet's atmosphere
to be investigated through spectroscopy. Because the transit method
requires that part of the planet's orbit intersect a line-of-sight between the
host star and Earth, the probability that an exoplanet in a randomly oriented
orbit will be observed to transit the star is somewhat small. The Kepler telescope uses
this method.

Discovered extrasolar planets per year and bydetection
method (as of September 2014):

```
       microlensing

```

```
       timing

```

·
Radial velocity or
Doppler method

As
a planet orbits a star, the star also moves in its own small orbit around the
system's center of mass. Variations in the star's radial velocity—that is, the
speed with which it moves towards or away from Earth—can be detected from
displacements in the star's spectral lines due to the Doppler effect. Extremely small
radial-velocity variations can be observed, of 1 m/s or even somewhat less.63 This method has
the advantage of being applicable to stars with a wide range of
characteristics. One of its disadvantages is that it cannot determine a
planet's true mass, but can only set a lower limit on that mass. However, if
the radial velocity of the planet itself can be distinguished from the radial
velocity of the star, then the true mass can be determined.64

·
Transit timing
variation (TTV)

When
multiple planets are present, each one slightly perturbs the others' orbits.
Small variations in the times of transit for one planet can thus indicate the
presence of another planet, which itself may or may not transit. For example,
variations in the transits of the planet Kepler-19b suggest the existence of a
second planet in the system, the non-transiting Kepler-19c.6566 If multiple
transiting planets exist in one system, then this method can be used to confirm
their existence.67 In another form
of the method, timing the eclipses in an eclipsing binary star can
reveal an outer planet that orbits both stars; as of August 2013, a few planets
have been found in that way with numerous planets confirmed with this method.

·
Transit
duration variation (TDV)

Animation showing
difference between planet transit timing of 1-planet and 2-planet systems

When
a planet orbits multiple stars or if the planet has moons, its transit time can
significantly vary per transit. Although no new planets or moons have been
discovered with this method, it is used to successfully confirm many transiting
circumbinary planets.68

·
Gravitational
microlensing

Microlensing
occurs when the gravitational field of a star acts like a lens, magnifying the
light of a distant background star. Planets orbiting the lensing star can cause
detectable anomalies in the magnification as it varies over time. Unlike most
other methods which have detection bias towards planets with small (or for
resolved imaging, large) orbits, microlensing method is most sensitive to
detecting planets around 1–10 AU away from Sun-like stars.

·
Astrometry

Astrometry
consists of precisely measuring a star's position in the sky and observing the
changes in that position over time. The motion of a star due to the
gravitational influence of a planet may be observable. Because the motion is so
small, however, this method has not yet been very productive. It has produced
only a few disputed detections, though it has been successfully used to
investigate the properties of planets found in other ways.

·
Pulsar timing

A pulsar (the small, ultradense remnant of
a star that has exploded as a supernova) emits radio waves extremely
regularly as it rotates. If planets orbit the pulsar, they will cause slight
anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was
made using this method. But as of 2011, it has not been very productive; five
planets have been detected in this way, around three different pulsars.

·
Variable
star timing (pulsation frequency)

Like
pulsars, there are some other types of stars which exhibit periodic activity.
Deviations from the periodicity can sometimes be caused by a planet orbiting
it. As of 2013, a few planets have been discovered with this method.69

·
Reflection/emission
modulations

When
a planet orbits very close to the star, it catches a considerable amount of
starlight. As the planet orbits around the star, the amount of light changes
due to planets having phases from Earth's viewpoint or planet glowing more from
one side than the other due to temperature differences.70

·
Relativistic
beaming

Relativistic
beaming measures the observed flux from the star due to its motion. The
brightness of the star changes as the planet moves closer or further away from
its host star.71

·
Ellipsoidal
variations

Massive
planets close to their host stars can slightly deform the shape of the star.
This causes the brightness of the star to slightly deviate depending how it is
rotated relative to Earth.72

·
Polarimetry

With
polarimetry method, a polarized light reflected off the planet is separated
from unpolarized light emitted from the star. No new planets have been
discovered with this method although a few already discovered planets have been
detected with this method.7374

·
Circumstellar
disks

Disks
of space dust surround many stars, believed to originate from collisions among
asteroids and comets. The dust can be detected because it absorbs starlight and
re-emits it as infrared radiation.
Features in the disks may suggest the presence of planets, though this is not
considered a definitive detection method.

Nomenclature

**Proper names**

Most exoplanets have catalog names which are explained in the
following sections, but in July 2014 the IAU launched
a process for giving proper names to exoplanets.75 The process
involved public nomination and voting for the new names,76 the results of
which were announced in December 2015.77 The planets so
named were AEgir (sic), Amateru, Arion, Arkas, Brahe, Dagon, Dimidium, Draugr, Dulcinea, Fortitudo, Galileo, Harriot, Hypatia,Janssen, Lippershey, Majriti, Meztli, Orbitar, Phobetor, Poltergeist, Quijote, Rocinante, Saffar, Samh, Smertrios, Sancho, Spe, Tadmor, Taphao Kaew, Taphao Thong and Thestias.

The decision to give the planets new names followed the private
company Uwingu's exoplanet naming contest, which the
IAU harshly criticized.78 Previously a few
planets had received unofficial names: notably Osiris (HD 209458 b), Bellerophon (51 Pegasi b),79 and Methuselah (PSR B1620-26 b).

**Multiple-star standard**

The convention for naming exoplanets is an extension of the one
used by the Washington Multiplicity Catalog (WMC) for multiple-star systems, and adopted by the International
Astronomical Union.80 The brightest
member of a star system receives the letter "A". Distinct components
not contained within "A" are labeled "B", "C",
etc. Subcomponents are designated by one or more suffixes with the primary
label, starting with lowercase letters for the 2nd hierarchical level and then
numbers for the 3rd.81 For example, if
there is a triple star system in which two stars orbit each other closely with
a third star in a more distant orbit, the two closely orbiting stars would be
named Aa and Ab, whereas the distant star would named B. For historical
reasons, this standard is not always followed: for example Alpha Centauri A, B and C are *not* labelled
Alpha Centauri Aa, Ab and B.

**Extrasolar planet**
standard

Following an extension of the above standard, an exoplanet's
name is normally formed by taking the name of its parent star and adding a
lowercase letter. The first planet discovered in a system is given the
designation "b" and later planets are given subsequent letters. If
several planets in the same system are discovered at the same time, the closest
one to the star gets the next letter, followed by the other planets in order of
orbital size.

For instance, in the 55 Cancri system the first planet – 55 Cancri b – was discovered in 1996; two
additional farther planets were simultaneously discovered in 2002 with the nearest
to the star being named 55 Cancri c and
the other 55 Cancri d; a
fourth planet was claimed (its existence was later disputed) in 2004 and named 55 Cancri e despite lying closer to the
star than 55 Cancri b; and the most recently discovered planet, in 2007, was
named 55 Cancri f despite
lying between 55 Cancri c and 55 Cancri d.82 As of April 2012
the highest letter in use is "j", for the unconfirmed planet HD 10180 j, and with "h" being the
highest letter for a confirmed planet, belonging to the same host star).3

If a planet orbits one member of a binary star system, then an uppercase
letter for the star will be followed by a lowercase letter for the planet.
Examples are 16 Cygni Bb83 and HD 178911 Bb.84 Planets orbiting
the primary or "A" star should have 'Ab' after the name of the
system, as in HD 41004 Ab.85 However, the
"A" is sometimes omitted; for example the first planet discovered
around the primary star of the Tau Boötis binary system is usually
called simply Tau Boötis b.86 The star
designation is necessary when more than one star in the system has its own
planetary system such as in case of WASP-94 A and WASP-94 B.87

If the parent star is a single star, then it may still be
regarded as having an "A" designation, though the "A" is
not normally written. The first exoplanet found to be orbiting such a star
could then be regarded as a secondary subcomponent that should be given the
suffix "Ab". For example, 51 Peg Aa is the host star in the system 51 Peg; and the first exoplanet is then 51 Peg
Ab. Because most exoplanets are in single-star systems, the implicit
"A" designation was simply dropped, leaving the exoplanet name with
the lower-case letter only: 51 Peg b.

A few exoplanets have been given names that do not conform to
the above standard. For example, the planets that orbit the pulsar PSR 1257 are
often referred to with capital rather than lowercase letters. Also, the
underlying name of the star system itself can follow several different systems.
In fact, some stars (such as Kepler-11) have only received their names due
to their inclusion in planet-search programs, previously only being referred to
by their celestial coordinates.

**Circumbinary planets and**
2010 proposal

Hessman et al. state that the implicit system for exoplanet
names utterly failed with the discovery of circumbinary planets.80 They note that
the discoverers of the two planets around HW Virginis tried to circumvent the
naming problem by calling them "HW Vir 3" and "HW Vir 4",
i.e. the latter is the 4th object – stellar or planetary – discovered
in the system. They also note that the discoverers of the two planets around NN Serpentis were confronted with
multiple suggestions from various official sources and finally chose to use the
designations "NN Ser c" and "NN Ser d".

The proposal of Hessman et al. starts with the following two
rules:

**Rule 1**. The formal name of
an exoplanet is obtained by appending the appropriate suffixes to the formal
name of the host star or stellar system. The upper hierarchy is defined by
upper-case letters, followed by lower-case letters, followed by numbers, etc.
The naming order within a hierarchical level is for the order of discovery
only. (This rule corresponds to the present provisional WMC naming convention.)

**Rule 2**. Whenever the leading
capital letter designation is missing, this is interpreted as being an informal
form with an implicit "A" unless otherwise explicitly stated. (This
rule corresponds to the present exoplanet community usage for planets around
single stars.)

They
note that under these two proposed rules all of the present names for 99% of
the planets around single stars are preserved as informal forms of the IAU
sanctioned provisional standard. They would rename Tau Boötis b formally as Tau
Boötis Ab, retaining the prior form as an informal usage (using Rule 2, above).

To
deal with the difficulties relating to circumbinary planets, the proposal
contains two further rules:

**Rule 3.** As an
alternative to the nomenclature standard in Rule 1, a hierarchical relationship
can be expressed by concatenating the names of the higher order system and
placing them in parentheses, after which
the suffix for a lower order system is added.

**Rule 4**. When in doubt (i.e.
if a different name has not been clearly set in the literature), the hierarchy
expressed by the nomenclature should correspond to dynamically distinct
(sub)systems in order of their dynamical relevance. The choice of hierarchical
levels should be made to emphasize dynamical relationships, if known.

They submit that the new form using parentheses is the best for
known circumbinary planets and has the desirable effect of giving these planets
identical sublevel hierarchical labels and stellar component names that conform
to the usage for binary stars. They say that it requires the complete renaming
of only two exoplanetary systems: The planets around HW Virginis would be
renamed HW Vir (AB) b & (AB) c, whereas those around NN Serpentis would be
renamed NN Ser (AB) b & (AB) c. In addition the previously known single circumbinary
planets around PSR B1620-26 and DP Leonis) can almost retain their names (PSR B1620-26 b and DP Leonis b) as unofficial informal forms of
the "(AB)b" designation where the "(AB)" is left out.

The discoverers of the circumbinary planet around Kepler-16 followed the naming scheme
proposed by Hessman et al. when naming the body Kepler-16 (AB)-b, or simply Kepler-16b when there is no ambiguity.54

**Other naming systems**

Another nomenclature, often seen in science fiction, uses Roman numerals in the order of planets'
positions from the star. (This was inspired by an old system for naming moons
of the outer planets, such as "Jupiter IV" forCallisto.) But such a system is impractical
for scientific use, because new planets may be found closer to the star,
changing all numerals.

Formation and evolution

*See also: Nebular hypothesis, Planetary migration, Formation*
and evolution of the Solar System and Future of the Earth

Planets form within a few tens of millions of years of their
star forming,888990 and there are
stars that are forming today and other stars that are ten billion years old, so
unlike the planets of the Solar System, which can only be observed as
they are today, studying exoplanets allows the observation of exoplanets at
different stages of evolution. When planets form they have hydrogen envelopes
that cool and contract over time and, depending on the mass of the planet, some
or all of the hydrogen is eventually lost to space. This means that even
terrestrial planets can start off with large radii.919293 An example is
Kepler-51b which has only about twice the mass of Earth but is almost the size
of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young
at a few hundred million years old.94

Planet-hosting stars

*Main article: Planetary system §*
Planet-hosting stars

There is at least one planet on average per star.9 About 1 in 5 Sun-like starsa have an
"Earth-sized"b planet in the habitable zone 11

The Morgan-Keenan
spectral classification

Most known exoplanets orbit stars roughly similar to the Sun,
that is, main-sequence stars of spectral categories F,
G, or K. Lower-mass stars (red dwarfs, of spectral category M)
are less likely to have planets massive enough to detect by the radial-velocity
method.9596 Although several
tens of planets around red dwarfs have been discovered by the Kepler spacecraft which uses the transit method which can detect smaller
planets.

Stars with a higher metallicity than the Sun are more likely
to have planets, especially giant planets, than stars with lower metallicity.97

Some planets orbit one member of a binary star system,98 and several circumbinary planets have
been discovered which orbit around both members of binary star. A few planets
in triple star systems are known99 and one in the
quadruple system Kepler 64.

Orbital parameters

Most known extrasolar planet candidates have been discovered
using indirect methods and therefore only some of their physical and orbital
parameters can be determined. For example, out of the six independent parametersthat define an orbit, the
radial-velocity method can determine four: semi-major axis, eccentricity, longitude of
periastron, and time of periastron. Two parameters remain unknown: inclination and longitude
of the ascending node.

**Distance from star,**
semi-major axis and orbital period

Diagram showing how a
planet and a star orbit their commoncenter of mass (red cross).

Scatterplot showing
masses and orbital radii (and period) of all extrasolar planets discovered
through September 2014, with colors indicating method of detection:

 timing
 direct imaging

For reference, Solar
System planets are marked as gray circles. The horizontal axis plots the log of
the semi-major axis, and the vertical axis plots the log of the mass.

There are exoplanets that are much closer to their parent star
than any planet in the Solar System is to the Sun, and there are also
exoplanets that are much further from their star.Mercury, the closest planet to the Sun at
0.4 astronomical units (AU),
takes 88 days for an orbit, but the smallest known orbits of exoplanets have
orbital periods of only a few hours, e.g.Kepler-70b. The Kepler-11 system has five of its planets
in smaller orbits than Mercury's. Neptune is 30 AU from the Sun and
takes 165 years to orbit it, but there are exoplanets that are thousands of AU
from their star and take tens of thousands of years to orbit, e.g. GU Piscium b.100

The orbit of a planet is not centered on the star but on their
common center of mass (see
diagram on right). For circular orbits, the semi-major axis is the distance between
the planet and the center of mass of the system. For elliptical orbits, the
planet–star distance varies over the course of the orbit, in which case the
semi-major axis is the average of the largest andsmallest distances between the planet and
the center of mass of the system. If the sizes of the star and planet are
relatively small compared to the size of the orbit and the orbit is nearly
circular and the center of mass is not too far from the star's center, such as
in the Earth–Sun system, then the distance from any point on the star to any
point on the planet is approximately the same as the semi-major axis. However,
when a star's radius expands when it turns into a red giant, then the distance between the
planet and the star's surface can become close to zero, or even less than zero
if the planet has been engulfed by the expanding red giant, whereas the center
of mass from which the semi-major axis is measured will still be near the
center of the red giant.

Orbital period is
the time taken to complete one orbit. For any given star, the shorter the semi-major
axis of a planet, the shorter the orbital period. Also comparing planets around
different stars but with the same semi-major axis, the more massive the star,
the shorter the orbital period.

Over the lifetime of a star, the semi-major axes
of its planets changes. This planetary migration happens
especially during the formation of
the planetary system when planets interact with the protoplanetary disk and
each other until a relatively stable position is reached, and later in the red-giant and asymptotic-giant-branch
phases when the star expands and engulfs the nearest planets
that can cause them to move inwards, and when the red giant loses mass as the
outer layers dissipate causing planets to move outwards as a result of the red
giant's reduced gravitational field.

The radial-velocity and transit methods are most sensitive to
planets with small orbits. The earliest discoveries such as 51 Peg b were gas giants with orbits of a few days.95 These "hot Jupiters" likely formed further out
and migrated inwards. The Kepler spacecraft has found planets with
even shorter orbits of only a few hours, which places them within the star's
upper atmosphere or corona, and these planets
are Earth-sized or smaller and are probably the left-over solid cores of giant
planets that have evaporated due to being so close to the star,101 or even being
engulfed by the star in its red-giant phase in the case of Kepler-70b. As well as evaporation, other
reasons why larger planets are unlikely to survive orbits only a few hours long
include orbital decay caused by tidal force, tidal-inflation instability, and Roche-lobe overflow.102 TheRoche limit implies that small planets
with orbits of a few hours are likely made mostly of iron.102

The direct imaging method
is most sensitive to planets with large orbits, and has discovered some planets
that have planet–star separations of hundreds of AU. However, protoplanetary
disks are usually only around 100 AU in radius, and core accretion models predict
giant planet formation to be within 10 AU, where the planets can coalesce
quickly enough before the disk evaporates. Very-long-period giant planets may
have been rogue planets that
were captured,103 or formed
close-in and gravitationally scattered outwards, or the planet and star could
be a mass-imbalanced wide binary system with the planet being the
primary object of its own separate protoplanetary disk.Gravitational
instability models might produce planets at multi-hundred AU
separations but this would require unusually large disks.104105 For planets with
very wide orbits up to several hundred thousand AU it may be difficult to
observationally determine whether the planet is gravitationally bound to the
star.

Most planets that have been discovered are within a couple of AU
from their host star because the most used methods (radial-velocity and
transit) require observation of several orbits to confirm that the planet
exists and there has only been enough time since these methods were first used
to cover small separations. Some planets with larger orbits have been
discovered by direct imaging but there is a middle range of distances, roughly
equivalent to the Solar System's gas giant region, which is largely unexplored.
Direct imaging equipment for exploring that region is being installed on the
world's largest telescopes and should begin operation in 2014. e.g. Gemini Planet Imager and VLT-SPHERE. The microlensing method
has detected a few planets in the 1–10 AU range.106 It appears
plausible that in most exoplanetary systems, there are one or two giant planets
with orbits comparable in size to those of Jupiter and Saturn in the Solar
System. Giant planets with substantially larger orbits are now known to be
rare, at least around Sun-like stars.107

The distance of the habitable zone from a star depends on the
type of star and this distance changes during the star's lifetime as the size
and temperature of the star changes.

·
The Fate of
Scattered Planets, Benjamin C. Bromley, Scott J. Kenyon, 10 October
2014

·
Planetary
Populations in the Mass-Period Diagram: A Statistical Treatment of Exoplanet
Formation and the Role of Planet Traps, Yasuhiro Hasegawa, Ralph E.
Pudritz, 8 October 2013

**Eccentricity**

The eccentricity of
an orbit is a measure of how elliptical (elongated) it is. All the planets of
the Solar System except for Mercury have near-circular orbits (e<0.1).108 Most exoplanets
with orbital periods of 20 days or less have near-circular orbits, i.e. very
low eccentricity. That is believed to be due to tidal circularization:
reduction of eccentricity over time due to gravitational interaction between
two bodies. The mostly sub-Neptune-sized planets found by the Kepler spacecraft
with short orbital periods have very circular orbits.55 By contrast, the
giant planets with longer orbital periods discovered by radial-velocity methods
have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have
eccentricities greater than 0.2, whereas 17% have eccentricities greater than
0.5.3) Moderate to high eccentricities
(e>0.2) of giant planets are *not* an observational selection
effect, because a planet can be detected about equally well regardless of the
eccentricity of its orbit. The prevalence of elliptical orbits for giant
planets is a major puzzle, because current theories of planetary formation strongly
suggest planets should form with circular (that is, non-eccentric) orbits.109

However, for weak Doppler signals
near the limits of the current detection ability the eccentricity becomes
poorly constrained and biased towards higher values. It is suggested that some
of the high eccentricities reported for low-mass exoplanets may be
overestimates, because simulations show that many observations are also
consistent with two planets on circular orbits. Reported observations of single
planets in moderately eccentric orbits have about a 15% chance of being a pair
of planets.110 This
misinterpretation is especially likely if the two planets orbit with a 2:1
resonance. With the exoplanet sample known in 2009, a group of astronomers has
concluded that "(1) around 35% of the published eccentric one-planet
solutions are statistically indistinguishable from planetary systems in 2:1
orbital resonance, (2) another 40% cannot be statistically distinguished from a
circular orbital solution" and "(3) planets with masses comparable to
Earth could be hidden in known orbital solutions of eccentric super-Earths and
Neptune mass planets".111

Radial velocity surveys found exoplanet orbits beyond 0.1 AU to
be eccentric, particularly for large planets. Kepler spacecraft transit data is
consistent with the RV surveys and also revealed that smaller planets tend to
have less eccentric orbits.112

**Inclination vs.**
spin–orbit angle

Orbital inclination is
the angle between a planet's orbital plane and another plane of
reference. For exoplanets the inclination is usually stated with respect to an
observer on Earth: the angle used is that between the normal to the planet's orbital plane and
the line of sight from Earth to the star. Therefore, most planets observed by
the transit method are
close to 90 degrees.113 Because the word
'inclination' is used in exoplanet studies for this line-of-sight inclination
then the angle between the planet's orbit and the star's rotation must use a
different word and is termed the spin–orbit angle or spin–orbit alignment. In
most cases the orientation of the star's rotational axis is unknown. The Kepler
spacecraft has found a few hundred multi-planet systems and in most of these
systems the planets all orbit in nearly the same plane, much like the Solar
System.55 However, a
combination of astrometric and radial-velocity measurements has shown that some
planetary systems contain planets whose orbital planes are significantly tilted
relative to each other.114 More than half
of hot Jupiters have orbital planes
substantially misaligned with their parent star's rotation. A substantial
fraction of hot-Jupiters even have retrograde orbits, meaning that they orbit in
the opposite direction from the star's rotation.115 Rather than a
planet's orbit having been disturbed, it may be that the star itself flipped
early in their system's formation due to interactions between the star's
magnetic field and the planet-forming disc.116

**Periastron precession**

Periastron precession is
the rotation of a planet's orbit within the orbital plane, i.e. the axes of the
ellipse change direction. Various factors cause the precession. In the Solar
System perturbations from other planets are the main cause, but for close-in
exoplanets the largest factor can be tidal forces between the star and planet.
For close-in exoplanets, the general relativistic contribution
to the precession is also significant and can be orders of magnitude larger
than the same
effect for Mercury. Some exoplanets have significantly eccentric orbits, which makes it easier to
detect the precession. The effect of general relativity can be detectable in
timescales of roughly 10 years or less.117

**Nodal precession**

Nodal precession is
rotation of a planet's orbital plane. This differs from periastron precession,
which is rotation of a planet's orbit within that plane. Nodal precession is
more easily seen as distinct from periastron precession when the orbital plane
is inclined to the star's rotation, the
extreme case being a polar orbit.

WASP-33 is a fast-rotating star that
hosts a hot Jupiter in
an almost polar orbit. The quadrupole mass moment and
the proper angular momentum of
the star are 1900 and 400 times, respectively, larger than those of the Sun.
This causes significant classical and relativistic deviations
from Kepler's laws. In
particular, the fast rotation causes large nodal precession because of the
star's oblateness and the Lense–Thirring
effect.118

Rotation and axial tilt

Plot of equatorial
spin velocity vs. mass for planets comparingBeta Pictoris b to the Solar System planets. (ESO/I. Snellen
(Leiden University)

In April 2014 the first measurement of a planet's rotation period was announced: the length
of day for the super-Jupiter gas
giant Beta Pictoris b is
8 hours (based on the assumption that the axial tilt of the planet is small.)119120121 With an
equatorial rotational velocity of 25 km per second, this is faster than
for the giant planets of the Solar System, in line with the expectation that
the more massive a giant planet, the faster it spins. Beta Pictoris b's
distance from its star is 9AU. At such distances the rotation of Jovian planets is not slowed by tidal
effects.122 Beta Pictoris b
is still warm and young and over the next hundreds of millions of years, it
will cool down and shrink to about the size of Jupiter, and if its angular momentum is preserved then as it
shrinks the length of its day will decrease to about 3 hours and its equatorial
rotation velocity will speed up to about 40 km per second.120 The images of
Beta Pictoris b do not have high enough resolution to directly see details but doppler spectroscopy techniques
were used to show that different parts of the planet were moving at different
speeds and in opposite directions from which it was inferred that the planet is
rotating.119 With the next
generation of large
ground-based telescopes it will be possible to use doppler
imaging techniques to make a global map of the planet, like the recent mapping
of the brown dwarf Luhman 16B.123124

**Origin of spin and tilt**
of terrestrial planets

Giant impacts have
a large effect on the spin of terrestrial planets.
The last few giant impacts during planetary formation tend
to be the main determiner of a terrestrial planet's rotation rate. On average
the spin angular velocity will
be about 70% of the velocity that would cause the planet to break up and fly
apart; the natural outcome of planetary embryo impacts at speeds
slightly larger than escape velocity. In later stages terrestrial
planet spin is also affected by impacts with planetesimals. During the giant impact stage,
the thickness of a protoplanetary disk is
far larger than the size of planetary embryos so collisions are equally likely
to come from any direction in three-dimensions. This results in the axial tilt of accreted planets ranging
from 0 to 180 degrees with any direction as likely as any other with both prograde and
retrograde spins equally probable. Therefore, prograde spin
with a small axial tilt, common for the Solar System's terrestrial planets
except Venus, is not common in general for terrestrial planets built by giant
impacts. The initial axial tilt of a planet determined by giant impacts can be
substantially changed by stellar tides if the planet is close to its star and
by satellite tides if the planet has a large satellite.125

**Tidal effects**

For most planets the rotation period and axial tilt (also called obliquity) are
not known, but a large number of planets have been detected with very short
orbits (where tidal effects are greater) and will probably have reached an equilibrium rotation
that can be predicted.

Tidal effects are the result of forces acting on a body
differing from one part of the body to another.122 For example, the
gravitational effect of a star varies with distance from one side of a planet
to another. Also heat from a star creates a temperature gradient between the day
and nightsides which is another source of tides. For example, on Earth, air
pressure variations on the ground are affected more by temperature differences
than gravitational ones.

Tides modify the rotation and orbit of planets until an
equilibrium is reached. Whenever the rotation rate is slowed, there is an
increase of the orbit semi-major axis due to the conservation of angular
momentum. Most of the large moons in the Solar System, including the Moon,
are tidally locked to
their host planet; the same side of the moon is always facing the planet. This
means the moons' rotation periods are synchronous with their orbital period.
However, when an orbit is eccentric,
as is the case with many exoplanets' orbits of their host stars, there are
equilibrium states such as spin–orbit resonances that
are far more likely than synchronous rotation. A spin–orbit resonance is when
the rotation period and the orbital period are in an integer ratio –
this is called a commensurability. Non-resonant equilibriums such as the retrograde
rotation of Venus can also occur when both gravitational and
thermal atmospheric tides are both significant.

A synchronous tidal lock isn't necessarily particularly slow –
there are planets with orbits that take only a few hours.

Gravitational tides tend to reduce the axial tilt to zero but
over a longer time-scale than the rotation rate reaches equilibrium. However,
the presence of multiple planets in a system can cause axial tilt to be
captured in a resonance called a Cassini state. There are small oscillations
around this state and in the case of Mars these
axial tilt variations are chaotic.

Hot Jupiters' close
proximity to their host star means that their spin–orbit evolution is mostly
due to the star's gravity and not the other effects. Hot Jupiters rotation rate
is not thought to be captured into spin–orbit resonance due to way fluid-body
reacts to tides, and therefore slows down to synchronous rotation if it is on a
circular orbit or slows to a non-synchronous rotation if on an eccentric orbit.
Hot Jupiters are likely to evolve towards zero axial tilt even if they had been
in a Cassini state during planetary migration when they were further from their
star. Hot Jupiters' orbits will become more circular over time, however the
presence of other planets in the system on eccentric orbits, even ones as small
as Earth and as far away as the habitable zone, can continue to maintain the
eccentricity of the Hot Jupiter so that the length of time for tidal circularization can
be billions instead of millions of years.

The rotation rate of planet HD 80606 b is predicted to be about 1.9
days. HD 80606 b avoids spin–orbit resonance because it is a gas giant. The
eccentricity of its orbit means that it avoids becoming tidally locked.

Physical parameters

*See also: Planet § Physical characteristics*

**Mass**

When a planet is found by the radial-velocity
method, its orbital inclination *i* is
unknown and can range from 0 to 90 degrees. The method is unable to determine
the true mass (*M*) of the planet, but
rather gives a lower limit for its mass, *M* sin*i*.
In a few cases an apparent exoplanet may be a more massive object such as a
brown dwarf or red dwarf. However, the probability of a small value of i (say
less than 30 degrees, which would give a true mass at least double the observed
lower limit) is relatively low (1−(√3)/2 ≈ 13%) and hence most planets will
have true masses fairly close to the observed lower limit.95

If a planet's orbit is nearly perpendicular to the line of
vision (i.e. *i* close to 90°), a planet can be detected through
the transit method.
The inclination will then be known, and the inclination combined with *M* sin*i* from
radial-velocity observations will give the planet's true mass.

Also, astrometric observations
and dynamical considerations in multiple-planet systems can sometimes provide
an upper limit to the planet's true mass.

The mass of a transiting exoplanet can also be determined from
the transmission spectrum of its atmosphere, as it can be used to constrain
independently the atmospheric composition, temperature, pressure, and scale height.126

Transit-timing
variation can also be used to find planets' masses.127

**Radius, density and bulk**
composition

Prior to recent results from the Kepler spacecraft most
confirmed planets were gas giants comparable in size to Jupiter or larger
because they are most easily detected. However, the planets detected by Kepler
are mostly between the size of Neptune and the size of Earth.55

If a planet is detectable by both the radial-velocity and the
transit methods, then both its true mass and its radius can be found. The
planet's density can then be calculated. Planets
with low density are inferred to be composed mainly of hydrogen and helium,
whereas planets of intermediate density are inferred to have water as a major
constituent. A planet of high density is inferred to be rocky, like Earth and
the other terrestrial planets of the Solar System.

Sizes of *Kepler* Planet
Candidates – based on 2,740 candidates orbiting 2,036 stars as of 4 November
2013 (NASA).

Comparison of sizes of planets with different compositions.

**Gas giants, puffy planets, and super-Jupiters**

Size comparison of WASP-17b (right) withJupiter (left).

*Main articles: Gas giant, Puffy planet and Super-Jupiter*

Gaseous planets that are hot because they are close to their
star or because they are still hot from their formation are expanded by the
heat. For colder gas planets there is a maximum radius which is slightly larger
than Jupiter which occurs when the mass reaches a few Jupiter-masses. Adding
mass beyond this point causes the radius to shrink.32128129

Even when taking heating from the star into account, many
transiting exoplanets are much larger than expected given their mass, meaning
that they have surprisingly low density.130 See the magnetic field section for one possible
explanation.

Plots of exoplanet density andradius.e Top: Density vs. Radius. Bottom: Diffusity=1/Density vs.
Radius. Units: Radius in RJup. Density in g/cm3. Diffusity in cm3/g. These plots show
that there are a wide range of densities for planets between Earth and Neptune
size, then the planets of 0.6RJup size are very low-density and there are very
few of them, then the gas giants have a large range of densities.

Besides those inflated hot Jupiters there is another type of
low-density planet: occurring at around 0.6 times the size of Jupiter where
there are very few planets. The planets around Kepler-5194 are far less
dense (far more diffuse) than the inflated hot Jupiters as can be seen in the
plots on the right where the three Kepler-51 planets stand out in the diffusity
vs. radius plot. A more detailed study taking into account star spots may modify these results to
produce less extreme values.94

**Ice giants and super-Neptunes**

*Main article: Ice giant*

Kepler-101b is
the first super-Neptune planet.
It has three times Neptune's mass but a Neptune-like composition with more than
60% heavy elements unlike hydrogen/helium-dominated gas giants.131

**Super-Earths, mini-Neptunes, and gas dwarfs**

*Main articles: Super-Earth, Mini-Neptune, Helium planet and Gas dwarf*

If a planet has a radius and/or mass between that of Earth and
Neptune then there is a question about whether the planet is rocky like Earth,
a mixture of volatiles and gas like Neptune, a small planet with a
hydrogen/helium envelope (mini-Jupiter), or of some other composition.

Some of the Kepler transiting planets with radii in the range
1–4 Earth radii have had their masses measured by radial-velocity or
transit-timing methods. The calculated densities show that up to 1.5 Earth
radii, these planets are rocky and that density increases with increasing
radius due to gravitational compression. However, between 1.5 and 4 Earth radii
the density decreases with increasing radius. This indicates that above 1.5
Earth radii planets tend to have increasing amounts of volatiles and gas.
Despite this general trend there is a wide range of masses at a given radius,
which could be because gas planets can have rocky cores of different masses or
compositions132 and could also
be due to photoevaporation of
volatiles.133 Thermal
evolutionary atmosphere models suggest a radius of 1.75 times that of Earth as
a dividing line between rocky and gaseous planets.134 Excluding
close-in planets that have lost their gas envelope due to stellar irradiation,
studies of the metallicity of
stars suggest a dividing line of 1.7 Earth radii between rocky planets and gas
dwarfs; then another dividing line at 3.9 Earth radii between gas dwarfs and
gas giants. These dividing lines are statistical trends and do not necessarily
apply to specific planets because there are many other factors besides
metallicity that affect planet formation, including distance from star – there
may be larger rocky planets formed at larger distances.135 An independent
reanalysis of the data suggests that there are no such dividing lines and that
there is a continuum of planet formation between 1 and 4 Earth radii and no
reason to expect that the amount of solid material in a protoplanetary disk
determines whether super-Earths or mini-Neptunes are formed.136

The discovery of the low-density Earth-mass planet Kepler-138d shows that there is an
overlapping range of *masses* in which both rocky planets and
low-density planets occur.137Low-mass low-density
planets could be ocean planets or super-Earths with a remnant hydrogen
atmosphere, or hot planets with a steam atmosphere, or mini-Neptunes with a
hydrogen-helium atmosphere.138 Other
possibilities for low-mass low-density planets are large atmospheres of carbon monoxide, carbon dioxide, methane, or nitrogen.139

**Massive solid planets and giant planets with**
massive cores

Size comparison of Kepler-10c with Earth and Neptune

In 2014, new measurements of Kepler-10c found that it is a
Neptune-mass planet (17 Earth masses) with a density higher than Earth's,
indicating that Kepler-10c is made mostly of rock with possibly up to 20% high-pressure
water ice but without a hydrogen-dominated envelope. Because this is well above
the 10-Earth-mass upper limit that is commonly used for the term 'super-Earth',
the term **mega-Earth** has
been coined.140141 A similarly
massive and dense planet could be Kepler-131b,
although its density is not as well measured as that of Kepler 10c. The next
most massive known solid planets are half this mass: 55 Cancri e and Kepler-20b.142

Gas planets can also have large solid cores: the Saturn-mass
planet HD 149026 b has
only two-thirds of Saturn's radius, so it may have a rock–ice core of 60 Earth
masses or more.32 Corot-20b has
4.24 times Jupiter's mass but a radius of only 0.84 that of Jupiter—it may have
a metal core of 800 Earth masses if the heavy elements are concentrated in the
core or a core of 300 Earth masses if the heavy elements are more distributed
throughout the planet.143144

Transit-timing
variation measurements indicate that Kepler-52b, Kepler-52c and
Kepler-57b have maximum-masses between 30 and 100 times the mass of Earth,
although the actual masses could be much lower. With radii about 2 Earth radii
in size, they might have densities larger than that of an iron planet of the same size. They orbit
very close to their stars, so they could be the remnant cores (chthonian planets) of evaporated gas giants or brown dwarfs. If cores are massive enough they
could remain compressed for billions of years despite losing the atmospheric
mass.145146

Solid planets up to thousands of Earth masses may be able to
form around massive stars (B-type and O-type stars;
5–120 solar masses), where the protoplanetary disk would
contain enough heavy elements. Also, these stars have high UV radiation and winds that could photoevaporate the gas in the disk,
leaving just the heavy elements.147 For comparison,
Neptune's mass equals 17 Earth masses, Jupiter has 318 Earth masses, and the 13
Jupiter-mass limit used in the IAU's working definition of an
exoplanet equals approximately 4000 Earth masses.147

Another way of forming massive solid planets is when a white dwarf in a close binary system
loses material to a companion neutron star. The white dwarf can be reduced
to planetary-mass, leaving just its crystallised carbon–oxygen core. A likely
example of this is PSR J1719-1438 b.

Cold planets have a maximum radius because adding more mass at
that point causes the planet to compress under the weight instead of increasing
the radius. The maximum radius for solid planets is smaller than the maximum
radius for gas planets.147

**Shape**

When the size of a planet is described using its radius this is
approximating the shape by a sphere. However, the
rotation of a planet causes it to be flattened at the poles so that the
equatorial radius is larger than the polar radius, making it closer to an oblate spheroid. The oblateness of transiting exoplanets will affect the
transit light curves. At the limits of current technology it has been possible
to show that HD 189733b is
less oblate thanSaturn.148 If the planet is
close to its star, then gravitational tides will elongate the planet in the
direction of the star, so that the planet will be closer to a triaxial ellipsoid.149 Because tidal
deformation is along a line between the planet and the star, it is difficult to
detect from transit photometry—it will have an order of magnitude less effect
on the transit light curves than that caused by rotational deformation even in
cases where the tidal deformation is larger than rotational deformation (such as
is the case for tidally locked hot Jupiters).148 Material
rigidity of rocky planets and rocky cores of gas planets will cause further
deviations from the aforementioned shapes.148 Thermal tides
caused by unevenly irradiated surfaces are another factor.150

Atmosphere

Sunset studies on Titan by*Cassini* help
understand exoplanetatmospheres (artist's
concept).

As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres
have been observed,151 resulting in
detection of molecular spectral
features; observation of day–night temperature gradients; and constraints on
vertical atmospheric structure.152 Also, an
atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.153154

Spectroscopic measurements
can be used to study a transiting planet's atmospheric composition,155 temperature,
pressure, and scale height, and
hence can be used to determine its mass.126

Stellar light is polarized by atmospheric molecules; this could
be detected with a polarimeter. HD 189733 b has been studied by polarimetry.

Extrasolar planets have phases similar to the phases of the Moon.
By observing the exact variation
of brightness with phase, astronomers can calculate
atmospheric-particle sizes.

**Atmospheric composition**

In 2001 sodium was detected in
the atmosphere of HD 209458 b.156

In 2008 water, carbon monoxide, carbon dioxide157 and methane158 were detected in
the atmosphere of HD 189733 b.

In 2013 water was detected in
the atmospheres of HD 209458 b, XO-1b, WASP-12b, WASP-17b, and WASP-19b.159160161

In July 2014, NASA announced finding very dry atmospheres on three exoplanets (HD 189733b, HD 209458b, WASP-12b) orbiting Sun-like stars.162

In September 2014, NASA reported that HAT-P-11b is the first Neptune-sized
exoplanet known to have a relatively cloud-free atmosphere and, as well, the
first time molecules of any
kind have been found, specifically water vapor, on such a relatively small
exoplanet.163

The presence of oxygen may be detectable by ground-based
telescopes,164 which, if
discovered, would suggest the presence of life on an exoplanet.

In June 2015, NASA reported that WASP-33b has a stratosphere. Ozone and hydrocarbons absorb large amounts of
ultraviolet radiation, heating the upper parts of atmosphere's that contain
them, creating a temperature inversion and
a stratosphere. However, these molecules are destroyed at the temperatures of
hot exoplanets, creating doubt if the hot exoplanets could have a stratosphere.
A temperature inversion, and stratosphere was identified on WASP-33b caused by titanium oxide, which is a strong absorber of
visible and ultraviolet radiation, and can only exist as a gas in a hot
atmosphere. WASP-33b is the hottest exoplanet known, with a temperature of
3,200 °C (5,790 °F)165 and is
approximately four and a half times the mass of Jupiter.166167

**Atmospheric circulation**

The atmospheric circulation of planets that rotate more slowly
or have a thicker atmosphere allows more heat to flow to the poles which
reduces the temperature differences between the poles and the equator.168

**Clouds**

In October 2013, the detection of clouds in the atmosphere of Kepler-7b was announced,169170 and, in December
2013, also in the atmospheres of GJ 436 b and GJ 1214 b.171172173174

**Precipitation**

Precipitation in
the form of liquid (rain) or solid (snow) varies in composition depending on
atmospheric temperature, pressure, composition, and altitude. Hot atmospheres could have iron
rain,175 molten-glass
rain,176 and rain made
from rocky minerals such as enstatite, corundum, spinel, and wollastonite.177 Deep in the
atmospheres of gas giants it could rain diamonds178 and helium
containing dissolved neon.179

**Abiotic oxygen**

*There are non-biological processes that produce oxygen, so the*
detection of oxygen is not necessarily an indication of life.

The processes of life result in a mixture of chemicals that are
not in chemical equilibrium but
there are also abiotic disequilibrium processes that need to be considered. The
most robust atmospheric biosignature is often considered to be
molecular oxygen O2 and its photochemical byproduct ozone O3. The photolysis of water H2O by UV rays followed by hydrodynamic escape of
hydrogen can lead to a build-up of oxygen in planets close to their star
undergoing runaway
greenhouse effect. For planets in the habitable zone it was believed that water
photolysis would be strongly limited by cold-trapping of
water vapour in the lower atmosphere. However, the extent of H2O cold-trapping
depends strongly on the amount of non-condensible gases in the atmosphere such
as nitrogen N2 and argon.
In the absence of such gases the likelihood of build-up of oxygen also depends in
complex ways on the planet's accretion history, internal chemistry, atmospheric
dynamics and orbital state. Therefore, oxygen on its own cannot be considered a
robust biosignature.180 The ratio of
nitrogen and argon to oxygen could be detected by studying thermal phase curves181 or by transit transmission spectroscopy
measurement of the spectral Rayleigh scattering slope
in a clear-sky (i.e. aerosol-free) atmosphere.182

Surface

**Surface composition**

Surface features can be distinguished from atmospheric features
by comparing emission and reflection spectroscopy with transmission
spectroscopy. Mid-infrared spectroscopy of exoplanets may detect
rocky surfaces, and near-infrared may identify magma oceans or high-temperature
lavas, hydrated silicate surfaces and water ice, giving an unambiguous method
to distinguish between rocky and gaseous exoplanets.183

**Surface temperature**

*See also: § Habitability*

Artist’s illustration
of temperature inversion in exoplanet’s atmosphere.184

One can estimate the temperature of an exoplanet based on the
intensity of the light it receives from its parent star. For example, the
planet OGLE-2005-BLG-390Lb is
estimated to have a surface temperature of roughly −220 °C (50 K).
However, such estimates may be substantially in error because they depend on
the planet's usually unknown albedo, and because factors
such as the greenhouse effect may introduce unknown complications. A few
planets have had their temperature measured by observing the variation in
infrared radiation as the planet moves around in its orbit and is eclipsed by
its parent star. For example, the planet HD 189733b has been found to have an
average temperature of 1205±9 K (932±9 °C) on its dayside and 973±33 K
(700±33 °C) on its nightside.185

General features

**Color and brightness**

*See also: Sudarsky's*
gas giant classification

This color–color diagram compares
the colors of planets in the Solar System to exoplanet HD 189733b. The exoplanet's deep blue color is
produced by silicate droplets,
which scatter blue light in its atmosphere.

In 2013 the color of an exoplanet was found for the first time.
The best-fit albedo measurements of HD 189733b suggest that it is deep dark
blue.186187

Visually, GJ 504 b would
have a magenta color.188

Kappa Andromedae b if
seen up close, would appear reddish in color.189

The apparent brightness (apparent magnitude)
of a planet depends on how far away the observer is, how reflective the planet
is (albedo), and how much light the planet
receives from its star, which depends on how far the planet is from the star
and how bright the star is. So, a planet with a low albedo that is close to its
star can appear brighter than a planet with high albedo that is far from the
star.190

The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of
the light from its star, making it less reflective than coal or black acrylic
paint. Hot Jupiters are expected to be quite dark due to sodium and potassium
in their atmospheres but it is not known why TrES-2b is so dark—it could be due
to an unknown chemical.191192193

For gas giants, geometric
albedo generally decreases with increasing metallicity or atmospheric
temperature unless there are clouds to modify this effect. Increased
cloud-column depth increases the albedo at optical wavelengths, but decreases
it at some infrared wavelengths. Optical albedo increases with age, because
older planets have higher cloud-column depths. Optical albedo decreases with increasing
mass, because higher-mass giant planets have higher surface gravities, which
produces lower cloud-column depths. Also, elliptical orbits can cause major
fluctuations in atmospheric composition, which can have a significant effect.194

There is more thermal emission than reflection at some
near-infrared wavelengths for massive and/or young gas giants. So, although
optical brightness is fully phase-dependent, this is not always the case
in the near infrared.194

Temperatures of gas giants reduce over time and with distance
from their star. Lowering the temperature increases optical albedo even without
clouds. At a sufficiently low temperature, water clouds form, which further
increase optical albedo. At even lower temperatures ammonia clouds form,
resulting in the highest albedos at most optical and near-infrared wavelengths.194

**Magnetic field**

In 2014, a magnetic field around HD 209458 b was inferred from the way
hydrogen was evaporating from the planet. It is the first (indirect) detection
of a magnetic field on an exoplanet. The magnetic field is estimated to be
about one tenth as strong as Jupiter's.195196

Interaction between a close-in planet's magnetic field and a
star can produce spots on the star in a similar way to how the Galilean moons produce aurorae on
Jupiter.197 Auroral radio emissions
could be detected with radio telescopes such as LOFAR.198199 The radio
emissions could enable determination of the rotation rate of a planet which is
difficult to detect otherwise.200

Earth's magnetic field results from its flowing liquid metallic
core, but in super-Earths the mass can produce high pressures with large
viscosities and high melting temperatures which could prevent the interiors
from separating into different layers and so result in undifferentiated
coreless mantles. Magnesium oxide, which is rocky on Earth, can be a liquid
metal at the pressures and temperatures found in super-Earths and could
generate a magnetic field in the mantles of super-Earths.201

Hot Jupiters have
been observed to have a larger radius than expected. This could be caused by
the interaction between the stellar wind and the planet's
magnetosphere creating an electric current through the planet that heats it
up causing it to expand. The more magnetically active a star is the greater the
stellar wind and the larger the electric current leading to more heating and
expansion of the planet. This theory matches the observation that stellar
activity is correlated with inflated planetary radii.202

**Plate tectonics**

In 2007 two independent teams of researchers came to opposing
conclusions about the likelihood of plate tectonics on larger super-earths203204 with one team
saying that plate tectonics would be episodic or stagnant205and the other team
saying that plate tectonics is very likely on super-earths even if the planet
is dry.206

If super-earths have more than 80 times as much water as Earth
then they become ocean planets with
all land completely submerged. However, if there is less water than this limit,
then the deep water cycle will move enough water between the oceans and mantle
to allow continents to exist.207208

**Volcanism**

Large surface temperature variations on 55 Cancri e have been attributed to
possible volcanic activity releasing large clouds of dust which blanket the
planet and block thermal emissions.209210

**Rings**

The star 1SWASP
J140747.93-394542.6 is orbited by an object that is circled by
a ring system much larger than Saturn's rings. However, the mass of the
object is not known; it could be a brown dwarf or low-mass star instead of a
planet.211212

The brightness of optical images of Fomalhaut b could be due to starlight
reflecting off a circumplanetary ring system with a radius between 20 to 40
times that of Jupiter's radius, about the size of the orbits of the Galilean moons.213

The rings of the Solar System's gas giants are aligned with
their planet's equator. However, for exoplanets that orbit close to their star,
tidal forces from the star would lead to the outermost rings of a planet being
aligned with the planet's orbital plane around the star. A planet's innermost
rings would still be aligned with the planet's equator so that if the planet
has a tilted rotational axis, then the different
alignments between the inner and outer rings would create a warped ring system.214

**Moons**

In December 2013 a candidate exomoon of a rogue planet was announced.215 No exomoons have
been confirmed so far.

**Comet-like tails**

KIC 12557548 b is
a small rocky planet, very close to its star, that is evaporating and leaving a
trailing tail of cloud and dust like a comet.216 The dust could
be ash erupting from volcanos and escaping due to the small planet's low
surface-gravity, or it could be from metals that are vaporized by the high
temperatures of being so close to the star with the metal vapor then condensing
into dust.217

In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a
giant cloud around the planet and, due to radiation from the host star, a long
trailing tail 14×106 km (9×106 mi) long.218

**Insolation pattern**

Tidally locked planets
in a 1:1 spin-orbit resonance would
have their star always shining directly overhead on one spot which would be hot
with the opposite hemisphere receiving no light and being freezing cold. Such a
planet could resemble an eyeball with the hotspot being the pupil.219 Planets with an eccentric orbit could
be locked in other resonances. 3:2 and 5:2 resonances would result in a
double-eyeball pattern with hotspots in both eastern and western hemispheres.220 Planets with
both an eccentric orbit and a tilted axis of rotation would have more
complicated insolation patterns.221

Habitability

*Main articles: Planetary*
habitability, Astrobiology and Drake equation

**Habitable zone**

The habitable zone around
a star is the region where the temperature is just right to allow liquid water
to exist on a planet; that is, not too close to the star for the water to
evaporate and not too far away from the star for the water to freeze. The heat
produced by stars varies depending on the size and age of the star so that the
habitable zone can be at different distances. Also, the atmospheric conditions
on the planet influence the planet's ability to retain heat so that the
location of the habitable zone is also specific to each type of planet: desert planets (also known as dry
planets), with very little water, will have less water vapor in the atmosphere
than Earth and so have a reduced greenhouse effect, meaning that a desert
planet could maintain oases of water closer to its star than Earth is to the
Sun. The lack of water also means there is less ice to reflect heat into space,
so the outer edge of desert-planet habitable zones is further out.222223 Rocky planets
with a thick hydrogen atmosphere
could maintain surface water much further out than the Earth–Sun distance.224 Planets with
larger mass have wider habitable zones because the gravity reduces the water
cloud column depth which reduces the greenhouse effect of water vapor thus
moving the inner edge of the HZ closer to the star.225

Planetary rotation rate is one of the major factors
determining the circulation of the atmosphere and hence
the pattern of clouds: slowly rotating planets create thick clouds that reflect more and so can be habitable much
closer to their star. Earth with its current atmosphere would be habitable in
Venus's orbit, if it had Venus's slow rotation, so Venus must have had a higher
rotation rate in the past if it lost its water ocean as a result of going
through arunaway
greenhouse effect, but if Venus never had an ocean because water
vapor was lost to space during its formation before it could cool to form an
ocean,226 Venus could have
had its slow rotation throughout its history.227

Tidally locked planets (a.k.a. "eyeball" planets228) can be habitable
closer to their star than previously thought due to the effect of clouds: at
high stellar flux, strong convection produces thick water clouds near the
substellar point that greatly increase the planetary albedo and reduce surface
temperatures.229

Habitable zones have usually been defined in terms of surface
temperature, however over half of Earth's biomass is from subsurface microbes,230 and the
temperature increases as you go deeper underground, so the subsurface can be
conducive for life when the surface is frozen and if this is considered, the
habitable zone extends much further from the star,231 even rogue planets could have liquid water at
sufficient depths underground.232 In an earlier
era of the universe the
temperature of the cosmic
microwave background would have allowed any rocky planets that
existed to have liquid water on their surface regardless of their distance from
a star.233 Jupiter-like
planets might not be habitable, but they could have habitable moons.234

**Ice ages and snowball**
states

*See also: Ice age and Snowball Earth*

The outer edge of the habitable zone is where planets will be
completely frozen but even planets well inside the habitable zone can
periodically become frozen. If orbital fluctuations or other causes produce
cooling then this creates more ice but ice reflects sunlight causing even more
cooling creating a feedback loop until the planet is completely or nearly
completely frozen. When the surface is frozen this stops carbon dioxide weathering resulting
in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This
creates a greenhouse effect which unfreezes the planet again. Planets with a
large axial tilt235 are less likely
to enter snowball states and can retain liquid water further from their star.
Large fluctuations of axial tilt can have even more of a warming effect than a
fixed large tilt.236237 Paradoxically
planets around cooler stars, such as red dwarfs, are less likely to enter
snowball states because the infrared radiation emitted by cooler stars is
mostly at wavelengths that are absorbed by ice which heats it up.238239

**Tidal heating**

If a planet has an eccentric orbit then tidal heating can provide another source
of energy besides stellar irradiation. This means that eccentric planets in the
radiative habitable zone can be too hot for liquid water (Tidal
Venus). Tides also circularize orbits
over time so there could be planets in the habitable zone with circular orbits
that have no water because they used to have eccentric orbits.240 Eccentric
planets further out than the radiative habitable zone would still have frozen
surfaces but the tidal heating could create a subsurface ocean similar to Europa's.241 In some
planetary systems, such as in the Upsilon Andromedae system,
the eccentricity of orbits is maintained or even periodically varied by
perturbations from other planets in the system. Tidal heating can cause
outgassing from the mantle, contributing to the formation and replenishment of
an atmosphere.242

**Potentially habitable**
planets

*See also: List*
of potentially habitable exoplanets and List of nearest terrestrial exoplanet candidates

A review in 2015 came to the conclusion that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best
candidates for being potentially habitable.243 These are at a
distance of 1200, 490 and 1,120 light-years away, respectively. Of these,
Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and
it is located towards the outer edge of the habitable zone around its red dwarf sun.

When looking at the nearest terrestrial exoplanet candidates, Tau Ceti e is merely 11.9 light-years
away. Its average surface temperature is estimated to be 68 °C
(154 °F).244

**Earth-size planets**

*See also: Earth analog*

·
In November 2013 it
was announced that 22±8% of Sun-likea stars have an
Earth-sizedb planet in the
habitablec zone.1011 Assuming 200
billion stars in the Milky Way,d that would be 11
billion potentially habitable Earths, rising to 40 billion if red dwarfs are included.12

·
Kepler-186f, a 1.2-Earth-radius planet in the
habitable zone of a red dwarf, announced
in April 2014.

·
In February 2013,
researchers calculated that up to 6% of small red dwarfs may have planets with
Earth-like properties. This suggests that the closest "alien Earth"
to the Solar System could be 13 light-years away. The estimated distance
increases to 21 light-years when a 95 percent confidence interval is
used.245 In March 2013 a
revised estimate based on a more accurate consideration of the size of the
habitable zone around red dwarfs gave an occurrence rate of 50% for Earth-size
planets in the habitable zone of red dwarfs.246

·
At 1.63 times Earth's
radius Kepler-452b is
the first discovered near-Earth-size planet in the “habitable
zone” around a G2-type Sun-like star (July 23, 2015).247

Cultural impact

On 9 May 2013, a congressional hearing by two United
States House of Representatives subcommittees discussed
"Exoplanet Discoveries: Have We Found Other Earths?",
prompted by the discovery of exoplanetKepler-62f, along with Kepler-62e and Kepler-62c. A related special issue of the
journal *Science*,

published earlier, described the discovery of the exoplanets.248

* List of exoplanets
* Exocomet
* Exomoon
* Extragalactic planet
* List of exoplanet extremes
* List of exoplanet research projects
* Nexus for Exoplanet System Science
* Planetary system
* List of planets

## Notes

1. ^ Jump up to:***a*** ***b*** ***c*** For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
2. ^ Jump up to:***a*** ***b*** ***c*** For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
3. ^ Jump up to:***a*** ***b*** For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
4. ^ Jump up to:***a*** ***b*** About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.
5. **Jump up^** Data from NASA catalog July 2014, excluding objects described as having unphysically high density

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117. **Jump up^** Observability of the General Relativistic Precession of Periastra in Exoplanets, Andres Jordan, Gaspar A. Bakos, 3 June 2008
118. **Jump up^** Classical and relativistic node precessional effects in WASP-33b and perspectives for detecting them, Lorenzo Iorio, 25 August 2010
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208. **Jump up^** Cowan, N. B.; Abbot, D. S. (2014). "Water Cycling Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds". *The Astrophysical Journal* **781**: 27. arXiv:1401.0720. Bibcode:2014ApJ...781...27C.doi:10.1088/0004-637X/781/1/27.
209. **Jump up^** Astronomers May Have Found Volcanoes 40 Light-Years From Earth
210. **Jump up^** Variability in the super-Earth 55 Cnc e, Brice-Olivier Demory, Michael Gillon, Nikku Madhusudhan, Didier Queloz, (Submitted on 1 May 2015)
211. **Jump up^** Scientists Discover a Saturn-like Ring System Eclipsing a Sun-like Star, Space Daily, 13 January 2012
212. **Jump up^** Mamajek, E. E.; Quillen, A. C.; Pecaut, M. J.; Moolekamp, F.; Scott, E. L.; Kenworthy, M. A.; Cameron, A. C.; Parley, N. R. (2012). "Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-Like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks". *The Astronomical Journal* **143** (3): 72.arXiv:1108.4070. Bibcode:2012AJ....143...72M. doi:10.1088/0004-6256/143/3/72.
213. **Jump up^** Kalas, P.; Graham, J. R.; Chiang, E.; Fitzgerald, M. P.; Clampin, M.; Kite, E. S.; Stapelfeldt, K.; Marois, C.; Krist, J. (2008). "Optical Images of an Exosolar Planet 25 Light-Years from Earth". *Science* **322** (5906): 1345–8. arXiv:0811.1994. Bibcode:2008Sci...322.1345K.doi:10.1126/science.1166609. PMID 19008414.
214. **Jump up^** Schlichting, Hilke E. and Chang, Philip (19 Apr 2011) Warm Saturns: On the Nature of Rings around Extrasolar Planets that Reside Inside the Ice Line. arxiv.org
215. **Jump up^** Bennett, D. P.; Batista, V.; Bond, I. A.; Bennett, C. S.; Suzuki, D.; Beaulieu, J. -P.; Udalski, A.; Donatowicz, J.; Bozza, V.; Abe, F.; Botzler, C. S.; Freeman, M.; Fukunaga, D.; Fukui, A.; Itow, Y.; Koshimoto, N.; Ling, C. H.; Masuda, K.; Matsubara, Y.; Muraki, Y.; Namba, S.; Ohnishi, K.; Rattenbury, N. J.; Saito, T.; Sullivan, D. J.; Sumi, T.; Sweatman, W. L.; Tristram, P. J.; Tsurumi, N.; et al. (2014). "MOA-2011-BLG-262Lb: A sub-Earth-mass moon orbiting a gas giant or a high-velocity planetary system in the galactic bulge". *The Astrophysical Journal* **785** (2): 155. arXiv:1312.3951.Bibcode:2014ApJ...785..155B. doi:10.1088/0004-637X/785/2/155.
216. **Jump up^** Evaporating exoplanet stirs up dust. Phys.org. 28 August 2012
217. **Jump up^** Woollacott, Emma (18 May 2012) New-found exoplanet is evaporating away. *TG Daily*
218. **Jump up^** Bhanoo, Sindya N. (25 June 2015). "A Planet with a Tail Nine Million Miles Long". *New York Times*. Retrieved 25 June 2015.
219. **Jump up^** Forget "Earth-Like"—We’ll First Find Aliens on Eyeball Planets, Nautilus, POSTED BY SEAN RAYMOND ON FEB 20, 2015
220. **Jump up^** Insolation patterns on eccentric exoplanets, Anthony R. Dobrovolskis, Icarus, Volume 250, April 2015, Pages 395–399
221. **Jump up^** Patterns of Sunlight on Extra-Solar Planets, Tony Dobrovolskis, 18 March 2014
222. **Jump up^** Choi, Charles Q. (1 September 2011) Alien Life More Likely on 'Dune' Planets. *Astrobiology Magazine*
223. **Jump up^** Abe, Y.; Abe-Ouchi, A.; Sleep, N. H.; Zahnle, K. J. (2011). "Habitable Zone Limits for Dry Planets". *Astrobiology* **11** (5): 443–460.Bibcode:2011AsBio..11..443A. doi:10.1089/ast.2010.0545.PMID 21707386.
224. **Jump up^** Seager, S. (2013). "Exoplanet Habitability". *Science* **340** (6132): 577–81. Bibcode:2013Sci...340..577S.doi:10.1126/science.1232226. PMID 23641111.
225. **Jump up^** Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass, Ravi kumar Kopparapu, Ramses M. Ramirez, James SchottelKotte, James F. Kasting, Shawn Domagal-Goldman, Vincent Eymet, 21 Apr 2014
226. **Jump up^** Hamano, K.; Abe, Y.; Genda, H. (2013). "Emergence of two types of terrestrial planet on solidification of magma ocean". *Nature* **497** (7451): 607–10. Bibcode:2013Natur.497..607H. doi:10.1038/nature12163.PMID 23719462.
227. **Jump up^** Yang, J.; Boué, G. L.; Fabrycky, D. C.; Abbot, D. S. (2014). "Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate" (PDF). *The Astrophysical Journal* **787**: L2.arXiv:1404.4992. Bibcode:2014ApJ...787L...2Y. doi:10.1088/2041-8205/787/1/L2.
228. **Jump up^** Real-life Sci-Fi World #2: the Hot Eyeball planet
229. **Jump up^** Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets, Jun Yang, Nicolas B. Cowan, Dorian S. Abbot, 1 Jul 2013
230. **Jump up^** Amend, J. P.; Teske, A. (2005). "Expanding frontiers in deep subsurface microbiology". *Palaeogeography, Palaeoclimatology, Palaeoecology* **219**: 131–155. doi:10.1016/j.palaeo.2004.10.018.
231. **Jump up^** Further away planets 'can support life' say researchers, BBC, 7 January 2014
232. **Jump up^** Abbot, D. S.; Switzer, E. R. (2011). "The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space". *The Astrophysical Journal***735** (2): L27. arXiv:1102.1108. Bibcode:2011ApJ...735L..27A.doi:10.1088/2041-8205/735/2/L27.
233. **Jump up^** Loeb, A. (2014). "The habitable epoch of the early Universe".*International Journal of Astrobiology* **13** (4): 337–339. arXiv:1312.0613.Bibcode:2014IJAsB..13..337L. doi:10.1017/S1473550414000196.
234. **Jump up^** Home, sweet exomoon: The new frontier in the search for ET, New Scientist, 29 July 2015
235. **Jump up^** Linsenmeier, Manuel *et al.* (21 January 2014) Habitability of Earth-like planets with high obliquity and eccentric orbits: results from a general circulation model. arxiv.org
236. **Jump up^** Kelley, Peter (15 April 2014) Astronomers: ‘Tilt-a-worlds’ could harbor life. www.washington.edu
237. **Jump up^** Armstrong, J. C.; Barnes, R.; Domagal-Goldman, S.; Breiner, J.; Quinn, T. R.; Meadows, V. S. (2014). "Effects of Extreme Obliquity Variations on the Habitability of Exoplanets". *Astrobiology* **14** (4): 277–291. arXiv:1404.3686. Bibcode:2014AsBio..14..277A.doi:10.1089/ast.2013.1129.
238. **Jump up^** Kelley, Peter (18 July 2013) A warmer planetary haven around cool stars, as ice warms rather than cools. www.washington.edu
239. **Jump up^** Shields, A. L.; Bitz, C. M.; Meadows, V. S.; Joshi, M. M.; Robinson, T. D. (2014). "Spectrum-Driven Planetary Deglaciation Due to Increases in Stellar Luminosity". *The Astrophysical Journal* **785**: L9.arXiv:1403.3695. Bibcode:2014ApJ...785L...9S. doi:10.1088/2041-8205/785/1/L9.
240. **Jump up^** Barnes, R.; Mullins, K.; Goldblatt, C.; Meadows, V. S.; Kasting, J. F.; Heller, R. (2013). "Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating". *Astrobiology* **13** (3): 225–250. arXiv:1203.5104.Bibcode:2013AsBio..13..225B. doi:10.1089/ast.2012.0851.
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243. **Jump up^** Paul Gilster, Andrew LePage (2015-01-30). "A Review of the Best Habitable Planet Candidates". Centauri Dreams, Tau Zero Foundation. Retrieved 2015-07-24.
244. **Jump up^** Giovanni F. Bignami (2015). *The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space*. Springer. ISBN 9783319170046.,Page 110
245. **Jump up^** Howell, Elizabeth (6 February 2013). "Closest 'Alien Earth' May Be 13 Light-Years Away". *Space.com*. TechMediaNetwork. Retrieved7 February 2013.
246. **Jump up^** Kopparapu, Ravi Kumar (March 2013). "A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs". *The Astrophysical Journal Letters* **767**: L8.arXiv:1303.2649. Bibcode:2013ApJ...767L...8K. doi:10.1088/2041-8205/767/1/L8.
247. **Jump up^** "NASA’s Kepler Mission Discovers Bigger, Older Cousin to Earth". Retrieved 2015-07-23.
248. **Jump up^** "Special Issue: Exoplanets". *Science*. 3 May 2013.

## Further reading

* Boss, Alan (2009). *The Crowded Universe: The Search for Living Planets*. Basic Books. ISBN 978-0-465-00936-7 (Hardback); ISBN 978-0-465-02039-3 (Paperback).
* Dorminey, Bruce (2001). *Distant Wanderers*. Springer-Verlag. ISBN 978-0-387-95074-7 (Hardback); ISBN 978-1-4419-2872-6 (Paperback).
* Jayawardhana, Ray (2011). *Strange New Worlds: The Search for Alien Planets and Life beyond Our Solar System*. Princeton, NJ: Princeton University Press. ISBN 978-0-691-14254-8 (Hardcover).
* Perryman, Michael (2011). *The Exoplanet Handbook*. Cambridge University Press. ISBN 978-0-521-76559-6.
* Seager, Sara (2010). *Exoplanet Atmospheres: Physical Processes*. Princeton University Press. ISBN 978-0-691-11914-4 (Hardback); ISBN 978-0-691-14645-4 (Paperback).
* Seager, Sara, ed. (2011). *Exoplanets*. University of Arizona Press. ISBN 978-0-8165-2945-2.
* Villard, Ray; Cook, Lynette R. (2005). *Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun*. University of California Press. ISBN 978-0-520-23710-0.
* Yaqoob, Tahir (2011). *Exoplanets and Alien Solar Systems*. New Earth Labs (Education and Outreach). ISBN 978-0-974-16892-0 (Paperback).

### Formation and evolution

* Co-evolution of atmospheres, life, and climate, John Lee Grenfell et al., 20 May 2010
* Phase Separation in Giant Planets: Inhomogeneous Evolution of Saturn, Jonathan J. Fortney, William B. Hubbard, 1 May 2003
* Magnetodynamo Lifetimes for Rocky, Earth-Mass Exoplanets with Contrasting Mantle Convection Regimes, Joost van Summeren, Eric Gaidos, Clinton P. Conrad, 9 Apr 2013
* The effect of evaporation on the evolution of close-in giant planets, I. Baraffe, F. Selsis, G. Chabrier, T. S. Barman, F. Allard, P.H. Hauschildt, H. Lammer, 5 Apr 2004
* Observational Evidence for Tidal Destruction of Exoplanets, Brian Jackson, Rory Barnes, Richard Greenberg, 7 Apr 2009
* A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects, Fred C. Adams, Gregory Laughlin, 18 Jan 1997

### Volcanism

* Detecting Volcanism on Extrasolar Planets, L. Kaltenegger, W. G. Henning, D. D. Sasselov, 7 September 2010
* Detecting planetary geochemical cycles on exoplanets: Atmospheric signatures and the case of SO2, L. Kaltenegger, D. Sasselov, 17 November 2009
* Geodynamics and Rate of Volcanism on Massive Earth-like Planets, Edwin S. Kite, Michael Manga, Eric Gaidos, 31 May 2009
* Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability, Brian Jackson, Rory Barnes, Richard Greenberg, 20 August 2008

### Interior structure

* Planetary internal structures, I. Baraffe, G. Chabrier, J. Fortney, C. Sotin, 19 January 2014

### Surface mapping

* Global Mapping of Earth-like Exoplanets from Scattered Light Curves, Hajime Kawahara, Yuka Fujii, 16 July 2010
* A Two-Dimensional Infrared Map of the Extrasolar Planet HD 189733b, C. Majeau, E. Agol, N. Cowan, 19 September 2012

### Climate and weather

* Patterns of Sunlight on Extra-Solar Planets, Tony Dobrovolskis, 18 March 2014
* Possible climates on terrestrial exoplanets, Francois Forget, Jeremy Leconte, 18 November 2013
* Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction, Dorian S. Abbot, Nicolas B. Cowan, Fred J. Ciesla, 8 August 2012
* Clouds and Hazes in Exoplanet Atmospheres, Mark S. Marley, Andrew S. Ackerman, Jeffrey N. Cuzzi, Daniel Kitzmann, 23 January 2013
* Atmospheric Circulation of Exoplanets, Adam P. Showman, James Y-K. Cho, Kristen Menou, 16 November 2009
* New Technique Could Measure Exoplanet Atmospheric Pressure, an Indicator of Habitability, Shannon Hall on 6 March 2014, www.universetoday.com

### Water

See also: § Habitability, § Plate tectonics, Ocean planet, Desert planet and Extraterrestrial liquid water

After hydrogen and helium, oxygen is the most common element in many planetary systems (in some systems carbon is more common than oxygen), and water H2O one of the most common compounds. Gas giants are composed mostly of hydrogen and helium, but most planets are between the size of Earth and Neptune, where many planets will have deep water oceans covering the entire surface in addition to a H–He envelope.

* Water: from clouds to planets, Ewine F. van Dishoeck, Edwin A. Bergin, Dariusz C. Lis, Jonathan I. Lunine, 25 February 2014
* Are Exoplanets Orbiting Red Dwarf Stars too Dry for Life?, Michael Schirber, Astrobiology Magazine, 27 August 2013
* Carbon-Rich Exoplanets May Lack Surface Water, 26 October 2013
* 'Water-Trapped' Worlds, Adam Hadhazy, Astrobiology Magazine, 18 July 2013
* Lobster-Shaped Extrasolar Oceans, 10 March 2014, Charles Q. Choi, Astrobiology Magazine
* Alien Moons Could Bake Dry from Young Gas Giants' Hot Glow, Adam Hadhazy, Astrobiology Magazine 25 March 2014
* The Longevity of Oceans on Terrestrial Exoplanets, Bullock, Mark Alan; Grinspoon, D. H.
* False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect, Nicolas B. Cowan, Dorian S. Abbot, Aiko Voigt, 4 May 2012

### Orbital dynamics

#### Eccentricity dynamics

See also: Tidal circularization and Kozai mechanism\*High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism, G. Takeda, F.A. Rasio, last revised 9 June 2005

* Extreme Climate Variations from Milankovitch-like Eccentricity Oscillations in Extrasolar Planetary Systems, David S. Spiegel, 11 October 2010
* Orbital Dynamics of Multi-Planet Systems with Eccentricity Diversity, Stephen R. Kane, Sean N. Raymond, 8 February 2014
* Type II migration of planets on eccentric orbits, Althea V. Moorhead, Eric B. Ford, 21 April 2009
* Evolution of Giant Planets in Eccentric Disks, Gennaro D'Angelo, Stephen H. Lubow, Matthew R. Bate, 1 December 2006

#### Inclination dynamics

* A Class of Warm Jupiters with Mutually Inclined, Apsidally Misaligned, Close Friends, Rebekah Dawson, Eugene Chiang, 9 October 2014

## External links

* The Extrasolar Planets Encyclopaedia (Paris Observatory)
* NASA Exoplanet Archive
* Open Exoplanet Catalogue
* The Habitable Exoplanets Catalog (PHL/UPR Arecibo)
* The Habitable Zone Gallery
* Exoplanets: Interactive Visual of XKCD 1071
* NASA's PlanetQuest
* A Zoo of Extra-Solar Planets (audio and transcript) —Astronomy Cast on 9 February 2009 with Pamela Gay and Chris Lintott
* Transiting Exoplanet Light Curves Using Differential Photometry
* Extrasolar Planets – D. Montes, UCM
* Exoplanets at Paris Observatory
* "Exoplanets in relation to host star's current habitable zone". *planetarybiology.com*.
* Doyle, Laurence R. (19 March 2009). "Naming New Extrasolar Planets". *SETI institute*. SPACE.com. Retrieved 2 June 2010.
* Exomol Project Spectroscopic database of molecules of importance for the characterization of exoplanets.
* Characterizing bulk composition of Solid Planets
* Graphical Comparison of Extrasolar Planets
* Video (86:49) – "Search for Life in the Universe" – NASA (14 July 2014).
* Kepler’s Tally of Planets

### News

* Arxiv: Earth and Planetary Astrophysics
* Extrasolar News and Discoveries
* astrobites the astro-ph reader's digest
* Virtual Planetary Laboratory