Extrasolar planet

An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system. A related concept is an exomoon, a natural satellite orbiting an exoplanet.

In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion to at least 144 billion. The smaller estimate studied planet candidates gathered by the Kepler space observatory. Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".Palmer, Jason 2013. Kepler telescope: Earth-sized planets 'number 17bn'. BBC News Science & Environment. 

Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".

Analogies with planets in the Solar System apply to few of the extrasolar planets known. Most are quite unlike any of our planets, for example the so-called "hot Jupiters".

 History 
 Early speculations 
In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition."Cosmos" in
The New Encyclopædia Britannica (15th edition, Chicago, 1991) 16:787:2a. "For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600."

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

Confirmed discoveries
The first published and confirmed discovery was made in 1988.Campbell B.; Walker G.A.H.; Yang S. 1988. A search for substellar companions to solar-type stars. Astrophysical Journal 331: 902–921. Bibcode 1988ApJ...331..902C. doi:10.1086/166608 It was finally confirmed in 2002.

In 1992, radio astronomers announced the discovery of planets around a pulsar.Wolszczan A. & Frail D.A. 1992. A planetary system around the millisecond pulsar PSR1257+12. Nature 355 (6356): 145–147. doi:10.1038/355145a0 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. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.

In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000."NASA's Kepler Mission Announces Largest Planet Collection Ever Discovered" May 10, 2016 https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered

 Types 
Extrasolar planets can have many different forms.
 They can be gas giants or rocky planets
 They can possibly be dwarf planets, i.e. planets smaller and less dense than regular planets
 They can orbit several different types of stars
 They can be free-floating or orbit a brown dwarf
 They may support life. One recently discovered exoplanet, Gliese 581g is thought to possibly support life, but the existence of this planet is not yet confirmed.

Classes of exoplanets include:

 A super-terran or super-Earth is a terrestrial planet that is bigger than Earth, but smaller than Neptune

 A water-world is a hypothetical class of planet between terrestrial and jovian. These worlds would have very very little, if any dry land.

 Hot Jupiters are gas planets orbiting closer to their stars than Mercury to the sun.

 Rogue planets orbit no stars at all.

 Nearest 
The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.Tau Ceti's planets nearest around single, Sun-like star. BBC News Science & Environment. Tuomi M. et al [2012]. Signals embedded in the radial velocity noise. Astronomy and Astrophysics (in press). 

 Most Earth-like 
Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8. For comparison, the four solar terrestrial planets are included in this list.

Name     Distance (ly)StatusYear ofdiscoveryRef Earth    warm terran           Kepler-438b    warm terran      472.9    2015 Kepler-1410b   warm superterran      1213.4    2011  Gliese 667 Cc    warm terran     23.6    2011 Kepler-442b  warm superterran      1291.6    2015Kepler-62e  warm superterran      1199.7    2013 Kepler-452b    warm superterran       1402.5    2015   Gliese 832 c    warm superterran      16.1    2014 Kepler-283c    warm superterran      1496.8    2011 Kepler-436b    warm superterran      1339.4    2015 Kepler-1229b    warm superterran      769.7    2016 Tau Ceti e    warm superterran      11.9    2012 Kepler-296f    warm superterran      1089.6    2011 Gliese 180 c    warm superterran      39.5    2014 Gliese 667 Cf    warm terran      23.6    2013 Gliese 581 g    warm superterran      20.2    2010 Gliese 163 c    warm superterran       48.9    2012 Gliese 180 b    warm superterran       39.5    2014 HD 40307 g    warm superterran      41.7     2012 Kepler-61b    warm superterran      1062.8    2013 Kepler-443b    warm superterran      2564.4    2015Gliese 422 b  warm megaterran       41.3    2014 Kepler-22b    warm superterran      619.4    2011 Kepler-440b    warm superterran      706.5    2015 Kepler-298d    warm superterran      1545    2012   Kepler-439b    warm superterran      1914.8    2015 Kapteyn b    warm superterran      12.7    2014 Kepler-62f    warm superterran      1199.7    2013 Kepler-186f    warm terran      492    2014 Kepler-174d    warm superterran      878.3    2011 Gliese 667 Ce    warm terran      23.6    2013 Gliese 682 c    warm superterran      16.6    2014 Gliese 581 d    warm superterran      20.2    2007  Reanalysis of data suggests ‘habitable’ planet GJ 581d really could exist – Astronomy Nowhttp://www.sciencemag.org/content/347/6226/1080.3.abstract Kepler-155c   hot superterran      965   2014Venus    warm terran                 Mars     warm subterran          Mercury    hot mercurian               

 Related pages 
 Planet
 List of exoplanets
 dwarf planet
 51 Pegasi b
 70 Virginis b

 References 

 Other websites 
 Search projects 
 University of California Planet Search Project
 The Geneva Extrasolar Planet Search Programmes
 PlanetQuest distributed computing project
 Extrasolar Planets 

 Resources 
 NASA's PlanetQuest 
 PlanetQuest 3D Atlas of extrasolar planets within 400 light years of our Solar System 
 Planetquest Flash 
 German Center for Exo-Planet Research Jena/Tautenburg
 Astrophysical Institute & University Observatory Jena (AIU)
 Table of known planetary systems
 Extrasolar Planet XML Database 
 Andrew Collier Cameron, Extrasolar planets, Physics World (January 2001). (See the online version .)
 searchable dynamic database of extrasolar planets and their parent stars 
 List of important exoplanets
 Extrasolar Planets - D. Montes, UCM
 Extrasolar Visions 

 News 
 6-8 Earth-Mass Planet Discovered orbiting Gliese 876 
 Newfound World Shatters Distance Record from space.com
 Oldest Known World from space.com
 Earth Sized Planets Confirmed from space.com
 Sunshade to Look for Distant Life from news.bbc.co.uk