Newsgroup: it.scienza.astronomia
Titolo: [LUNGO] IAU - Planet Definition Q&A

Planet Definition Questions & Answers Sheet
(da 
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_Q_A.html
)
The following Question and Answer sheet may help to interpret the "IAU
Resolution 5 for GA-XXVI" provided this is passed during the 2 nd Session of
the 2006 IAU General Assembly.
Q: What is the origin of the word "planet"?
A: The word "planet" comes from the Greek word for "wanderer", meaning that
planets were originally defined as objects that moved in the sky with
respect to the background of fixed stars.
Q: Why is there a need for a new definition for the word "planet"?
A: Modern science provides much more knowledge than the simple fact that
objects orbiting the Sun appear to move with respect to the background of
fixed stars. For example, recent new discoveries have been made of objects
in the outer regions of our Solar System that have sizes comparable to and
larger than Pluto. (Noting that historically Pluto has been recognized as
"the ninth planet.") Thus these discoveries have rightfully called into
question whether or not they should be considered as new "planets."
Q: How did astronomers reach a consensus for a new definition of "planet"?
A: The world's astronomers, under the auspices of the International
Astronomical Union, have had official deliberations on a new definition for
the word "planet" for nearly two years. The results of these deliberations
were channelled to a Planet Definition Committee comprising seven persons
who were astronomers, writers, and historians with broad international
representation. This group of seven convened in Paris in late June and early
July 2006. They culminated the two year process by reaching a unanimous
consensus for a proposed new definition of the word "planet."
Q: What new terms are proposed as official IAU definitions?
A: There are two new terms being proposed for use as official definitions of
the IAU. The terms are: "planet" and "pluton".
Q: What is the proposed new definition of "planet"?
A: The new definition of "planet" recognizes and utilizes the capabilities
of modern science. The new definition is based on the principle that we no
longer need to rely on the simple definition from past millennia that an
object is a "planet" if it moves against the background of fixed stars.
Instead we can utilize our modern ability to measure the physical properties
of an object to determine its true nature. An object is thus defined as a
planet based on its intrinsic physical nature. Two conditions must be
satisfied for an object to be called a "planet." First, the object must be
in orbit around a star, while not being itself a star. Second, the object
must be large enough (or more technically correct, massive enough) for its
own gravity to pull it into a nearly spherical shape. The shape of objects
with mass above 5 x 10 20 kg and diameter greater than 800 km would normally
be determined by self-gravity, but all borderline cases would have to be
established by observation.
Q: What is the exact wording of the official IAU proposed definition of
"planet" in "Resolution 5 for GA-XXVI"?
A:
"A planet is a celestial body that (a) has sufficient mass for its
self-gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round) shape, and (b) is in orbit around a star, and is
neither a star nor a satellite of a planet."
Q: Does an object have to be in orbit around a star in order to be called a
"planet"?
A: Yes.
Q: Does a body have to be perfectly spherical to be called a "planet"?
A: No. For example, the rotation of a body can slightly distort the shape so
that it is not perfectly spherical. Earth, for example, has a slightly
greater diameter measured at the equator than measured from pole to pole.
Q: Based on this new definition, how many planets are there in our Solar
System?
A: There are currently 12. Eight are the classical planets Mercury through
Neptune. Three are in a newly defined (and growing in number) category
called "plutons", for which Pluto is the prototype. One is Ceres, which may
be described as a dwarf planet.
Q: What are the 12 planets?
A: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune,
Pluto, Charon and 2003 UB313 (provisional name).
Q: How are these 12 planets categorized?
A: There are eight "classical planets": Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. Ceres is a planet, but because it is
smaller than Mercury, one may describe it as a "dwarf planet". A new
category of planet is now defined: "plutons". Pluto, Charon, and 2003 UB313
fall into the growing category of planets called "plutons".
Q: What are the classical planets?
A: The classical planets are those recognized by sky watchers and
astronomers starting from the beginning of human history until the year 1900
A.D. The classical planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune. Note that the term "classical planet" is only a historical
reference and not an IAU definition.
Q: What is a dwarf planet?
A: A dwarf planet is a term generally used to describe any planet that is
smaller than Mercury. Note that the term "dwarf planet" is simply a
descriptive category and not an IAU definition. Terms such as "terrestrial
planets" and "giant planets" are additional examples of descriptive
categories that are not IAU definitions.
Q: What is a "pluton"?
A: A pluton is a new category of planet now being defined by the IAU. A
"pluton" is an object satisfying the technical (hydrostatic equilibrium
shape in the presence of self-gravity) definition of "planet." Plutons are
distinguished from classical planets in that they reside in orbits around
the Sun that take longer than 200 years to complete (i.e. they orbit beyond
Neptune). Plutons typically have orbits that are highly tilted with respect
to the classical planets (technically referred to as a large orbital
inclination). Plutons also typically have orbits that are far from being
perfectly circular (technically referred to as having a large orbital
eccentricity). All of these distinguishing characteristics for plutons are
scientifically interesting in that they suggest a different origin from the
classical planets.
Q: Where does the name "pluton" come from?
A: The name "pluton" comes from Pluto itself. Pluto is the first object
discovered that is a large spherical body, and therefore a planet, orbiting
beyond Neptune.
Q: Is a "pluton" a planet?
A: Yes.
Q: Is Pluto a "pluton"?
A: Yes.
Q: Is Pluto a planet?
A: Yes. In fact, Pluto's large companion named Charon is also large enough
and massive enough to satisfy the definition of "planet". Because Pluto and
Charon are gravitationally bound together, they are actually now considered
to be a "double planet."
Q: Is Pluto a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet",
then Pluto could be called a dwarf planet. Note that in terms of an IAU
definition, Pluto is a "pluton." "Dwarf planet" is simply a descriptive
category like "terrestrial planet" and "giant planet." None of these
descriptive categories are formally defined by the IAU.
Q: What is a "double planet"?
A: A pair of objects, which each independently satisfy the definition of
"planet" are considered a "double planet" if they orbit each other around a
common point in space that is technically known as the "barycentre". In
addition, the definition of "double planet" requires that this "barycentre"
point must not be located within the interior of either body.
Q: What is a "satellite" of a planet?
A: For a body that is large enough (massive enough) to satisfy the
definition of "planet", an object in orbit around the planet is called a
"satellite" of the planet if the point that represents their common centre
of gravity (called the "barycentre") is located inside the surface of the
planet.
Q: The Earth's moon is spherical. Is the Moon now eligible to be called a
"planet"?
A: No. The Moon is a satellite of the Earth. The reason the Moon is called a
"satellite" instead of a "planet" is because the common centre of gravity
between the Earth and Moon (called the "barycentre") resides below the
surface of the Earth.
Q: Jupiter and Saturn, for example, have large spherical satellites in orbit
around them. Are these large spherical satellites now to be called planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and
Saturn (for example, Titan) orbit around a common centre of gravity (called
the "barycentre") that is deep inside of their massive planet. Regardless of
the large size and shapes of these orbiting bodies, the location of the
barycentre inside the massive planet is what defines large orbiting bodies
such as Europa, Titan, etc. to be "satellites" rather than planets.
Q: Why is Pluto-Charon a "double planet" and not a "planet with a satellite"
?
A: Both Pluto and Charon each are large enough (massive enough) to be
spherical. Both bodies independently satisfy the definition of "planet". The
reason they are called a "double planet" is that their common centre of
gravity is a point that is located in free space outside the surface of
Pluto. Because both conditions are met: each body is "planet-like" and each
body orbits around a point in free space that is not inside one of them, the
system qualifies to be called a "double planet."
Q: Pluto has at least two recently discovered additional satellites that are
smaller than Charon. If these smaller satellites also orbit the "barycentre"
, does this make Pluto a "quadruple planet"?
A: No. The two newly discovered smaller bodies in orbit around Pluto are too
small and not massive enough for their self-gravity to force them in to a
spherical shape. Therefore neither of these bodies independently satisfies
the definition of "planet." The fact that their size (mass) and shapes does
not qualify them as planets implies that they must be called satellites,
even though the center of gravity (called the "barycentre") about which they
orbit is located outside the surface of Pluto.
Q: Can there be triple planets or quadruple planets?
A: Yes, but none are currently known.
Q: Is Ceres a planet?
A: Yes.
Q: Didn't Ceres used to be called an asteroid or minor planet?
A: Historically, Ceres was called a "planet" when it was first discovered
(in 1801) orbiting in what is known as the asteroid belt between Mars and
Jupiter. Because 19 th century astronomers could not resolve the size and
shape of Ceres, and because numerous other bodies were discovered in the
same region, Ceres lost its planetary status. For more than a century, Ceres
has been referred to as an asteroid or minor planet.
Q: Why is Ceres now being called a "planet"?
A: An object in orbit around a star is now being called a "planet" if it is
large enough (or more technically, massive enough) for its own gravity to
pull it into a nearly spherical shape. Recent Hubble Space Telescope images
that resolve the size and shape of Ceres show it to be nearly spherical.
More technically, Ceres is found to have a shape that is in a state of
hydrostatic equilibrium under self-gravity. Therefore Ceres is a planet
because it satisfies the IAU definition of "planet." [Published reference
for shape of Ceres: P. Thomas et al. (2005), Nature 437, 224-227. Dr. Peter
Thomas is at Cornell University.]
Q: Is Ceres a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet",
then Ceres could be called a dwarf planet. Note that "dwarf planet" is
simply a descriptive category like "terrestrial planet" and "giant planet."
None of these descriptive categories are formally defined by the IAU.
Q: Is Ceres a "pluton"?
A: No.
Q: What is 2003 UB313?
A: "2003 UB313" is a provisional name given to a large object discovered in
2003 that resides in an orbit around the Sun beyond Neptune.
Q: Is 2003 UB313 a planet?
A: Yes.
Q: Why is 2003 UB313 a planet?
A: Recent Hubble Space Telescope images have resolved the size of 2003 UB313
showing it to be as large as, or larger than Pluto. Any object having this
size, and any reasonable estimate of density, is understood to have
sufficient mass that its own gravity will pull it into a nearly spherical
shape determined by hydrostatic equilibrium. Therefore, 2003 UB313 is a
planet because it satisfies the IAU definition of "planet." [Published
reference: M. Brown et al. (2006). Astrophysical Journal643, L61-L63. Dr.
Michael Brown is at the California Institute of Technology.]
Q: Will the new planet 2003 UB313 receive a name? When?
A: Yes. The International Astronomical Union has the official authority to
assign names to objects in space. This object has been popularly called
"Xena", but this is not an official IAU name. A decision and announcement of
the new name are likely not to be made during the IAU General Assembly in
Prague, but at a later time.
Q: Has the IAU ever named a planet?
A: No, so far not.
Q: Is 2003 UB313 a "pluton" ?
A: Yes.
Q: Is 2003 UB313 a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet",
then 2003 UB313 could be called a dwarf planet. Note that "dwarf planet" is
simply a descriptive category like "terrestrial planet" and "giant planet."
None of these descriptive categories are formally defined by the IAU.
Q: What is an object called that is too small to be a "planet"?
A: All objects that orbit the Sun, which are too small (not massive enough)
for their own gravity to pull them into a nearly spherical shape are now
collectively referred to as "small Solar System bodies." This collection
includes the category of objects we continue to call asteroids and comets.
This collection also currently includes, near-Earth objects (NEOs), Mars-
and Jupiter-Trojan asteroids, most Centaurs and most Trans-Neptunian Objects
(TNOs). In the new system of IAU definitions, the term "minor planet" is no
longer used.
Q: Is the term "minor planet" still to be used?
A: No. The term "minor planet" is no longer to be used for official IAU
purposes. The term will be replaced by "small Solar System bodies."
Q: Why is the term "minor planet" being replaced by "small Solar System
bodies"?
A: Under the new definition of "planet", nearly all objects currently called
"minor planets" are not planets. For IAU purposes, a definition and name is
needed that clearly distinguishes between objects that are officially
recognized as planets and those that are not.
Q: For any newly discovered object, how will a decision be reached on
whether or not to officially call it a "planet."
A: The decision on whether or not an object is officially a "planet" will be
made by a review committee within the IAU. The review process will be an
evaluation, based on the best available data, of whether or not the physical
properties of the object satisfy the definition of "planet." It is likely
that for many objects, a period of time of several years may be required in
order for sufficient data to be gathered.
Q: Are there additional "planet" candidates currently being considered?
A: Yes.
Q: Does this mean there will be more than 12 planets in our Solar System?
A: Almost certainly yes.
Q: When will additional new planets likely be announced?
A: When the responsible committee has had time to work on the issues after
the resolution has been voted upon. Most likely any further new planet
announcements will not be made until after the General Assembly in Prague.
Q: How many more new planets are there likely to be?
A: Perhaps as many as a dozen or two new planets in the IAU category called
"plutons" remain to be discovered. We estimate this number based on our
understanding of the current discovery statistics. This understanding is
subject to change as new data continue to be gathered.
Q: When is an object too large to be called a "planet"?
A: The new definitions proposed by the IAU seek only to define the lower
boundary between an object that is a "planet" or a "small Solar System
 body." At this time there is no official IAU definition in place or
proposed that defines the upper limit for when an object is, for example a
"planet" or a "brown dwarf." This limit is generally thought to be about 13
times more massive than Jupiter, but is subject to discussion.
Q: Is the new definition for "planet" intended to apply also to objects
discovered in orbit around other stars?
A: Yes.
Q: Are objects that have planetary sizes and masses, but which are free
floating in space (and not orbit a star) officially "planets" by the IAU
definition?
A: No. At this time there is no official IAU definition in place that
addresses this class of objects.
Q: Is a "pluton" a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet",
then any pluton smaller than Mercury could be called a dwarf planet. Note
that "dwarf planet" is simply a descriptive category like "terrestrial
planet" and "giant planet." None of these descriptive categories are
formally defined by the IAU.
Q: What is the difference between a "pluton" and a "plutino"?
A: A "plutino" is a small body that has a similar orbit to Pluto, that is,
it has the same 248 year orbital period as Pluto. Plutino is not an official
IAU description. All currently known plutinos are too small to be planets.
Plutinos are not plutons.
Q: Is Pluto the ninth planet?
A: Historically it was indeed the ninth planet to be discovered, but now
Pluto is also known as the first pluton, with its moon Charon being the
second pluton. The classical planets can be numbered by their distance from
the Sun, and there is no change in their order. Plutons, on the other hand,
may due to their high eccentricity change their relative distances from the
Sun with time (and hence their order).
Q: In summary how will the Solar System look if the Resolution is passed?
A: If the Resolution is successfully passed, there will be 12 currently
known planets in our Solar System:
Table 1: Overview of the planets in the Solar System as per 24 August 2006
if "Resolution 5 for GA-XXVI" is passed.
Object IAU definition IAU planet category Descriptive category Unofficial
mean diameter estimate(1)
Mercury Planet  Classical 4,879 km
Venus Planet  Classical 12,104 km
Earth Planet  Classical 12,746 km
Mars Planet  Classical 6,780 km
Jupiter Planet  Classical 138,346 km
Saturn Planet  Classical 114,632 km
Uranus Planet  Classical 50,532 km
Neptune Planet  Classical 49,105 km
Ceres Planet  Dwarf 952 km
Pluto Planet Pluton Dwarf 2306±20 km
Charon Planet Pluton Dwarf 1205±2 km
2003 UB313 Planet Pluton Dwarf 2400 ±100 km(2)
Other objects that appear large enough so that their shape satisfies the
definition of "planet" will be further considered on a case by case basis.
Examples of these are listed below.
Table 2: Planet candidates as per 24 August 2006 to be given future
consideration if "Resolution 5 for GA-XXVI" is passed.
Object Unofficial diameter estimate
2003 EL61 2000×1000×1200 km(3)
2005 FY9 1500±300 km(4)
(90377) Sedna 1200-1800 km(5)
(90482) Orcus 1000±200 km(6)
(50000) Quaoar ~1000 km(7)
(20000) Varuna 600 ± 150 km(8)
(55636) 2002 TX300 <700 km(9)
(28978) Ixion 500±100 km(10)
(55565) 2002 AW197 700±100 km(11)
(4) Vesta 578×560×458 km(12)
(2) Pallas570×525×500 km (13,14)
(10) Hygiea 500×400×350 km(15,16)
1 Brown, Binzel, private communication (2006)
2 Reference: Brown et al. (2006). Astrophys. J. 643, L61-L63.
3 Rabinowitz et al. (2006), Astrophys. J. 639, 1238-1251.
4 Based on Spitzer results
5 Based on Spitzer and Hubble results
6 Brown, Binzel, private communication (2006)
7 Mean of Spitzer and Hubble results
8 Stansberry et al. (2005), BAAS 37, 737
9 Uper limit from Spitzer results
10 Stansberry et al. (2005), BAAS 37, 737
11 Stansberry et al. (2005), BAAS 37, 737.
12 Thomas et al. (1997), Science 277, 1492.
13 Drummond and Cooke (1989), Icarus 78, 323.
14 Dunham et al. (1990), Astron J. 99, 1636.
15 Tedesco et al. (1992). IRAS Minor Planet Survey
16 Kaasalainen et al. (2002), Icarus 159, 369.


Mamma mia, che casino stanno facendo! :-)
Ma Cerere non è nel mezzo della Fascia Asteroidale? Per anni si è detto che 
**l'intera** Fascia Asteroidale era un pianeta mancato a causa di Giove, e 
adesso promuovono a pianeta un suo "pezzetto" di 950 km ? Totalmente 
illogico.
Secondo me la cosa più logica era tenere come limite minimo il diametro di 
Plutone. Per ragioni storiche più che fisiche.
La distinzione fra sistema pianeta-satellite e pianeta doppio è 
soddisfacente anche se il caso peculiare di Plutone si poteva evitare 
declassificando Plutone a KBO (cosa, IMHO, più che logica, viste le 
dimensioni, l'orbita, la composizione...)
Seguendo la distinzione in base alle orbite, mi sembra di capire inoltre, 
che un oggetto di diametro planetario ma che non ruota attorno ad alcuna 
stella (perchè fiondato fuori da qualche influenza gravitazionale o 
semplicemente perchè la stella genitrice non esiste più come tale), allora 
non è più un pianeta... Strano!
Al di là dell'imposibilità tecnica di rilevare questi "pianeti vaganti", la 
definizione proposta mi sembra appartenente ad una visione ancora troppo 
eliocentrica dell'universo.
Se il prossimo futuro è quello di osservare centinaia di pianeti 
extrasolari, la IAU dovrebbe pensare meno a cambiare il sistema solare (la 
storia ha comunque un peso di cui bisognerebbe tenerne conto!) e pensare 
maggiormente a dare definizioni *univoche* che tengano conto anche dei 
pianeti extrasolari!
Per ora non mi sembra che siano sulla strada giusta... e a voi?
Ciao
-- 
Lorenzo Preti
http://digilander.libero.it/luranz