The IAU definitions of 'planet', 'dwarf planet', 'plutoid' and 'small solar system bodies'
- A "planet" [1] is a celestial body that: (a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
- A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that
it assumes a hydrostatic equilibrium (nearly round) shape [2],
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
- "Plutoids" are celestial bodies in orbit around the sun at a distance greater than that of
Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that
they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the
neighborhood around their orbit.
- All other objects [3] orbiting the Sun shall be referred to collectively as
"Small Solar System Bodies".
Footnotes:
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either "dwarf planet"
and other categories.
[3] These currently include most of the Solar System asteroids, most
Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Note that the IAU is clearly concentrating on the Solar System here,
and has not taken account of extrasolar planets, or brown dwarfs
(bodies of a size between planets and stars).
Extrasolar Objects
(By Extrasolar we mean anything outside our own Solar System)
In 2003 the IAU defined extrasolar planets, brown dwarfs and sub-brown dwarfs as follows:
- 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 our Solar System.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of
deuterium are "brown dwarfs", no matter how they formed nor 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).
[Note that 'substellar object' means anything below the size of a star
- in other words, of too small a mass to burn hydrogen in its core - this mass limit
itself varies depending upon the amount of heavier elements in the body, varying between about
75 and 90 times the mass of Jupiter]
It appears that the term 'planemos', or more strictly, 'free floating planemos' is being
increasingly used instead of "sub-brown dwarfs". For instance, in 2006 a number of articles
appeared relating to the discovery of two such objects circling each other -
the Oph 1622 system - with most articles about the discovery by ESOs Very Large Telescope
preferring the term 'planemos'.
What is a 'Planemo'?
'Planemo' is short for Planetary Mass Object.
The term has no official standing, but generally covers anything that is of planetary mass -
i.e. is smaller than a brown dwarf, and has sufficient mass to be approximately spherical.
Borrowing from the definitions above, maybe we can define 'Planemo' as follows:
- "Planemos" are celestial bodies that:
(a) have a true mass below the limiting mass
for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses
for objects of solar metallicity), and
(b) have sufficient mass for their self-gravity to overcome rigid body forces so that
they it assume a hydrostatic equilibrium (nearly round) shape,
no matter how they formed nor where they are located.
So 'planemo' covers planets, dwarf planets, any satellites of planets or dwarf planets
with planetary mass, and sub-brown dwarfs (which we might term 'free-floating planemos').
The 'missing' definition?
It does not appear clear from the IAU definitions whether something that meets
the definition of planet or dwarf planet, except that it is going round a
brown dwarf, instead of a star, should be called a planet (or
dwarf planet, as the case may be).
For completeness:
What is a 'Fusor'?
The term has no official standing, but is sometimes used to refer collectively to
brown dwarfs and stars, so again borrowing from the definitions above, we might say:
- "Fusors" are celestial bodies with true masses above the limiting mass
for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses
for objects of solar metallicity).
So maybe the solution (to the 'missing' definition)
is that planets and dwarf planets are celestial bodies going round a fusor?
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