International Astronomical Union (IAU), the official
nomenclature, defines a "dwarf planet" as a
celestial body that, within the
- (a) is in
around the Sun;
- (b) has sufficient
rigid body forces so that it assumes a
hydrostatic equilibrium (near-spherical) shape;
- (c) has not
cleared the neighbourhood around its orbit; and
- (d) is not a
The term "dwarf planet" was
adopted in 2006 as part of a three-way classification of bodies orbiting the
Sun. Objects that are large enough to have cleared the neighbourhood of their
orbit are defined as "planets",
while those which are too small to be in hydrostatic equilibrium are defined as
solar system bodies". As defined, the term "dwarf planet" does not apply to
Three dwarf planets are currently recognized:
Eris. Other bodies might be added to the list once it has been determined
whether criterion (b) is fulfilled.
List of dwarf planets
The IAU has officially identified three celestial bodies that have
immediately received "dwarf planet" classification:
|Mass in kg
compared to Earth
|~1.5×1022 kg (est.)
|Mean equatorial radius*
gravity (in m/s2)
Escape velocity (in km/s)
|Rotation period (d)
|Orbital radius* (AU)
|Mean orbital speed
|Inclination of the
|Mean surface temperature (in K)
|Date of discovery
||February 18, 1930
*Measured relative to the Earth.
Additionally, there are several bodies potentially qualifying as "dwarf
planets". Among these, the following are known or thought to be greater than
around 750 km in
Possible dwarf planets
2005 FY9 ("Easterbunny")
||1600 – 2000? km
||840 - 1880 km
||6.2 - 7.0 × 1020 kg
||1.7-6.1 × 1021 kg
2003 EL61 ("Santa")
||~ 1500 km
||~4.2 × 1021 kg
||989 - 1346? km
||1.0-2.6 × 1021 kg
||1207 km ± 3 km
Scattered disc object
||≤ 1200 km
||~5.9 × 1020 kg
||~7.9 × 1020 kg
The status of
Charon, currently regarded as a satellite of Pluto, remains uncertain, as
there is presently no clear definition of what distinguishes a satellite system
from a binary (double
planet) system. The original draft resolution (5)
presented to the IAU stated that Charon could be considered a planet because:
- Charon independently would satisfy the size and shape criteria for planetary
status (and in the terms of the final resolution, for the status of dwarf
- Charon, on account of its large mass relative to Pluto, revolves with Pluto
around a common
located in space between Pluto and Charon rather than around a point located
This definition, however, was not preserved in the IAU's final resolution. It
is unknown if it will be taken up at a future date. If a similar definition were
to be adopted, Charon would be added to the list of dwarf planets.
The second, third, and fourth largest asteroids (Vesta,
could be classified as dwarf planets if it is shown that their shape is
hydrostatic equilibrium. At present this has not been demonstrated
Size and mass of dwarf planets
The upper and lower limits to the size and mass of dwarf planets are not
specified in IAU resolution 5A. There is strictly no upper limit, and an object
larger or more massive than
Mercury that is considered not to have "cleared the neighborhood around its
orbit" may still be classified as a dwarf planet.
The lower limit is determined by the concept of hydrostatic equilibrium
shape, but the size or mass at which an object attains this shape is undefined,
and empirical observations suggest that it may vary according to the composition
and history of the object. The original draft of IAU resolution 5 defined
hydrostatic equilibrium shape as applying "to objects with mass above 5×1020
kg and diameter greater than 800 km",
but this language was not retained in the final resolution 5A that was passed.
According to some astronomers, the new definition could mean the addition of
up to 45 new dwarf planets.
Using a parameter developed by
Alan Stern and
Harold F. Levison,
Soter and other astronomers have argued for a distinction between dwarf
planets and the other eight
on their inability to "clear
the neighborhood around their orbits", that is, to remove smaller bodies
whose orbits bring them nearby by collision, capture, or gravitational
disturbance. This concept is combined with a concept of orbital dominance
measured in terms of the ratio of the mass of a planetary candidate to the
combined mass of all other objects in its vicinity. Dwarf planets are too small
in mass to significantly alter their environment in the manner of a planet.
There are several other theories that try to differentiate between planets
and dwarf planets, but the current definition of what constitutes a planet uses
Stern et al. introduce a parameter Λ, expressing the probability of an
encounter resulting in a given deflection of orbit. The value of this parameter
in Stern’s model is proportional to the square of the mass and inversely
proportional to the period. Following the authors, this value can be used to
estimate the capacity of a body to clear the neighbourhood of its orbit. Stern
and Levison found a gap of five orders of magnitude in Λ between the smallest
terrestrial planets and the largest asteroids and KBOs:
*ME in Earth masses.
**Λ/ΛE = M2/P, in Earth masses squared per year.
***µ = M/m, where M is the mass of the body, and m is the aggregate mass
of all the other bodies that share its orbital zone.
A number of scientists expressed their disagreement
with the currently adopted IAU definition of "dwarf planet" by means of car
While accepting the characterization of "dwarf planet" for
Eris (dwarf-planet in this case meaning just a "small planet"), Stern
rejects the current IAU definition of planet, both in terms of defining "dwarf
planets" as something other than a type of planet, and in using orbital
characteristics (rather than intrinsic characteristics) of objects to define
them as dwarf planets.
Thus, he and his team will still refer to Pluto as the ninth planet. One should
also note, that it will be in pages hosted by NASA and controlled by Stern's
team, that the upcoming information and the first photographs of Pluto will be
unveiled to the world. However, NASA has announced that it will use the new
guidelines established by the IAU.
Types of dwarf planets
IAU's Resolution 6a
recognizes Pluto as "the prototype of a new category of trans-Neptunian
objects". The name and precise nature of this category are not specified, but in
the debate leading up to the resolution, the members of the category were
variously referred to as "Plutons" and "Plutonian objects". The former name was
and was abandoned in the final draft resolution (6b);
the latter name failed to win majority approval on a 180–186 vote in the IAU
General Assembly on
August 24, 2006.
The category, while established,
At an earlier stage in the definition process, the category (then described
as "pluton") was defined to be a
whose orbit took more than 200
Julian years to complete and whose orbit was more
highly inclined and elliptical than a traditional planetary orbit.
This category of Pluto-like objects only applies to dwarf planets which meet
the conditions of being trans-Neptunian and "like Pluto" in terms of period,
inclination and eccentricity. A dwarf planet may or may not be a member of this
category, but all members of the category must be dwarf planets.
The membership of this class, other than Pluto itself, remains obscure.
Pluto's largest satellite, Charon would qualify if it were to be classed as a
dwarf planet in its own right.
Eris and the objects listed in the table "Possible dwarf planets" (above)
also qualify in terms of the minimum period, and most exhibit orbital
eccentricity and inclination that are significant, though not always equal to or
greater than Pluto's.
however, has a much smaller eccentricity and inclination, and so possibly does
not qualify as a Pluto-like object.