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What's a Planet?
Donald K. Yeomans and Paul W. Chodas (October 2006)
Jet Propulsion Laboratory/California Institute of Technology

In August 2006, astronomers attending a meeting of the International Astronomical Union (IAU) voted on the definition of the term “planet”. The vote was precipitated by recent discoveries of several large objects beyond Pluto, one of which was found to be even larger than Pluto. After much debate, it was decided that a planet should be defined as an object which (a) orbits the sun, and (b) is massive enough not only to coalesce itself into a nearly spherical shape, but also to gravitationally dominate its region of the solar system. Specifically, a planet is required to be massive enough to have cleared away similarly sized objects in the neighborhood of its orbit. Using this new definition, there are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

A new category of object called “dwarf planet” was also defined. A dwarf planet is an object which shares the characteristics of a planet but is not massive enough to gravitationally dominate its region, and therefore has not cleared its neighborhood of similarly sized objects. The IAU designated three dwarf planets including Ceres (previously the largest so-called minor planet or asteroid), Pluto (previously the ninth planet) and Eris, currently the largest of the so-called trans-Neptunian objects – a population of icy bodies located in a region of the solar system beyond the orbit of Neptune.

The formal resolutions passed at the IAU meeting were worded as follows:

(1)   A “planet”1 is a celestial body that:
  a.   is in orbit about the sun
  b.   has sufficient mass for its self-gravity to overcome its rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, and
  c.   has cleared the neighbourhood around its orbit.
(2)   A ”dwarf planet” is a celestial body that
  a.   is in orbit about the sun
  b.   has sufficient mass for its self-gravity to overcome its rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  c.   has not cleared the neighbourhood around its orbit, and
  d.   is not a satellite
(3)   All other objects3, except satellites, orbiting the sun shall be referred to collectively as “Small Solar-System Bodies”.

Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

 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 planets or other categories.
 3    These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Without commenting upon the wisdom of these definitions, we offer the following notes in an effort to clarify these changes.

The IAU agreed that the categories of planet and dwarf planet are distinct, and more dwarf planets are expected to be announced by the IAU in the coming months and years. An IAU committee has been established to facilitate this process and to maintain a list of objects that are candidates to become future dwarf planets as new objects are discovered and the characteristics and physics of the existing candidates become better known.

Once its orbit is reasonably well known, a minor planet is assigned an official number. As the first minor planet discovered (Jan. 1, 1801), Ceres has been known as 1 Ceres and other minor planets have received consecutive numbers in their order of discovery. Thus the second, third and fourth minor planets discovered are known as 2 Pallas, 3 Juno and 4 Vesta. Once more is known about the largest minor planets, some of them may be designated dwarf planets at some point in the future. At the time of this writing, the largest known dwarf planet is 136199 Eris and the second largest dwarf planet is 134340 Pluto. Pluto was numbered only recently since it had been designated a planet, rather than a dwarf planet, until August 2006.

Thus the three designated dwarf planets (1 Ceres, 134340 Pluto, and 136199 Eris), as well as dwarf planets designated in the future, will have numbers assigned to them. Minor planets whose orbits are well known but do not qualify for dwarf planet status will also receive numbers.

While there is no specific upper or lower bound for the mass of a planet or dwarf planet, a rocky body like an asteroid would be expected to form into a round shape if its diameter exceeds about 800 km, while a less dense icy body would be expected to achieve roundness if its diameter exceeds 400 km or so.

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