11 June 2008

Plutoids Defined

We all have new astronomy terminology to learn:

The International Astronomical Union announced June 11 that it has accepted the name plutoid to distinguish all dwarf planets lying beyond Neptune. . . . A plutoid is defined as a type of dwarf planet that orbits the Sun at a distance greater than Neptune and has enough mass for its self-gravity to give it a near-spherical shape. This characterization means “we have two known and named plutoids: Pluto and Eris,” says Marsden of the Harvard-Smithsonian Astrophysical Observatory.

Ceres, which is a mostly round rocky body about a third the diameter of Earth's moon (about 950 km) "is in the asteroid belt between Mars and Jupiter," is a dwarf planet which is not a plutoid. It is the largest celestial body in this category. Arguably, the category of dwarf planet which is not a plutoid is a category of one.

Vesta, Pallas, and Hygiea are also large bodies in the asteroid belt, but it isn't clear to me if they are dwarf planets, rather than a lesser designation. Their dimensions are as follows:

* Vesta 578×560×458 km
* Pallas 570×525×500 km
* Hygiea 500×400×350 km

The dimensions suggests that because they aren't spherical, that they would count merely as the largest asteroids, rather than as dwarf planets.

These four asteroid belt bodies are a significant part of the overall Main asteroid belt (citations omitted):

The vast majority of known asteroids are found within the main asteroid belt, between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e., not very elongated) orbits. This belt is estimated to contain more than 750,000 asteroids larger than 1 kilometer across, and millions of smaller ones. . . . The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 3.0-3.6×1021 kg, or about 4 percent of the mass of the Moon. Of this, Ceres comprises 0.95×1021 kg, some 32 percent of the total. Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%), and 10 Hygiea (3%), brings this figure up to 51%; while the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

Every body in the solar system larger than Ceres is a star (there is just one in our solar system), planet (there are eight in the solar system), satellite of a planet (about fifteen larger than Ceres), plutoid (about six to eight larger than Ceres) or satellite of a plutoid (Charon is the only one known that is larger than Ceres). Some plutoids, such as Eris, have satellites that are all smaller than Ceres.

Jupiter's largest satellite, Ganymede, and Titan's largest satellite, Titan, are the only satellites larger than the smallest terrestrial planet, Mercury, although Jupiter's second largest planet, Callisto, comes very close.

Prior posts here, and here.

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