Planets are bodies orbiting a star that are not stars themselves, and that are not too small. How small a body can be and still deserve to be called a planet is somewhat arbitrary. Small objects without atmospheres have been called minor planets, or more commonly, asteroids. Pluto has a diameter of 2,300 kilometers; Ceres, which used to be called the largest of the asteroids, has a diameter of 940 kilometers.
Because of their size, all planets are roughly spherical. Even if it is made of a relatively strong material like Earth’s silicate rocks, any body more than 400 kilometers in diameter will be pulled into a roughly spherical shape by its own gravity.
Planets cannot be larger than about 1.33 × 1029 kilograms, about 70 times the size of Jupiter. The force of gravity on a body larger than that is so strong that it compresses the body’s interior so much that the temperature rises to the point at which nuclear fusion of hydrogen atoms begins, and the body ignites to become a star.
Not any more. In 2006 the International Astronomical Union General Assembly decided (Resolutions 5A and 6A), that besides orbiting the sun and being big enough for its gravity to make it round, a planet had to have swept all the miscellaneous debris out of its orbit.¹ By this definition, Pluto does not qualify as a planet, and it became a “dwarf planet”, along with Ceres and Eris (née Xena).
In 2008, the IAU Executive Committee decided that an object orbiting the sun beyond the orbit of Neptune, with enough gravity to make it round but which had not cleared its orbit of debris, would be called a “plutoid”.² Pluto and Eris thus became plutoids. In 2008 Makemake (née Easterbunny) was added to the list of plutoids.
Ceres, which is in the asteroid belt between Mars and Jupiter, was left in an unnamed class by itself. Astronomers think more objects like Pluto and Eris will be discovered, but nothing else like Ceres.
1. The IAU press release describing the introduction of the term “dwarf planet”: www.iau.org/public_press/news/release/iau0603/
2. The IAU press release describing the introduction of the term “plutoid”: www.iau.org/public_press/news/release/iau0804/
at the equator, km
|4 878||3.3022 × 1023||none|
|12 104||4.8690 × 1024||none|
|12 756||5.9742 × 1024||the Moon|
|6 787||6.4191 × 1023||Deimos, Phobos|
Jupiter (and Ganymede)
|143 800||1.8988 × 1027||63 satellites. The four largest, seen by Galileo in 1610, are called the Galilean
satellites. They are
Callisto, Europa, Ganymede and Io. For a complete list and plot of their
orbits, visit Scott Sheppard's page at
For Galileo's role:
If you have a telescope or binoculars and want to see the Galilean satellites for yourself, these web sites will draw a diagram to help you look for them:
Saturn (and Tethys and Dione)
|120 660||5.6850 × 1026||60 satellites. The biggest are Rhea and Titan. Visit|
|52 290||8.6625 × 1025||27 satellites, the biggest being Ariel, Miranda, Oberon, Titania and Umbriel. Visit|
|49 500||1.0278 × 1026||13 satellites, the biggest being Triton. Visit|
Eliot Young (SwRI) et al, NASA
|2302||1.5 × 1022||3 satellites, the biggest being Charon. Since 2008 a plutoid, not a planet.|
4. The length of the planet's day (the period of rotation, in relation to distant stars) in Earth days. Planets that spin in the opposite direction from Earth's spin are marked with an asterisk. (back)
5. The length of a year (one trip around the sun, in relation to distant stars) in Earth years. (back)
6. The distance to the sun at the point in its orbit at which the planet is farthest from the sun, where Earth is 1. (back)
F. W. Taylor.
The Cambridge Photographic Guide to the Planets.
Cambridge (UK): Cambridge University Press, 2002.
How the sun's planets got their names: http://planetarynames.wr.usgs.gov/append7.html
Copyright © 2000 Sizes, Inc. All rights reserved.
Last revised: 20 July 2008.