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.


Courtesy NASA

The asteroid Gaspra. Its greatest dimension is only 19 km, so its gravity is too weak to pull it into a spherical shape.

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.

Planets whose diameters are more than twice that of Earth's are likely to be “gas giants” like Neptune, rather than rocky like Earth (though they may have a solid core).

Is Pluto a planet?

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 (nèe 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 (nèe 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”:

2. The IAU press release describing the introduction of the term “plutoid”:

Relative diameters

drawing showing relative diameters of planets

Diameters, masses and satellites

Planet Average diameter
at the equator,
4 878 3.3022
× 10²³
12 104 4.8690
× 10²⁴
12 756 5.9742
× 10²⁴
the Moon
6 787 6.4191
× 10²³
(and Ganymede)
143 800 1.8988
× 10²⁷

79 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 and NASA's.

Saturn and 2 moons
(and Tethys and Dione)
120 660 5.6850
× 10²⁶

82 satellites. The biggest are Rhea and Titan. Visit Sheppard's page.

52 290 8.6625
× 10²⁵

27 satellites, the biggest being Ariel, Miranda, Oberon, Titania and Umbriel. Visit Sheppard's page.

49 500 1.0278
× 10²⁶

14 satellites, the biggest being Triton. Visit Sheppard's page.


NASA/Johns Hopkins APL/SRI/Alex Parker

2302 1.5
× 10²²

5 satellites, the biggest being Charon. Since 2008 a plutoid, not a planet.


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:


Sky and Telescope.

Surface gravity and orbit relative to Earth

Planet Surface
of day
(note 4)
of year
(note 5)
to sun
(note 6)
Mercury 0.38 58.65 0.24 0.38
Venus 0.91 243.01* 0.61 0.72
Earth 1 1.00 1.00 1.00
Mars 0.38 1.03 1.88 1.52
Jupiter 2.53 0.41 11.86 5.20
Saturn 1.07 0.44 29.46 9.53
Uranus 0.92 0.65* 84.07 19.19
Neptune 1.19 0.77 64.82 30.06
Pluto 0.09 6.39* 248.6 35.53


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 a planet's orbit at which the it 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:

Minor planet names.


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