Which two planets have the most moons?

Question posed by Maria

[Important edit: This post is a little out of date, with an updated version for 2020 right here. This post will be kept for posterity, and as a reminder that our understanding of the universe is not static but develops over time. There is always plenty more to discover!]

Astronomers have discovered over 170 moons in our solar system, and new ones are still discovered from time to time; the most recent (as far as I can find out) was the satellite designated S/2009 S 1, first imaged on July 26th 2009 as part of the Cassini-Huygens mission, but this has not been directly observed by a human being (via a telescope, for example), which is usually regarded as the true 'discovery' date.

The top two planets in our solar system with regards to number of natural satellites are listed below. The results are not that surprising; it makes sense that the most massive bodies in the system should either capture new bodies or retain the material required for moons to form more than less massive planets.

1. Jupiter

Jupiter has 63 known moons. The four Galilean moons (Ganymede, Io, Callisto and Europa, in size order, and from left to right in the image above (click to visit originator)) were the first natural satellites to be discovered in orbit around a body other than the Earth or the Sun. All four are larger than any of the dwarf planets, and Ganymede is the largest natural satellite in the solar system, and is even larger than the planet Mercury. Between them, these four moons represent more than 99.99% of the mass of matter in orbit around Jupiter.

The other (less than) 0.01% of matter includes the other 59 discovered moons, which include the regular inner satellites, and smaller, more distant objects with more elliptical orbits known as irregular satellites.

2. Saturn

Saturn has 62 moons with confirmed orbits, seven of which having a large enough mass to be in hydrostatic equilibrium*. The largest of these is Titan, the second largest moon in the solar system. 24 of Saturn's moons are regular, and all bar one of the irregular satellites (Phoebe) are small.

The other planets (including dwarf planets) in the solar system are listed below, along with their current number (to the best of my knowledge) of discovered natural satellites:

3. Uranus - 27 moons, with five massive enough to have achieved hydrostatic equilibrium.
The image above depicts, from left to right, Puck, Miranda, Ariel, Umbriel, Titania and Oberon and compares their relative sizes (the big blue blob on the left is a portion of Uranus). The original photographs were taken by Voyager 2. Click image to visit originator.

Neptune's largest moon, Triton - Click to visit originator
4. Neptune - 13 moons including Triton which, although containing 95% of the mass in orbit around Neptune, is an irregular moon, suggesting that it was captured rather than forming in its present orbit. This makes it unique among all of the other large moons in our solar system.

5. Pluto - 3 moons: Charon (which is more than half the size of Pluto), Nix and Hydra.

=6. Mars - 2 natural satellites, Phobos and Deimos, both small, potato-shaped moons.

=6 Haumea - 2 moons, Namaka and Hi'iaka

7. Earth - 1 moon, imaginatively named 'The Moon'.

Mercury, Venus, Ceres and Makemake have no discovered moons.

243 Ida with its moon Dactyl (imaged by the Galileo spacecraft). Click for originator
As of October 2009, a total of 153 'minor planet moons' have been discovered. Minor planet moons are minor planets (e.g. asteroids) that are in orbit around another minor planet. 190 of these are in the asteroid belt, and 63 are in a trans-neptunian orbit.

* This means that they have enough gravity to form a roughly spherical shape, which means that if they weren't in orbit around another body, they'd qualify as dwarf planets.


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