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Why do the outer planets have more moons than the inner planets?

Deanna Rostic, St. Catharines, Ontario, Canada
Moons
The number of moons a planet has depends mostly on where in the solar system the planet formed. While the terrestrial worlds (Mercury, Venus, Earth, and Mars) have a total of three, the giant planets (Jupiter, Saturn, Uranus, and Neptune) combined possess 169 known moons. This collage shows only the solar system's largest moons — Earth's Moon is the only satellite of the inner solar system illustrated. // NASA
The giant planets formed in a drastically different environment from the terrestrial planets. It was much colder in the outer solar system, so water was ice, but in the region near where Earth formed, water turned to steam. Water is one of the most common chemical compounds, and its ice form accretes much more easily. This encouraged the outer planets to grow massive.
 
The larger volume of space and lower gravity of the Sun in the giant-planet region allowed Jupiter, Saturn, Uranus, and Neptune to form like solar systems in miniature. We think each of them pulled in lots of material and had a disk of gas, dust, and ice surrounding it during formation. Any dust and ice not incorporated into the planet itself would have coalesced into the regular moons we see today around the giant planets. These satellites are in nearly circular orbits near the equators of their host planets, such as the four Galilean satellites around Jupiter and Titan around Saturn, which are all composed of large fractions of ice.

Because the giant planets gravitationally dominate a very large area, they also can efficiently capture passing objects as moons, which we call irregular satellites. These irregular moons are relatively small and have orbits with large inclinations, eccentricities, and semimajor axes.

In contrast, the terrestrial planets formed much closer to the Sun with much less material around them. They grew slowly by accreting many small rocky planetesimals and never grew massive enough to capture a disk of gas and dust. Earth’s Moon probably formed through a direct collision. Any large object striking a planet would have thrown material off the world into orbit, allowing it to coalesce into a moon.
 
Mars’ moons are still a bit of a mystery — they could result from a giant collision, or they could be captured asteroids. Mercury and Venus might have had moons in the past, but their close proximity to the Sun means most satellite orbits around these planets would be unstable over the solar system’s age. — Scott Sheppard, Carnegie Institution of Washington, D.C.
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