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Old October 3rd 03, 04:52 PM
Henry Spencer
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Default Hubble Uncovers Smallest Moons Yet Seen Around Uranus

In article ,
Russell Wallace wrote:
Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

The two types of planets form differently (as is obvious anyway, given
their radically different compositions). The major moons of the gas-giant
planets probably formed in place, as part of the formation of the planet,
although there are one or two probable exceptions and the details are not
well understood yet.

A lot of the smaller odds and ends are probably captured asteroids, and
there again the outer planets are favored: their large masses combine
with their greater distance from the Sun to give large gravitational
fields (more properly, areas of influence) with quite fuzzy edges, thus
making it much more likely that a passing object will wander through the
fuzzy area and possibly end up captured.

The formation of the inner planets doesn't seem to leave any room for
parts of them to remain in orbit. And their areas of gravitational
influence are small, with relatively sharp edges, so it's rare for them to
capture anything. (Not impossible, but rare.)

Earth's moon is the result of a huge impact late in Earth's formation, a
relatively unlikely accident.

Mars's moons seem to be captured asteroids, and just how that happened is
deeply mysterious. Their orbits are fairly close and fairly circular,
which is not at all what you'd expect for captured objects. Worse, they
seem to be *outer-belt* asteroids, and how they ended up at Mars is
another puzzle. Again, this is probably the result of some unusual
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