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Old October 8th 03, 08:19 PM
Hop David
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Default Hubble Uncovers Smallest Moons Yet Seen Around Uranus



Gordon D. Pusch wrote:
Hop David writes:


Gordon D. Pusch wrote:

(Russell Wallace) writes:


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?

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."


Do Uranus' moons lie in her equatorial plane?



Yes. Tidal friction tends to damp out an inner moon's orbital inclination
relative to the planetary equator on timescales short compared to the
lifetime of the solar system. High orbital inclination satellites
usually have fairly large orbital radii, and are generally thought
to be relatively recent "captures." (One exception to this is if
their inclinations are being "forced" by some orbital resonance.)


At first I had thought Uranus and her moons must have formed from a
subdisk perpendicular to the solar system. But I guess if a later event
had knocked Uranus' axis off kilter, the moons would've moved into her
equatorial plane over a short time.

Hop
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