QUESTION:
I would like too know why do some planets have rings?

ANSWER from Rosaly Lopes-Gautier on January 18, 1996:
This is a very good question - I cannot say that anyone knows for
sure why some planets have rings and others don't, but here is
what we know so far:

First we have to consider how the planets formed. To give you a
short answer, about 4.6 billion years ago the Sun was surrounded
by a spinning cloud of dust and gas which we call the solar
nebula. Material in the cloud collided, stuck together, and
eventually formed the planets. The larger the clump of material
was, the more matter it could attract towards it, because of its
gravitational atraction. Now, within these clumps, the material
was also spinning and in a way each was like a miniature solar
system: the larger pieces formed satellites. But material very
close to the planet - in a region called the Roche limit - could
not stick together, because the tidal forces of the planet
prevented it. So, in this region, the planet would be surrounded
by a thin ring of small particles. These rings may have survived
around the outer solar system planets. Jupiter, Saturn, Uranus
and Neptune all have rings. Some astronomers think that these
rings they have today may not be the original ones. They could
also have formed at a later stage, when satellites which were dragged
within the Roche limit collided with other objects, for example,
comets, and disintegrated.

Now, the gas giant planets all have rings and many satellites,
while the inner planets (and Pluto) have few satellites and no
rings. What this probably means is that their formation processes
were somewhat different. The process I described above seems
logical for the gas giants, but other things must have happened
in the formation of the inner planets to make them different.

What exactly is this difference is not really well understood,
but many theoreticians are thinking about this problem. It is
possible that it is an effect of tides which are raised on the
solid planets. For example, we think the Sun's tidal effects on
Venus and Mercury is what made their rotation periods so slow.
One effect of slow rotations is that planetary tides could have
pulled satellites into the planets, and on their way in, the
satellites could have swept up any rings.

Other factors which could have had an influence are: the material
of the outer planets' satellites is mostly icy rather than rocky,
and icy material would be easier to break up. Also, the actual
size  of the Roche limit region is larger for the larger planets,
so it is more likely that objects would get into the region.

I hope this gives you something to think about. I don't research
planetary formation and cannot claim to be totally up to date
with the research but those are some of the ideas my colleagues
in that field have discussed.

By the way, if all the above was too complicated, here is the
short answer: the gas giant planets are very different from the
"terrestrial" planets (Earth, Venus, etc) and they were formed in
somewhat different ways. The way outer planets formed favored the
formation of rings around them.