Question

I have been reading about exoplanets from various sources and I have noticed that generally all giant planets like jupiter and saturn are composed of gas and small planets like earth and mars are generally solid. Is there an explanation for this? and also is there any evidence of giant rocky planets as big or bigger than jupiter for instance.

In many cases we do not know anything about the composition of exoplanets, just their mass. In a few cases there is additional information, which indicates that the giant exoplanets are composed in large part of gas, like Jupiter and Saturn in our solar system. This is probably the case because as a planet accretes from mostly solid matter, it grows large enough to capture the much larger quantities of gas that are available. As far as small (terrestrial) planets are concerned, we have not yet discovered these in other planetary systems, so we know nothing about their composition. The Kepler mission, to be launched next year, should give us our first inventory of small planets. We expect them to be composed of rock and metal, but we will see.

David Morrison
NAI Senior Scientist
September 12, 2008