Question

Are planets that are 1-10 earths masses rocky planets?

This is not an easy question to answer, because in our solar system we have planets only at the bottom of this size range. Earth and Venus both have about 1 earth mass, and they are certainly rocky (with metal cores). From what we understand about planet formation, we expect all planets near this size of Earth to be composed primarily of a mixture of rock and metal, and this is likely true even for planets several times the Earth's mass. Up near 10 Earth masses this may not be true. If such a planet formed where temperatures are low and ice is available as a building block, they might well be composed in large part of ice. The cores of Uranus and Neptune are thought to be about 10 earth masses, and composed of a mixture of metal, rock and ice. But I don't know if you could have a stable planet this size or whether, like the giant planets in our own solar system, these massive cores would grow further by attracting huge atmospheres. Probably the Kepler mission will tell us how common planets are in the size range of 1-10 earth masses. David Morrison
NAI Senior Scientist
June 19, 2007