This is the process of differentiation and it is something evident in the fully formed terrestrial planets like Earth and Mercury.

BBC: A fleeting encounter with an 'inbetweener'

Several panel members have favored dividing planets into categories: terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto.

NPR: Dwarf Planets May Finally Get Respect

"It happened probably right at the end of the formation of the four terrestrial planets - Mercury, Venus, Earth and Mars, " said Craig Agnor, a co-author on the Francis Nimmo study.

BBC: Mars' two-faced riddle 'solved'

Mr. ALAN STERN (Southwest Research Institute): And, you know, whereas we know of four terrestrial planets and four gas giants, we expect that there are hundreds if not thousands of these ice dwarfs, and so this class of planets which we have not yet reconnoitered is in reality the most populous class of planetary body in our solar system.

NPR: NASA Sends Spacecraft to Pluto

Dr. THOMMES: We had have ordered a few percent of systems that where the giant planets at least resemble the architecture in our own system, as to whether we would then have terrestrial planets in the habitable zone, the chances of that actually aren't too bad, because the terrestrial planets, we think, they kind of sprout like mushrooms pretty much anywhere.

NPR: What Makes Our Solar System Special?

The kilometre-wide separation of the component satellites of Terrestrial Planet Finder should allow it to see planets in distant solar systems a problem akin to looking for moths flying around a football-stadium arc light that is several hundred kilometres away.

ECONOMIST: Birds of a feather

In the second decade of this century, Esa plans to launch the Darwin flotilla - a fleet of four or five spacecraft that will hunt for Earth-like planets around other stars and analyse their atmospheres for signs of extra-terrestrial life.

BBC: Corot will be launched just after Christmas (Image: Cnes)

Dr Murray stressed there was no direct evidence these stars did have Earth-sized planets in orbit about them, but modelling had shown that if sufficient terrestrial material existed in a system it would eventually clump together into bodies as big as the Earth.

BBC: 'Hungry' stars reveal planet presence