There is a new class of planet out there that astronomers are calling the "mega-Earth". It is an object with a hard surface like our own world but much, much bigger. The necessity for the new designation follows the discovery of a planet which has a mass some 17 times that of Earth. Known as Kepler-10c, it orbits a star about 560 light-years away.
Scientists described its properties at an American Astronomical Society meeting in Boston. They confess it is something of a head-scratcher. Theorists had always thought that any planet that large would pull so much hydrogen on to itself that it would look more like a Neptune or a Jupiter.