BBC NEWS | Science/Nature | Smallest extrasolar planet found: "The researchers have measured a minimum mass for the planet of 5.9 Earth masses. It orbits Gliese 876 with a period of 1.94 days at a distance of 0.021 astronomical units (AU), or 3.2 million km (2 million miles).
Though the team has no direct proof the planet is rocky, its low mass precludes it from holding on to gas in the way that Jupiter does.Three other supposed rocky planets have been reported, but they orbit a pulsar, the corpse of an exploded star.
'This planet answers an ancient question,' said team leader Geoffrey Marcy, professor of astronomy at the University of California, Berkeley.
'Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star.'"