Water world?: New calculations suggest that Gliese 581 d (pictured here in an artist's illustration) may lie in the habitable zone of its star and could even have oceans of liquid water.
Credit: ESO
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Distant planets, even big ones, are too small to be directly observed, and are usually detected by measuring their impact on the movement of the stars they orbit. Almost all those discovered to date are large gas giants.
"It is amazing to see how far we have come since [the early 90s]," said lead researcher Michael Mayer, also of Geneva University. "The mass of Gliese 581 e is 80 times less than that of 51 Pegasi b," the first exoplanet discovered, he said.
Planets are formed from a disc of gas and dusty debris left over from the creation of a star. Just how long this process takes is still a matter of debate.
Earth is believed to be about 4.5 billion years old, and the Sun about 100 million years older.
