Paul Butler of the Carnegie Institute of Washington, Geoffrey Marcy of the University of California at Berkeley and colleagues discovered one of the new planets orbiting around a small star called Gliese 436 every two-and-a-half days. Barbara McArthur of the University of Texas in Austin discovered the second new planet orbiting around a nearby star called 55 Cancri with a period of just under three days. It is the fourth planet to be discovered around this star. The results have been accepted for publication in the Astrophysical Journal.

The latest discoveries come a week after a group led by Michel Mayor of the Geneva Observatory announced that it had found a planet 14 times as big as the Earth orbiting around the star mu Arae.

Astronomers think that the new planets will be made of rock, or rock and ice, like the Earth, rather than gas. The new planets were all discovered using the radial velocity technique, which relies on measuring tiny Doppler shifts in light from the parent star, caused by its motion around the centre of mass of the combined star-planet system.

"These Neptune-sized planets prove that Jupiter-sized, gas giants aren't the only planets out there," says Marcy. "We are beginning to see smaller and smaller planets," adds Butler. "Earth-like planets are the next destination."