Smallest extrasolar planet found

The planet was discovered using the "wobble" technique (Artist's impression: T.Schindler/NSF)

Astronomers have detected the smallest extrasolar planet yet: a world about seven and a half times as massive as Earth, orbiting a star much like ours.

All of the 150 or so exoplanets found orbiting normal stars are larger than Uranus, itself 15 times Earth's mass.

The new find may be the first rocky world found around a star like our Sun.

The newly discovered "super-Earth" orbits the star Gliese 876, located 15 light-years away in the direction of the constellation Aquarius.

Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets Geoffrey Marcy, UC Berkeley

This star also has two larger, Jupiter-size planets orbiting it.

The new planet whips around the star in a mere two days, and is so close to the star's surface that its temperature probably tops 200-400C 400-750F) - oven-like temperatures, far too hot for life as we know it.

The planet was discovered using the familiar "wobble technique": the planet's gravitational tug on its parent star produces changes in the star's velocity. This can be picked up in the light spectrum emitted by the star.

The nature of that signal can reveal details such as the mass and orbital period of the planet.

"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," said team member Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz.

Indirect evidence

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."

Professor Marcy, Dr Vogt, Dr Paul Butler of the Carnegie Institution of Washington, and other team members carried out the study at the Keck Observatory in Hawaii.

They have submitted a scientific paper to the Astrophysical Journal.