Page last updated at 16:07 GMT, Monday, 16 June 2008 17:07 UK

Trio of 'super-Earths' discovered

Artist's impression of super-Earth trio (Eso)
The planets are rocky worlds between two and 10 times the size of Earth

Astronomers have identified a trio of so-called "super-Earths" - rocky planets between two and 10 times the mass of Earth.

The three new planets were detected using the Harps instrument at the La Silla Observatory in central Chile.

The star they circle is slightly smaller than our Sun, and is located 42 light-years away near the southern Doradus and Pictor constellations.

The discoveries were announced at an astronomy conference in Nantes, France.

When a planet orbits its star, it exerts a gravitational pull which causes the parent star to "wobble" around its centre of mass.

The High Accuracy Radial velocity Planet Searcher (Harps) spectrograph was able to measure this wobble to a very high precision over a period of five years.

This was vital because the perturbations caused by the planets were tiny.

"The mass of the smallest planet is one hundred thousand times smaller than that of the star," said co-author Francois Bouchy, from the Astrophysics Institute of Paris, France.

Chances are

The new worlds, which circle the star HD 40307, are 4.2, 6.7 and 9.4 times the size of Earth. They are named super-Earths because they are more massive than the Earth but less massive than Uranus and Neptune (which are about 15 Earth masses).

Using Harps data, the astronomers also counted a total of 45 candidate planets with a mass below 30 Earth masses.

This implies that one solar-like star out of three harbours such planets.

Astronomer Michel Mayor from the Geneva Observatory in Switzerland commented: "Does every single star harbour planets and, if yes, how many?

"We may not yet know the answer but we are making huge progress towards it."

Since the discovery in 1995 of a planet around the star 51 Pegasi by Michel Mayor and his colleague Didier Queloz, more than 270 exoplanets have been found - mostly around Sun-like stars.

The majority of these planets are gas giants, a bit like Jupiter or Saturn in our own Solar System. Current data shows that about one in 14 stars harbours this kind of planet.

The Harps instrument is attached to the La Silla 3.6m telescope in Chile. The facility is run by the European Southern Observatory (Eso) organisation.

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SEE ALSO
Tiniest extrasolar planet found
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Planet hunters find 'super-Earth'
14 Mar 06 |  Science & Environment
New 'super-Earth' found in space
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