Lightest exoplanet is discovered

Astronomers have announced the discovery of the lightest planet ever detected outside our Solar System.

Situated in the constellation Libra, it is only about twice as massive as the Earth, whereas most other exoplanets identified have been far bigger.

The scientists say the planet's orbit takes it far too close to its star Gliese 581 for life to be possible.

The detection was made by an international team of researchers using a 3.6m telescope at La Silla, Chile.

"This is by far the smallest planet that's ever been detected," said group member Michel Mayor, from the Geneva Observatory, Switzerland.

"This is just one more step in the search for the twin of the Earth.

"At the beginning, we discovered Jupiter-like planets several hundred times the mass of the Earth; and now we have the sensitivity with new instruments to detect very small planets very close to that of the Earth,".

The planet joins three others previously detected around its star and takes the designation Gliese 581 e.

As with the previous discoveries, its presence was picked up using the so-called wobble technique. This is an indirect method of detection that infers the existence of orbiting planets from the way their gravity makes a parent star appear to twitch in its motion across the sky.

Astronomy is working right at the limits of the current technology capable of detecting exoplanets and most of those found so far are Jupiter scale and bigger.

To discover one so small is a major coup. The previous record holder was about four times as massive as the Earth.

The HARPS instrument is attached to a telescope at La Silla
Because Gliese 581 e takes just 3.15 days to orbit its host star, it lies beyond what scientists call the habitable, or "Goldilocks", zone, where it is neither too hot nor too cold for liquid water to exist.

But one of the other planets in this system does appear to be. Gliese 581 d was first discovered in 2007. The latest research has allowed scientists to refine details of its orbit.

The team now believes planet d (which is about seven Earth-masses in size) circles Gliese 581 in 66.8 days.

"This planet is probably not just rocky; it's very probably an icy planet - but relatively close to the star so at the surface, we should have some big ocean," said Professor Mayor.

"Maybe, it's the first candidate in a new class of planet called an 'ocean planet'."

The exoplanet discovery was announced at the JENAM conference during the European Week of Astronomy & Space Science, which is taking place at the University of Hertfordshire, UK.

A scientific paper detailing the research has been submitted for publication in the journal Astronomy & Astrophysics.