By Paul Rincon and Jonathan Amos
Science reporters, BBC News
An artist's impression of Gliese 581 e with its star in the distance
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," he told BBC News.
THE GLIESE 581 'SOLAR SYSTEM'
From closest in to furthest out
Planet e is 1.9 Earth masses; Planet b is 16 Earth masses; Planet c is 5 Earth masses; Planet d is 7 Earth masses
The first planet to be found is always given the 'b' designation
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.
The US space agency (Nasa) recently launched its Kepler telescope dedicated to finding Earth-size planets. It will use a different approach to the HARPS/La Silla set-up.
Kepler will look for the tiny dip in light coming from a star as a planet crosses its face as viewed from Earth.
Michel Mayor commented: "The challenge in coming years will be to find Earth-mass planets in the habitable zones of stars."
He added: "I'm absolutely confident that in one year or two years, we will arrive at [a planet with] the mass of the Earth."
In the future, some of these planets could be imaged in some detail by the next generation of ground telescopes.
The E-ELT will dwarf all previous telescopes
One of these projects, the European Extremely Large Telescope (E-ELT) is currently coming to the end of its design process.
"One of the interesting things about today's announcement is that some of the planets in this system would actually be imageable with the next generation of telescopes," said Isobel Hook, from Oxford University and the UK project scientist on the E-ELT.
"The type of technology coming along, such as extreme adaptive optics, will allow you to produce very sharp images. The seven-Earth-mass planet we think could be imaged directly. You would be able to see it go around its star and see what it was made of," she told BBC News.
Tim de Zeeuw, director-general of the European Southern Observatory (Eso) organisation, which will operate the E-ELT, told BBC News: "The E-ELT will make it possible to take images of (Earth-mass planets) and indeed find evidence for many of them.
"This then leads to very interesting questions: do we find many Solar Systems like our own? Or is there only one like us?"
He added: "I don't follow this field daily... (but) the number of cases we have is steadily growing to a size where we can start asking this question and there are some indications that perhaps our Solar System is a little unusual."
The 42m E-ELT comprises five large mirrors. Its adaptive optics system will compensate for the distortions to images of the sky caused by turbulence in the Earth's atmosphere.
Construction of the ground observatory could begin in 2011 if all goes to plan. Eso intends to select a location for the telescope by the end of this year.
Six sites have been shortlisted: three in Chile; one in the Canary Islands, Spain; one in Morocco; and one in Argentina.
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