Artist's impression of the planetary system around the red dwarf Gliese 581. Using the instrument HARPS on the ESO 3.6-m telescope, astronomers have uncovered 3 planets, all of relative low-mass: 5, 8 and 15 Earth masses. The five Earth-mass planet makes a full orbit around the star in 13 days, the other two in 5 and 84 days. (c) ESO

This exoplanet - as astronomers call planets around a star other than the Sun – is the smallest ever found up to now [1] and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581 [2], is smaller and colder than the Sun – and thus less luminous – the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid!

“We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid,” explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. “Moreover, its radius should be only 1.5 times the Earth’s radius, and models predict that the planet should be either rocky – like our Earth – or covered with oceans,” he adds.

“Liquid water is critical to life as we know it,” avows Xavier Delfosse, a member of the team from Grenoble University (France). “Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X.”

The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra (“the Scales”). It has a mass of only one third the mass of the Sun. Such red dwarfs are intrinsically at least 50 times fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars to the Sun, 80 belong to this class.

“Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun,” emphasizes Xavier Bonfils, a co-worker from Lisbon University. Planets lying in this zone are then more easily detected with the radial-velocity method [3], the most successful in detecting exoplanets.

Two years ago, the same team of astronomers already found a planet around Gliese 581 (see ESO 30/05). With a mass of 15 Earth-masses, i.e. similar to that of Neptune, it orbits its host star in 5.4 days. At the time, the astronomers had already seen hints of another planet. They therefore obtained a new set of measurements and found the new super-Earth, but also clear indications for another one, an 8 Earth-mass planet completing an orbit in 84 days. The planetary system surrounding Gliese 581 contains thus no fewer than 3 planets of 15 Earth masses or less, and as such is a quite remarkable system.

The discovery was made thanks to HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. Located on the ESO 3.6-m telescope at La Silla, Chile, HARPS is able to measure velocities with a precision better than one metre per second (or 3.6 km/h)! HARPS is one of the most successful instruments for detecting exoplanets and holds already several recent records, including the discovery of another ‘Trio of Neptunes’ (ESO 18/06, see also ESO 22/04).

The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.

“HARPS is a unique planet hunting machine,” says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. “Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!”

HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets – HD 69830 and Gl 581 – were discovered by HARPS.

“And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach,” affirms Mayor.

Re: Astronomers Find First Habitable Earth-like Planet Astronuc    April 25th, 2007 - 9:22 PM I read about this early this morning.  New 'super-Earth' found in space http://news.bbc.co.uk/1/hi/sci/tech/6589157.stm http://en.wikipedia.org/wiki/Gliese_581_c Quote The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra (“the Scales”). Only 20.5 ly away!  Practically in the neighborhood. Too bad about the 5g's though.  I'd weigh over 850 lbs on that planet. I imagine that it is quite cool there with a red dwarf nearby. I wonder if any Librans are looking back.

Re: Astronomers Find First Habitable Earth-like Planet smich    May 11th, 2007 - 3:32 PM Hi all, I'm interested in this one from a weather point of view! A 13 day orbit....a bit of axis tilt, and you get 4 seasons in 2 weeks! Also - tides. Given the size of the two other companions, they would be almost tsunami-like. And another - if it's tidally locked and the orbit isn't too eccentric, possibility of life on the terminator? If the atmosphere is substantial enough, it might just spread the heat out too...rather than evaporating on the inward side and freezing on the other. One sobering thought though. From 20 light years away, I expect Venus would look like a very life supporting world... ...but then, perhaps it was once?! Steve M

Re: Astronomers Find First Habitable Earth-like Planet Astronuc    June 8th, 2007 - 7:55 PM Earth-Like Planet Discovered in Libra http://www.npr.org/templates/story/story.php?storyId=9796321 · Morning Edition, April 25, 2007 Quote Scientists have discovered a new planet in the constellation Libra. The small, rocky planet is special because it appears to have mild temperatures, like Earth. Researchers believe it looks like the first planet outside of our solar system that could be home to liquid water, and maybe even life. Our solar system has only eight planets — nine if you count Pluto. But outside of our solar system, around other stars, scientists have found dozens and dozens of planets. "We have discovered more than 100 planets, here in Geneva," says Michel Mayor, a planet hunter at the University of Geneva. Almost all of these known "extrasolar" planets are giant balls of gas, much like Jupiter or Saturn. Such massive planets are relatively easy to find. They have a gravitational pull that makes their stars wobble, and when scientists see that wobble, they know there is a planet. Small, rocky planets cause less of a wobble, making them harder to find. Still, Mayor and his colleagues have had some luck using the European Southern Observatory's big telescope at La Silla, Chile. They recently pointed it at a nearby star called Gliese 581, in the constellation Libra.

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