AP: First detection of extra-solar terrestial sized planet?

WASHINGTON -- For the first time astronomers have discovered a planet outside our solar system that is potentially habitable, with Earth-like temperatures, a find researchers described Tuesday as a big step in the search for "life in the universe."

The planet is just the right size, might have water in liquid form, and in galactic terms is relatively nearby at 120 trillion miles away. But the star it closely orbits, known as a "red dwarf," is much smaller, dimmer and cooler than our sun.

There's still a lot that is unknown about the new planet, which could be deemed inhospitable to life once more is known about it. And it's worth noting that scientists' requirements for habitability count Mars in that category: a size relatively similar to Earth's with temperatures that would permit liquid water. However, this is the first outside our solar system that meets those standards.

"It's a significant step on the way to finding possible life in the universe," said University of Geneva astronomer Michel Mayor, one of 11 European scientists on the team that found the planet. "It's a nice discovery. We still have a lot of questions."

The results of the discovery have not been published but have been submitted to the journal Astronomy and Astrophysics.

Alan Boss, who works at the Carnegie Institution of Washington where a U.S. team of astronomers competed in the hunt for an Earth-like planet, called it "a major milestone in this business."

The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds.

What they revealed is a planet circling the red dwarf star, Gliese 581. Red dwarfs are low-energy, tiny stars that give off dim red light and last longer than stars like our sun. Until a few years ago, astronomers didn't consider these stars as possible hosts of planets that might sustain life.

The discovery of the new planet, named 581 c, is sure to fuel studies of planets circling similar dim stars. About 80 percent of the stars near Earth are red dwarfs.

The new planet is about five times heavier than Earth. Its discoverers aren't certain if it is rocky like Earth or if its a frozen ice ball with liquid water on the surface. If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, as Mayor suggests, it would be even bigger.

Based on theory, 581 c should have an atmosphere, but what's in that atmosphere is still a mystery and if it's too thick that could make the planet's surface temperature too hot, Mayor said.

However, the research team believes the average temperature to be somewhere between 32 and 104 degrees and that set off celebrations among astronomers.

Until now, all 220 planets astronomers have found outside our solar system have had the "Goldilocks problem." They've been too hot, too cold or just plain too big and gaseous, like uninhabitable Jupiter.

The new planet seems just right _ or at least that's what scientists think.

"This could be very important," said NASA astrobiology expert Chris McKay, who was not part of the discovery team. "It doesn't mean there is life, but it means it's an Earth-like planet in terms of potential habitability."

Eventually astronomers will rack up discoveries of dozens, maybe even hundreds of planets considered habitable, the astronomers said. But this one _ simply called "c" by its discoverers when they talk among themselves _ will go down in cosmic history as No. 1.

Besides having the right temperature, the new planet is probably full of liquid water, hypothesizes Stephane Udry, the discovery team's lead author and another Geneva astronomer. But that is based on theory about how planets form, not on any evidence, he said.

"Liquid water is critical to life as we know it," co-author Xavier Delfosse of Grenoble University in France, said in a statement. "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 extraterrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."

Other astronomers cautioned it's too early to tell whether there is water.

"You need more work to say it's got water or it doesn't have water," said retired NASA astronomer Steve Maran, press officer for the American Astronomical Society. "You wouldn't send a crew there assuming that when you get there, they'll have enough water to get back."

The new planet's star system is a mere 20.5 light years away, making Gliese 581 one of the 100 closest stars to Earth. It's so dim, you can't see it without a telescope, but it's somewhere in the constellation Libra, which is low in the southeastern sky during the midevening in the Northern Hemisphere.

"I expect there will be planets like Earth, but whether they have life is another question," said renowned astrophysicist Stephen Hawking in an interview with The Associated Press in Orlando. "We haven't been visited by little green men yet."

Before you book your extrastellar flight to 581 c, a few caveats about how alien that world probably is: Anyone sitting on the planet would get heavier quickly, and birthdays would add up fast since it orbits its star every 13 days.

Gravity is 1.6 times as strong as Earth's so a 150-pound person would feel like 240 pounds.

But oh, the view. The planet is 14 times closer to the star it orbits. Udry figures the red dwarf star would hang in the sky at a size 20 times larger than our moon. And it's likely, but still not known, that the planet doesn't rotate, so one side would always be sunlit and the other dark.

Distance is another problem. "We don't know how to get to those places in a human lifetime," Maran said.

Two teams of astronomers, one in Europe and one in the United States, have been racing to be the first to find a planet like 581 c outside the solar system.

The European team looked at 100 different stars using a tool called HARPS (High Accuracy Radial Velocity for Planetary Searcher) to find this one planet, said Xavier Bonfils of the Lisbon Observatory, one of the co-discoverers.

Much of the effort to find Earth-like planets has focused on stars like our sun with the challenge being to find a planet the right distance from the star it orbits. About 90 percent of the time, the European telescope focused its search more on sun-like stars, Udry said.

A few weeks before the European discovery earlier this month, a scientific paper in the journal Astrobiology theorized a few days that red dwarf stars were good candidates.

"Now we have the possibility to find many more," Bonfils said.

And the actual press release:

www.spaceref.com/news/viewpr.html?pid=22469

Image: Artist's impression of the system of three planets surrounding the red dwarf Gliese 581. One of them is the first rocky planet lying in the habitable zone to have been... Click here for more information.

Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf, already known to harbour a Neptune-mass planet. The astronomers have also strong evidence for the presence of a third planet with a mass about 8 Earth masses.

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.

High-resolution images and broadcast quality material is available. During the embargo period, please contact Henri Boffin. Once the embargo is lifted, the material will be available from www.eso.org/outreach/press-rel/pr-2007/pr-22-07.html

More Information

This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system", by S. Udry et al.)

The team is composed of Stéphane Udry, Michel Mayor, Christophe Lovis, Francesco Pepe, and Didier Queloz (Geneva Observatory, Switzerland), Xavier Bonfils (Lisbonne Observatory, Portugal), Xavier Delfosse, Thierry Forveille, and C.Perrier (LAOG, Grenoble, France), François Bouchy (Institut d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aéronomie du CNRS, France)

Notes

[1] Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system. Two other systems have a mass close to this. The icy planet around OGLE-05-390L, discovered by microlensing with a network of telescopes including one at La Silla (ESO 03/06), has a (real) mass of 5.7 Earth masses. It, however, orbits much farther from its small host star than the present one and is hence much colder. The other is one of the planets surrounding the star Gliese 876. It has a minimum mass of 5.89 Earth masses (and a probable real mass of 7.53 Earth masses) and completes an orbit in less than 2 days, making it too hot for liquid water to be present.

[2] Gl 581, or Gliese 581, is the 581th entry in the Gliese Catalogue, which lists all known stars within 25 parsecs (81.5 light years) of the Sun. It was originally compiled by Gliese and published in 1969, and later updated by Gliese and Jahreiss in 1991.

[3] This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows deducing the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.

Thank you for this interesting article, how amazing this is, how hugely far away it is also.

Water is the most abundant molecule on the Earth, “H2O”
Hydrogen and Oxygen exist in huge quantities in the Universe, but not always together and so far only on Earth as a liquid on the surface.
Here on the Earth water exists in all three states Solid- Ice, Liquid- and Vapor- Steam
We are so lucky that our host star is far enough away that it does not boil our water and burn off our atmosphere and make life unsustainable.
We are so lucky that liquid water is here in vast quantities.
We are so lucky that our planet is in a such a fine balance in orbit around our star.
I think that we should be more aware of this uniqueness and take more care of this planet.
We should take care of our water, and never take it for granted.
This super earth sounds like it’s in a balance of it’s own in its orbit around Gliese 581 what a wonderful discovery.

Deirdre Kelleghan

A great discovery and hopefully the first of many.

I really think the astrobiologists have outdone themselves this time though. they have not even found water and already they are talking about life on this new planet.

Before you know it we will have reverted to the spontaneous generation of life theory.

One has to smile at the words "only 20 light years".
It would take hundreds of thousands,if not millions,of years for our fastest space probes to cross that vast gulf.

Peter.

This is great news! Lets hope this is the first of many to come soon!

Bart.