Astronomers Find First Habitable Earth-like Planet (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Stephane Udry, Michel Mayor
Observatory of Geneva University, Switzerland
Phone: +41 22 379 22 00

Xavier Delfosse, Thierry Forveille
LAOG, France
Phone: +33 476 51 42 06

Xavier Bonfils
Lisbonne Observatory, Portugal
Phone: +351 21 361 67 43

For Immediate Release: 25 April 2007

ESO Science Release 22/07

Astronomers Find First Habitable Earth-like Planet

The Dwarf Carried Other Worlds Too!

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
Sthane 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
extraterrestrial 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.

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 paper is available from
http://obswww.unige.ch/~udry/udry_preprint.pdf

The team is composed of Stephane 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), Franis Bouchy (Institut
d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aonomie
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-2005-BLG-390L,
discovered by microlensing with a network of telescopes including one at La
Silla (ESO 03/06), has a (real) mass of 5.5 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.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vornle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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