Well-known exoplanet researcher Michel Mayor today announced the
discovery of the lightest exoplanet found so far. The planet, “e”, in
the famous system Gliese 581, is only about twice the mass of our
Earth. The team also refined the orbit of the planet Gliese 581 d,
first discovered in 2007, placing it well within the habitable zone,
where liquid water oceans could exist. These amazing discoveries are
the outcome of more than four years of observations using the most
successful low-mass-exoplanet hunter in the world, the HARPS
spectrograph attached to the 3.6-metre ESO telescope at La Silla,
Chile.

“The holy grail of current exoplanet research is the detection of a
rocky, Earth-like planet in the ‘habitable zone’ — a region around
the host star with the right conditions for water to be liquid on a
planet’s surface”, says Michel Mayor from the Geneva Observatory, who
led the European team to this stunning breakthrough.

Planet Gliese 581 e orbits its host star — located only 20.5
light-years away in the constellation Libra (“the Scales”) — in just
3.15 days. “With only 1.9 Earth-masses, it is the least massive
exoplanet ever detected and is, very likely, a rocky planet”, says
co-author Xavier Bonfils from Grenoble Observatory.

Artist’s impression of the newly discovered planetary system Gliese 581.

Being so close to its host star, the planet is not in the habitable
zone. But another planet in this system appears to be. From previous
observations — also obtained with the HARPS spectrograph at ESO’s La
Silla Observatory and announced two years ago — this star was known
to harbor a system with a Neptune-sized planet (ESO 30/05) and two
super-Earths (ESO 22/07). With the discovery of Gliese 581 e, the
planetary system now has four known planets, with masses of about 1.9
(planet e), 16 (planet b), 5 (planet c), and 7 Earth-masses (planet
d). The planet furthest out, Gliese 581 d, orbits its host star in
66.8 days. “Gliese 581 d is probably too massive to be made only of
rocky material, but we can speculate that it is an icy planet that has
migrated closer to the star,” says team member Stephane Udry. The new
observations have revealed that this planet is in the habitable zone,
where liquid water could exist. “‘d’ could even be covered by a large
and deep ocean — it is the first serious ‘water world’ candidate,”
continued Udry.

The gentle pull of an exoplanet as it orbits the host star introduces
a tiny wobble in the star’s motion — only about 7 km/hour,
corresponding to brisk walking speed — that can just be detected on
Earth with today’s most sophisticated technology. Low-mass red dwarf
stars such as Gliese 581 are potentially fruitful hunting grounds for
low-mass exoplanets in the habitable zone. Such cool stars are
relatively faint and their habitable zones lie close in, where the
gravitational tug of any orbiting planet found there would be
stronger, making the telltale wobble more pronounced. Even so,
detecting these tiny signals is still a challenge, and the discovery
of Gliese 581 e and the refinement of Gliese 581 d’s orbit were only
possible due to HARPS’s unique precision and stability.

“It is amazing to see how far we have come since we discovered the
first exoplanet around a normal star in 1995 — the one around 51
Pegasi,” says Mayor. “The mass of Gliese 581 e is 80 times less than
that of 51 Pegasi b. This is tremendous progress in just 14 years.”

The astronomers are confident that they can still do better. “With
similar observing conditions an Earth-like planet located in the
middle of the habitable zone of a red dwarf star could be detectable,”
says Bonfils. “The hunt continues.”

Notes

This discovery was announced today at the JENAM conference during the
European Week of Astronomy & Space Science, which is taking place at
the University of Hertfordshire, UK. The results have also been
submitted for publication in the research journal Astronomy &
Astrophysics (“The HARPS search for southern extra-solar planets:
XVIII. An Earth-mass planet in the GJ 581 planetary system”, by Mayor
et al., 2009).

The team is composed of M. Mayor, S. Udry, C. Lovis, F. Pepe and D.
Queloz (Geneva Observatory, Switzerland), X. Bonfils, T. Forveille ,
X. Delfosse, H. Beust and C. Perrier (LAOG, France), N. C. Santos
(Centro de Astrofisica, Universidade de Porto), F. Bouchy (IAP, Paris,
France) and J.-L. Bertaux (Service d’Aeronomie du CNRS,
Verrières-le-Buisson, France).

ESO, the European Southern Observatory, is the foremost
intergovernmental astronomy organisation in Europe. It is supported by
14 countries: Austria, Belgium, the Czech Republic, Denmark, France,
Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden,
Switzerland and the United Kingdom. ESO carries out an ambitious
programme focused on the design, construction and operation of
powerful ground-based observing facilities enabling astronomers to
make important scientific discoveries. ESO also plays a leading role
in promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in the Atacama
Desert region of Chile: La Silla, Paranal and Chajnantor.

Science Contacts:
Michel Mayor
Geneva University, Switzerland
E-mail: michel.mayor@unige.ch

Prof. Mayor will attend the JENAM conference from 20 to 21 April and
can be reached by phone through the JENAM press centre.

Xavier Bonfils, Thierry Forveille
Grenoble Observatory, France
Phone: +33 476 63 55 27, +33 4 76 51 42 15
E-mail: xavier.bonfils@obs.ujf-grenoble.fr,
thierry.forveille@obs.ujf-grenoble.fr

Stephane Udry
Geneva University, Switzerland
Phone: +41 22 379 2467
E-mail: stephane.udry@unige.ch

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