A milestone was set in astronomy in 1995 with the discovery of 51 Peg b, the first planet known to orbit another star than our Sun. Today more than 230 planets have been found, and the progress in instrumentation pushed the detection limit further toward low-mass planets like our Earth. Very recently, we 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 possibly with liquid water on its surface. This super-Earth has about 5 times the mass of the Earth and orbits a red dwarf already known to harbour a Neptune-mass planet. In addition, we have strong evidence for the presence of a third planet with a mass about 8 Earth masses.

The existence of other worlds is an idea intriguing humanity for centuries. We finally got the first proof with the discovery of 51 Peg b (M. Mayor, D. Queloz, Nature, 1995). Since 51 Peg b is much more massive than Jupiter and its orbit radius 20 times smaller than the distance Earth to Sun, theories had to be completely reformulated to explain the existence of this kind of planets.

But would all extra-solar planets be similar, or are there other types of planets, and how would they look like? The discovery of 51 Peg b initiated strong search activities for exoplanets to answer these questions. More than 230 planets have been found by different research groups during the last 10 years with strong varying features like very short, but also very long periods. Some of them have extremely elliptical, others almost circular orbits; some have masses up to 15 times Jupiter’s mass, others with masses smaller than that of Saturn. But very light planets could not yet be found, since the applied detection technologies like the radial-velocity technique [1] were optimized more towards high-mass planets or short orbital periods, and therefore were not sensitive enough to detect Earth-mass planets.

In spite of the remarkable progress of increasing the precision of initial 15 ms^-1 in 1995 to about 3 ms^-1, a real ‘break through’ was needed. This occurred 2003 with the introduction of HARPS, an ultra-stabilized spectrograph for the 3.6-m telescope of the European Southern Observatory (ESO) at the La Silla Observatory in Chile. Now sub-meter-per-second velocity changes, i.e. the speed of a walking person, can be measured on a 1.5 million-km large star, several tens of light-years away from us. The first Neptune-mass planet µAra c was discovered in 2004 (Santos et al., A&A) with this new instrument. But the real capability of HARPS was demonstrated last year with the discovery of a system of three Neptunes, orbiting the same star HD69830 (Lovis et al. Nature, 2006), where a complicated velocity-signal superposition of only few meters-per-seconds, induced by the three planets, has to be disentangled. Today, 13 Neptune-mass planets are known, 11 of them discovered with HARPS.

Neptune-mass planets are still giants, 10 to 20 times more massive and 2-3 times larger than the Earth. They are likely to be built up of an icy core and a gaseous envelope. The few exoplanets, which by chance could be discovered by the transit method [3], confirmed this hypothesis. Furthermore, most of these planets orbit solar-type stars. As we know from our own system, the habitable zone [4] is located at about 1 AU, the distance between Earth and Sun. An Earth-mass planet at this distance from the Sun infers, however, an annual radial-velocity change of only 8 cms^-1, a value below today’s detection limit.

Fortunately among the stars, which are candidates for exoplanets, there are also M-dwarf stars besides objects like our Sun (G dwarfs). These stars are much colder and lighter than our Sun, and therefore react stronger to the pull of an orbiting planet that even Earth-mass planets become accessible. Due to the lower star temperature, the habitable zone is consequently located much closer to the star.

Gliese 581 is one of these stars, and harbours a Neptune-mass extra-solar planet, Gl 581 b (Bonfils et al., A&A, 2005). The analysis of the radial-velocity data of GI 581 indicated two additional low-mass planets, Gl 581 c and Gl 581 d. The first one is the smallest exoplanet found up to now. It completes a full orbit in 13 days and is 14 times closer to its star than the Earth from the Sun. Its minimum mass [2] is about 5 times that of the Earth, and from formation models we can estimate its diameter to be about 50% larger than that of the Earth. We deduce from these parameters that the gravity at the planet’s surface is twice that on Earth. The second one, GI 581 d with 8 Earth-masses completes its orbit in 84 days. The planetary system of Gl 581 contains thus at least 3 planets of less than 15 Earth masses.

Since its host star Gliese 581, a red dwarf [5], is smaller and colder than the Sun – and thus less luminous – the planet GI 581 c lies in the habitable zone. We estimated a mean temperature between 0°C and 40°C, thus water would be liquid. Moreover, its radius should be only 1.5 times the Earth radius, and models predict that the planet should be either rocky or covered with oceans. Since liquid water is necessary for life, Gl 581 c is considered to be the first known exoplanet able to harbour Life.

Due to HARPS efficiency in finding planetary systems, we are confident to find more Earth-mass planets around red dwarfs. The challenges will be to better characterize the physical conditions governing on these planets, for example the temperature distributions, the composition of the atmosphere, and possible signatures of Life. Powerful instruments of the next generation will allow to investigate these planets in more detail. Because of its temperature and relative proximity, Gl 581 will most probably be a prominent candidate for future space missions on the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with a high-lighted X.

Explanation notes:

[1] The radial-velocity method measures velocity variations of the central star, caused by the changing direction of the gravitational pull of the (unseen) orbiting exoplanet. The velocity change is deduced from the Doppler shift variation of the stellar absorption lines, and allows to determine the planet's orbit, in particular the period and the distance from the star.
[2] The radial-velocity technique leads to a minimum mass value, since the sine of the inclination of the orbital plane to the line of sight is unknown. This, however, is often close to the real mass of the system from a statistical point of view.
[3] If, by chance, the planetary orbital plane lies in the line of sight connecting the Earth with the star, the planet will pass in front of the star and produce an tiny decrease of the star light - a mini eclipse. From the measurement of this flux reduction, one can estimate the ratio between the planet’s and star’s diameter. Knowing the stellar diameter, one can finally compute the planetary diameter and, consequently, using the mass provided by the radial velocities, its density.
[4] The region around a star where water could be liquid is considered as habitable zone. This defines the planet surface temperature range and thus the distance range to the star, in which the planet must be located.
[5] Gl 581, or Gliese 581, is the 581^th entry in the Gliese Catalogue, listing all known stars within 25 parsecs (81.5 light years) away from the Sun. It was compiled by Gliese in 1969 and updated in 1991. The host star Gl 581 is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra (“the Scales”). Its mass is one third of the mass of the Sun. Such red dwarfs are at least 50 times intrinsically fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars 80 belong to this class.