The Dwarf that Carries a World

December 03, 2005 - A team of French and Swiss astronomers have discovered one of the lightest extrasolar planets ever found using the HARPS instrument on ESO's (European Southern Observatory) 3.6-m telescope at La Silla (Chile).

The new planet orbits a little star belonging to the class of red dwarfs. These type of stars is the most frequent in the galaxy and the new findings could mean that more extrasolar planets exist than previous thought.

 "Our finding possibly means that planets are rather frequent around the smallest stars," says Xavier Delfosse, from the Laboratoire d'Astrophysique de Grenoble (France). "It certainly tells us that red dwarfs are ideal targets for the search for exoplanets."

The host star, Gl 581 is located 20.5 light-years away in the Libra constellation and has a mass of only 1/3 the mass of the Sun. Such red dwarfs are at least 50 times fainter than the Sun and among the 100 closest stars to the Sun, 80 belong to this class.

Previous surveys were rather unsuccessful. Observations of about 200 red dwarfs revealed only 2 with planets.

"But previous surveys may have missed many planets due to their insufficient precision," says Stéphane Udry, from the Geneva Observatory. "This is why we decided to make use of the ultra-precise, second generation, HARPS spectrograph. Our new result indicates this was the right strategy."

The newly found planet is about 17 times the Earth's mass, or about the mass of Neptune. It is therefore one of the smallest extrasolar planets ever found. The planet is very close to its host star (only 6 million kilometres away) and completes a full circle in only 5.4 days. By comparison, Mercury, the closest planet to the Sun, is at a distance of 58 million kilometres and completes an orbit in 88 days. Being so close, this alien world must be very hot, about 150 degrees.

The planet was revealed by the wobble due to the mutual attraction of mass. With the HARPS very precise measurements, the astronomers found the star to move back and forth with a maximum velocity of 13 metres/second, or a little bit less than 50 km/h.