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A harvest of low mass planets has been discovered with the High Accuracy Radial Velocity Planet Searcher (HARPS) at the ESO La Silla Observatory, and 45 other candidate planets with masses below 30 Earth masses have also been identified. The announcement was made by a team of European astronomers at the three day international conference ‘Extra-solar Super-Earths’ which began today. The team made very precise measurements of the velocities of many stars over the last five years, which revealed the planets by inducing a wobble in the star’s motion. “The perturbations induced by the planets are really tiny – the mass of the smallest planets is one hundred thousand times smaller than that of the star – and only the high sensitivity of HARPS made it possible to detect them," says Francois Bouchy of the Institut d'Astrophysique de Paris. "With the advent of much more precise instruments such as the HARPS spectrograph on ESO's 3.6 metre telescope at La Silla, we can now discover smaller planets, with masses between 2 and 10 times the Earth's mass," adds Stephane Udry from Geneva Observatory.
Artist impression of the new 'triple-Earth' system discovered by European astronomers using the HARP instrument at ESO's La Silla Observatory. The system orbits a star known to astronomers as HD 40307. Image: ESO. The three super-Earths, which have masses 4.2, 6.7 and 9.4 times the mass of the Earth, orbit a star only slightly less massive than our own Sun with periods of 4.3, 9.6 and 20.4 days respectively. The system is located 42 light years away towards the southern Doradus and Pictor constellations. In a second system, one 7.5 Earth mass planet orbits its parent star HD 181433 in 9.5 days and has a Jupiter-like companion orbiting at a relatively leisurely pace of 3 years. Yet another system highlighted by the research team is reported to have a 22-Earth mass planet orbiting with a period of 4 days and a Saturn-like planet orbiting with a three year period.
More remarkably, analysis of all the stars studied so far with HARPS shows that about one third of all solar-like stars have either super-Earth of Neptune-like planets with orbits shorter than 50 days. It is even possible that more planets are lurking in these identified systems, awaiting discovery, because a planet in a tight, short period orbit is easier to find than one in a wide, long-period orbit. "It is most probable that there are many other planets present: not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet. Add to it the Jupiter-like planets already known, and you may well arrive at the conclusion that planets are ubiquitous," says Udry.
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