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Planetary News: Extrasolar Planets (2008)Trio of Super-Earths Found to Orbit Nearby StarJune 17, 2008
An international team of astronomers working from the European Southern Observatory in La Silla, Chile, have discovered a trio of low-mass planets orbiting a single star 42 light years away. Ranging in size from 4.2 to 9.4 Earth masses, the three are among the lowest mass planets yet discovered, and belong to a class known informally as "Super-Earths" – more massive than Earth but less than Uranus and Neptune. While the planets are scorching hot and unlikely to support life, their unexpected discovery around a star thought to possess no planets is intriguing. "Does every single star harbor planets and, if yes, how many?" wonders veteran planet hunter Michel Mayor of the Geneva Observatory, who led the team that discovered the first exoplanet over a decade ago. "We may not yet know the answer but we are making huge progress towards it." Mayor and his colleagues discovered the three super-Earths with the aid of HARPS – the High Accuracy Radial Velocity Planet Searcher – which is attached to the 3.6 meter telescope at La Silla. HARPS is designed to detect the slight back and forth motion of a star as it rocks to the tug of an orbiting planet, and is so sensitive it can register a movement as slight as 1 meter per second – equivalent to a leisurely stroll. To date HARPS has detected a total of 45 candidate planets with a mass below 30 Earth masses and an orbital period shorter than 50 days. Most of the more than 270 exoplanets found so far are gas giants, similar in mass and composition to Jupiter and Saturn. But with HARPS and a few other instruments of comparable sensitivity, smaller rocky worlds of between 2 and 10 Earth masses are being detected as well. "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," said co-discoverer Francois Bouchy of the Institut d'Astrophysique de Paris, France.
The discovery did not happen overnight, but is the result of years of accumulated data on the spectrum of the star HD 40307. "We have made very precise measurements of the velocity of the star . . . over the last five years, which clearly reveal the presence of three planets," says Mayor. And what does the discovery mean for the search for exoplanets? "Clearly these planets are only the tip of the iceberg," said Mayor. "The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days." "It is most probable that there are many other planets present" agreed Stephane Udry, Mayor's colleague at the Geneva Observatory. "Not only super-Earths 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." Did you like this story? Send
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