By Jonathan Amos

Science reporter, BBC News

Artist's impression: Astronomers are finding smaller and smaller planets

Astronomers have announced a haul of planets found beyond our Solar System. The 32 "exoplanets" ranged in size from five times the mass of Earth to 5-10 times the mass of Jupiter, the researchers said. They were found using a very sensitive instrument on a 3.6m telescope at the European Southern Observatory's La Silla facility in Chile. The discovery is exciting because it suggests that low-mass planets could be numerous in our galaxy. "From [our] results, we know now that at least 40% of solar-type stars have low-mass planets. This is really important because it means that low-mass planets are everywhere, basically," explained Stephane Udry from Geneva University, Switzerland. "What's very interesting is that models are predicting them, and we are finding them; and furthermore the models are predicting even more lower-mass planets like the Earth." Size selection The discovery now takes the number of known exoplanets - planets outside our Solar System - to more than 400. These have been identified using a range of astronomical techniques and telescopes, but this latest group was spotted as a result of observations made with the Harps spectrometer at La Silla. The High Accuracy Radial Velocity Planet Searcher instrument employs what is sometimes called the "wobble technique". This is an indirect method of detection that infers the existence of orbiting planets from the way their gravity makes a parent star appear to twitch in its motion across the sky. Astronomy is working right at the limits of the current technology capable of detecting exoplanets and most of those found so far are Jupiter-scale and bigger. Harps, however, has focussed its efforts on small, relatively cool stars - so-called M-class stars - in the hope of finding low-mass planets, ones most likely to resemble the rocky planets in our own Solar System. Of the 28 planets known with masses below 20 Earth-masses, Harps has now identified 24 of them - and six of those are in the newly announced group. "We have two candidates at five Earth-masses and two at six Earth-masses," Professor Udry told BBC News. Combined approach Harps has previously identified an object which is only twice as massive as the Earth (announced in April). Scientists are confident this planet harbours no life, though, because it orbits so close to its parent star that surface temperatures would be scorching. In revealing the new collection of planets on Monday, the Harps team-members said they expected to confirm the existence of another batch, similar in number, during the coming six months. The ultimate goal is to find a rocky planet in a star's "habitable zone", an orbit where temperatures are in a range that would support the presence of liquid water. Scientists believe the introduction of newer, more sensitive technologies will allow them to identify such an object within just a few years from now. The US space agency (Nasa) recently launched its Kepler telescope. This hopes to find Earth-size planets by looking for the tiny dip in light coming from a star as an object crosses its face as viewed from Earth. To properly characterise a planet, different observing techniques are required. The Kepler "transit" method reveals the diameter of an object, but a Harps-like measurement is needed to resolve the mass. Jonathan.Amos-INTERNET@bbc.co.uk



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