The planets were found by Australian, American and British astronomers using the Anglo-Australian Telescope in New South Wales and the Keck Telescope in Hawai'i. They employed the "Doppler wobble" technique, which measures how stars are tugged around by their planets' gravity.
Three planets with masses ranging from 5.3 to 24.9 Earth masses orbit the star 61 Virginis, which is virtually a twin of the Sun.
"These planets are particularly exciting," said team member Professor Chris Tinney of the University of NSW. "Neptune in our Solar System has a mass 17 times that of the Earth. It looks like there may be many Sun-like stars nearby with planets of that mass or less. They point the way to even smaller planets that could be rocky and suitable for life."
61 Virginis can be seen with the naked eye. It lies 28 light-years from Earth in the constellation of Virgo, which at this time of year can be seen rising a few hours before the Sun. The findings for 61 Virginis are to be published in The Astrophysical Journal.
The fourth planet the research team found is a Jupiter-mass planet orbiting the Sun-like star 23 Librae. 23 Librae lies 84 light-years away in the constellation of Libra. Another planet was found around this star in 2006: this new one is the second.The new planet has a 14-year orbit. This makes it very like Jupiter, which has a 12-year orbit.
"In fact, what we detect from this star system is very like what we'd detect from our own Solar System if we were observing it from a distance, because Jupiter has the strongest gravitational effect of all our Sun's planets," said Dr Simon O'Toole of the Anglo-Australian Observatory, a member of the planet-hunting team.
"We are now in a position to quantify how common planets like Jupiter are around stars like our Sun," said team member Hugh Jones of University of Hertfordshire. "Compared to the Solar System, most extrasolar systems look odd, with planets in very small or very elliptical orbits. In contrast, this new planet has an orbit that is both large, and nearly circular—and for the first time we are beginning to see systems that resemble our own."
"These detections are truly at the current state-of-the-art," said team member Dr Paul Butler of the Carnegie Institute of Washington, "The inner planet of the 61 Vir system is among the two or three lowest-amplitude planetary signals that have been identified with confidence. We've found there's a tremendous advantage to be gained from combining data from two world-class observatories, and it's clear that we'll have an excellent shot at identifying potentially habitable planets around the very nearest stars within just a few years."
Chris Tinney | EurekAlert!
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