Dec 15 2009
Two nearby stars have been found to harbor "super-Earths"¯ rocky planets larger than the Earth but smaller than ice giants such as Uranus and Neptune. Unlike previously discovered stars with super-Earths, both of the stars are similar to the Sun, suggesting to scientists that low-mass planets may be common around nearby stars.
"Over the last 12 years or so nearly 400 planets have been found, and the vast majority of them have been very large¯Jupiter mass or even larger," says researcher Paul Butler of the Carnegie Institution's Department of Terrestrial Magnetism. "These latest planets are part of a new trend of finding much smaller planets – planets that are more comparable to Earth."
The international team of researchers, co-led by Butler and Steven Vogt of the University of California, Santa Cruz, was able to detect the new planetary systems by combining data from observations spanning several years at the W. M. Keck Observatory in Hawaii and the Anglo-Australian Telescope in New South Wales, Australia. The researchers used the subtle "wobbling" of the stars caused by the planets' gravitational pull to determine the planets' size and orbits. Greg Henry at Tennessee State University independently monitored the brightness of the stars to rule out stellar "jitter"¯roiling of gases on a star's surface that can be confused with a planet-induced wobble.
The bright star 61 Virginis, visible with the naked eye in the constellation Virgo, is only 28 light-years from Earth and closely resembles the Sun in size, age and other properties. Earlier studies had eliminated the possibility of a Jupiter-sized planet orbiting 61 Virginis. In this study, the researchers found evidence of three low-mass planets, the smallest of which is five times the mass of Earth and speeds around the star once every four days.
Butler points out that the signal produced by this planet was one of the smallest ever detected. "One has to be very cautious when you claim a discovery," he says. "What gives us confidence is that we see the signal from two separate telescopes, and the two signals match up perfectly."
The other newly-discovered system orbits the star HD 1461, located 76 light-years from Earth. HD 1461 also closely resembles the Sun and is visible in the constellation Cetus. The researchers found clear evidence for one planet 7.5 times the mass of Earth and possible indications of two others. The 7.5-Earth-mass planet, designated HD 1461b, is intermediate in size between Earth and Uranus. It orbits its star once every six days.
These planets have orbits close to their stars and so they would be too hot to support life or liquid water. But Butler says that they point the way toward finding similar planets in similar orbits around nearby M-dwarfs, stars that are typically less than half the mass of the Sun and typically put out less than two percent the Sun's energy. "These sorts of planets around M dwarfs actually would be in a liquid water zone," he says. "So we are knocking on the door right now of being able to find habitable planets."