Rocky planets discovered around nearby stars

Physics Today : Three papers indicate new candidates for rocky Earth-type planets around other star systems. David Charbonneau and colleagues published in Nature the clearest evidence yet for an Earth-type planet only 2.7 times larger than the Earth.Two nearby stars, similar to the Sun, have been found to harbor “super-Earths"—rocky planets larger than Earth but smaller than ice giants such as Uranus and Neptune—of five to seven times to the size of the Earth. These results, to be published in the Astrophysical Journal and in the Monthly Notices of the Royal Astronomical Society , suggest that low-mass planets may be common around nearby Sun-like stars."Over the last 12 years or so nearly 400 planets have been found, and the vast majority of them have been very largeâJupiter 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.” A new techniqueCharbonneau and his colleagues discovered their new low-mass planet by using eight 40-cm diameter telescopes, coupled with IR CCD light detectors. With these telescopes they looked at stars to see which ones regularly dimmed for periodic intervals, indicating the passage of a planet. The dimness is proportional to the size of the planet relative to the star. By checking small stars they were able to tease out smaller planets from the background noise. One star, a red dwarf called GJ 1214 , which is about 42 light-years from Earth, repeatedly dimmed for 52 minutes every 1.6 days, indicating a planet with an orbit of 1.6 days. The group calculated that it had a radius of 12% of that of the star, which is roughly 2.7 times the size of Earth.The drag caused by the planet on GJ 1214 allowed the group to calculate its density, of 1.9 g cm ^-3, compared with Earth’s, which is 5.5 g cm ^-3, which suggests that because water’s density is 1 g cm ^-3 , the planet is a mixture of rock and water, with other trace gases."Despite its hot temperature, this appears to be a waterworld,” said Zachory Berta , a graduate student at the Harvard-Smithsonian Center for Astrophysics, who first spotted the hint of the planet among the data. “It is much smaller, cooler, and more Earthlike than any other known exoplanet."Berta added that some of the planet’s water should be in the form of exotic materials like Ice VII —a crystalline form of water that exists at pressures greater than 20,000 times Earth’s sea-level atmosphere.Renowned planet hunter Geoffrey Marcy from the University of California, Berkeley, writes in Nature : The next step for astronomers is to try to directly detect and characterize the atmosphere, which will require the Hubble Space Telescope."Since this planet is so close to Earth, Hubble should be able to detect the atmosphere and determine what it’s made of,” said Charbonneau. “That will make it the first super-Earth with a confirmed atmosphere—even though that atmosphere probably won’t be hospitable to life as we know it.” Rocky planets round Sun-like starsThe 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"âroiling 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 2% 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."Paul Guinnessy Related papers A super-Earth transiting a nearby low-mass star Nature 61 Vir paper (to appear in The Astrophysical Journal) 23 Lib paper (submitted to the Monthly Notices of the Royal Astronomical Society) Related links Paul Butler’s earthbound planet search web page Related news stories Extrasolar planets: Water world larger than Earth Nature Super-earth found close by, may host water Science News ‘Waterworld’ planet six times the size of Earth discovered The Daily Telegraph Nearby Super-Earth may be a waterworld Space.com Super-Earth found close by, may host water sciencenews