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Two Planets Identified as Most Similar to Earth By Thomas H. Maugh II, Los Angeles Times April 22, 2009 Both orbit the same star, 20 light-years from Earth, called Gliese 581. The larger of the two orbits in the 'habitable zone,' and could have oceans. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. AFP/Getty Images An illustration shows two exoplanets, Gliese 581ƒ|e in the left foreground and Gliese 581ƒ|d just to its right, orbiting a star that is 20 light-years from Earth. After locating more than 340 planets orbiting other stars, astronomers have identified two that are the most similar to Earth so far. The most recently discovered one is almost twice as large as Earth, making it the smallest exoplanet -- for extra-solar planet -- found to date. The second one was found in 2007, but new observations have shown that it is the only exoplanet to date that orbits its star in the so-called habitable zone, where water remains a liquid. Thus, it is the only exoplanet discovered that is likely to have oceans. Intriguingly, both orbit the same star, a dwarf 20 light-years from Earth called Gliese 581, European researchers said Tuesday. The identification of the small planet "is a remarkable discovery and bodes well for our eventual discovery of a true Earth-like, habitable planet," astronomer Alan Boss of the Carnegie Institution of Washington wrote in an e-mail. It "is the most exciting discovery in exoplanets so far," added astronomer Geoffrey W. Marcy of UC Berkeley via e-mail. "It shows that nature makes such small planets, probably in large numbers." The small planet is the fourth discovered circling Gliese 581 by a team of astronomers working with the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. They identified the planets by detecting and analyzing slight wobbles in the star's path as the planets orbit it. The small planet, called Gliese 581 e, has an estimated mass equal to 1.9 Earths and orbits its sun every 3.15 days, the team reported at an astronomical meeting at the University of Hertfordshire in Britain. Because it is so close to Gliese 581, it is blisteringly hot, and any gases or liquids that it might have carried have long since dissipated, leaving only uninhabitable rock. In February, French astronomers said they had discovered an even smaller planet, called CoRoT-Exo-7b, that has an estimated mass equal to 1.7 Earths, circling a different star. But experts said the data for Gliese 581 e is more convincing. The other three planets in the Gliese system have masses of 16, five and seven Earths. The one with a mass of seven Earths, called Gliese 581 d, was initially thought to have an orbital period of 80 days, which would put it just on the outer edge of the habitable zone. Recent refinements of the data, however, show that it has an orbit of only 66.8 days, which places it well within the habitable zone, astronomer Stephane Udry of Geneva University told the meeting. Because of its distance from Gliese 581, moreover, it must have a significant amount of water and other gases, he added. It could have oceans thousands of meters deep, he said. The team is continuing to monitor Gliese 581 in hopes that the orbital planes of the planets will bring them between the star and Earth, which will allow astronomers to learn more about their composition. [email protected] Astronomers Find Planet Closer to Size of Earth By Dennis Overbye, The New York Times April 21, 2009 European astronomers said Tuesday that they had discovered the smallest planet yet found orbiting another star. The planet could be as little as only 1.9 times as massive as the Earth and belongs to a dim red star known as Gliese 581, which lies about 20 lightyears from Earth in the constellation Libra. The star was already know to harbor at least three more massive planets. The new planet, known as Gliese 581e, is probably rocky like the Earth, but it lies in such a close orbit — only three million miles from its star — that it is surely blasted with too much radiation and heat to be livable. Michel Mayor, of Geneva Observatory, and his colleagues announced their results at a conference at the University of Hertfordshire in Britain and in a paper submitted to the journal Astronomy and Astrophysics. 2 Astronomers said the discovery was more encouragement that the galaxy was full of small-mass planets and that with more time and improved instruments like the Kepler satellite, recently launched by NASA, they would eventually find Earth-like planets in orbits suitable for life around other stars. “Finding Earth-like planets with lukewarm temperatures is the next great goal,” Geoff Marcy, of the University of California, Berkeley, a planet-hunting rival of Dr. Mayor’s, said in an e-mail message. “This is the most exciting discovery in exoplanets so far,” Dr. Marcy said. Dr. Mayor’s group also discovered the first exoplanet, a gas giant 160 times the mass of the Earth, in 1995, using a technique known popularly as the “wiggle” method that detects planets by a slight gravitational tug they give their stars. The method is most sensitive to massive planets in close orbits. In a statement, Dr. Mayor noted that the new planet is only one-eightieth of the mass of the first one, saying, “This is tremendous progress in 14 years.” The discovery also cements the Gliese system as one of the most promising exoplanet systems. Two years ago, the third planet from that star was hailed as a ”Goldilocks” planet, where liquid water and thus life might be possible, until calculations showed that the greenhouse effect would broil it. But the new data also shifted the orbit of the star’s outermost planet inward so that it now appears to revolve in the so-called habitable zone where liquid water is possible, according to Stéphane Udry of Geneva University, one of the team members. That planet, 581d, is about seven times as massive as the Earth, Dr. Udry explained, which is too big to be just rock. It probably formed as a combination of ice and rock farther out in the Gliese system and then migrated inward, according to various planetary formation models, and melted. He called it the first serious “water world candidate.” Sara Seager, a planet theorist at the Massachusetts Institute of Technology, said in an e-mail message that the Gliese planetary system “is like the gift that keeps on giving.” 3