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Planets 65 Mind-Stretcher. There could be alien planets in our own solar system. Billions of years ago, our Sun and another star may have swapped their most distant companions during an encounter that disrupted orbits in the outer parts of both systems. Each might have captured thousands of Sedna-like bodies from the other. One theory suggests that Sedna is a true alien planet, tugged away from the outer disk of a low-mass star that passed relatively close to our Sun. Theorists predicted that objects whose orbits are highly inclined to the plane of our own solar system (the ecliptic) would have come from outside. In 2005, astronomers found a minor planet larger than Pluto, but twice as far from the Sun—presently 9 billion miles out. Tentatively named Xena, this object is in an orbit inclined 44 degrees to the ecliptic. The new plan- etoid could have been discovered much sooner if anyone had looked that far away from the narrow band in which the classic planets revolve.*° Observatories planned for the near future will improve our ability to detect bodies orbiting other stars, possibly enabling us to identify Earth- sized planets. The Large Binocular Telescope Interferometer, designed to image exoplanets and to study circumstellar disks, is scheduled to go into operation soon on Arizona’s Mount Graham.*” The European orbiting observatory Corot, to be launched this year, and NASA’s Kepler satellite, scheduled for launch in 2008, will monitor thousands of stars for slight dips in brightness caused by planets crossing their faces. The first space-based observatories able to detect Earth-like bodies orbiting other stars, these BA ace de Af 1 nt missions may find thousands of planets. NASA’s Space Interferometry Mission, to be launched some years later, will survey 2000 nearby stars for indirect signatures of medium and large planets. For 200 closer stars, this mission should reveal hints of Earth-sized bodies. The proposed Terrestrial Planet Finder could have a good chance of finding terrestrial-type planets within 50 light-years of the Sun if a good fraction of stars in our neighborhood have such companions (planners assume that 10% do).** The most important discovery that astrobiology can realistically make in the next couple of decades, suggested Darling, is to find an out-of- equilibrium atmosphere on an extrasolar planet.*” We could use spectro- scopic analysis of the atmospheric composition of planets orbiting other stars to establish the existence of even nontechnological forms of life. Oxygen at any appreciable abundance would almost certainly indicate biology comparable to that of modern Earth; methane also would suggest some form of life. The first steps have been taken. Less than a decade after finding the first planet beyond our solar system, observers were able to study the atmo- sphere of a “hot Jupiter” that passed in front of its sun.*° Scientists have tested our ability to detect biosignatures by aiming the detectors of Mars missions at the Earth. They were able to identify several