Extrasolar Planets

All of the extrasolar planets detected so far are gas giants with masses on the order of Jupiter's; however, this is probably an artifact of the methods being used, which are only capable of detecting large planets. The orbits observed vary wildly—some planets are closer to their stars than Mercury is to the Sun, with orbits lasting mere days, while others are separated from their stars by many times the Earth-Sun distance. Planets with nearly round, Earth-like orbits have been discovered as well as planets whose orbits more closely approximate the eccentric elliptical shape of cometary paths. The stars around which planets have been discovered include dying stars, twin stars, Sun-like stars, and pulsars, with locations ranging from a Barnard's Star to more than 150 light years.

In 1998, astronomers discovered a protoplanet (planet in the process of formation) apparently in the midst of being ejected from its star system. Infrared images from the Hubble Space Telescope showed a pinpoint object with a 130 billion-mile-long filamentary structure trailing behind it toward a pair of binary stars. Although some astronomers speculate that the object could be a brown dwarf, others believe that it is a planet flung into deep space by a gravitational "slingshot" effect from its parent stars. This suggests the possibility that rogue planets unattached to any star may also be roving the Universe.

In the spring of 1999, astronomers announced the discovery of a second multiple-planet solar system (not counting our own), detecting three planets circling the star Upsilon Andromedae, some 44 light years away. Though the objects detected are Jupiter-like gas giants, the data does not rule out Earth-type planets, which would not provide sufficient gravitation effect to be detected by the techniques used so far.

Between 2000 and 2003, more than a dozen additional extrasolar planets were discovered.

Although the radial-velocity technique has been responsible for all extrasolar-planet discoveries in recent years, this is expected to change as the transmit method is applied more thoroughly. The advantage of the transit method is that the light from many stars can be monitored for telltale brightness simultaneously; a certain fraction of solar systems are bound to be oriented edge-on to us, allowing for their detection by this means. In 2007, the Kepler spacecraft, a space telescope especially designed to scan large areas of the sky for transits by planets as small as Earth, will be launched by the U.S. National Aeronautics and Space Adminstration. By 2011, Kepler should have gathered enough data to pinpoint hundreds of extra-solar planets and to determine how typical our own solar system is in the Universe. This is of interest to scientists because estimates of the probability that life exists elsewhere in the Universe depend strongly on the existence of planets not too different from our own. Intelligent life is unlikely to evolve on large gas giants or on bodies of any type that orbit very near to their stars or follow highly eccentric, bake-and-freeze orbits. If solar systems like our own are rare in the Universe, then life (intelligent or otherwise) may be correspondingly rare. Theoretical models of the formation of solar systems have been in a state of rapid change under the pressure of the rush of extrasolar planet discoveries, and revised models indicate that solar systems like our own may be abundant. However, these models supply only educated guesses, and must be checked against observation.