An exoplanet, or extrasolar planet, is a planet in orbit around a star other than the Sun. More than 3,000 exoplanets have been confirmed. Until recently, the great majority of exoplanets detected (see exoplanets, detection), with the exception of the pulsar planets, had small orbital radii and masses similar to or greater than that of Jupiter. This apparent bias arose simply because high-mass objects in compact orbits produce comparatively large effects on their host stars which put them within reach of the methods, and the sensitivity of those methods, currently available. However, over the past two or three years, more and more planets of lower mass – down to just a few times the mass of Earth – have been found.
The existence of giant planets in sub-Mercurian orbits (see epistellar jovians) and in highly elongated orbits (see eccentric jovians) has come as a surprise and forced theorists to revise their understanding of how young planetary systems evolve (see planetary systems, formation). The challenge now is to develop more sensitive techniques capable of finding Earth-class planets, especially those in orbit within the habitable zones of their host stars. These are potentially the most important worlds in the search for life, although, as in the case of Europa and Callisto, large moons of giant planets may also prove to be biologically interesting.
The discovery of (a) exoplanets around roughly 12% of the target stars searched (remembering that many lower mass worlds around these stars almost certainly await detection), (b) a planetary system in the case of upsilon Andromedae, and (c) circumstellar disks around stars of widely differing spectral type, has led to heightened optimism that planetary systems are common throughout the Universe. However, some researchers have urged caution. In 1997, Guillermo Gonzalez of the University of Washington, Seattle, and his colleagues, pointed out1 that almost all of the stars around which planets have been detected to date are richer in heavy elements than is the Sun (see heavy element concentration, related to the occurrence of planets). The relative scarcity of such stars might imply strict limits, too, on the numbers of planetary systems. Moreover, argued Gonzalez, the phenomenon of inward orbital migration which might account for the presence of giant planets in orbits close to their host stars would tend to scatter any terrestrial planets that had formed between the migrating planet and the star, resulting in a dearth of Earthlike worlds.2 The next decade should see a clarification of such issues.
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III: Rho1 Cancri Revisited," Astronomy & Astrophysics, 334, 221 (1998).
2. Gonzalez, G. "Extrasolar Planets and ETI," Astronomy & Geophysics, 39, 68 (1998).
3. Black, D. "Completing the Copernican Revolution: The Search for Other Planetary Systems," Annual Review of Astronomy and Astrophysics, 33, 359 (1995).
4. Boss, Alan, "Forming a Jupiter-like Companion for 51 Peg," Lunar and Planetary Science, 27, 139 (1996).
5. Clark, Stuart. Extrasolar Planets, London: Wiley (1999).
6. Croswell, Ken. Planet Quest: The Epic Discovery of Alien Solar Systems. New York: Free Press (1997).
7. Goldsmith, Donald. Worlds Unnumbered: The Search for Extrasolar Planets. Sausalito, Calif.: University Science Books (1997).
8. Halpern, Paul. The Quest for Alien Planets. New York: Plenum (1997).
9. Marcy, G. W. "Extrasolar Planets," Nature, 391, 127 (1998).