Because the solar system was the only planetary system known, all models of planetary systems were based on its characteristics—several small planets close to the star, several large planets at greater distances, and circular planetary orbits. Most of the extrasolar planets discovered so far, however, are larger than earth, and many of those are much larger than Jupiter, the largest of the solar planets; many of the rocky planets are much larger than earth, often up to 10 times more massive (one, Kepler 10c, is 17 times as massive); many orbit their star at distances less than that of Mercury, the solar planet closest to the sun—in one system found by the Kepler space telescope, five planets orbit a star more closely than Mercury does the sun—though a few planets orbit their star at many times the distance of Pluto from the sun; and many have elliptical rather than circular orbits. Planets have also been found orbiting binary stars. All of this has caused planetary scientists to revisit accepted theories of planetary formation. Future theories will be measured against stars surrounded by a ring of gas and dust, such as Beta Pictoris, which are thought to be young adult stars with a planetary system forming around them.
An increasing number of planets with masses between one and seven times the earth's have been found in the 21st cent. In 2013 it was announced that astronomers reviewing the data from the Kepler space telescope had identified a planetary system (Kepler 37) that included a planet (Kepler 37c) somewhat smaller than the earth and another (Kepler 37b) that was smaller than Mercury. Kepler 37b is the smallest extrasolar planet discovered so far; neither of the two planets was in the habitable zone. In 2014, Kepler scientists announced the discovery of a habitable-zone planet (Kepler 186f) with a radius estimated to be 10% larger than the earth's, that orbited a cool dwarf star with four other planets; because of its size, Kepler 186f was believed to be a rocky planet with the potential to have liquid water. The Trappist-1 system, whose discovery by astronomers using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile was first announced in 2016, is now known to contain at least seven earth-sized plants, three of which are definitely in the habitable zone. It is possible that some of the bodies that have been discovered are not planets in the solar-system sense but a new class of celestial bodies or even brown dwarfs.
Because stars are so distant and bright and an extrasolar planet, no matter how large, is relatively small and dim, it cannot be seen or photographed directly in visible light. Several techniques have been used to infer the presence of such planets. Astrometry is based on the slight gravitational disturbance, or wobble, that the planet causes in the motion of the star. Photometry, also called the transit method, is to measure the distinct dimming of light from the star as the planet's orbit brings it between the star and the earth. Using photometric techniques it also has been possible to photograph extrasolar planets in infrared light. Doppler spectroscopy is based on the fact that a planet periodically pulls its star closer to and farther from the earth as it orbits the star; this motion has a measurable effect on the spectrum of light coming from the star. In pulsar timing, planets orbiting a
dead star, rather than a main-sequence star like the sun, this tends to be of less interest in the search for an earthlike extrasolar planet.
See A. Boss, Looking for Earths: The Race to Find New Solar Systems (1998); J. K. Beatty, ed., The New Solar System (1999).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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