Extrasolar planets are planets that orbit stars other than the sun. A planet is defined generally as an object too small for gravitational pressure at its core to ignite the deuterium-fusion reaction that powers a star. In August 2006, the International Astronomical Union (IAU) officially defined a planet as a “celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.” As such, smaller planets, such as Pluto, are now defined as dwarf planets.
As of 2006, there is one direct method (direct imaging) and six indirect methods (astrometry, radial velocity, pulsar timing, transit method, gravitational microlensing, and circumstellar disks) to discover extrasolar planets.
The existence of extrasolar planets has been suspected since at least the time of Dutch astronomer Christian Huygens (1629–1695). The ancient Greek astronomer Aristarchus of Samos (310–230 BC) may have developed the concept over 2,000 years ago, although this is not known certainly. However, extrasolar planets remained hypothetical until recently because there was no way to detect them. Extrasolar planets are difficult to observe directly because planets shine by reflected light and so are only about a billionth as bright as the stars they orbit. Their light is either too dim to see at all with many older techniques, or is lost in their stars’ glare. The direct imaging method, with the use of current telescopes, may be capable of directly imaging planets when the planet is very large (many sizes larger than Jupiter) or very far away from its central star.
Since October 2006 (according to the IAU), thanks to new, indirect observational techniques, 208 extrasolar planets have been discovered, with masses ranging from that of Jupiter to the upper size limit for a planet (about 15 Jupiter masses). Since 2002, more than 20 extrasolar planets have been discovered, on average, each year.
In 1943, Danish-born U.S. astronomer K.A. Strand (1907–2000) reported a suspected companion to one of the components of the double star 61 Cygni, based on a slight wobble of the two stars’ orbital motions (using radial velocity, also known as Doppler method or wobble method). This seems to be the earliest report of an extrasolar planet; recent data have confirmed the presence of a planet with about eight times the mass of Jupiter orbiting the brighter component of 61 Cygni. Strand’s estimate of a period of 4.1 years for the planet’s orbit has also been confirmed.
In 1963, Dutch-born U.S. astronomer Peter van de Kamp (1901–1995) reported the detection of a planet orbiting Barnard’s star (Gliese 699) with a 24-year period of revolution. Barnard’s star, which is 5.98 light years away, is the second-closest star system to Earth’s solar system after the Alpha Centauri triple-star system. Van de Kamp suggested that Barnard’s star’s wobbling proper motion could be explained by two planets in orbits with periods of revolution around Gliese 699 of 12 years and 26 years, respectively.
Independent efforts to confirm Strand’s planet (or planets) orbiting Barnard’s star failed, however. By the 1970s, most astronomers had concluded that, instead of discovering extrasolar planets, both Strand’s and van de Kamp’s studies had only detected a slight systematic change in the characteristics of the telescope at the observatory where they had made their observations.
Astronomers kept looking, but for many years the search for extrasolar planets was marked by exciting possible advances followed by summary retreats. No sooner would a group of astronomers announce the discovery of a planet outside of the solar system, than an outside group would present evidence refuting the findings discovery. A first breakthrough came in 1991, when it was shown that three approximately Earth-size planets orbit the pulsar PSR1257.12. Subtle timing shifts in the flashes of the pulsar revealed the existence of the two planets; this is the first time this technique (pulsar timing) has detected extrasolar planets. No more discoveries were made for several years; then, in 1995, the dam broke. Using the radial-velocity technique for detection of extrasolar planets (to be explained below), group after group of astronomers announced extrasolar planet findings, findings quickly confirmed by other independent researchers. Suddenly, extrasolar planets went from rare to commonplace.
The recent rush of discoveries has been made possible by new methods of search. Direct visual observation of extrasolar planets remains difficult; all the recent discoveries have been made, therefore, by indirect means, that is, by observing their effects on either the motions or brightness of the stars they orbit.
Apart from the been detected by analyzing the perturbations (disturbances) they cause in their star’s motions. A planet does not simply orbit around its star; rather, a star and its planet both orbit around their common center of gravity. Because a star weighs more than a planet, it follows a tighter orbit, but if a planet (or other companion) is massive enough, the orbital motion of a star—its wobble—may be detectable from the Earth. Several techniques have been and are being developed to detect the orbital wobbles caused by planet-size bodies.
Most extrasolar planets so far detected have been detected by the radial-velocity technique. This uses spectroscopy (analysis of the electromagnetic spectra emitted by stars) to detect perturbations of stars orbited by planets. The mutual orbital motions of a star and a planet around each other manifest in the star’s light via the Doppler effect. In this technique, spectroscopic lines from a light source such as a star are shifted to longer wavelengths in the case of a source moving away from the observer (red shift) or to shorter wavelengths in the case of a source approaching an observer (blue shift). These shifts are measured relative to the wavelengths of spectral lines for a source at rest. Small changes in the wavelengths of spectroscopic lines in the star’s spectrum indicate changes in its line-of-sight (radial) velocity relative to the observer; periodic (regularly repeated) spectral shifts probably indicate the presence of a planet (or planets) perturbing the star’s motion by swinging it toward the Earth and then away from it. (This is true only if the planetary orbits happen to be oriented flat-on to the Earth-sun in space.)
Another new technique for detecting extrasolar planets involves searching for transits: the transit method. A transit is the passage of a planet directly between its star and the Earth, and can occur only when the planet’s orbit happens to be oriented edge-on the Earth-sun. When transit does occur, the star’s apparent brightness dims for several minutes—perhaps by only a percent or so. The amount of dimming and the speed with which dimming occurs as the planet begins to move across the star’s disk reveal the planet’s diameter, which the wobble method cannot do. Furthermore, since a planet with an atmosphere does not block all the light from the star behind it but allows some of that light to filter through its atmosphere, precise measurements of changes in the star’s spectrum during transit can supply information about the chemical composition of the transiting planet’s atmosphere. In 2001, the Hubble Space Telescope detected sodium in the atmosphere of a transiting extrasolar planet approximately 150 light years away. This was the first information ever obtained about the composition of an extrasolar planet. This study was limited to the wavelengths in which sodium absorbs light, and were not expected to detect other chemicals; using different wavelengths, astronomers intend to search for potassium, water vapor, methane, and other substances in the atmosphere of this and other transiting extrasolar planets.
Most of the extrasolar planets detected so far are gas giants with masses on the order of Jupiter’s. 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, Earthlike 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, sunlike 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 Earth’s system), detecting three planets circling the star Upsilon Andromedae, some 44 light years away. Though the objects detected are Jupiterlike 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.
Other methods for observing extrtasolar planets also exist. The astrometry method measures a star’s position by observing how it changes position over time. If the star has a planet orbiting about it, then the gravitational field of the planet will cause the star to move slightly in its orbit.
The gravitational microlensing method is used based on the ability of the gravitational field of a star to act like a magnifying lens. It magnifies the light coming from a star that is further away than it is to Earth. When this magnifying star has a planet around it, the planet’s gravitational field helps with the magnifying effect, which can be detected by astronomers on Earth.
Circumstellar disks method uses the disks of space dust that envelopes many stars. These disks absorb starlight and, then, re-emits it as infrared radiation that can be detected on Earth. When detected, it often implies that planets are present.
Although the radial-velocity technique has been responsible for most 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 is bound to be oriented edge-on to Earth, allowing for their detection by this means.
In November 2008, the Kepler space observatory, a space telescope especially designed to scan large areas of the sky for transits by planets as small as Earth, is scheduled to be launched by the National Aeronautics and Space Adminstration. By 2012 or 2013, Kepler should have gathered enough data to pinpoint hundreds of extrasolar planets and to determine how typical the Earth’s 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 Earth. 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 Earth’s 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
Spectrometer— An instrument to separate the different wavelengths (colors) of light or other radiation. The separating (dispersing) element in an astronomical spectrometer is usually a grating, sometimes a prism.
Terrestrial planets— Planets with Earth-like characteristics relatively close to the sun. The terrestrial planets are Mercury, Venus, Earth, and Mars.
supply only educated guesses, and must be checked against observation.
Within the middle years of the 2000s, astronomers estimate that at least 10% of all stars similar in size and characteristics to the sun have one or more planets orbiting them. In 2005 and 2006, some of the extrasolar planets discovered have included: Gliese 876d, the third planet around the red dwarf star Gliese 876; HD 149026 b, the largest planet core ever found; HD 188753 Ab, a planet within a triple star system; and OGLE-2005-BLG-390Lb, the most distant (near the center of the Milky Way galaxy) and coldest extrasolar planet ever discovered.
See also Binary star.
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