Extrasolar planet

Extrasolar planets, or exoplanets, are planets that exist outside our solar system. These planets may orbit stars other than our Sun or move independently through interstellar space.

The existence of extrasolar planets has been suspected since the time of ancient Greece. For centuries, however, extrasolar planets existed only in theory because they are extremely difficult to observe directly. Planets shine only by reflected light from the stars they orbit. Because they are so far away from Earth, the faint light they reflect is lost in the scattered light from nearby stars.

The modern search

In the twentieth century, astronomers first tried to detect extrasolar planets by viewing stars that wobble. The motion of celestial bodies is affected by their closeness to other bodies. The gravitational force of one body will "pull" another to it as they pass close to each other. The orbits of the planets in our solar system have a direct effect on the motion of the Sun as it travels through the Milky Way galaxy. Seen from another part of the galaxy, the Sun would appear to wobble as moved along its path. This method only can be used for stars nearest to the Sun because the farther away the star is, the smaller its wobble.

A more accurate method for detecting extrasolar planets is the use of a spectroscope, a device that breaks down light into its component frequencies. A change in the color of a star (meaning a change in the wavelength of light it emits) would show that the star is moving toward or away from Earth. This movement might be the result of the gravitational pull of an orbiting planet.

In late 1995 and early 1996, three planet-sized objects were discovered. The first planet, discovered by Swiss astronomers Michel Mayor and Didier Queloz of the Geneva Observatory, orbits a star in the constellation Pegasus, about forty light-years away from Earth. The next two planets were discovered by American astronomers Geoffrey Marcy and R. Paul Butler. One is in the constellation Virgo and the other is in Ursa Major. By late 2000, astronomers had found evidence of more than 40 additional planets outside of our solar system.

In 1999, astronomers announced they had discovered the first planetary system outside of our own. They detected three planets circling the star Upsilon Andromedae, some 44 light-years away. Two of the three planets are at least twice as massive as Jupiter. The innermost lies extremely close to Upsilon Andromedae—about one-eighth the distance at which Mercury circles the Sun.

In early 2001, stunned astronomers disclosed they had found two more planetary systems in the universe. Each bears little resemblance to the other or to our solar system. In one, a star like our Sun is accompanied by a massive planet and an even larger object 17 times as massive as Jupiter. Astronomers believe this large object could be a dim, failed star or an astronomical object that simply has not been seen before. In the second system, two planets of a more standard size orbit a small star. However, their orbits around the star are perplexing: the inner planet goes around twice (it has an orbital period of 30 days) for each orbit of the outer planet (it has an orbital period of 61 days). These discoveries have left astronomers wondering just what a normal planetary system is in the universe.

[See also Binary star; Solar system; Star ]

extrasolar planet A planet orbiting a star other than the Sun, also known as an exoplanet. Planets of other stars are too faint to be seen directly with existing instruments, so indirect methods of detection are necessary. The first extrasolar planet, found in 1992, orbits a pulsar, PSR 1257+12. A cyclic change in the timing of the pulsar's radio emissions was interpreted as motion of the pulsar about its centre of mass with an orbiting planet. A similar approach, known as the radial velocity method, has been used to find other extrasolar planets. The technique looks for a cyclical Doppler shift in light from a star that would occur as it orbited its common centre of mass with one or more planets. Using this technique, astronomers in 1995 detected the first known extrasolar planet around an ordinary star, 51 Pegasi, 50 l.y. away. The planet has a mass about half that of Jupiter and an orbital period of 4.2 days. The first multi-planet system, also discovered by the radial velocity method, was found in 1999 around Upsilon Andromedae. An alternative approach, the transit method, is to look for dips in the brightness of a star as an orbiting planet moves in front of it, although this is restricted to planets whose orbital plane lies close to our line of sight. The first transit of an extrasolar planet across the face of a star, HDE 209458 in Pegasus, was observed in 1999. A third technique, which has also borne fruit, looks for a spike on the light curve of a gravitational microlensing event that would be caused by a planet accompanying the lensing star. Masses of the first 100 known extrasolar planets range from 13 Jupiters (the theoretical maximum for a planet) down to 0.1 Jupiter. Orbital radii are 0.04 to 5.9 AU from their parent stars and orbital periods are 3 days to nearly 15 years. Planet-finding spacecraft such as COROT and Kepler will use the transit technique to detect smaller drops in light, and hence smaller planets, than is possible from the ground. http://exoplanet.eu\