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.
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.