First Extrasolar Planet Found Around Sun-Like Star
(From the July/August 1999 issue of StarDate magazine)
An extrasolar planet discovered three years ago has company. Two teams of astronomers, working independent of each other, have detected two additional planets orbiting Upsilon Andromedae, making it the first known Sun-like star with a family of planets.
The prolific planet-finding team of Geoffrey Marcy and R. Paul Butler discovered the new planets with observations made at Lick Observatory near San Jose, California. Astronomers at the High-Altitude Observatory (HAO) and the Smithsonian's Whipple Observatory reached the same conclusion. Both new planets are larger than Jupiter and orbit closer to their star then Jupiter does to the Sun.
Though the discovery may mean planetary systems are common, it also muddles an already shaken theory of planetary formation. Until the first extrasolar planets orbiting Sun-like stars were discovered in 1995, astronomers's ideas about how planets form were based entirely on our solar system. The system around Upsilon Andromedae bears little resemblance to Earth and its eight neighbors.
When Marcy and Butler first detected a planet orbiting Upsilon Andromedae in 1996, anomalies in the data indicated that one or more additional planets also might be present. The wobble of the star as seen from Earth could not be explained by the gravitational pull of the first planet alone, which lies 5.5 million miles (8.8 million km) from the star and orbits it every 4.6 days. With 11 years of observations in hand, Marcy and Butler first considered a two-planet scenario before deciding that the three-planet system best explained the data.
The middle planet orbiting Upsilon Andromedae is the size of two Jupiters and orbits every 242 days at a distance comparable to Venus' distance from the Sun. The outer planet, with the mass of four Jupiters, lies 2.5 times the Earth-Sun distance from the star and orbits it once every 3.5 to four years. Smaller Earth-size planets also may be present, but cannot be detected with the method used by Marcy and Butler.
No one expected that three giant planets could exist in stable orbits so close to a star. Planet-formation theory long held that the temperature closer than four times the Earth-Sun distance is too warm for such planets, composed mostly of frozen gases, to survive.
Some astronomers also had questioned whether single, large bodies orbiting so close to a star were planets at all, or instead were failed companions in a binary-star system. Robert Noyes, an astronomer with the Smithsonian-HAO team, says the discovery of additional planets puts those notions to rest.
Upsilon Andromedae is a fourth-magnitude star in the autumn constellation Andromeda. It lies about 44 light-years from Earth and is thought to be about three billion years old. -- Doug Addison
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