April 18, 2007

Planets Like Binary Stars, Too

In our solar system, planets, comets, asteroids, and dwarf-planets all orbit around a single star, our Sun. But imagine if we could witness two Suns in our sky. NASA's Spitzer Space Telescope has helped astronomers see that planetary systems are at least as common around double-stars. These binary stars make up more than half of all the stars that we know of, and this conclusion is suggesting that planets may be more common than we previously thought of them.

Astronomers knew that planets could form in binary star systems, where the two stars were really wide apart, about 1000 AU (remember that one Astronomical Unit (AU) is equal to the distance from the Earth to the Sun). The planets that we have found that exist in this situation, number to about 50, and they orbit one of these stars in the system.

This new information, however, focuses on binary stars that are much closer together, between 0 and 500 AU. The question the astronomers were asking was whether the fact that the two stars are so close together affects how the planets grow. To seek the answer to this question, the astronomers had to develop a new technique to hunt for planets. Astronomers use the Spitzer Space Telescope's infrared cameras to look for debris disks, not planets, which are leftover pieces of rock that could not form into rocky planets. This can prove that planet building has occurred in the binary star system, with planets orbiting one, or maybe even two, of the stars in the system.

When these astronomers surveyed binary stars between 50 and 200 light-years from Earth, about 40 percent of them had disks. This number is a little higher than that of single stars. The number gets higher (about 60 percent) for binary stars that are closer together. These stars are only about 0 to 3 AU apart. However, stars between 3 to 50 AU do not harbor as many disks, suggesting that binary stars either have to be either very far apart or really close together to contain planets.

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