How often do gas giants appear in other solar systems? In our search for planets outside our solar system, all the planets discovered to date are gas giants like Jupiter and Saturn. This does not mean that all the planets to be found are gas giants, however. An inherent bias in the search method is that it can only detect very massive planets.

Over 100 Jupiter-mass planets have been found to date, from a survey of over 1,000 stars - nearly all the solar-type stars within 30 parsecs. However, Jupiters that are further away from their stars take longer to complete an orbit, and therefore require longer periods of astronomical observation. These stars might harbor many more Jupiters that have not yet been detected.

Several extrasolar giants are extremely close to their stars, and many scientists believe, due to the conditions necessary for gaseous planet formation, that they must have formed further away from their stars and then migrated inward. Such behavior would be a death warrant for life on inner terrestrial planets like Earth, causing the planets to be flung outside of the solar system, away from the heat and light of their star. The highly elliptical orbits of many extrasolar Jupiters are thought to be caused by such orbital turf battles.

Estimating the number of Jupiter-mass planets in the galaxy greatly relies on our understanding of planet formation. For a long time, it was thought that gas giants formed the same way terrestrial planets did – by the slow accretion of matter over many millions of years. However, the accretion model has a fundamental problem – how do gas giants accumulate enough gas before the gaseous disk around a young star dissipates?

"(Jupiter-mass plants) are made of gas, and gas is lost very early in some solar systems due to intense ultraviolet irradiation from nearby stars," says Brownlee. "The success of Jupiter formation may vary with location and time due to the build up of heavy elements in the galaxy."

Boss’s disk instability model suggests a different manner of gas giant formation. In his model, the disk of gas and dust that swirl around a young star develop points of instability. These disturbed areas become gravitational wells, accumulating more and more matter until they form the gas giant planets.

A recent computer model by Lucio Mayer and Thomas Quinn of the University of Washington, which was based on Boss’s disk instability model, found that gas giants like Jupiter can form in only 1,000 years. Their computer simulation produced planets 2 to 12 Jupiter masses, with elongated orbits rather than the more circular orbits of the planets in our own solar system.

"If the disk instability mechanism can work, then most planetary systems should have gas giant planets," says Boss. "However, if core accretion is the mechanism that forms gas giants, then most planetary systems may only have failed cores that grew too slowly to accrete enough gas to become gas giants. In other words, they would be full of ice giants like Neptune instead. Only observations will prove which is right."

Understanding the role that Jupiter plays in our own solar system can help astronomers narrow their search for habitable planets around other stars. While current planet search techniques are limited to the detection of very massive planets, finding a planet similar to Jupiter in mass and orbital distance might indicate places where Earth-like planets could be found.

After 15 years of observations, an extrasolar planet with an orbital distance similar to Jupiter’s was detected in 2002. This planet, orbiting the star 55 Cancri in the constellation Cancer, orbits at approximately 5 AU. However, the planet has a mass about 4 times that of Jupiter, and this larger mass may affect the inner solar system in ways our own Jupiter does not.

In addition, this solar system contains two other Jupiter-mass planets in tight orbits around their star: one at .115 AU, the other at .241 AU. These inner gas giants suggest there probably aren’t any terrestrial planets within the star’s habitable zone.

Still, scientists are optimistic about finding other Jupiter-mass planets at similar orbital distances in the near feature. Such solar systems will be prime candidates for NASA’s Terrestrial Planet Finder, a space-born telescope designed to take visual images of Earth-mass planets. The Terrestrial Planet Finder is due to be launched sometime between 2012 and 2015.

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Editor’s Note: This story is presented in cooperation with Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.