Wednesday, November 07, 2007

Scientists go into orbit over solar system similar to ours

Astronomers scanning the heavens for planets spinning around distant stars have found one solar system in particular that looks a lot like home.

For the first time, scientists say, they have found a star with at least five planets in orbit. The star, 55 Cancri, is similar to the sun in both size and age and lies about 41 light years from Earth.

Our solar system has eight planets, not counting Pluto.

"This discovery has me jumping out of my socks," said Geoff Marcy a University of California, Berkeley, astronomer and co-leader of a team that made the discovery. "The significance is marvelous, as we now know our sun and its family of planets are not unique."

Astronomers have now found more than 250 planets orbiting other stars.

Interesting orbit

But scientists have found only a few stars with more than one planet. That's because their technique of finding distant planets — measuring the gravitational tug that it exerts upon its star — is not ideal for finding more than the biggest planets.

Before Tuesday's announcement, scientists knew of four planets surrounding 55 Cancri. With additional observations and data-crunching, however, the research team found a fifth planet. This one is fourth from the star and is about 45 times more massive than Earth.

Its location is tantalizing. At 72.5 million miles from its star, the planet is just a bit nearer to its star than Earth is to the sun and, therefore, is probably only slightly warmer.

The mass of the planet means it has a crushing gravitational field. However, if our solar system is any guide, big planets like this should have moons, and its moons could be habitable with Earth-like gravity and water. Moreover, scientists say, it's possible that more planets orbit 55 Cancri.

To find distant planets, astronomers use the fact that stars wobble slightly as their planets circle around and tug on them. This produces tiny changes in the light given off by stars.

The wobble factor

As scientists have refined the technique, they have found smaller planets but have yet to find Earth-sized bodies. That doesn't mean they don't exist.

Finding planets around a star has been a challenge, since astronomers must differentiate the tugs made by several planets. Scientists use computer models that attempt to explain all of a star's wobble by adding in planetary tugs until the right configuration is found.

Yet even with five planets, residual wobbles that the mathematical models cannot account for still come from 55 Cancri. Scientists are intrigued, because there is a large gap between the fourth and fifth planet, a possible nesting place in the star's habitable zone for a small, rocky, Earth-sized planet that may be discovered later.

"Because of the residuals they have left, they're guessing there's another planet, and it's entirely possible" said Barbara McArthur, an astronomer at the University of Texas at Austin, who found one of the planets orbiting 55 Cancri but was not involved in the current study.

Fritz Benedict, another UT-Austin astronomer, said the 55 Cancri system is rich in metals. The theory of planet formation suggests such systems should produce a lot of planets.

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