A Sister Solar System?

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Time was, being an earthling was something special. In a universe full of apparently planetless stars, our colorful solar system seemed like one of a cosmic kind. The planet club became a little less elite last week, however, when astronomers announced the discovery of a Jupiter-like world orbiting a not-too-distant star, a star that could well have an Earth-like planet in its brood too.

The star that's causing all the buzz is 55 Cancri, about the size and age of our sun, located just 41 light-years away. This is not the first time a planet has been seen orbiting the star. In 1996 Geoffrey Marcy, a professor of astronomy at the University of California, Berkeley, and astrophysicist Paul Butler of the Carnegie Institution of Washington spotted a different Jupiter-size world circling 55 Cancri in a close-up orbit just 10 million miles from the solar fires — closer than little Mercury orbits our own sun. All told, astronomers have found about 90 big planets circling many other stars, either clinging close to their parent's skirt or whipping in and out in irregular, slingshot-like orbits — including what seems to be another planet closely orbiting 55 Cancri.

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The world revealed by Marcy, Butler and their team last week, however — yet another around 55 Cancri — is an entirely different beast. About 3.5 to 5 times the mass of Jupiter — close enough by planetary standards to be a sister — it orbits at a distance of 510 million miles, remarkably like Jupiter's 480 million. The new world takes 13 years to complete a single orbit; Jupiter takes almost 12--again, all but identical.

The close similarity to Jupiter has a special significance to astronomers. One of the things that made life on Earth possible in the first place was the looming presence of our sun's fifth planet, whose massive gravity vacuumed up a lot of incoming comets and asteroids, keeping the small inner worlds relatively safe in the shooting gallery that was the early solar system. The clear lane Jupiter created became a sort of orbital sweet spot, where temperatures could remain moderate and volatile substances such as water could persist, giving life a chance to arise. While plenty of stars beyond 55 Cancri and our sun may have their own Jupiter-size planets, their orbits often carry them through this habitable zone, preventing small Earth-size worlds from gaining a toehold. "An Earth-like planet can't get established when a Jupiter-size wrecking ball is sailing through," explains Butler.

Whether there's a lush little Earth spinning around 55 Cancri is still anybody's guess. Spotting even a giant planet in the glare of its sun is so hard — astronomers compare it to looking for a firefly next to a searchlight — that no one really sees any of these extra-solar worlds. Instead, investigators look for tiny perturbations in the position of the mother star that may suggest that the gravity of a planet of a particular size and distance is tugging at it.

NASA hopes, however, to launch within the next 20 years two new spacecraft that will circle our sun in unusual Earth-trailing orbits, studying the gravitational wobble of stars and trying to filter out some of their light. This, astronomers hope, will allow the hidden planets to pop into view. When the ships go online, 55 Cancri will be one of the first stars to receive a close look — a prospect that last week became more tantalizing than ever.