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Solar System Like Ours Is Called 'Missing Link'


Astronomers searching for worlds around distant stars announced the discovery Thursday of the first solar system like Earth's, boosting hopes that there are other habitable spots in the universe.

"One of the big questions in science is, 'Are we alone?' " said Anne Kinney, who directs the astronomy and physics division at NASA headquarters. This "brings us one step closer to answering that."

While scientists did not find an Earth, they found a close cousin: a Jupiter. It is the first planet scientists have found with a roughly circular orbit that is a healthy distance from its star, like many of Earth's neighbors.

"It's got the smell of our own solar system," said Geoff Marcy, the UC Berkeley astronomer who leads the planet-hunting team. "In a sense this solar system is a missing link."

Since the first extrasolar planet was discovered seven years ago, 91 have been discovered. But many have been so odd--many times the size of Jupiter, so close to their suns they'd be permanently scorched or on wild, elliptical orbits--scientists began to wonder whether our home solar system was unique. Looks like it's not.

The planet, a gas giant known as "55 Cnc d," circles around the star 55 Cancri, about 41 light-years from Earth. The middle-aged star is about the same size as our sun and is visible to the naked eye.

The new planet is about four times the size of Jupiter and is about the same distance from its sun as Jupiter is from ours. While that planet looks comfortingly familiar, the solar system also contains some strange elements: two other large planets hundreds of times larger than Earth that circle very close to the sun.

Those oddities carry the "wacky stink of some of the strange solar systems we've been finding over the past few years," Marcy said. They underscore that while Earth's orderly solar system is not unique, neither is it the norm.

Finding planets is difficult work. They are not visible, even to the powerful Hubble Space Telescope, because they give off only a faint glow of reflected light, light that is imperceptible in the glare coming from the stars they circle. Instead, Marcy's team detects planets using a sensitive technique that measures the slight wobble of stars caused by the gravitational yank of planets circling them.

The technique has a bias that explains why most findings so far have been of big, close-in planets--"oddballs" that are easiest to find because they perturb their stars the most. The smallest planet discovered so far, one of 14 others also announced Thursday, is about half the size of Saturn, or 40 times the size of Earth.

Planets the size of Earth would not be visible. For example, if the team looked at our own solar system from afar with its technique, it could detect only the two largest planets, Jupiter and Saturn.

The reason many of the early discoveries have been of planets close to stars is because their full orbits take only days. The wobbles caused by the planets cannot be analyzed until at least one full orbit has been made. A planet farther away, that takes years to orbit its star, takes years of monitoring to detect.

The newly discovered Jupiter-like planet took 13 years to orbit its sun, and therefore took 13 years to detect.

"There's a tremendous amount of persistence required for this work," said Alycia Weinberger, an astronomer at the Carnegie Institution of Washington who was not involved in the research. "It's an experiment that doesn't necessarily give its most exciting results right away."

Astronomers are debating whether a smaller planet could exist in a temperate zone among 55 Cancri's three Jupiter-like planets.

The outer Jupiter might serve, as ours does, to absorb the blows of comets and asteroids that could otherwise explode into our planet and wipe out life.

Theories suggest that gas giant planets form in the outer part of the solar system and exist near stars only if they migrate there. Behemoths spiraling toward the core of a solar system could pummel smaller planets out of existence. But Weinberger said enough dusty material might survive this transit to allow an Earth-sized planet to form after the monsters passed by. UC Santa Cruz theoretical astronomer Greg Laughlin has shown that a small planet could survive in a stable orbit between two giants.

Marcy and Paul Butler, an astronomer at the Carnegie Institution, have spent 17 years perfecting the process of planet hunting and waited nearly a decade before finding their first planet. Now, their system works so smoothly, "we're actually drowning in planets," Marcy said. Last week, the team had planned to announce 13 new planets at its news conference. It found two more over the weekend.

Given the length of time the astronomers have been monitoring some stars, they expect to find more and more "normal" planets in these distant regions from their suns that have never been probed. "We're entering virgin territory," Butler said.

The team plans to conduct a census of the 2,000 stars within 150 light-years of Earth in an attempt to see how common our solar system is. "Are we one in a hundred? One in a thousand? We have no idea right now," Butler said.

The scientists do not expect that they--or anyone else--will find another Earth for at least a decade. That will have to wait for new technology now being developed at the Jet Propulsion Laboratory.

The NASA center working on several space telescopes to be launched this decade and next will continue the search for new planets with far more powerful tools and attempt to take the first images of these distant worlds.

The Jupiter-like planet announced Thursday is a good candidate for the first portrait, scientists said, because it is relatively close to Earth and far from the blinding light coming from its home star.

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