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Two Planets Discovered Around Star in 'Big Dipper' - 2001-08-17

Astronomers have found the first distant solar system with characteristics similar to ours. It has a sun like ours and planets in almost circular orbits like our own.

Since 1995, astronomers have detected more than 70 planets orbiting distant stars. Now, the California team that has found most of them has discovered two more around a faint star in the constellation known as the "Big Dipper," or Ursa Major. But this pair is different.

At the U.S. National Science Foundation in Washington, the agency that helped fund the study, astronomer Morris Aizenman says they exist in a solar system reminiscent of ours. "It's essentially the first results we have of a solar system that's almost a sister solar system to our own," he said. "All of the other stars where we found planets have orbits that are rather unusual compared to the orbits we find in our own solar system. In this case, we're finding something that's strikingly parallel to our own solar system."

The key similarity is that the two planets follow nearly circular orbits around the star, at distances between those of Mars and Jupiter in our system. In contrast, the previously discovered planets have eccentric orbits, too close to their stars for life as we know it to exist.

Mr. Aizenman notes other similarities in the new system. "The sun is similar to our own," he says. "The two planets that are in this system their masses are very similar to Jupiter and Saturn, and the ratio of the masses is the same as the ratio of the masses of Jupiter and Saturn in our own system."

The findings demonstrate that solar systems like ours may not be unusual after all. "Until now, our solar system looked like the oddball," says Debra Fischer of the University of California at Berkeley, one of the astronomers who detected the two planets. "Right now, we truly stand on the threshold of breaking new scientific ground, and finding more systems that look a lot more like our own solar system, and will allow us to answer the question: Is our own solar system typical or is it unusual in some way?"

The progress is the result of better optics. The California astronomers did not see the planets directly. Instead, they inferred their presence by the wobble in the star's orbit induced by the pull of their gravity.

They had detected the bigger planet in 1996, but only later found the smaller, more distant one, because the improved optics allowed them to measure the star's motion more precisely. "The technique does have to be more precise for the more distant planets, because the star is being tugged by the planet and, as you can imagine, the amount of that tug is going to scale depending on both the mass of the star and on the separation between the star and the planet," Ms. Fischer says. "So those planets that are much farther away from the star exert a much smaller tug."

So far, the technique has detected only massive gas planets, such as Jupiter and Saturn. Ms. Fischer says the discovery of smaller, rocky Earth-like bodies in the Big Dipper solar system and others must await more sensitive instruments.

Those are being designed. The U.S. space agency NASA plans a 2009 launch of a package of telescopes that will pioneer a technique to block the light of bright stars to make dim planets more visible. That is to be followed by launch of a telescope capable of taking pictures 100 times more detailed than the Hubble Space Telescope, in an effort to trace the formation of rocky planets.

At the National Science Foundation, Morris Aizenman is following these developments closely. "So who knows? There might be Earth-like planets around us. Someday, perhaps, we might find out," he says.