Johny Setiawan

Baby fat.
This artist's impression shows a very young and still-forming massive planet just discovered orbiting its parent star.

A Baby Planet's First Steps?

Astronomers think they have found the first "baby" exoplanet--a world so young it could provide insight into the earliest stages of planet formation.

Over the past decade and a half, astronomers have identified 270 planets orbiting stars beyond our solar system (ScienceNOW, 10 January 2007). Using ever-more-powerful telescopes, they have also detected hundreds of nascent stars surrounded by clouds of gas and dust. Astronomers have presumed that these protoplanetary disks, as they're called, coalesce into rocks, asteroids, and eventually planets, but so far direct evidence has been lacking.

Now a team from the Max Planck Institute for Astronomy in Heidelberg, Germany, claims to have landed the proof. After scanning disks surrounding about 200 stars, the researchers detected a periodic wobble in the motion of TW Hydrae, located about 180 light-years away in the constellation Hydra. The wobble means that something with significant gravity is tugging at the still-forming star on a regular basis. Further analysis suggested a planet with about 10 times the mass of Jupiter tightly orbiting its parent star about every 4 days.

More striking, the team reports in the 3 January issue of Nature, the data suggest that the planet's age is less than 10 million years--only about a tenth as old as any other extrasolar planet found so far. "Before this discovery, it was not clear what the real time scale of planet formation was," says astronomer and lead author Johny Setiawan. But the detection of this young planet around TW Hydrae suggests a much faster process than scientists had thought, and it "shows us that what we call protoplanetary disks are indeed protoplanetary," he says.

Perhaps, but the object may not be a planet at all, says planetary scientist Jack Lissauer of NASA's Ames Research Center at Moffett Field, California. He's not convinced by the data regarding the size of the object, and if it ends up being just a little larger than the German team estimates--say, 13 times the size of Jupiter--it could instead be a starlike body known as a brown dwarf.

If the object does turn out to be a planet, it will allow researchers to rigorously test current hypotheses about planet formation, says planetary scientist Joseph Harrington of the University of Central Florida in Orlando. Astronomers can gain a much better sense of the planet-forming process now, he says; however, "this one observation will not lay the arguments to rest."

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