Image: TW Hydrae system
Johny Setiawan  /  Max Planck Institute
An artist's conception shows the sunlike star known as TW Hydrae, with a young planet in a close-in orbit. The star's dusty protoplanetary disk swirls farther out around the star.
updated 1/2/2008 2:40:24 PM ET 2008-01-02T19:40:24

Astronomers say they have discovered the youngest planet to date circling a sunlike star, a find that will be a boon to the field of planet-formation theory.

The extrasolar planet is an estimated 8 million to 10 million years old, a mere toddler compared to Earth, which is 4.5 billion years old. Until now, the researchers say, no planet younger than 100 million years old has been detected circling a sunlike star.

"It means we're opening up a new field of trying to find planets around very young stars," said Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington. "So it's the very first example, and we hope there will be a lot more." Boss was not involved in the discovery.

The newly found world is so infantile that it resides in the star's "protoplanetary disk," a ring of gas and dust circling the star. It has been cataloged as TW Hydrae b.

"This demonstrates that planets can form within 10 million years, before the disk has been dissipated by stellar winds and radiation," the researchers write in Thursday's issue of the journal Nature.

Weighing in at nearly 10 Jupiter masses, the planet circles at a distance of 0.04 Astronomical Units from its host star, TW Hydrae, in the constellation Hydra. One AU is the average distance between Earth and sun.

The gassy "hot Jupiter" takes 3.56 days to orbit its star. The host star is located 180 light-years away from Earth.

Planets are thought to form within disks of dust and gas around newly born stars. Catching a planet in its childhood can give astronomers lots of information about how planets materialize.

"The discovery shows that what we always call as 'protoplanetary' disks are indeed protoplanetary; they form planets," study researcher Johny Setiawan of the Max-Planck Institute for Astronomy in Germany told "There are many 'protoplanetary' disks detected around young stars, but no planets so far have been detected within such young systems."

Around some young star systems, however, astronomers have found signs of planets by noting clear lanes of dust within the disks. In these cases, it's presumed that young planets are forming and have scooped up the dust, but the planets themselves have not been detected.

Setiawan and colleagues discovered their new world by measuring a wobble in the host star due to the gravitational tug from the orbiting planet. This so-called radial-velocity method is great at detecting extrasolar planets, but it also can produce false positives — suggesting a planet is there when in fact the data owe to some other object or phenomenon.

That's particularly true in young star systems. For one, nascent stars are incredibly active, and their changing outer atmospheres can at the very least make for background noise. In addition, if the star rotates about its axis, that can be problematic.

"There are lots of other things going on in these young stars that could give you a false positive, where you think you're seeing a planet but you're actually seeing some other stellar activity," Boss said in a telephone interview.

Boss thinks the discoverers ruled out these non-planet signals. "They've done a good job of trying to address those worries," he said.

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