Newborn Planet Found Orbiting Young Star

Anne Minard
for National Geographic News
January 3, 2008
A newly formed planet orbiting a young star offers the first observational evidence for the long-held theory that planets form early, within the first ten million years of a parent star's life, according to a new study.

Until recently all of the 250-plus planets outside our solar system have been found around much older stars—a hundred million years of age or more.

Now a research team led by Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, has found a newborn extrasolar planet—or exoplanet—around a star that's between eight million and ten million years old.

The exoplanet, which is ten times more massive than Jupiter, is still linked to the dusty disk of material surrounding its parent star.

With this new find, Setiawan said, so-called protoplanetary disks have at last earned their name.

"It is very exciting to know that things we called 'protoplanetary' disks are indeed protoplanetary—they form planets!"

Setiawan and colleagues describe the findings in this weeks' issue of the journal Nature.

Planetary Tug

The new planet's star, known as TW Hydrae, sits about 180 light-years from Earth in the constellation Hydra.

Although TW Hydrae is our galactic neighbor, the young planet it hosts is too small and distant to be seen with modern instruments.

Instead astronomers spotted the exoplanet using an indirect technique called radial velocity.

This method measures the effect of the gravitational tug of a planet on its host star's movement.

Larger planets are easier to detect, because their influence isn't as easily drowned out by the activity of the star itself, such as sunspots.

Thus, today's list of exoplanets contains many more Jupiter-size bodies than Earth-size ones.

What's more, astronomers using the radial velocity technique to search for other worlds have often shunned young stars.

"They are very active stars, and the signature of the planet would be obscured by the signature of starspots, et cetera," Setiawan explained.

"If one wants to find a planet easily and quickly, she or he will avoid young stars."

But young planets are increasingly hot targets for study because of what they can teach scientists about planet formation.

Mysterious Movement

Marina Romanova, an astrophysicist at Cornell University, said the newfound planet may indeed shed light on how planets take shape and migrate.

"[It] is an important test point for checking theoretical and numerical models of such processes," she said.

Jack Lissauer, of the NASA Ames Research Center, was a co-discoverer in 2005 of a more Earthlike planet near the star Gliese 876 in the constellation Aquarius.

He agreed that it's "nice to have observational confirmation" of a new planet around a young star.

"However, I do think that the authors have substantially underestimated the uncertainties in the mass of the object," he said.

In his opinion, the body could turn out to be as massive as a brown dwarf or as puny as just a few Jupiters rather than ten.

Setiawan, lead author of the new study, said there's another mystery about the newly discovered planet.

The body orbits very close to its host star, making the trip roughly every three and a half Earth days.

But there's another disruption in its parent star's movement that occurs on a slightly longer timescale, about once every nine days.

"There is still a mystery about the nine-day period," Setiawan said, noting that his team is already looking for an explanation.

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