Exoplanet Hunters Finally Catch One in a Star’s Debris

Image: ESO/A.-M. Lagrange. This image shows the dust disk around the star (blue light at edges), and the observed position of beta Pictoris b in 2003 and autumn 2009. The light from the star has been blocked out to make the planet visible.


A giant planet lurks in the dust and debris surrounding a young, nearby star — and astronomers have finally seen it in action.

Using the Very Large Telescope in Chile, astronomers took infrared images of the planet in two different positions around its star in 2003 and late 2009.

“It’s so exciting that we can see it,” said astronomer Paul Kalas of the University of California, Berkeley, who was not involved in the new work. “We’ve been looking a long time.”

The discovery, announced June 10 in Science, proves that giant planets can form quickly around young stars and suggests that dust disks are signposts for stars hosting giant planets.

Beta Pictoris, a star almost twice the mass of the sun and located 63 light-years away, has been a celebrity among planet hunters since the 1984 discovery of a wide halo of dust and rocky debris that could eventually coalesce into planets. Later observations showed that the disk was oddly warped, and that it had a big hole near the center.

Theoretical models predicted that a planet around five to 10 times the mass of Jupiter could make both the warp and the hole. But when Anne-Marie Lagrange of the Grenoble Observatory in France, first author of the new paper, and her colleagues observed the star in 2003, they saw nothing.

“The tools we had in 2003 were not precise enough,” she said.

After Kalas’s group and another team released images of planets around the stars Fomalhaut and HR 8799 in November 2008, Lagrange and colleagues tried again. They used newer techniques to cancel out the light from the star, allowing the planet to shine through.

The image showed a bright object next to Beta Pictoris, but whether it was a planet or another star in the background was unclear.

“Frankly, if I had a bet on whether or not they’d actually seen a planet back in 2003 … I would definitely bet it was not a planet,” said astronomer Ben Zuckerman of the University of California, Los Angeles, who was involved in imaging the planets around HR 8799.

Follow-up observations in late 2008 and early 2009 also came up empty. Finally, in October 2009, the planet re-emerged on the other side of the star. Lagrange and colleagues kept taking images until March 2010 to confirm that the object was a planet.

“We spent a really long time, nights and days, to check it,” she said. “It really shows that when we see disks, we have to look at every detail, because they can indicate the presence of a planet.”

Because Beta Pictoris is such a young star — about 10 million years old, or two thousandths the age of the solar system — studying its planetary system can help astronomers decide between competing models of planet formation. For instance, earlier theoretical work showed that debris disks around stars broke up fairly quickly, within a few million years. Some theorists worried that massive planets wouldn’t be able to form fast enough, but the planet around Beta Pictoris is proof that they can.

“It’s taking a snapshot of another solar system right after it’s born,” Kalas said. “The other alternative is to invent a time machine and go back 4.5 billion years and look at our own Jupiter when it just formed. But obviously we can’t do that.”

The planet weighs in between six and 12 times the mass of Jupiter, similar to the models’ predictions. It orbits its star at about the orbit of Saturn, between eight and 13 times the distance from the Earth to the sun, making it the closest planet to a star ever imaged. It also means the planet makes a complete circuit around its star every 17 to 30 Earth years, well within human lifetimes.

“Eventually, we’ll have a movie of this planet going around Beta Pic,” Kalas said. By contrast, the planets around HR 8799 and Fomalhaut take between 100 and 870 years to complete an orbit.

The next step is to observe the planet in more wavelengths to get an idea of what its atmosphere is made of, Lagrange said. And with new instruments like the Gemini Planet Imager coming online, the next few years should see even more direct images of extrasolar planets.

“The future is really bright,” said astronomer Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics. “It’ll be a really interesting field in the next two or three years.”