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Snapshot of a planet beyond the solar system

The image is possibly the first of an extrasolar planet orbiting a normal star, but some of its features counter current thinking.

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9:28am, September 15, 2008
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After years of searching, astronomers may finally have recorded the first image of a planet orbiting a sunlike star beyond the solar system. The body, about eight times Jupiter’s mass, lies exceptionally far from its presumed parent star — roughly 11 times Neptune’s average distance from the sun.

“If this object is a planet at such a wide separation it would challenge our conceptions of planet and companion formation,” says theorist Adam Burrows of Princeton University.

In an article posted online September 10, codiscoverers David Lafrenière, Ray Jayawardhana and Marten H. van Kerkwijk of the University of Toronto caution there’s a small chance that the object, small enough to be classified as a planet, merely resides in the same part of the sky as the star but is not gravitationally bound to it. But if the body does turn out to orbit the young sunlike star, which has the unwieldy name 1RXS J160929.1-210524, it could pose a problem for planet formation theories. A widely accepted model suggests that the planet-forming disks of gas, dust and ice that surround newborn stars concentrate most of their material close to their stars. “The bulk of the material from which planets might form is significantly closer to the parent star,” Burrows says. The outermost parts of such disks wouldn’t contain enough material to assemble a Jupiter-mass planet at the distance from the star — 330 astronomical units (AU), or 330 times the separation between Earth and the sun — at which the Toronto team found the faint object.

“At hundreds of astronomical units from the star, the density of material in the disk is so low that any small seed of planet would not be able to grow [large] enough before the disk vanishes in a few million years,” says Lafrenière. He and his colleagues found the new planet earlier this year by using a special optics system on the Gemini North telescope atop Hawaii’s Mauna Kea. The team scoured the vicinity of some 85 stars belonging to the Upper Scorpius association. Stars in this grouping lie 500 light-years from Earth and are only about 5 million years old. The sun, by comparison, is 4.56 billion years old. Any planets these young stars harbor would also be youthful and therefore still warm, making it easier to image them at infrared wavelengths. In addition, planets that are widely separated from their parent star are easiest to spot because the brilliant glare of the star is less likely to hide them. The new planet was one of two candidate planets in Upper Scorpius. Additional images taken at Gemini North, along with spectroscopy to better identify mass, composition and age, revealed that one candidate was merely a background star, while the body near 1RXS J160929.1-210524 is both young and has a low mass. Objects less than 13 times Jupiter’s mass are generally classified as planets, thought to be formed from material within disks around stars. Bodies between 13 and 80 Jupiter masses are called failed stars or brown dwarfs — objects that form as stars do, from the collapse of a cold, dense cloud of gas, but which are too puny to sustain nuclear burning at their core. Of the more than 300 extrasolar planets astronomers have discovered since 1995, most have been detected only indirectly, by the tiny wobble they induce in the motion of a parent star or by minieclipses generated as they pass in front of a star.

In 2004, researchers imaged an object two to five times as massive as Jupiter and located at a distance from a brown dwarf greater than Pluto’s average distance from the sun (SN: 9/18/2004, p. 179).

But no one has recorded an image of a planet orbiting a sunlike star. Astronomers will need to track the motion of the newly discovered body across the sky for one to two years to determine whether it moves in sync with the star, Jayawardhana says. But even assuming that Upper Scorpius has as many free-floating planets — planetary objects unattached to any star — as stars, the team calculates only about a 0.03 percent chance that a planetary-mass object would lie within 330 AU of 1RXS J160929.1-210524 without orbiting it.

Which begs the question, “if this object really is what they think it is, what the hell is it doing out there,” 330 AU from the star, says theorist Alan Boss of the Carnegie Institution for Science in Washington, D.C. One possibility is that the planet formed much closer to the star and then got kicked out, either through gravitational interactions with other planet-forming material in what would have been an unusually large disk, or through the gravity of an as yet undetected massive planet. “It will thus be very interesting to search for the presence of other, closer-in giant planets around this star,” says Lafrenière. If the planet had been ejected, it should have a highly elongated orbit, Boss notes. “If it’s actually on a more circular orbit, then we’re really puzzled,” he adds. It’s also conceivable that the planet didn’t form in a disk in the first place. Instead, the orb and the sunlike star might have arisen together, from the collapse and fragmentation of the same cloud of gas and dust, the Toronto team suggests. But Jayawardhana and Boss agree that it’s hard to imagine that one cloud would have given birth to two objects of such different masses. For now, says Boss, the finding is fascinating simply because it’s an image of an extrasolar planet. “It’s been a challenge,” he says, “just to know what a young Jupiter should look like.”

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