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Extrasolar planetary system makes pictorial debut

First images of planetary system beyond the solar system released

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1:07pm, November 13, 2008
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They’ve got the whole world in their hands. Four worlds, actually.

Two teams of extrasolar planet-hunters report that they have achieved a long-sought milestone: obtaining the first undisputed images of planets orbiting stars beyond the solar system.

One team, using the Hubble Space Telescope, has recorded a single planet around the massive star Fomalhaut, which lies just 25 light-years from Earth. The other team, using two large ground-based telescopes, has taken images of three planets orbiting a star — the first portrait of an entire planetary system outside the solar system. Details of both findings appear online November 13 in separate articles in Science.

One of the teams, led by Christian Marois of the Herzberg Institute of Astrophysics in Victoria, Canada, used the Gemini North and Keck 2 telescopes atop Hawaii’s Mauna Kea to image the region around a massive star called HR 8799. Marois was already carrying a tightly held secret when he boarded an airplane in July to Mauna Kea. Images his team had taken with Gemini North nine months earlier had revealed a faint point of light — a possible planet — near HR 8799, which lies about 130 light-years from Earth.

During the flight, Marois pulled out his laptop and did a quick-and-dirty analysis of images of the same star taken at Keck. To his amazement, he found evidence of a second planet.

Then, against all odds, Marois’ team attained a trifecta. During the July observing run at Keck, the team photographed a candidate third planet around HR 8799.

Marois and his collaborators kept the findings under wraps until they could take a look at archival data on the star from a few years back. A comparison of old and new images proved that the newly discovered orbs are not mere background objects, but rather true planets, gravitationally bound to and circling their parent star. The team relied on both a coronagraph — a device that blocks the brilliant light from the parent star — and telescope mirrors that flex hundreds of time a second to remove the blurriness created by Earth’s turbulent atmosphere.

The discovery of this planetary trio, a stretched-out version of the outer solar system recorded at seven different infrared wavelengths, is “spectacular,” says Ray Jayawardhana, an astronomer from the University of Toronto. It also adds to the evidence that massive stars like HR 8799 form heavier planet-making disks that give birth to more massive planets, he adds.

Astronomers have been hunting for images of extrasolar planets for decades, but had yet to find a body gravitationally bound to an object massive enough to be a star.

“There is good reason to hope that that this is indeed the first true image of an extrasolar planetary system,” says theorist Alan Boss of the Carnegie Institution for Science in Washington, D.C. “This one might well be the real enchilada.”

The two innermost planets are the heaviest, about 10 times as massive as Jupiter, while the outer planet is the lightest, about seven times as massive. That size relationship suggests the planets coalesced from a vast disk of gas, dust and ice particles that once encircled the star. The planets reside at roughly 25, 40 and 70 astronomical units from HR 8799. (One AU is the average Earth-sun distance.) By comparison, the solar system’s most distant planet, Neptune, has an average separation of 30 AU from the sun.

Intriguingly, the HR 8799 system’s outermost planet lies just inside a remnant disk of dusty debris that hasn’t been directly imaged but is believed to surround the star. The disk is similar to the solar system’s Kuiper Belt, which was generated by comet collisions.

The faint, tiny images reveal no detailed features of the planets, but the strong reddish colors of the orbs and their overall brightness suggest they have dusty atmospheres, says study coauthor Travis Barman of Lowell Observatory in Flagstaff, Ariz.

In a separate report, Paul Kalas of the University of California, Berkeley and his colleagues unveil what appears to be another historic extrasolar planet image. Using the Hubble Space Telescope, Kalas and his collaborators have captured a portrait of a body orbiting the well-known massive star Fomalhaut, which is only 25 light-years from Earth.

The planet lies at a distance from Fomalhaut that is about four times Neptune’s average distance from the sun, or 120 AU. Kalas and his collaborators proposed in 2005 that a vast disk of debris surrounding Fomalhaut, first imaged in 1998, was being shaped by the gravity of an unseen planet. The disk’s location, slightly off-center from the star, also hints at the handiwork of a planet. Indeed, the newly imaged body lies next to the sharp, inner edge of the disk, which contains more dust than the mass of three Earths — enough to easily supply the solid core of the giant, Jupiter-like body Kalas’ team has photographed. Moreover, visible-light observations taken 21 months apart by Hubble’s Advanced Camera for Surveys revealed that the body is circling the star. Because Fomalhaut’s disk is relatively undisturbed, the planet can be no heavier than three Jupiters, Kalas’ team reports. The planet’s brightness further constrains the mass to about the same as Jupiter’s, the team concludes in an upcoming Astrophysical Journal and also at http://arxiv.org/abs/0811.1994 .

The body’s proximity to the remnant disk strongly suggests that it is a planet, comments Jonathan Fortney of the University of California, Santa Cruz.

However, two other aspects of the study give researchers pause. Fomalhaut is a young, roughly 200-million-year-old star, but a search for the orb at infrared wavelengths, where a young planet is expected to radiate the bulk of its heat, failed to spot the body. Water clouds in the planet’s atmosphere, which would trap some of the infrared radiation, could explain why the object hasn’t been seen at an infrared wavelength of 2 micrometers, according to a model by Fortney and Mark Marley of NASA’s Ames Research Center in Mountain View, Calif. But if searches at longer infrared wavelengths also fail to detect the body, the finding would be at odds with widely accepted models of planet formation and evolution, Fortney says.

In addition, the orb appears much brighter at the shorter of the two visible-light wavelengths recorded by Hubble. A planet can’t radiate that much visible light, notes Fortney. Instead, Kalas’ team proposes that the unexpected brightness comes from starlight reflected off of huge ice or dust rings that might surround the planet. The rings would dwarf the icy necklaces surrounding Saturn.

There’s an extremely remote possibility that the body his team has detected is actually an orbiting dust blob, rather than an orbiting planet, Kalas says. But such a blob, created by the chance collision of two asteroids, would last only for the astronomically short time of a thousand years. It would be highly unlikely that the Hubble camera happened to observe Fomalhaut at just the right time to catch such an ephemeral blob.

For these reasons, the finding “is not really as ironclad as theorists would want it to be,” says Fortney, “but nature can be messy.” Says Jayawardhana, “It’s an intriguing object, especially because of where it is relative to the dust disk, but also quite puzzling.”

Jayawardhana and his collaborators recently released an infrared image — as well as spectra — of a possible planet around another nearby star (SN: 10/11/08 p. 8), but the researchers don’t yet know if the faint body is orbiting the star or merely resides in the same patch of sky.

Marois says he never expected to find planet images before he began operating a new device on the Gemini telescope tailored to find images of extrasolar planets (SN: 7/5/2008, p. 16). Because all of the newfound orbs lie far from their parent stars, none could have been found by the standard technique — inferring the presence of a planet by the wobble it induces in the motion of its star.

With the new findings, “the era of direct imaging of extrasolar planets is here at last,” Jayawardhana says. The discoveries “are revealing an entirely new population of planets — massive ones in wide orbits — that couldn't be found with other methods.” The snapshots, he adds “remind us yet again the importance of using the full array of detection methods in order to build up a complete picture of the astonishingly diverse planet population out there.”

A study in the Nov. 20 Astrophysical Journal Letters may add to the diversity. Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his collaborators describe a survey of the dusty environs of young A stars — the same class of massive star that includes Fomalhaut and HR 8799. Infrared studies with NASA’s Spitzer Space Telescope show that the dusty disks swaddling these stars have the right temperature to have formed from the collision of icy planetary bodies about as small as Pluto. The finding suggests that about half of all A stars have have tiny, icy planets.

The results indicate that the Jupiter-like, presumably gaseous planets imaged around Fomalhaut and HR 8799 are just the tip of the iceberg. Farther out, each star may harbor a retinue of smaller and much colder planets.

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