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Newfound planet stirs old debate

Discoverers of a fourth body orbiting nearby star call it hard to explain

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1:59pm, December 3, 2010
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Astronomers who previously imaged three giant planets around a single star have once again hit the jackpot by finding a fourth large body in the star system. More massive than Jupiter, the newly discovered planet lies considerably closer to the nearby star HR 8799 than the other three planets, raising questions about how all four giants formed.

The planet’s discoverers think so at least. But some other researchers disagree, saying that the situation may not be all that difficult to explain with current planet formation theories.

The star HR 8799 and its planetary retinue lie just 130 light-years from Earth. Captured in a series of telescope images over 15 months, the newly found planet lies about 14.5 astronomical units from its sun. That puts the planet about midway between where Saturn and Uranus sit in the solar system and much closer than its three companions, which the team unveiled two years ago (SN: 12/6/08, p. 5).

The planets orbiting HR 8799, with masses estimated at between five and 10 times that of Jupiter, form a scaled-up version of the planets in the outer solar system, according to codiscoverer Christian Marois of the Herzberg Institute of Astrophysics in Victoria, Canada, and his colleagues. They describe their findings, based on observations with the Keck II telescope atop Hawaii’s Mauna Kea in 2009 and 2010, in an article posted on arXiv.org on November 23 (http://arxiv.org/abs/1011.4918).

Marois declined to discuss details of the posted article because it’s scheduled to appear in an upcoming issue of Nature.

At issue is how gas giants arise. Scientists have two leading models for the evolution of large planets like those in the HR 8799 system. Both models start with a disk of gas, dust and ice that swaddles young stars. In one recipe, known as the gravitational instability model, a large mass of both gas and dust in the disk suddenly fragments and clumps together, forming a giant planet in one fell swoop.

The other model, known as core accretion, requires two steps. First, bits of dust in the disk coalesce to form a rocky core. Then the core snares a vast envelope of helium and hydrogen gases from the disk. However, the core must form quickly enough — in about 10 million years — to capture the helium and hydrogen before the gases spiral into the star.

The three outer planets, if they formed where they now reside, must have been produced by gravitational instability. Core accretion wouldn’t work because the orbs would have arisen from the outer part of the disk, which has such a small smattering of dust that it would take too long to produce a rocky core.

In contrast, the newly discovered, innermost planet might be able to form by core accretion. Marois’ team argues that that the planet lies too close to its parent star to form by gravitational instability.

But Alan Boss of the Carnegie Institution for Science in Washington, D.C., who developed the gravitational instability model, vehemently disagrees. “I have been publishing models for 10 years showing disk [instability] models that form gaseous protoplanets at these distances and even closer,” he says.

Researchers are “trapped in the mindset of the disk-instability deniers” who believe that the inner part of a planet-forming disk, because it lies closer to the parent star, would be too hot to fragment, Boss says. “This really is more of a religious split than a scientific one,” he adds.

Although Marois’ team doesn’t like the idea of both core accretion and gravitational instability working in the same system, both may be possible, says Sara Seager of MIT.

There’s also the possibility, favored by Sean Raymond of the Laboratoire d'Astrophysique de Bordeaux in France, that the four planets moved around after they were born. The “smoking gun” for that migration, he says, is that the three closest planets orbiting the star — the newly found HR 8799e, along with planets called d and c — are likely to have a special orbital synchrony, according to Marois and his collaborators. Each time planet e goes around the star four times, planet d may go around twice and planet c may circle once. That synchrony is most easily achieved if the planets traveled from their original positions. 

Migration muddies the picture, says Seager, because it makes it more difficult to connect the properties of a planet-forming disk with the type of planet formation model required for a particular planet.

Regardless of how the debate shakes out, says Raymond, “this kind of system is excellent for [understanding] planet formation because it reminds us that we don't know as much as we would like, and it's a new challenge.”

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