Published online 23 September 2011 | Nature | doi:10.1038/news.2011.555

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Astronomers argue over object's status after it unexpectedly veers off course.

An artist's impression of the planet Fomalhaut b. ESA/ NASA/L. Calcada

As astronomers continue to rack up exoplanet discoveries by the dozen (see Nature 477 , 383–384; 2011), the precise status of just one may not seem like much to fret over. But Fomalhaut b is different.

Unveiled in 20081, the tiny dot spotted circling Fomalhaut, a star just 7.7 parsecs from our own Solar System, was billed as the first exoplanet to be directly imaged at optical wavelengths. Now Fomalhaut b's identity is being questioned after data, presented last week at an exoplanet conference in Grand Teton National Park, Wyoming, showed that it has moved in an unexpected way.

Until now, Fomalhaut b had all the makings of a picture-perfect exoplanet. Two Hubble Space Telescope images, from 2004 and 2006, were used to show how the planet is tracing a textbook orbit just inside a luminous ring of dust encircling Fomalhaut. The implication was that the planet's gravity was helping to sweep away dust that was closer than the ring, giving it a sharp inner edge.

Paul Kalas, an astronomer at the University of California, Berkeley, and lead author on the 2008 study, says the latest image indicates that the planet's orbit crosses into the dust disk. And that has led Ray Jayawardhana, an astronomer at the University of Toronto in Canada, to question the planet's existence. On such a trajectory, the planet's gravitational influence would have previously disrupted the clearly delineated disk. "It's quite clear that the original story cannot stand anymore," Jayawardhana says.

Stirring controversy

Kalas acknowledges that the latest data point is puzzling, but he says he remains confident that Fomalhaut b is a planet. "You have one scientist trying to create a controversy out of nothing," he says. Hundreds of exoplanets have been detected indirectly by measuring their gravitational influence on the stars they orbit or by recording brightness changes as they cross in front of their stars. Only a handful of planets have been imaged directly. For the lucky astronomers who have bagged one of these, bragging rights have accrued — but so has the backbiting.

Fomalhaut b is already known as an oddball among exoplanets. It is far too bright in visible light for something expected to be only a few times the size of Jupiter. And ground-based follow-up observations in the infrared have been fruitless, even though this is the part of spectrum where hot, youthful planets are supposed to be brightest.

Kalas says one explanation may be that the Fomalhaut system is older than previously thought, and therefore cooler and fainter in the infrared. And, he says, the excessive optical brightness can be explained if the planet is surrounded by bright material, just as Saturn is surrounded by a system of rings, which would increase its overall reflectivity.

Jayawardhana says this alone should bump Fomalhaut b from the list of directly imaged planets, because the light would be coming from the dust, not the planet's surface. "They continue to call it a directly imaged planet," he says. "I think it's time to stop doing that."

Two successive images of Fomalhaut b suggest it is following a conventional planetary orbit (white line). A 2010 image, however, shows the object straying from this course. NASA/ ESA/Ref. 1

The new data point, and the potential disk-cutting orbit, adds to the mystery. Kalas says it might just be a problem with the latest image. The pictures in 2004 and 2006 were taken using a high-resolution channel on the Hubble's Advanced Camera for Surveys that failed in 2007 and was not restored when the camera was brought back online in 2009.

For the latest image, Kalas had to resort to another Hubble instrument, the Space Telescope Imaging Spectrograph. He says the switch to a different detector may explain the planet's slight deviation from its expected position. He has been allocated time to take another picture with that same instrument next summer. If the planet's unexpected motion persists, he says it is still possible to explain why the disk around the star is undisturbed: perhaps his team is seeing the planet just as some dynamic instability in the star system is knocking it off course.

Fighting words

Christian Marois, of the Herzberg Institute of Astrophysics in Victoria, Canada, doesn't like arguments that rely on coincidence. With an orbital period of about 800 years, Fomalhaut b would have had to have changed course quite recently.

Marois says it's far more likely that Kalas is adjusting his analysis to the different instrument on Hubble, and that the original orbit will hold. If anything, he says, the fact that Kalas spotted Fomalhuat b at all in 2010 is "another confirmation that this thing is real".

Jean Schneider, an astronomer at the Paris Observatory who maintains the exoplanet.eu database, says that Fomalhaut b will remain on the list. But on 22 September, he added a comment to the planet's entry saying that doubts have been raised (go.nature.com/ojgjjd).

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In an e-mail to Schneider today, Kalas said that, to be scrupulously fair, Schneider should also mention that there are doubts associated with 1RXJ1609, a planet imaged directly at infrared wavelengths that Jayawardhana co-discovered and announced in 2008, a few months before the unveiling of Fomalhaut b.

The dispute has all the makings of a celestial grudge match: Jayawardhana suggests that fierce professional competition and the glare of the media spotlight can fuel 'planet mania', leading astronomers to overstate their discoveries. Kalas notes that he coined the term 'planet mania' in a 1998 article in Science2, in which he criticized Jayawardhana for making inflated claims about observing planets in the process of forming.

For its part, Fomalhaut b seems to know what it's doing, even if no one else does.