New doubts about 'poster child' of exoplanets

When it comes to alien worlds, even poster children are fickle. Fomalhaut b – one of the first and only exoplanets to be photographed directly – may not be what it seems.

Astronomers expected to find a planet orbiting the young, nearby star Fomalhaut since 2005, when Hubble Space Telescope images showed that the star's spectacular dust disc lies a bit off-centre.

Sure enough, in 2008, Paul Kalas of the University of California, Berkeley, and colleagues spotted a wandering speck of light in two separate images taken by the Hubble Space Telescope's Advanced Camera for Surveys in 2004 and 2006.

These were combined to form the image shown (right), which contains two positions for the planet. Fomalhaut b became known as one of the only directly imaged exoplanets.

Disc disruption?

The candidate planet lies about 120 times as far from its star as the Earth lies from the sun, and apparently nestles neatly into a gap in the dust disc. Astronomers suggested that the planet's gravity created an opening in the disc at that spot, the same way that some of Saturn's moons open gaps in its rings.

The trouble is, in the three years since the 2008 discovery, no other telescope had detected the planet.

The Hubble instrument used to take the first images broke in 2007, and will not be repaired. So Kalas and colleagues used an older Hubble camera to take another peek in 2010. As they reported on 12 September, at the Extreme Solar Systems II conference in Moran, Wyoming, this revealed the bright speck again.

This time, however, it was in an unexpected place. "That's a problem," says Steinn Sigurdsson of Pennsylvania State University in University Park, who helped organise the conference. To make sense of it, the planet would need an elliptical orbit that takes it across the dust disc, yet its brightness suggests that it is too big to do so without disrupting the disc.

Hidden planet

That's causing Ray Jayawardhana of the University of Toronto, Canada, to doubt whether the initial images really were of an exoplanet. Jayawardhana was not involved in the planet's discovery but he gave a talk at the same meeting about the problems with imaging exoplanets. "I would say the evidence is confusing and contradictory at best," he says.

Kalas is not giving up. Possible explanations include a second, hidden planet that holds the ring steady and perturbs Fomalhaut b's orbit.

Or it may be that the speck in the newest image is not a planet at all, but a transient dust cloud within the disc, a background star, or a tiny protostar that failed to ignite.

"It will all be solved with observations, probably within the next year," says Kalas. His team is scheduled to observe the Fomalhaut system with Hubble again in 2012.

Data squeeze

The controversy highlights the problems in claiming first dibs on exoplanet discoveries.

"There's a little bit of tension about priorities; who did what first. Things are happening so rapidly that discoveries are overlapping, and sometimes people are trying to get in the door before it slams," Sigurdsson says. "We're squeezing the data till it bleeds. Some of it's going to be wrong, no question."

That concerns Jayawardhana. "I feel a little bit uncomfortable that the poster boy for directly imaged planets is the weakest case or least secure case that we have," he says.

It's not the first exoplanet discovery to be potentially fickle. Just two weeks after the first life-friendly exoplanet was discovered – Gliese 581g – its existence was called into question by members of another team, who didn't see the planet in their data. Gliese 581g's existence still awaits confirmation.