Fomalhaut b regains planetary status

Posted by Kate Taylor

The nearby star Fomalhaut does host a massive exoplanet after all, say NASA scientists following a second look at Hubble data.

Back in 2008, astronomers announced that they'd discovered a planet, named Fomalhaut b, shrouded by dust as it orbited Fomalhaut some 25 light-years away.

More recent studies, though, have claimed that this interpretation is wrong. Based on the object's apparent motion and the fact that NASA's infrared Spitzer Space Telescope failed to detect it, they argue that the object is a short-lived dust cloud unrelated to any planet.

Now, though, NASA says that the original theory was probably correct.

"Although our results seriously challenge the original discovery paper, they do so in a way that actually makes the object's interpretation much cleaner and leaves intact the core conclusion, that Fomalhaut b is indeed a massive planet," says Thayne Currie, now at the University of Toronto.

The first study reported that Fomalhaut b's brightness varied by about a factor of two, and cited this as evidence that it was a planetaccreting gas. Others, though, claimed that it actually showed that the object was a transient dust cloud instead.

Currie and his team have now reanalyzed Hubble observations of the star from 2004 and 2006 - and found it was easily spotted at visible wavelengths near 600 and 800 nanometers. They also made a new detection in violet light near 400 nanometers. And, in contrast to the earlier research, the team found that the planet remained at constant brightness.

While they too were unable to detect Fomalhaut b in the infrared using the Subaru Telescope in Hawaii, they say this simply implies that Fomalhaut b must have less than twice the mass of Jupiter.

And they say they've also settled the contentious issue of the object's orbit. Fomalhaut b is moving with a speed and direction consistent with the original idea that the planet's gravity is modifying the ring, they say.

"What we've seen from our analysis is that the object's minimum distance from the disk has hardly changed at all in two years, which is a good sign that it's in a nice ring-sculpting orbit," says University of Arizona graduate student Timothy Rodigas.

And there are reasons to believe that Fomalhaut b can't be a compact dust cloud not gravitationally bound to a planet.

Near Fomalhaut's ring, orbital dynamics would spread out or completely dissipate such a cloud in as little as 60,000 years, they say.

"Given what we know about the behavior of dust and the environment where the planet is located, we think that we're seeing a planetary object that is completely embedded in dust rather than a free-floating dust cloud," says John Debes of the Space Telescope Science Institute.