LONG BEACH, CALIFORNIA—Something is orbiting the bright star Fomalhaut in the constellation known as the Southern Fish, but no one knows exactly what it is. New observations carried out last year with the Hubble Space Telescope confirm that the mysterious object, known as Fomalhaut b, is traveling on a highly elongated path, but they haven't convincingly nailed down its true nature. But if it is a planet, as one team of astronomers thinks, we may be in for some celestial fireworks in 2032, when Fomalhaut b starts to plough through a broad belt of debris that surrounds the star and icy comets within the belt smash into the planet's atmosphere.
The new Hubble observations, presented here on Tuesday at the 221st meeting of the American Astronomical Society, reveal that Fomalhaut b "is definitely moving outwards," says Paul Kalas of the University of California (UC), Berkeley. Based on the four measurements carried out since 2004, he and his Berkeley colleague James Graham conclude that Fomalhaut's "planet" orbits the star every 2000 years. The elongated path could be the result of a close encounter with another planet in the system.
Fomalhaut b has an intriguing history. Kalas first recognized it in Hubble photos taken in 2004 and 2006 as a faint, slowly moving speck of light. Because of problems with one of Hubble's cameras, however, he could not take another Hubble snapshot of Fomalhaut until 2010. This third photo was the first one to suggest, quite unexpectedly, that the planet was on a noncircular orbit—something now confirmed by the fourth set of Hubble data.
Back in 2010, Kalas says, people thought that the oddly shaped orbit was proof that the moving speck couldn't be a planet. "But the fact that something is surprising doesn't mean it can't be true," he says. "Personally, I was never worried that the planet interpretation might be wrong." Other researchers were, though. To settle the matter as quickly as possible, the Space Telescope Science Institute moved up the fourth Hubble observation of Fomalhaut b by a few months. Kalas says: "Our new data now constitute a strong affirmation of the planet."
One mystery remains, though. No heat radiation is detected from Fomalhaut b, as would be expected for a large planet, meaning it must be smaller and less massive than Jupiter. But the moving speck of light puts out more light than you would expect for such a relatively small object. The team believes this contradiction could be explained if the planet was embedded in a large cloud of dusty material, which scatters the light of the star.
However, other astronomers are still not convinced that the object qualifies as a planet. A team led by Raphaël Galicher of the Dominion Astrophysical Observatory in Victoria, Canada, reanalyzed the earlier Hubble observations, and although they confirm that "something" is moving through the Fomalhaut system, they offer an alternative explanation. The "something" might be a cloud of debris resulting from the collision of two icy objects some 100 kilometers across. They have submitted their results for publication in The Astrophysical Journal.
"Our analysis suggests that the speck of light is slightly extended in the 2010 Hubble images," says Ben Zuckerman of UC Los Angeles, a member of Galicher's team. If the object continues to "grow" over time, that would support the debris-from-collision interpretation. However, Galicher's team hasn't been able yet to analyze the new 2012 data. "This object is very strange, and it acquires additional strangeness all the time," Zuckerman says. "Frankly, I don't know what to believe."
Even if Fomalhaut b is not a planet, Kalas believes that there could be one or more planets in the system. Tidal effects and gravitational disturbances from relatively large bodies are required to explain the sharp inner edge of Fomalhaut's debris belt, astronomers believe. Also, a newly discovered gap in the dust belt suggests that a planet might have passed through. Later this year, Kalas's team will carry out new Hubble Space Telescope observations of Fomalhaut b, to learn more about its orbit. Future observations with the Atacama Large Millimeter/submillimeter Array in Chile should produce very detailed maps of the dust distribution around the star—and better predictions of when the fireworks might begin.
Twenty years from now, the precise way in which Fomalhaut b interacts with the dust belt might reveal the object's true nature. Then again, Zuckerman says, if nothing happens it won't tell you a lot. "The orbit could be inclined with respect to the plane of the dust belt," he says, "in which case you wouldn't see anything."