Shining at 1st magnitude in the southern constellation Piscis Austrinus, Fomalhaut is the 18th brightest star in the night sky. At a distance of only 25 light-years, this dazzling beacon is one of the best-studied stars, and the subject of numerous sci-fi stories. With that kind of background, one might think that astronomers have learned everything they wanted to know about Fomalhaut.
But when Paul G. Kalas (University of California, Berkeley) and two colleagues pointed the Hubble Space Telescope at Fomalhaut last year, they were in for a huge surprise. Hoping to detect the feeble glow of orbiting planets, Kalas instead found a belt of cold dust orbiting far from the star. The dust comes from small bodies colliding and grinding themselves down to tiny grains. Kalas and his colleagues estimate that the belt — known as a debris disk — contains 50 to 100 Earth masses of material.
As Kalas's group reports in the June 23rd Nature, Fomalhaut's belt is analogous to our solar system's Kuiper Belt (which contains Pluto and nearly a thousand other known small, icy bodies), but with several important differences. Fomalhaut's belt is much farther from its star: 133 to 158 astronomical units (1 a.u. is the average Earth-Sun distance), compared to the 30 to 50 a.u. for our solar system. Perhaps more important, Fomalhaut's belt is slightly oval, and the geometric center of the ellipse is offset from the star by a whopping 15 a.u. Previous theoretical studies show that one or more massive planets on highly eccentric orbits would produce exactly this kind of asymmetry.
Adding to the evidence for planets is the fact that like our solar system's Kuiper Belt, Fomalhaut's belt has a sharp inner edge. In our solar system, gravitational interactions with Neptune sculpt the Kuiper Belt's sharp inner edge. "Instead of having a Neptune at 30 a.u., maybe Fomalhaut has a massive planet at 50 to 70 a.u.," says Kalas. "It's fascinating to see an extrasolar Kuiper Belt directly and see what's shaping it. That might lead to answers about what's shaping our Kuiper Belt and its 50-a.u. outer edge."
Observations at longer wavelengths in the 1990s had already indicated that a torus of material surrounded Fomalhaut, which is a relatively young star with an estimated age of 200 million years. In recent years, astronomers have also imaged or indirectly detected debris disks around other young stars. But never before have they seen a disk in such stunning detail, and with a directly measured stellar offset.
"It's a fantastic image and in combination with recent imaging of similar systems, this will help us to understand debris disk evolution much better," says Michael C. Liu (University of Hawaii). "For many years, the number of spatially resolved debris disks could be counted on one hand, and each one seemed to tell a different story. With the recent discovery boom in spatially resolved systems, we're finally seeing how to connect the dots — namely what are the common features in disks around stars of different masses and ages."
Liu notes that there is still no "smoking gun" linking a debris disk to a known planet. But Kalas and his colleagues plan to acquire even deeper images of Fomalhaut with Hubble and ground-based telescopes equipped with adaptive optics. Such images could reveal planets, but they could also expose even more intricate structure in the disk such as gaps and clumps. Such features will help theorists narrow the range of possibilities for planet masses and orbits. Kalas and his colleagues also hope to learn more about a soft glow interior to the belt, which might be Fomalhaut's version of zodiacal light — which would come from rocky asteroids grinding themselves down to dust.
"We want to see if the color of the main belt is the same as the color of the interior nebulosity," says Kalas. "Do they have the same dust-grain population, or is the dust interior to the belt different from the belt? With an age of 200 million years, Fomalhaut is comparable to the solar system when Earth and the other planets were being bombarded by comets and asteroids as they were sweeping up small objects."