Astronomers have released the first new science results from the Atacama Large Millimeter/submillimeter Array (ALMA), a still-under-construction network of 66 antennas in northern Chile. The new observations suggest the contentious Fomalhaut star system may have two small planets shepherding its gigantic ring.
The first new science results are out from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and they’re worth bragging about. The observations reveal the debris ring around the star Fomalhaut in exceptional detail, as shown in the comparison image at right, which overlays the ALMA data on top of observations at a similar wavelength taken in 2003. (The blobs are the ring, 2003-style.)
Only 15 of the planned 66 antennas contributed to the new observations, but when considered along with the first test images released last fall, the new data are enough to confirm ALMA should live up to the extraordinary expectations set for it.
“It’s as if you’re witnessing the type of breakthrough image first provided by the Hubble Space Telescope,” says Paul Kalas (University of California, Berkeley), who has used that telescope to study the Fomalhaut system before.
“It’s really an amazing instrument,” agrees Aaron Boley (University of Florida), principle investigator on the new study, which will appear in the Astrophysical Journal Letters. Part of that amazingness is ALMA’s superb resolution and sensitivity, unprecedented even in the modern astronomy era. But part of it, too, is the effort ALMA staff invest working with astronomers and their data to make sure the observations are as well-calibrated as possible, ready for analysis. “It’s a user-friendly interferometer,” Boley says, “which is actually a very odd statement.”
The Fomalhaut disk Boley’s team observed is a prime target for understanding planet formation. At 25 light-years away and about 20 times the Sun’s luminosity, the star is an easy, bright target that has received lots of attention. Much of that focus centers on a proposed planet, Fomalhaut b, announced by Kalas and his colleagues in 2008 using visible light images from the Hubble Space Telescope that revealed a moving spot just inside the star’s icy ring. Since then Fom b’s existence has been called into question, and it’s currently in scientific limbo.
The new observations confirm the disk is offset from the star, not a perfect circle, and far-out, starting 135
astronomical units away from the star. It’s also remarkably thin compared to its size, between 13 and 19 a.u. wide. But what are really interesting are the disk’s sharp edges: the disk doesn’t peter out to either side, but cuts off rapidly, as though carved.
Boley and his team think the best explanation is that two “shepherding planets” corral the disk, much as the moons Cordelia and Ophelia herd Uranus’s epsilon ring. These exoplanets would be less than 3 Earth masses, with one lying to either side of the debris.
It’s unclear whether the inner proposed planet could be the elusive Fom b. The ALMA results suggest the radius of the inner planet’s orbit differs from Fom b’s by about 10 a.u., but that’s not based on an actual exoplanet detection. Recent infrared observations with the Spitzer Space Telescope failed to find Fom b. Disagreement persists over whether that’s because the object was actually a transient dust cloud or because reflective debris enshrouds the planet, making it bright in optical but invisible in infrared. Planned HST and ALMA observations may help unravel the puzzle, perhaps by revealing planet-caused structure in the debris disk.