Unexplained ripples have been found racing outward in a dusty disk around a star.
Nearby debris disks — the dusty, sometimes rocky planes circling young stars — have only recently become the hunting grounds of astronomers, who search for the telltale signs of forming planets: gaps, clumps or warped features in these disks. But thus far, very few disks have revealed planets hidden inside. The majority remain a mystery waiting to be unfolded.
SPHERE, an instrument mounted on the Very Large Telescope in Chile, is designed to directly image debris disks and reveal their secrets. Its coronagraph blocks the light of the host star, while the instrument’s adaptive optics reveals details around the star to a resolution of 0.5 arcseconds, rivaling the Hubble Space Telescope’s imaging prowess.
In 2014, Anthony Boccaletti (Paris Observatory) and his colleagues pointed SPHERE at a test target known as AU Microscopii, a young star 32 light-years away in the southern constellation Microscopium. But what they found was something utterly unexpected: wave-like arches on one side of the disk. Were they real? The team turned to data gathered by the Hubble Space Telescope in 2010 and 2011, and sure enough, the features were there too.
What’s more, the arching waves had moved at a breakneck pace through the disk, moving away from the central star at 4 to 10 kilometers per second (between 9,000 and 22,000 mph).
“This is a fascinating result,” says Richard Nelson (Queen Mary University of London), who was not involved in the study. “But interpreting the observations is a real puzzle.” Not only have astronomers never seen anything like it, they really can’t find a viable explanation.
The five bright smears in AU Mic’s disk lie within 10 and 60 times the Earth-Sun distance of the star, and are probably clumps or clouds of dust shining in near-infrared light. The Hubble photos allowed the team to track the ripples over a 4-year baseline, revealing their immense speed. The outer waves moved much faster than the inner ones, and at least three of the features are moving so fast that they could easily slip beyond the star’s gravitational pull.
No Good Explanation
Such high speeds rule out any classic scenarios caused by orbiting planets. A warp carved in the disk by a nearby planet, for example, would move at speeds too lethargic compared to the observed ripples. Boccaletti and his colleagues searched high and low for a planet with no luck. “If there was a planet in there and it was larger than 6 Jupiter masses, we'd be able to find it,” says co-author Dean Hines (Space Telescope Science Institute). “If there's something in there stirring up the pot, which there almost certainly is, it's going to be smaller than that.”
So maybe, the authors propose, two smaller as-yet unseen planets collided within the disk. After all, most astronomers expect that all forming planetary systems are extremely violent. Our solar system, for example, is still scarred by the collisions of its youth, which should have been readily visible to an extraterrestrial observer.
“If you grind up chalk, put it in a bag and pop it, the chalk dust goes everywhere,” Hines says. “It’s really hard if you're in the back of the room to see that piece of chalk, but once you explode it, it has a huge surface area and it's easy to see.” Although a collision might explain the asymmetries in brightness from one side of the disk to the other, it couldn’t cause material to move so fast.
The most promising scenario requires an even more violent interaction. Young stars, while promising abodes for life, are wildly active. They emit giant flares — huge eruptions of charged particles — that can wreak havoc on a circling planetary disk. If a flare hits a forming planet, it could easily strip material away from the planet and propagate it outward at rapid speeds. Nelson, however, doubts whether even these speeds would be fast enough to match those found within AU Microscopii’s disk.
The team plans to continue to observe the system with SPHERE and other facilities. “It's not often that you see something changing on human timescales,” says Hines, who is excited to see further changes that will allow them to narrow down the range of possibilities.
"We wish we knew what it was,” says co-author John Debes (Space Telescope Science Institute). “But sometimes you just have to throw up you hands and say 'We don't know what it is yet and we'll keep looking and keep thinking to try to come up with the answer.'”
Anthony Boccaletti et al. “Fast-Moving Structures in the Debris Disk Around AU Microscopii.” Nature. October 8, 2015.