A new, more detailed look at the star AU Microscopii has revealed some baffling behavior. The star was first discovered from a ground-based telescope back in the 1980s, before images from the Hubble Space Telescope showed that it was orbited by a debris disk. Some new images bring an unprecedented level of detail to the system—even more than Hubble, despite being another ground-based observation.

Previous observations of Microscopii’s debris disk, which is thought to be the remains of planet-forming material, discovered a peculiar series of variations in the intensity of the light passing through that disk. These variations were thought to be structures, clumps of stuff, all on one side of the disk as seen from Earth.

This unusual structure inspired a lot of interest in the system, as it raised suspicion that there might be planets there, perturbing the clumps. It became one of the first targets of the new SPHERE instrument on the Very Large Telescope (VLT). SPHERE specializes in high-contrast imaging, allowing the VLT to point toward a bright star and block out most of its light, revealing a more detailed view of the environment near the star.

“It is very satisfying that SPHERE has proved to be very capable at studying discs like this in its first year of operation,” said Jean-Luc Beuzit, a coauthor of the study who also led the development of SPHERE.

The new observations detected the same structure, more clearly delineating its components. There are five clumps of material, labeled A through E by the researchers, which lie at distances of about 10 to 60 astronomical units (Earth-Sun distances) from Microscopii. And they look nothing like anything that’s been seen, or even predicted, before in this kind of a system.

“Our observations have shown something unexpected,” explained Anthony Boccaletti, LESIA (Observatoire de Paris/CNRS/UPMC/Paris-Diderot), France, and the paper’s lead author. “The images from SPHERE show a set of unexplained features in the disc which have an arch-like, or wave-like, structure, unlike anything that has ever been observed before.”

The earlier observations, not being as detailed, missed some of the clumps that were the closest to the star and those that were the farthest, and thus only captured the range from about 20-40 AU from Microscopii.

The researchers went back and reanalyzed the data from Hubble and discovered that the clumps have moved over the past few years, and at a rapid pace: about four to ten kilometers per second.

That speed seems to increase with distance from the star. The farthest clumps, D and E, seem to be moving fast enough to escape Microscopii’s gravity, although their velocities aren't very certain. This might indicate that they’re being blown out to interstellar space. C also might be exceeding escape velocity, though this is even less certain.

Material being blown out of a star system isn’t entirely unexpected. Stars produce a constant stream of charged particles, known as the stellar wind, that can push dust and other particles away over time. What’s unexpected is the way it’s happening in Microscopii. Stellar wind should push each particle of dust at about the same rate, creating a smooth continuum of dust moving away. Here, we're seeing discrete clumps moving in a coherent pattern.

The researchers discuss a few possible explanations for this strange behavior, such as massive (but rare) asteroid-like bodies colliding and releasing dust, as well as spiral waves triggered by instabilities in the system’s gravity.

One explanation has the star itself directly involved, as coauthor Glenn Schneider of Steward Observatory explains: “One explanation for the strange structure links them to the star’s flares. AU Mic is a star with high flaring activity—it often lets off huge and sudden bursts of energy from on or near its surface. One of these flares could perhaps have triggered something on one of the planets—if there are planets—like a violent stripping of material which could now be propagating through the disc, propelled by the flare’s force.”

But none of these explanations are completely solid or satisfying, so what's left is a mystery. As the researchers explain in their paper: “the distinct morphology of the features, their high apparent speeds incompatible with low-eccentricity orbits, and their spatial localization on only one side of the disk are at odds with most scenarios.”

The hunt is on to find answers to this conundrum. The Atacama Large Millimeter/submillimeter Array (ALMA) may shed some light on the situation by getting a better estimate of how much gas is left in the system; monitoring of flares from the star may reveal whether coronal mass ejections are linked to the clumps; and finally, hydrogen-alpha differential imaging may reveal traces of any planets in the system. And finally, SPHERE will continue to study the system, looking for more clues.

For the time being, we’re left to wonder.

Nature, 2015. DOI: doi: 10.1038/nature15705 (About DOIs)