Bilder från VLT och Hubble av skivan runt stjärnan AU Microscopii. Foto: ESO
Images from VLT and Hubble of the disc surrounding the star AU Microscopii. Photo: ESO


AU Microscopii, or AU Mic for short, is a young, nearby star surrounded by a large disc of dust. Studies of such debris discs can provide valuable clues about how planets, which form from these discs, are created.

Astronomers have been searching AU Mic’s disc for any signs of clumpy or warped features, as such signs might give away the location of possible planets. And in 2014 they used the more powerful high-contrast imaging capabilities of ESO’s newly installed SPHERE instrument, mounted on the Very Large Telescope for their search — and discovered something very unusual.

“Our observations have shown something unexpected,” explains Anthony Boccaletti, LESIA (Observatoire de Paris/CNRS/UPMC/Paris-Diderot), France, and lead author on the paper. “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.”

Like ripples in water

Five wave-like arches at different distances from the star show up in the new images, reminiscent of ripples in water. After spotting the features in the SPHERE data the team turned to earlier images of the disc taken by the NASA/ESA Hubble Space Telescope in 2010 and 2011 to see whether the features were also visible in these. They were not only able to identify the features on the earlier Hubble images — but they also discovered that they had changed over time. It turns out that these ripples are moving — and very fast!

“We reprocessed images from the Hubble data and ended up with enough information to track the movement of these strange features over a four-year period,” explains team member Christian Thalmann from ETH Zürich, Switzerland. “By doing this, we found that the arches are racing away from the star at speeds of up to about 40 000 kilometres/hour!”

Escaping from the gravitational attraction of the star?

The features further away from the star seem to be moving faster than those closer to it. At least three of the features are moving so fast that they could well be escaping from the gravitational attraction of the star. Such high speeds rule out the possibility that these are conventional disc features caused by objects — like planets — disturbing material in the disc while orbiting the star. There must have been something else involved to speed up the ripples and make them move so quickly, meaning that they are a sign of something truly unusual.

Markus Janson Foto: Alexis Brandeker
Markus Janson.
Photo: Alexis Brandeker

”We have looked at a number of phenomena, that could possibly explain the ripples we see, but they have, in general, been ruled out since they don’t match our available data”, says Markus Janson at the Department of Astronomy, Stockholm University and one of the authors behind the article in Nature.

The team plans to continue to observe the AU Mic system with SPHERE and other facilities, including ALMA, to try to understand what is happening. But, for now, these curious features remain an unsolved mystery.

Hear more about the discoveries on Astronomins dag och natt on 10 October

On 10 October, Markus Janson, from the research team behind the study, will talk about the research at Vetenskapens hus in Stockholm as part of the theme day Astronomins dag och natt. See www.astronominsdag.se/stockholm for more information (in Swedish).

The research results are presented in an article titled "Fast-Moving Structures in the Debris Disk Around AU Microscopii", published in the scientific journal Nature 8 October 2015.