Free Astronomy Magazine November-December 2015

SPACE CHRONICLES in the SPHERE data the team turned to earlier images of the disc taken by the NASA/ESA Hubble Space Tele- scope in 2010 and 2011 to see wheth- er the features were also visible in these. They were not only able to identify the features on the earlier Hubble images — but they also dis- covered that they had changed over time. It turns out that these ripples are moving — and very fast! “We re- processed images from the Hubble data and ended up with enough in- formation to track the movement of Mysterious ripples found racing through planet-forming disc AU Microscopii, or AU Mic for short, is a young, nearby star surround- ed by a large disc of dust. The disc essentially comprises asteroids that have collided with such vigour that they have been ground to dust. Studies of such debris discs can pro- vide valuable clues about how plan- ets, 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 pos- sible planets. And in 2014 they used the 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 some- thing unexpected,” explains Antho- ny Boccaletti of the Observatoire de Paris, France, lead author on the study. “The images from SPHERE show a set of unexplained features in the disc which have an arch-like, or wave-like, structure, unlike any- thing that has ever been observed before.” Five wave-like arches at different dis- tances from the star show up in the new images, reminiscent of ripples in water. After spotting the features by ESO T he top row shows a Hubble image of the AU Mic disc from 2010, the middle row Hubble from 2011 and the bottom row is an image taken with the SPHERE instrument, mounted on the Very Large Telescope, from 2014. The black central circles show where the brilliant light of the central star has been blocked off to reveal the much fainter disc, and the position of the star is indicated schematically. The scale bar at the top of the picture indicates the diameter of the orbit of the planet Neptune in the Solar Sys- tem (60 AU). Note that the brightness of the outer parts of the disc has been artificially brightened to reveal the faint structure. [ESO, NASA & ESA]