Astronomers baffled by mysterious waves in planet-forming disc

Fast-moving waves racing through a dusty disc surround a young star have baffled scientists who have never seen a feature like it before, but hope it may give them clues about how planets form.

The phenomenon was spotted by astronomers using NASA’s Hubble Space Telescope and the European Southern Observatory’s (ESO) Very Large Telescope in Chile in the planet-forming disc surround the star AU Microscopii, which is 32 light-years away in the southern constellation Microscopium.

NASA says the star is ideal for observation for signs of planet formation as the surrounding disc of dust is tilted edge-on to our view from Earth.

Scientists discovered the unusual ripples near the star by using ESO’s SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument, mounted on the Very Large Telescope.

“The images from SPHERE show a set of unexplained features in the disk, which have an arc-like, or wave-like structure unlike anything that has ever been observed before,” said Anthony Boccaletti of the Paris Observatory, the lead author of a paper on the subject published in Nature.

After seeing the features in the SPHERE data the team found they had missed them in Hubble images of the disk, taken in 2010 and 2011. Inspection of the earlier images shows the waves had changed over time and are moving very fast.

“We ended up with enough information to track the movement of these strange features over a four-year period,” explained team member Christian Thalmann of the Swiss Federal Institute of Technology in Zurich, Switzerland. “By doing this, we found that the arches are racing away from the star at speeds of up to 10 kilometres per second (35,400 kilometres per hour).”

Co-investigator Carol Grady of Eureka Scientific in Oakland, California, said she did know what was causing the waves. “We can only hypothesize when it comes to what we are seeing and how it came about,” she said.

The ripples farther 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 are escaping from the gravitational attraction of the star. Such high speeds rule out the possibility that these features are caused by objects, like planets, gravitationally disturbing material in the disk. The team has also ruled out a series of phenomena as explanations, including the collision of two massive and rare asteroid-like objects releasing large quantities of dust and spiral waves triggered by instabilities in the system’s gravity.

“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,” said co-author Glenn Schneider of Steward Observatory in Phoenix, Arizona.

“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 disk, propelled by the flare’s force.”

Bill Condie

Bill Condie

Bill Condie is a science journalist based in Adelaide, Australia.

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