Based on current knowledge, they're unexplainable.
Astronomers have an excellent new mystery on their hands. Some 32.3 light-years away, a small star named AU Microscopii sits planet-less but not alone. Within its deep debris disc—a tight platter of dust and Moon-sized planetesimals—are strange, unexplained structures.
These mysterious ripples are described in a paper published this week in Nature by Anthony Boccaletti of the Observatoire de Paris and others. Their work is based on data from the Hubble Space Telescope and the ESO Very Large Telescope's newly installed SPHERE instrument.
"Giant planets form in disks of gas and dust that surround newborn stars, over timescales of a few million years," Boccaletti and his team write. "Some stars retain a dust disk significantly longer, which is taken as evidence for an unseen population of asteroid-like bodies (planetesimals) whose destructive collisions continually replenish the system with dust."
These discs wind up being excellent observational tools, functioning as markers for planet formation in the form of "eccentric, clumpy or warped features." And this is just what a group of astronomers found in 2004: a general asymmetry along with a sharp change in structure within AU Mic's innermost debris disc. If planets are indeed hiding within the star's warped rings, however, they've so-far remained hidden, though Hubble observations in 2010 and 2011 confirmed the debris disc's structural asymmetry.
When the SPHERE planet-hunting instrument was first installed on the Very Large Telescope in 2014, AU Mic was one of its initial test targets. "While the general shape agrees with previous observations, the new SPHERE images show the morphology of the whole disk with unprecedented resolution and detail," Boccaletti and his group write in the current study.
This increased resolution is what allowed the group to identify the "enigmatic patterns" in question. The patterns consist of five structures in total, each separated by 10 to 55 astronomical units (AUs, or roughly the distance from the Sun to the Earth) and each between 5 and 10 AU in size. Crucially, they're moving together, appearing as arches or the crests of waves all in a row.
The researchers were able to locate the same structures from earlier Hubble data from 2010 and 2011, finding that not only were they present in those previous images, AU Mic's strange ripples appear to be hauling ass. Calculations show them traveling at between 4 and 11 kilometers per-second faster than what should be expected for objects moving within the star's orbit. This is to say that they're exceeding the local escape velocity and are being expelled from star's system.
No explanation for the structures really fits properly and it seems likely this is for-reals something we've never seen before. Explanations such as dust scattering from colliding asteroids and gravity-spawned spiral waves have been ruled out.
"One explanation for the strange structure links them to the star's flares," offers study co-author Glenn Schneider in a statement. "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."
That is a completely bad-ass explanation, yes. We'll have to wait for the real answer, while astronomers continue spying on the system with the SPHERE instrument as well as others, such as the ALMA telescope in Chile. Sounds like it will be worth the wait.