Okay, this is pretty bizarre. Astronomers have spotted strange “ripples” in the debris disk of a distant, young planetary system traveling away from its central star at 40,000 kilometers (25,000 miles) per hour. And at the moment, no one knows what’s causing them.
The ripples – which are essentially areas of lower density debris in the disk – were spotted by the European Southern Observatory’s (ESO) new Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument on the Very Large Telescope (VLT) in Chile. They were found in the large disk of dust around the young red dwarf star AU Microscopii (AU Mic), 32 light-years from Earth and just 23 million years old. The research is published in the journal Nature.
Young planetary systems are thought to have large amounts of swirling debris – a protoplanetary disk – which ultimately clumps together to form planets and other objects. Our own Solar System is thought to have begun in this manner.
However, this latest discovery is hugely surprising. Ripples in a debris disk have not even been theorized before, let alone observed. "Our observations have shown something unexpected," explains Anthony Boccaletti of the Observatoire de Paris, France, lead author on the paper, in a statement. "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."
In total, five wave-like arches were seen at different distances from the star, moving outwards through the disk, and reexamination of previous Hubble images showed they had been present since 2010. These earlier observations allowed astronomers to estimate the speed of the ripples to be up to a blistering 40,000 kilometers per hour – fast enough to escape the gravitational pull of the star. Those further away seem to be moving faster than those closer. The features vary in distance from the star from 10 to 60 astronomical units (one AU is the Earth-Sun distance).
Planetary systems are thought to begin with debris discs like this. NASA / FUSE / Lynette Cook.
Several theories for their formation have been ruled out. The fact that the ripples are moving at high speeds indicates they are not simply gaps in the disk where planets have formed. Their speed has also seemingly ruled out the possibility they were the result of a massive collision between asteroid-like objects. Thus, the researchers seem to be favoring the "solar flare" theory.
"Our suspicion that the star is involved somehow comes from the nature of the star itself, which is a cool, small star, with strong activity releasing flares and mass ejections, hence producing a ‘wind’ in the system," Boccaletti told IFLScience. "This wind may interact with an object somewhere in the disk, which by some mechanism that we don't know could produce a visible structure. As a result we also think that the structures are relatively recent, 15 years or so old."
This debris disk is thought to have already formed planets, with the remaining debris being left over. What effect the ripples are having on any planets in the system is unknown, though. "It may be having no effect at all," added Boccaletti.
Using SPHERE, the team plans to continue observing this system, alongside searching others for similar ripples. What’s clear, though, is that there is still much we don’t understand about protoplanetary disks.