There may be more ways to make a planet than we realized, at least when it comes to timing. Red dwarfs have been found with disks apparently taking much longer to coalesce into planets than was previously thought possible. Coincidently, the announcement comes only days after the announcement of exceptionally rapid planet formation.
It’s been a big week for astronomers studying planet formation. The first video of a planet orbiting another star was released in conjunction with new data about the disk that orbits the same star, which is expected to eventually condense into additional planets. A star that hasn't finished forming was found to already have a planet, suggesting formation can be more rapid than anyone expected.
Now it seems that the opposite is also true, with two stars reported whose planets appear to be forming at a surprisingly leisurely pace.
Dr. Simon Murphy of the Australian National University is part of a team that were seeking stars from the Octans Association. This is one of a number of collections of stars that formed together but have now dispersed while maintaining some distinctive features. In the process, Murphy came across two stars he told IFLScience “look like Octans members, but their radial velocity is way off.” These stars’ characteristics mark them as belonging to the Scorpius-Centaurus Association of stars, which Murphy says are spread across 10% of the southern sky.
2M1239-5702, one of the two newly discovered red dwarfs with protoplanetary disks, is located in this red box, near the head of the Southern Cross. Credit: Akira Fujii
In the Monthly Notices of the Royal Astronomical Society, Murphy reports that these two faint red dwarfs are bright in the wavelengths of light associated with protoplanetary disks. This was unexpected, as Scorpius-Centaurus stars have an average age of 17 million years, well past the point where such disks can usually be seen. This is because they are thought to have aggregated into planets by then.
Murphy told IFLScience the exact age of the stellar pair is difficult to measure. The concentration of lithium in their outer atmosphere marks them as young, since lithium is absorbed into stars' cores early in their lives. Nevertheless, Murphy says that if they are indeed Scorpius-Centaurus Association members, they are “unusually old to have disks, among the oldest known.”
Murphy said long-lasting disks might shape the eventual nature of the solar system they spawn. So called Hot Jupiters have puzzled astronomers since they were first discovered, as it is considered impossible for them to have formed so close to their parent stars. One possible explanation is that drag from whatever part of the disk that survives the first planet's formation causes gas giants to spiral in towards the parent star. The discovery of long-lasting disks lends credibility to this theory.
“Red dwarf discs seem to live longer than those of hotter stars like the sun,” Murphy said in a statement. “We don't understand why.” However, he added to IFLScience that evidence is growing for an inverse relationship between the mass of a star and its disk's longevity.
Murphy predicts the new discoveries will attract astronomers to devote valuable telescope time to study them, as "young red dwarves are ideal places to directly pick out recently formed planets.”