Circumstellar discs are believed to be key components in planetary formation. However, we have very little actual evidence of planets growing in the “rings” that surround young stars. So planet formation theorists were ecstatic to learn that two new papers in Astrophysical Journal Letters describe a planet that is actively forming in the gap it most likely created in the ring system of a young, Sun-like star.
The system, which was also newly discovered, is called WI Separation Planets in Time (WISPIT) 2. WISPIT was a survey conducted on the SPHERE instrument of the Very Large Telescope (VLT) in Chile, and in this case focused on a cluster of young stars about 434 light years away in the constellation Aquila. Previous searches for this kind of system only turned up one other system with a confirmed planet actively shepherding a circumstellar disc - PDS 70 is a “multi-planet” system discovered in 2018 where two planets reside in the innermost ring cavity.
In contract, WISPIT has four distinct “rings”, each with a gap in between them. The gap between the third and fourth rings held a faint object that, when observed with SPHERE, showed a bright point of light. This planetary candidate was observed twice by SPHERE, and then twice again by the Magellan Adaptive Optics eXtreme (MagAO-X) on the Magellan telescope, which was looking at the planet’s H-alpha line.
The H-alpha line is used to find superheated hydrogen, which happens when it is being accretted into a planet. The researchers, in this case led by Laird Close of the University of Arizona, found the planet was indeed accreting hydrogen at a rate of about the equivalent of the weight of Earth’s Moon every 30,000 years - which is relatively slow in planetary formation terms.
That might be because the planet itself is an astonishing 57 AU away from its host star. That’s more than twice the distance from our Sun to Neptune - our farthest planet. It does weigh in at about 5 times the mass of Jupiter, but being so far away, and already having cleared out its orbital path, there’s probably not a whole lot of material left to build a planet out of.
There were several other interesting discoveries in the system, including a possible second planet. A faint object about the size of 9 Jupiters showed up closer to the star, at 16 AU, but wasn’t confirmed as part of these papers as it was only seen in the observations described in one of the two papers - other researchers will have to take another look at the system in order to confirm it.
Lecture on how circumstellar discs play a critical role in shaping planets.Another was an interesting observation that might show some observational bias for systems like these. So far, four protoplanetary systems have exhibited H-alpha radiation, and all four of them are at a very similar viewing “angle” to Earth - between 37 and 52 degrees inclination. This might just be coincidence - four isn’t a very large number to do any significant statistics on - but they calculated the probability of this happening to only be about 1%. Given that statistical chance, the authors suggest there might be another reason in play, such as those types of system providing a clearer, unobstructed view of that critical wavelength.
Ultimately the impact of this paper is to form a new benchmark system for the study of how protoplanets form out of circumstellar disc systems. That alone will undoubtedly attract more observational time from even more powerful telescopes, and potentially confirm the existence of other planets in the other gaps of the rings. While waiting for that confirmation, astronomers will continue to look for other systems like WISPIT2 in their quest to expand our understanding of how planets form.
Learn More:
Astronomie.nl - Discovery of the first ring-shaping embedded planet around a young solar analog
R. F. van Capelleveen et al - WIde Separation Planets In Time (WISPIT): A Gap-clearing Planet in a Multi-ringed Disk around the Young Solar-type Star WISPIT 2
L. M. Close et al - Wide Separation Planets in Time (WISPIT): Discovery of a Gap Hα Protoplanet WISPIT 2b with MagAO-X
UT - New Evidence For Fomalhaut Planets
UT - Rings in the Early Solar System Kept our Planet From Becoming a Super-Earth