In 2021, a network of ground-based telescopes scanning the sky spotted a Sun-like star dimming rapidly. An amateur astronomer looking at the same system in data from NASA’s now-decommissioned space-based NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer) telescope noticed that it had brightened by 4% at infrared wavelengths just a few years before.
Upon closer inspection, researchers proposed that the brightening in 2021 was the emission from two planets colliding and creating a new planetary body. The ensuing dimming was likely due to postcollision dust and debris passing in front of the star, named ASASSN-21qj.
Now, with new data collected in February and November with the James Webb Space Telescope (JWST), researchers can start to understand what happened after the collision.
A Lucky Spot
The original observations of ASASSN-21qj came from sky surveys, which are conducted by networks of telescopes that scan large portions of the sky regularly. That’s a lot of sky to sift through, so catching this collision was a matter of chance, said Simon Lock, a planetary scientist at the University of Bristol and a study coauthor. The Las Cumbres Observatory Global Telescope Network captured images in visible light, and the Wide-field Infrared Survey Explorer observed the system in the infrared.
“To look at the direct emission from a newly formed planet, that’s really cool.”
“We’ve never directly observed something like this before. We’ve seen bits like the debris passing in front of stars. We’ve seen disks of material suddenly get a bit fatter and hotter, but actually to look at the direct emission from a newly formed planet, that’s really cool,” Lock said.
There are several theories for how planets are formed, and these findings seem to support the hypothesis that they are formed through the collision of other celestial bodies, Lock said. Given the size and temperature of the event, one possible set of circumstances is that the colliding bodies were Neptune-sized ice giants.
After the team realized what the sky surveys had accidentally observed, they followed up the detection with observations from JWST. But the JWST infrared images and spectra of this system have been challenging to interpret, Lock explained, in part because the signal they are searching for is so faint in comparison to the whole system, especially the star.
The observations capture the star, a postcollision body, and potentially a dust cloud, said Richelle van Capelleveen, a doctoral candidate studying planetary science at Universiteit Leiden and a study coauthor. “It’s been quite a challenge to properly subtract the star off of the combined spectrum,” she said.
“It really is that we’re at a point of not knowing, and that’s quite exciting as a scientific point, but it’s also kind of terrifying.”
Astronomers have encountered this challenge before, for example, with direct observations of exoplanets, but in this case, the signal was even fainter than expected. The team thinks the reason is that the postimpact body cooled faster than models predicted.
What’s more, some parts of the spectrum returned by JWST are more intense in the November data than in the February readings, whereas others are less intense, Lock said. This behavior doesn’t match what astronomers would expect from rapidly cooling postcollision material. Though there are more calculations still to be run, the initial results indicate to Lock that there is something happening in this system that doesn’t fit with our current understanding.
“It really is that we’re at a point of not knowing, and that’s quite exciting as a scientific point, but it’s also kind of terrifying,” Lock said. He added that he is hoping that with input from more experts they can decode what could cause the unexpected readings. The team will receive more Webb data in June.
The researchers presented their findings on 9 December at AGU’s Annual Meeting 2024 in Washington, D.C.
A Not So Unique Event?
The uniqueness of this observation is a weakness, said Jack Lissauer, a planetary scientist at NASA’s Ames Research Center who was not involved in the study. Statistically, researchers can’t learn much from a single observation, he said. “Part of the problem with one [event] is when you are looking for a lot of things, you’re going to find some unusual things,” Lissauer said.
Right now it’s difficult to assess whether events like these are happening frequently, but from a theoretical standpoint, this type of event is expected, said stellar astrophysicist Carl Melis from the University of California, San Diego. Melis was not involved in the original paper but is now helping with further exploration.
The search for historical examples of this type of collision could help solidify the implications of these findings, said Matthew Kenworthy, an astronomy professor also at Universiteit Leiden who has been involved in this finding from the beginning.
“There’s plenty of evidence that violent interactions happen,” Kenworthy said. “To see one of these things actually caught in the act is pretty spectacular, and currently, it’s only one of them, but we discovered it by a very, very wonderful accident. Now we know what to look for. In fact, our suspicions are quite strong that this isn’t the only thing out there.”
—Marta Hill (@martajhill), Science Writer
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