A seemingly regular, Sun-like star began acting really weird, flickering haphazardly as its light faded in and out. The cause of the star’s strange behavior may have been a cosmic catastrophe that’s rarely captured by telescopes: two planets violently colliding with one another.

Anastasios (Andy) Tzanidakis, a graduate student at the University of Washington, was casually skimming through old telescope data when he noticed the star, named Gaia20ehk, flickering wildly. “The star’s light output was nice and flat, but starting in 2016 it had these three dips in brightness. And then, right around 2021, it went completely bonkers,” Tzanidakis said in a statement. “I can’t emphasize enough that stars like our sun don’t do that. So when we saw this one, we were like ‘Hello, what’s going on here?’”

It turns out, what was going on may have had nothing to do with the star itself. Instead, massive planetary chunks were flying around the star, blocking its light from reaching ground-based telescopes. The findings from the rare observation are detailed in a study published in The Astrophysical Journal Letters.

Stellar mystery

The star, located about 11,000 light-years from Earth near the constellation Pupis, was fluctuating in a way that had never been observed before. That lead Tzanidakis and a team of astronomers to use data from a different telescope that captured Gaia20ehk in infrared light.

“The infrared light curve was the complete opposite of the visible light,” Tzanidakis said. “As the visible light began to flicker and dim, the infrared light spiked. Which could mean that the material blocking the star is hot—so hot that it’s glowing in the infrared.”

The cause of the flickering was likely due to huge quantities of rocks and dust coming out of nowhere and passing in front of the star. As the material orbited the system, it would dim the star’s light reaching Earth.

A planetary collision would not only create that much debris, it would also produce enough heat to explain the infrared energy. Additionally, those initial dips in light may have been caused by the two planets spiraling closer and closer to each other.

“At first, they had a series of grazing impacts, which wouldn’t produce a lot of infrared energy. Then, they had their big catastrophic collision, and the infrared really ramped up,” Tzanidakis said.

When two worlds collide

Planets form from material such as dust, gas, ice, and rocky debris that orbits around young stars in the aftermath of their birth. During its early years, collisions between planets are more common as the star system starts to settle into its final form. Some worlds collide together, while others are thrown out of the star system over tens of millions of years.

Although these collisions are fairly common in the cosmos, observing one from Earth is extremely difficult. In order to be able to witness a planetary collision using ground-based telescopes, the debris must pass directly between Earth and the star and block some of its light. The star’s dimming can also play out over several years, making it more challenging for astronomers to take notice.

Thankfully, the astronomer behind the discovery is known for his patience when observing cosmic events. “Andy’s unique work leverages decades of data to find things that are happening slowly—astronomy stories that play out over the course of a decade,” James Davenport, astronomy professor at the University of Washington and senior author of the new study, said in a statement. “Not many researchers are looking for phenomena in this way, which means that all kinds of discoveries are potentially up for grabs.”

The researchers behind the study believe the NSF-DOE Vera C. Rubin Observatory, funded by the U.S. National Science Foundation and the U.S. Department of Energy’s Office of Science, is well-fitted to spot planetary collisions when it begins its 10-year, wide-field astronomical survey later this year.

“It’s incredible that various telescopes caught this impact in real time,” Tzanidakis said. “There are only a few other planetary collisions of any kind on record, and none that bear so many similarities to the impact that created the Earth and moon. If we can observe more moments like this elsewhere in the galaxy, it will teach us lots about the formation of our world.”