Astronomers think they just witnessed two planets colliding

"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," said Tzanidakis, a doctoral candidate in astronomy at the University of Washington. "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?'"

Evidence Points to a Massive Planetary Collision

Researchers eventually determined that the strange behavior was not coming from the star itself. Instead, huge amounts of rock and dust were passing in front of the star as they orbited the system, partially blocking the light traveling toward Earth. The debris appeared to have been created by an extraordinary event: a violent collision between two planets.

"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."

The team's analysis was published March 11 in The Astrophysical Journal Letters.

Why Planetary Collisions Happen

Planet formation is a chaotic process. Around young stars, gravity pulls together material such as dust, gas, ice, and rocky debris that orbit the star. In the early stages of a solar system, collisions between growing planetary bodies are common. Some worlds smash together, while others are thrown outward into space. Over tens of millions of years, this process gradually shapes and stabilizes planetary systems like our own.

Even though these collisions are likely common in the universe, witnessing one from Earth is extremely difficult. To detect it, the orbiting debris must pass directly between us and the star, blocking some of its light. The resulting dimming pattern can unfold slowly, sometimes over several years.

"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," said senior author James Davenport, a UW assistant research professor of astronomy. "Not many researchers are looking for phenomena in this way, which means that all kinds of discoveries are potentially up for grabs."

Infrared Signals Reveal Hot Debris

Tzanidakis, the study's lead author, focuses on stars that show dramatic changes in brightness over time. Earlier research at the University of Washington helped identify a system where a binary star and a large dust cloud produced an eclipse lasting seven years.

Gaia20ehk, however, presented a completely different puzzle. Its brightness first dipped briefly and then became extremely chaotic. Scientists struggled to explain the pattern until Davenport suggested examining observations taken in infrared light instead of visible 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."

A violent planetary collision could easily generate that level of heat. Such an event would also explain the earlier dips in brightness that astronomers observed.

"That could be caused by the two planets spiraling closer and closer to each other," Tzanidakis said. "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."

A Possible Echo of the Earth Moon Formation

There are also hints that this collision may resemble the event that formed the Earth and the moon roughly four and half billion years ago. The debris cloud around Gaia20ehk appears to orbit the star at about one astronomical unit, which is roughly the same distance between Earth and the sun.

At that location, the scattered material could eventually cool and combine into new planetary bodies, potentially forming something similar to an Earth moon type system. However, scientists will need to wait for the debris cloud to settle before knowing what ultimately forms. That process could take a few years, or even millions of years.

Future Telescopes Could Detect Many More Collisions

For now, the discovery highlights the importance of searching for more planetary impacts. The Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory is expected to play a major role once it begins its Legacy Survey of Space and Time later this year. According to Davenport's rough estimates, the Rubin Observatory could detect around 100 similar collisions over the next decade.

Finding more events like this could improve scientists' understanding of how planetary systems evolve and help narrow the search for habitable worlds beyond our solar system.

"How rare is the event that created the Earth and moon? That question is fundamental to astrobiology," Davenport said. "It seems like the moon is one of the magical ingredients that makes the Earth a good place for life. It can help shield Earth from some asteroids, it produces ocean tides and weather that allow chemistry and biology to mix globally, and it may even play a role in driving tectonic plate activity. Right now, we don't know how common these dynamics are. But if we catch more of these collisions, we'll start to figure it out."