Astronomers Just Saw Two Planets Collide Around a Distant Star

Astronomers have found evidence of a huge cosmic crash around a star far away. This event might be two planets colliding.

A Star's Strange Behavior

In 2020, astronomer Anastasios (Andy) Tzanidakis was looking at old telescope data. He noticed a star, Gaia20ehk, was acting strangely. Stars like it should shine steadily, but Gaia20ehk was flickering.

This star is about 11,000 light-years from Earth in the constellation Pupis. It's a "main sequence" star, much like our sun. Its light should not change much over time.

"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. He is a doctoral student 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?’"

Dust and Debris from a Collision

Scientists soon realized the flickering wasn't from the star itself. Instead, a lot of rocky material and dust was moving in front of it. This debris blocked some of the starlight coming to Earth.

The most likely reason for such a large cloud of debris is a dramatic event. Researchers think two planets might have crashed into each other. This collision would have sent fragments and dust into orbit around the star.

"It’s incredible that various telescopes caught this impact in real time," Tzanidakis noted. "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 research on Gaia20ehk was published on March 11 in The Astrophysical Journal Letters.

How Planets Form

Planetary systems begin from clouds of material around young stars. Over time, gravity pulls together dust, gas, ice, and rocks to form larger bodies. In these early stages, planets often crash into each other.

Young solar systems are often chaotic. Planets frequently collide, break apart, or are thrown into space. After tens of millions of years, these interactions reduce the number of planets. The system then settles into a stable arrangement.

Collisions like this are probably common in the universe. But it's hard to see one happening from Earth. The debris must pass directly between the star and our telescopes. Even then, the changes in brightness can take years to appear.

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

Solving the Mystery with Infrared Light

Tzanidakis, the lead author, studies stars that change brightness over long periods. He previously helped find a system with a binary star and a large dust cloud that caused a seven-year eclipse.

Gaia20ehk was a new challenge. The star first showed several short drops in brightness. Then, it had chaotic changes. Astronomers had never seen this pattern before.

The solution came when Davenport suggested looking at 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 collision between planets would create intense heat. This could explain the strong infrared signal. This type of event could also explain the earlier dips in brightness.

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

Clues to Earth's Formation

Researchers also see similarities between this event and the ancient collision that formed Earth and its moon. That happened about four and a half billion years ago.

The debris cloud around Gaia20ehk seems to orbit about one astronomical unit from the star. This is roughly the same distance between Earth and the sun. At that distance, the debris could cool and clump together again. This might form a system like Earth and the moon.

Scientists don't know the final outcome yet. The debris needs to settle and change over time. This process could take years, or even millions of years.

Future Telescopes and More Collisions

This discovery shows how important it is to look for more events like this. The Simonyi Survey Telescope at the NSF–DOE Vera C. Rubin Observatory will help a lot. It will begin its Legacy Survey of Space and Time later this year.

Davenport estimates that Rubin could find about 100 similar planetary collisions in the next decade. Finding more of these events would help scientists understand how planetary systems evolve. It would also show how common Earth-like worlds might be.

"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."

Deep Dive & References

Gaia-GIC-1: An Evolving Catastrophic Planetesimal Collision Candidate - The Astrophysical Journal Letters, 2026

This research was funded by Breakthrough Initiatives.