Astronomers Just Saw Two Planets Collide Around a Distant Star

Astronomers have uncovered evidence of a dramatic cosmic crash unfolding around a distant star.

While reviewing archived telescope observations from 2020, astronomer Anastasios (Andy) Tzanidakis noticed something unusual. A star that normally should behave in a predictable way was showing strange changes in brightness.

The star, called Gaia20ehk, lies about 11,000 light-years from Earth near the constellation Pupis. It is classified as a stable “main sequence” star similar to our sun, meaning its light should remain steady over time. Instead, the star began flickering dramatically.

“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?’”

Dust and Debris From a Planetary Collision

Scientists soon realized the strange flickering was not coming from the star itself. Instead, large amounts of rocky material and dust were moving across our line of sight as they orbited the star. This debris intermittently blocked some of the starlight traveling toward Earth.

The most likely explanation for such a massive cloud of debris is dramatic. Researchers believe two planets may have collided, sending fragments and dust into orbit around the star.

“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 research analyzing Gaia20ehk was published on March 11 in The Astrophysical Journal Letters.

How Planetary Systems Form

Planetary systems form from clouds of material surrounding young stars. Gravity gradually pulls together dust, gas, ice, and rocky fragments into larger bodies. During the early stages of this process, collisions between growing planets are common.

Young solar systems can be chaotic environments. Planetary bodies frequently collide, break apart, or are ejected into deep space. Over tens of millions of years, these interactions eventually reduce the number of planets and allow the system to settle into a stable arrangement.

Although such collisions are likely common in the universe, catching one in action from Earth is difficult. The debris must pass directly between the star and our telescopes so that it partially blocks the starlight. Even then, the resulting brightness changes may take years to unfold.

“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.”

A Mystery Solved With Infrared Observations

Tzanidakis, the study’s lead author, specializes in studying stars that change brightness over long periods of time. In earlier research at the University of Washington, he helped identify a system containing a binary star and a large dust cloud that caused an eclipse lasting seven years.

However, Gaia20ehk presented a new puzzle. The star first showed several short drops in brightness, followed by chaotic fluctuations. Astronomers had never observed this pattern before.

The breakthrough came when Davenport suggested examining observations taken in infrared light rather than 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 produce intense heat, which could explain the strong infrared signal. This type of event could also account for the earlier dips in brightness.

“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.”

Clues Linked to the Birth of the Earth-Moon System

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

The debris cloud around Gaia20ehk appears to orbit at roughly one astronomical unit from the star, which is approximately the same distance between Earth and the sun. At that distance, the debris could eventually cool and begin clumping together again, potentially forming a system resembling Earth and the moon.

Scientists cannot yet determine the final outcome. The debris must settle and evolve over time before researchers know what new structures might form. This process could take years, or even millions of years.

Future Telescopes May Discover Many More Planetary Collisions

The discovery highlights the importance of searching for more events like this. The Simonyi Survey Telescope at the NSF–DOE Vera C. Rubin Observatory is expected to help significantly when it begins its Legacy Survey of Space and Time later this year.

According to Davenport’s rough estimates, Rubin could detect around 100 similar planetary collisions within the next decade. Finding more of these events would help scientists better understand how planetary systems evolve and 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.”

Reference: “Gaia-GIC-1: An Evolving Catastrophic Planetesimal Collision Candidate” by Anastasios Tzanidakis and James R. A. Davenport, 11 March 2026, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ae3ddc

This research was funded by Breakthrough Initiatives.

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