Astronomers Catch Two Planets Colliding 11,000 Light-Years Away in a Replay of the Impact That Formed Earth's Moon
A University of Washington team identifies signs of a catastrophic planetary collision around star Gaia20ehk, offering the first real-time analog of the giant impact that created Earth's Moon 4.5 billion years ago.
Overview
Astronomers have identified what appears to be the aftermath of a violent collision between two planets orbiting the Sun-like star Gaia20ehk, located roughly 11,000 light-years from Earth near the constellation Puppis. The findings, published on March 11 in The Astrophysical Journal Letters, represent one of only a handful of planetary collisions ever observed and bear a striking resemblance to the giant impact hypothesized to have formed Earth’s Moon approximately 4.5 billion years ago.
What We Know
The discovery emerged from an analysis of archival brightness data for Gaia20ehk. Beginning around 2016, the star exhibited three distinct dips in visible light, a pattern inconsistent with normal stellar behavior. By 2021, the fluctuations had become far more dramatic: visible-light brightness dropped erratically while infrared emissions surged, a signature consistent with vast quantities of hot dust and rocky debris circling the star.
“Stars like our Sun don’t do that. So when we saw this one, we were like, ‘Hello, what’s going on here?’” said lead author Anastasios (Andy) Tzanidakis, a doctoral candidate in astronomy at the University of Washington.
The research team, led by Tzanidakis and senior author James Davenport, a UW assistant research professor of astronomy, concluded that the most plausible explanation was a catastrophic collision between two planets. The resulting debris cloud now orbits Gaia20ehk at roughly one astronomical unit, the same distance that separates Earth from the Sun.
The parallel to Earth’s own history is difficult to miss. The leading theory for the Moon’s origin holds that a Mars-sized body called Theia struck the proto-Earth 4.5 billion years ago, ejecting enough material to coalesce into our natural satellite. The debris field around Gaia20ehk sits at a comparable orbital distance and could, over geological timescales, aggregate into an exomoon.
“It’s incredible that various telescopes caught this impact in real time,” Tzanidakis noted.
What We Don’t Know
Several key details remain unresolved. The masses and compositions of the two colliding bodies have not been directly measured; the team inferred the collision scenario from the light-curve behavior and spectral signatures rather than from direct imaging. Whether the debris will ultimately form an exomoon, disperse into a ring, or fall back onto the remaining planetary core is an open question that will require years of continued observation.
The frequency of such collisions across the galaxy is also poorly understood. Recorded events of this type remain extremely rare, though that scarcity may reflect observational limitations rather than actual rarity. “If we catch more of these collisions, we’ll start to figure it out,” Davenport said.
The upcoming Vera C. Rubin Observatory and its Simonyi Survey Telescope are expected to help close that gap. Researchers estimate the instrument could detect on the order of 100 similar collisions over the next decade, transforming planetary collision science from a field of isolated case studies into one with a meaningful statistical sample.