Around a sunlike star 11,000 light-years from Earth, near the constellation Puppis, something that stars like our sun simply do not do began happening in 2021. The star’s light dipped. Then dipped again. Then a third time — and then descended into what one astronomer describes as going “completely bonkers.”
The star, Gaia20ehk, had looked entirely ordinary as recently as 2016. Five years later, its visible light buckled repeatedly while its infrared signal surged. That combination — dimmer but hotter — suggested the material blocking the starlight was itself radiating heat. The team’s conclusion, published March 11 in The Astrophysical Journal Letters, is that two planets collided in front of it.
A signal unlike anything on record
“I can’t emphasize enough that stars like our sun don’t do that,” said Anastasios Tzanidakis, a doctoral candidate in astronomy at the University of Washington and lead study author, in a statement. “So when we saw this one, we were like ‘Hello, what’s going on here?'”
Tzanidakis first noticed the anomaly while working through telescope data that included observations from NASA‘s SPHEREx mission. The three short brightness dips followed by broader chaos had no precedent. Visible light alone could not rule out simpler explanations — floating dust, a stellar outburst, or even a planet being shredded by a black hole’s gravity. The infrared data changed that. As the star’s optical brightness fell and became erratic, its infrared emissions climbed sharply, indicating that the blocking material was hot enough to glow on its own.
That pointed toward a violent collision rather than passive debris. Planetary impacts generate exactly this kind of superheated ejecta — material blasted outward at high temperature, capable of both obscuring starlight and emitting strongly in infrared.
What it might tell us about the moon
Planetary collisions are not considered rare events in young star systems. Catching one is. The geometry has to be nearly perfect: the colliding bodies must pass directly across the face of their host star from the telescope’s line of sight, so the resulting debris cloud intercepts enough starlight to produce a measurable signal. Most impacts go unseen.
According to the announcement, only a handful of planetary collisions of any kind have been recorded. None, the researchers say, carry as many similarities to the giant impact hypothesis — the widely held theory that a Mars-sized body struck the early Earth and threw off the material that became the moon. The pattern at Gaia20ehk offers what the team describes as a rare window into how that kind of event actually unfolds in real time.
“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.”
The star sits near the constellation Puppis and was flagged originally by the Gaia space observatory — the source of its designation. Its 2021 behavior, cross-referenced against archival data stretching back to its stable 2016 baseline, gave the research team a before-and-after record that no previous collision candidate had provided.
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