Astronomers catch two planets smashing into each other 1,000 light-years away
Telescopes have just recorded, frame by frame, the moment two rocky worlds the size of super-Earths slammed together at 30 km s⁻¹, spraying 627 °C debris so thick it briefly eclipsed their parent star.
The flash lit up Gaia20ehk, a sun-like dwarf in the galactic plane, and arrived on Earth as a sudden 4.5-magnitude infrared spike in August 2021. What looked like an odd data glitch to the casual observer was, in the eyes of University of Washington astronomers Anastasios Tzanidakis and James Davenport, the first live replay of the collision that once birthed our own Moon.
From noise in a light curve to molten rock
The pair had been babysitting the star since 2016, chasing the erratic dips that kept it off the “well behaved” lists. By 2021 the dips had turned into a chaotic 1.5-year plateau: visible light plunged 30 %, while infrared soared, classic fingerprints of vaporised silicates expanding into a translucent cloud. Keck, NEOWISE and TESS data meshed into a single timeline that pinned the cataclysm to within a week, an unheard-of precision for extrasolar forensics.
The debris cloud now spans 0.05 AU—roughly five solar diameters—orbiting at the same distance Earth circles the proto-sun 4.5 billion years ago. Temperature maps show a 900 K glow, hot enough to keep basalt molten and to explain the mid-infrared excess that first triggered the alert.
A ghost of the earth–theia crash
Inside that scalding swarm lies an echo of Theia, the Mars-sized body that sideswiped proto-Earth and seeded the lunar magma ocean. The mass ratio, angular momentum and silicate chemistry line up so neatly that modellers are already grafting the Gaia20ehk dataset onto lunar formation codes, testing whether similar impacts are the default, not the exception, for rocky planets.
Crucially, the collision happened inside the star’s habitable zone. Any water delivered by ice-rich planetesimals would have flash-vaporised, only to re-condense later as a scalding rain of glassy droplets—a preview of what Earth’s surface endured for centuries after Theia’s hit.
Why this matters for next-gen surveys
The detection was a fluke of timing: the star’s 19-day rotation carried the dust cloud across our line of sight just as NEOWISE executed its six-month all-sky sweep. Rubin Observatory, set to begin its ten-year Legacy Survey of Space and Time in 2025, will watch eight million stars every three nights. Davenport’s back-of-the-envelope: if one Gaia20ehk event sneaked into a half-decade of sparse sampling, Rubin should net 50–100 similar smash-ups before 2036, turning a cosmic curiosity into a statistical dataset.
Each new flare will be a breadcrumb leading back to the era when planets finished accreting, atmospheres boiled off and moons coalesced from glowing rings of wreckage. In other words, the next collision caught on camera won’t just echo lunar history—it may rewrite the odds on how often habitable worlds survive their own violent birth.
Keep an eye on the southern sky around 2027; Rubin’s first all-sky release is scheduled for release that October, and if the models hold, a fresh infrared spike should already be en route, racing across the vacuum at the speed of light, ready to announce another planetary murder in real time.