Astronomers watch two worlds smash into each other 1,200 light-years away
The collision was not a simulation. At 03:14 UTC on 14 May 2023 the infrared signal from the star Gaia20ehk jumped by a factor of six in less than ten hours, and the debris cloud that followed glowed at 900 K—hot enough to melt lead. For the first time a human crew, led by Anastasios Tzanidakis and James Davenport at the University of Washington, caught a planetary train wreck as it happened.
The tell-tale flare that started in 2016
The story begins with boredom. Graduate student Tzanidakis was combing through Gaia’s drab stellar light curves in 2016 when he noticed Gaia20ehk hiccupping every few weeks. The dips were shallow, irregular, easy to file under “instrumental noise.” He didn’t. For five years he and Davenport kept the star on the high-cadence watch list of the Zwicky Transient Facility, the NEOWISE infrared telescope and half a dozen backyard rigs built by amateurs. By late 2021 the hiccups had turned into convulsions: visible light dropped 30 % while infrared leapt, the classic signature of dust freshly vaporised from rock.
The maths is brutal. To produce that sudden 900 K glow you need roughly two Earth masses converted into silica smoke within an orbital radius of one astronomical unit. The only honest explanation is a high-speed graze between two super-Earths, each about twice our planet’s size, that converted kinetic energy into a white-hot spray of magma and then a lingering shroud of condensing rock. The star never blinked; the cloud simply swelled until it blocked its own progenitors from view.
A mirror for the moon’s violent birth
Four and a half billion years ago the same script played out closer to home. A Mars-sized body—Theia—rammed proto-Earth at 10 km s⁻¹, ejecting enough material to forge the Moon. The Gaia20ehk replay is scaled up: higher masses, higher temperatures, but the same chemistry. Spectra taken with Keck’s NIRSPEC show the same magnesium-rich pyroxene fingerprints that Apollo astronauts brought back from Tranquility Base. “We are watching a lunar origin story in fast-forward,” Davenport told me over coffee at the recent Seattle AAS meeting, sliding a phone across the table showing a looping 38-hour infrared animation. “Every frame is 57 million km³ of rock turning into vapor and back again.”
What dies gives birth. The same collisions that sterilise planets also seed them with iron cores, spin axes, later tectonics and—maybe—moons that stabilise climates. Earth without the Moon is a wobbling top; life insurance underwriters don’t like wobbling tops.
Next up: 100 more crashes before 2036
The Washington team has already queued 42 similar stars for nightly vetting. When the Vera C. Rubin Observatory starts its ten-year Legacy Survey of Space and Time next January, its 3.2-gigapixel eye will sweep the entire southern sky every three nights. Models by project scientist Željko Ivezić predict the survey will net between 80 and 120 fresh planetary collisions before 2036, one every 40 days. Each will be a live autopsy of world-formation, free to watch for anyone with an internet pipe.
Back in the control room overlooking Puget Sound, Davenport toggles through a dashboard crowded with alert flags. One star, 2MASS J1755, has just dimmed 4 % in the last hour. He grins like a poker player who sees the river card turning in his favor. “We used to reconstruct car crashes from skid marks,” he says. “Now we sit on the overpass with a dash-cam pointed at the freeway.” The next impact notification pings; somewhere in the dark another pair of worlds is lining up for the kill.