Astronomers catch two exoplanets mid-crash, replaying earth's lunar birth
Telescopes have frozen the instant when worlds collided. At 3 a.m. Pacific on 29 August 2021 the brightness of the obscure star Gaia20ehk leapt by a factor of 4.3, then dimmed for 30 hours until the star vanished entirely behind a cloud of 900-kelvin debris. That flash, decoded by a University of Washington team, is the first live recording of a planetary smash-up beyond the Solar System—and the geometry is eerily familiar.
What the light curve whispers
Anastasios Tzanidakis and James Davenport started tracking the star in 2016 for the all-sky Gaia survey, expecting routine flickers. Instead they got chaos: random 5–10 % dips that deepened each orbit. By 2020 the dips had become 60 %; by last year the star was blinking out. Infrared data from NEOWISE confirmed the glow of freshly vaporised rock. The only model that fits, accepted this week in The Astrophysical Journal Letters, is a grazing collision between two Neptune-sized bodies at 1 AU from their sun, spraying silicate vapour that now spreads across 0.05 AU—wide enough to swallow the star from our line of sight.
The temperature, mass and angular momentum match the canonical Theia-Earth impact 4.5 billion years ago that spun off our Moon. Nature, it seems, recycles the same brutal script.
Vera rubin will see dozens more
Davenport is already betting on encores. The upcoming Vera C. Rubin Observatory will image the entire Southern sky every three nights for a decade, capturing transient flickers down to 0.1 % accuracy. Extrapolating from Gaia20ehk’s five-year fade, the survey should log roughly one fresh planetary collision per month—enough, he claims, to build a statistical sample of how often rocky planets grow moons, shed atmospheres or simply obliterate each other. The first 100 events are forecast before 2036.
Each flash is a fossil of planetary adolescence. With enough light curves, astronomers can reverse-engineer the mass, speed and spin of the doomed pair, turning rubble into data and data into origin stories for every Earth-like world we will ever call home.