Planet Crumbles to Dust 140 Light-Years Away

In the constellation Pegasus, a tiny world is vanishing before astronomers’ eyes. MIT scientists have detected a Mercury-sized planet that’s literally evaporating as it orbits its star, leaving behind a spectacular trail of planetary debris stretching millions of kilometers through space.

The doomed planet, designated BD+05 4868 Ab, circles its star every 30.5 hours at blistering temperatures reaching 1,600 degrees Celsius — hot enough to boil away surface minerals into space. Even more remarkable, the dying world leaves behind what researchers describe as a “comet-like tail” of unprecedented proportions.

“The extent of the tail is gargantuan, stretching up to 9 million kilometers long, or roughly half of the planet’s entire orbit,” says Marc Hon, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research.

Astronomers made the discovery using NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission that watches nearby stars for telltale dips in brightness signaling orbiting planets. What caught researchers’ attention was a peculiar transit signal that fluctuated in depth with each orbit.

The planet’s demise is happening at an astonishing pace. Each orbit sheds material equivalent to Mount Everest, with complete disintegration projected within 1 to 2 million years — a mere blink in astronomical timescales.

“We got lucky with catching it exactly when it’s really going away,” notes Avi Shporer, a collaborator at the TESS Science Office. “It’s like on its last breath.”

BD+05 4868 Ab wasn’t discovered through targeted research but rather during routine analysis of TESS data. “We weren’t looking for this kind of planet,” Hon explains. “We were doing the typical planet vetting, and I happened to spot this signal that appeared very unusual.”

Unlike typical exoplanet signatures that show brief, uniform dips in starlight, this planet’s transit pattern revealed something strange: the brightness took longer to return to normal, suggesting a trailing structure, and the depth changed with each orbit, indicating variable blocking of starlight.

“The shape of the transit is typical of a comet with a long tail,” Hon says. “Except that it’s unlikely that this tail contains volatile gases and ice as expected from a real comet — these would not survive long at such close proximity to the host star. Mineral grains evaporated from the planetary surface, however, can linger long enough to present such a distinctive tail.”

The planet’s low mass — somewhere between Mercury and our moon — creates a deadly feedback loop. With minimal gravitational strength to hold itself together, it continually loses material, which further weakens its gravity.

“This is a very tiny object, with very weak gravity, so it easily loses a lot of mass, which then further weakens its gravity, so it loses even more mass,” Shporer explains. “It’s a runaway process, and it’s only getting worse and worse for the planet.”

Of nearly 6,000 confirmed exoplanets, only three other disintegrating worlds have been identified, all discovered over a decade ago by NASA’s Kepler telescope. BD+05 4868 Ab stands out among this rare group, featuring the longest tail and deepest transits.

“That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other planets,” notes Hon.

The scientific team plans to observe the system using NASA’s James Webb Space Telescope this summer, hoping to determine the mineral composition of the dust tail. Hon suggests these observations could offer unprecedented insights: “This will be a unique opportunity to directly measure the interior composition of a rocky planet, which may tell us a lot about the diversity and potential habitability of terrestrial planets outside our solar system.”

Meanwhile, researchers will continue searching through TESS data for more examples of these rare disappearing worlds. “Sometimes with the food comes the appetite, and we are now trying to initiate the search for exactly these kinds of objects,” says Shporer.

The research will be published in the Astrophysical Journal Letters, with support from NASA.