Astronomers Discover Planet With a Tail

Astronomers have discovered a doomed, disintegrating planet with a comet-like tail that is shedding a Mount Everest's worth of material each time it orbits its star.

The small, rocky world—given the catchy name BD+05 4868 Ab—lies some 140 light-years from Earth in the constellation of Pegasus.

The planet is orbiting so close to its star that researchers estimate it has a surface temperature around 3,000 degrees Fahrenheit, giving it a molten surface that is boiling off material into space, where this cools to form a long, dusty tail.

"The extent of the tail is gargantuan, stretching up to 9 million kilometers [5.6 million miles] long, or roughly half of the planet's entire orbit," said study lead and astronomer Marc Hon of the Massachusetts Institute of Technology (MIT) in a statement.

Tails are rare amid the 6,000-odd exoplanets discovered to date—BD+05 4868 Ab is only the fourth example ever found. It also has the longest tail and the strongest signal of the four.

"That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other planets," added Hon, who estimates that the world could completely vanish in 1 to 2 million years.

BD+05 4868 Ab was detected in data collected by NASA's Transiting Exoplanet Survey Satellite (TESS).

Led by MIT researchers, the TESS mission hunts for exoplanets around nearby stars by looking for the regular dips in light that occur when a planet passes, or "transits," in front of its host relative to us.

The transit signal the team spotted from BD+05 4868 Ab, however, was something rather curious—with a strength that changed between different orbits.

Furthermore, while a transit appeared every 30.5 hours, the host star's brightness took time to return to normal, suggesting the presence of a long, trailing structure behind the planet that was continuing to block the star light.

"We weren't looking for this kind of planet. We were doing the typical planet vetting, and I happened to spot this signal that appeared very unusual," said Hon.

"The shape of the transit is typical of a comet with a long tail. 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."

According to the researchers, the planet's slow demise can be attributed to its small mass, which lies between that of Mercury and the Moon—alongside its proximity to star, which is about 20 times closer than the distance Mercury orbits around the sun.

With such little gravitational influence to hold the planet together, it is very easy for its host star to strip it of its tail.

"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," said paper co-author and MIT astrophysicist Avi Shporer in a statement.

"It's a runaway process, and it's only getting worse and worse for the planet. We got lucky with catching it exactly when it's really going away. It's like on its last breath."

With their initial study complete, the researchers are planning for upcoming observations of BD+05 4868 Ab using the James Webb Space Telescope, which should be able to determine the composition of the tail based on which colors of infrared light it absorbs.

"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," said Hon.

The team is also planning to comb through TESS data in search of signs of other disintegrating planets.

"These are weird objects and the shape of the signal changes over time, which is something that's difficult for us to find," said Shporer.

However, he added, "it's something we're actively working on."

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about exoplanets? Let us know via science@newsweek.com.

Reference

Hon et al. (2025). A Disintegrating Rocky Planet with Prominent Comet-like Tails Around a Bright Star. The Astrophysical Journal Letters.