Evrim Yazgin
Cosmos science journalist
Astronomers have discovered an exoplanet 140 light-years from Earth which is disintegrating, leaving a 9-million-km-long comet-like tail.
For now, the disintegrating world is about the same size as Mercury. It orbits its host star every 30.5 hours at about 3 million km – about 20 times closer than Mercury is to our Sun. The planet’s surface is likely made of magma that is boiling off into space. It is evaporating away.
The discovery, published in the Astrophysical Journal Letters, was made using NASA’s Transiting Exoplanet Survey Satellite (TESS).
TESS sits in orbit around Earth looking for exoplanets using the “transit method” which is when exoplanets cause a faint dimming of their host star as they pass in front of it. So far, TESS has identified more than 7,500 possible exoplanets, 620 of which have been confirmed.
The disintegrating planet, dubbed BD+05 4868 Ab, orbits around a K-dwarf star – a type of orange dwarf a little smaller than our Sun.
The astronomers studying TESS data noticed a peculiar fluctuation in addition to the usual dip in the star’s light when the planet passed in front of it. They confirmed that this was caused by the tightly-orbiting rocky planet’s long, comet-like tail of debris.
“We weren’t looking for this kind of planet,” says first author Marc Hon, from the Massachusetts Institute of Technology (MIT) Kavli Institute for Astrophysics and Space Research. “We were doing the typical planet vetting, and I happened to spot this signal that appeared very unusual.”
“The extent of the tail is gargantuan, stretching up to 9 million km long, or roughly half of the planet’s entire orbit,” Hon says.
Analysis shows that the planet is shedding material equivalent to Mount Everest every time it orbits its star. At this rate, it may completely disintegrate in 1 to 2 million years.
“We got lucky with catching it exactly when it’s really going away,” says co-author Avi Shporer, also at MIT and the TESS Science Office. “It’s on its last breath.”
The signature of the planet’s transit most resembles that of a comet, the researchers say.
“Except 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,” Hon explains. “Mineral grains evaporated from the planetary surface, however, can linger long enough to present such a distinctive tail.”
BD+05 4868 Ab joins just 3 other exoplanets known to be disintegrating. All these doomed planets have a distinctive comet-like tail, but the new one is the longest.
“That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other planets,” Hon says.
The host star is relatively close and bright, making the planet a prime candidate for further observations using NASA’s James Webb Space Telescope.
“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,” Hon says.
