First planet of its kind with ‘gargantuan' comet-like tail discovered

Independent

23-04-2025

Astronomers have spotted a first-of-its-kind rapidly crumbling planet outside the Solar System that is leaving behind a 'gargantuan' comet-like tail of debris.
The Mercury -sized planet, dubbed BD+05 4868 Ab, is 140 light years away from Earth and located in the constellation of Pegasus, according to a new study published in The Astrophysical Journal Letters.
It is so close to its star that it is covered in magma and is boiling off into space, say researchers from the Massachusetts Institute of Technology (MIT).
While observing a section of space using Nasa's exoplanet surveying satellite, scientists found a peculiar periodic dip in starlight that tipped them off about the presence of a rocky planet with a long, comet-like tail of debris.
'The extent of the tail is gargantuan, stretching up to 9 million kilometres long, or roughly half of the planet's entire orbit,' said Marc Hon, one of the authors of the study from MIT.
Inspecting the planet further, they found that it is disintegrating at a dramatic rate, shedding a Mount Everest -equivalent of debris each time it orbits its star about every 30 hours.
Given the planet's mass, researchers suspect it may completely disintegrate in about a million years.
'We got lucky with catching it exactly when it's going away ... It's like on its last breath,' said Avi Shporer, another author of the study.
Typically, astronomers identify planets orbiting stars by looking for a brief dip in a light curve that repeats regularly.
This is the signal one might expect from a star as it passes in front of, and temporarily blocks, the light from its star.
However, when looking at the host star of BD+05 4868 A, they found that its brightness dip took much longer to return to normal.
This suggested that a long trailing structure was still blocking starlight.
Researchers found it even more intriguing that the depth of the light dip changed with each orbit, meaning the object passing in front of the star wasn't always the same size or shape.
'The shape of the transit is typical of a comet with a long tail,' said Marc Hon, another author of the study.
'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 proximity to the host star,' Dr Hon explained.
Scientists then came to the conclusion that what they were seeing was actually mineral grains evaporating from the surface of a planet that tend to linger long enough to form a distinctive 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. It's a runaway process, and it's only getting worse and worse for the planet,' Dr Shporer said.
While planets with such comet-like tails have been spotted before, these past observations were at least a decade ago.
Among the three other disintegrating planets spotted outside our solar system, BD+05 4868 Ab has the longest tail to date, researchers say.
'That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other planets,' scientists concluded.