If we need more evidence that our Solar System is not representative of other solar systems, take a look at BD+05 4868. It’s a binary star consisting of a K-dwarf and an M-dwarf about 140 light-years away. It’s not just the binary star sets the system apart from ours. A small rocky planet is so close to the primary star that it’s being vaporized, leaving a trail of debris like a comet.
The discovery of the planet is presented in a paper titled “A Disintegrating Rocky Planet with Prominent Comet-like Tails around a Bright Star.” It’s published in The Astrophysical Journal Letters, and the lead author is Marc Hon, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research.
The planet, BD+05 4868 Ab, is an extreme example of a disintegrating exoplanet. Astronomers know of only three others of its type. Compared to the others, BD+05 4868 Ab is suffering catastrophic mass loss. It’s 20 times closer to its star than Mercury is to the Sun, and completes an orbit in only 30.5 hours. It’s about the same mass as the Moon and is losing a Mt. Everest-size amount of material each orbit. That adds up to losing 10 Earth masses in a billion years. At that rate, the planet will cease to exist in only one or two million years.
“It’s a runaway process, and it’s only getting worse and worse for the planet.” – Avi Shporer, Kavlie Institute.
The exoplanet is also different because it generates two extended debris tails: one leading and one trailing. Unlike other disintegrating exoplanets found with the transit method, BD+05 4868 Ab generated deep transits, indicating the presence of extended dust tails. The trailing debris tail is much longer than the leading debris tail.
“The extent of the tail is gargantuan, stretching up to 9 million kilometers long, or roughly half of the planet’s entire orbit,” Hon said in a press release.
The researchers ran simulations to visualize the dust tails. The trailing tail is about five times longer than the leading tail. Image Credit: Hon et al. APJ letters 2025.
Avi Shporer, a co-author from the Kavli Institute, said, “We got lucky with catching it exactly when it’s really going away. It’s like on its last breath.”
The team of researchers weren’t going out of their way to find disintegrating planets. They were vetting exoplanet data from NASA’s TESS spacecraft. TESS spots planet candidates when they transit in front of their stars, causing a dip in the starlight. When the same dip repeats regularly, it indicates the presence of an exoplanet.
In this case, the dip in starlight was unusual.
“We weren’t looking for this kind of planet,” said lead author Hon. “We were doing the typical planet vetting, and I happened to spot this signal that appeared very unusual.”
The exoplanet’s dip appeared every 30.5 hours, indicating a planet orbiting the star. However, it took much longer for the starlight to return to normal than during most exoplanet transits. This long-lived, fading transit suggested that a trail of debris was coming from the planet.
The transit displayed some other intriguing characteristics. The depth of the dip, meaning the fraction of the starlight blocked, was different for each orbit. That means that whatever was blocking the light was different each time.
“The shape of the transit is typical of a comet with a long tail,” Hon explains. “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 researchers ran simulations to visualize the dust trails. They simulated the release of 50,000 dust grains in random directions, mimicking the loss of material from the exoplanet. “We assumed a nominal planet with a mass of 0.02 Earth masses and a radius of 2000 km that is orbiting a 0.7 solar mass host star,” the researchers wrote in their paper. They ran the simulation for 30 planetary orbits, eliminating any grains that fell back onto the planet.
The simulations showed that radiation pressure dictates the size of the tails. Grains with lower pressure are released into faster orbits, forming the leading tail, while grains with higher radiation pressure form the trailing tail.
The simulations also showed that while the leading tail is more uniform, the trailing tail varies in height, leading to the unusual transit depth.
This figure from the research shows how the trailing dust tail varies in height, creating a transit that takes a long time to fade. Image Credit: Hon et al. APJ letters 2025.
The planet is so hot that its surface is lava. The researchers think it reaches about 1,600 Celsius, or 3,000 Fahrenheit. That boils away minerals from the rocky planet’s surface, which then condense into dust, forming the tails.
The planet is not nearly massive enough to hold itself together. Its gravity is too weak. As it loses mass, its gravity becomes even weaker, hastening its demise.
“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.”
“That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other <disintegrating> planets,” Hon explains.
The planet’s loss is scientists’ gain, though. By examining the dust grains in the tails with the JWST, scientists can learn about the planet’s composition. That could prove to be invaluable evidence in the drive to understand other solar systems. Those observations will take place this summer.
“BD+05 4868 Ab presents a compelling target for exoplanet mineralogy studies because its dusty effluents form from the mineral vapour that sublimated off the planetary surface,” the researchers write in their paper. Previous research indicates that the mineral composition of the dust tails likely reflects the composition of the planet’s mantle. “Transmission spectroscopy of the dusty effluents, therefore, provides an opportunity to directly measure the surface composition of a small, rocky exoplanet,” they write.
“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.
“Beyond ground-based efforts, observations of BD+05 4868 Ab from the James Webb Space Telescope (JWST) may reveal exceptional new insights on planetary interiors,” the authors write in their paper.
The astronomers weren’t searching for evaporating planets, but now that they’ve found one, their appetites have been whetted. They intend to scour TESS data for more of these unusual, telltale transits.
“Sometimes with the food comes the appetite, and we are now trying to initiate the search for exactly these kinds of objects,” Shporer says. “These are weird objects, and the shape of the signal changes over time, which is something that’s difficult for us to find. But it’s something we’re actively working on.”
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