This unusual planet features an enormous tail resembling that of a comet.
Astronomers have accidentally found a planet with a debris trail resembling that of a comet, stretching up to 5.5 million miles in space. This unusual exoplanet, located 140 light-years away from Earth in the constellation Pegasus, appears to be rapidly disintegrating. Its blistering host star is so close that the radiation is effectively melting the planet, leaving behind a long tail of dust. Scientists say this rocky world, labeled BD+05 4868 Ab, is losing about one Mount Everest’s worth of rock material with each orbit. At this rate, it could completely disintegrate within 1 million to 2 million years. A team of researchers from MIT discovered the planet using NASA’s TESS mission, which stands for Transiting Exoplanet Survey Satellite, a probe designed to identify new worlds as they transit in front of their host stars. The findings are published in the Astrophysical Journal Letters. “We weren’t looking for this kind of planet,” said Marc Hon, the paper’s lead author. “I happened to spot this signal that appeared very unusual.”
Only three other planets with debris tails, all discovered over a decade ago, are known. However, this one is notable for the length of its tail and the brightness of its star, making it easier to study with advanced telescopes. The research team plans to follow up with observations from the James Webb Space Telescope this summer to investigate the tail’s features and determine its mineral composition. One notable aspect of the tail is that the dust particles are surprisingly large—comparable to sand grains. Without this massive tail, scientists might not have realized the planet was deteriorating. It had been blocking some of the star’s light after the planet passed in front of it, causing the star to dim unexpectedly. These dimmings were uneven and appeared to change with each orbit of the planet. “We got lucky with catching it exactly when it’s really going away,” said Avi Shporer, one of the co-authors of the paper. “It’s like on its last breath.”
Exoplanet BD+05 4868 Ab is roughly the size of Mercury and orbits its star once every 30.5 hours—meaning its year is only slightly longer than a day on Earth. This is possible because the planet is about 20 times closer to its star than Mercury is to the Sun. At that proximity, the planet’s surface temperature is approximately 3,000 degrees Fahrenheit and covered in magma. The team believes that over billions of years, the star has eroded the planet. Its small size means it doesn’t have enough gravity to retain its material, thereby exacerbating the effect. Researchers are now analyzing TESS data, hoping to find more planets with debris tails like BD+05 4868 Ab. They may have uncovered a novel method to directly study the interior composition of a rocky planet. Such research could enhance astronomers’ understanding of the diversity of terrestrial planets similar to Earth throughout the galaxy. “Sometimes with the food comes the appetite,” Shporer remarked.
