This weird planet sports a giant tail like a comet

Astronomers have accidentally found a planet with a debris trail like a comet that stretches up to 5.5 million miles in space. 

This strange exoplanet, located 140 light-years away from Earth in the constellation Pegasus, appears to be rapidly falling apart. Its blistering host star is so close, the radiation is effectively melting it down, leaving behind a long tail of dust. 

Scientists say the rocky world, labeled BD+05 4868 Ab, is losing about one Mount Everest's worth of rock material with each orbit. At that rate, it may completely disintegrate in 1 million to 2 million years.

A team of MIT-led researchers discovered the planet with NASA's TESS mission — short for Transiting Exoplanet Survey Satellite — a probe designed to find new worlds as they pass 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, in a statement. "I happened to spot this signal that appeared very unusual."

Only three other planets with tails, all discovered more than a decade ago, are known. But this one stands out for the length of its tail and how bright its star is, making it easier to study with powerful telescopes. The research team intends to follow up with James Webb Space Telescope observations this summer to study the tail's features and decipher what minerals make up the planet. 

One thing they already know about the tail is that its dust particles are surprisingly large — the size of sand grains. Without this gargantuan tail, scientists may not have known the planet was crumbling. The tail had been blocking some of the star's light after the planet passed in front of it, which caused the star to dim in unexpected ways. Those dimmings were uneven and appeared to change each time the planet zipped by.

"We got lucky with catching it exactly when it's really going away," said Avi Shporer, one of the paper's co-authors, in a statement. "It's like on its last breath."

Exoplanet BD+05 4868 Ab is about the size of Mercury and wraps around its star once every 30.5 hours — meaning its year is only slightly longer than a day on Earth. That's possible in part because the planet is roughly 20 times closer to its star than Mercury is to the sun. At that distance, the planet’s surface is perhaps 3,000 degrees Fahrenheit and covered in magma. 

The team believes the star has whittled away the planet over billions of years. Its diminutive size means it doesn’t have enough gravity to hold onto its material, thus compounding the effect.

Researchers are now sifting through TESS data, hoping to find more planets with tails like BD+05 4868 Ab. They may have stumbled upon a unique way to directly study the interior composition of a rocky planet. Such work could give astronomers a better understanding of the diversity of other terrestrial planets like Earth in the galaxy.

"Sometimes with the food comes the appetite," Shporer said.