MIT scientists discover a disintegrating planet with a comet-like tail

Astronomers at the Massachusetts Institute of Technology (MIT) have discovered a planet about 140 light-years from Earth that is disintegrating at an accelerated rate.

This crumbling world is roughly the mass of Mercury, yet it orbits about 20 times closer to its star than Mercury orbits the Sun, completing an orbit every 30.5 hours. Being so close to its star, the planet is likely covered in magma boiling into space.

As the planet orbits its star, it sheds a huge amount of minerals from its surface and evaporates, according to the authors of the study, whose results are published in The Astrophysical Journal Letters.

Furthermore, researchers suspect that this planet's gravity is very weak, so it doesn't have enough gravitational pull to hold all its parts together. According to astronomers, all these factors combined would explain its rapid disintegration rate.

The team detected the planet thanks to NASA's Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission that monitors nearby stars for transits— periodic dips in starlight that could indicate the presence of orbiting exoplanets.

“The tail’s length is enormous: it reaches 9 million kilometers in length, or about half the length of the planet’s orbit,” explains Marc Hon, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research.

The planet appears to be disintegrating at a spectacular rate, shedding an amount of material equivalent to Mount Everest each time it orbits its star. At this rate, and given its small mass, researchers predict the planet could disintegrate completely in about 1 to 2 million years.

“We were lucky to 'catch' it when it was actually disappearing,” says Avi Shporer, a collaborator on the discovery who also works in the TESS Science Office. “It's like it's taking its last breath.”

The new planet, which scientists have named BD+05 4868 Ab, was detected almost by chance. “We weren’t looking for this type of planet,” Hon says. “We were doing typical planet research, and I just happened to detect this signal that seemed very unusual.”

The typical signal of an orbiting exoplanet appears as a brief, regularly repeating dip in the light curve, indicating that a compact body, such as a planet, briefly passes in front of its host star and temporarily blocks its light.

This typical pattern is not what Hon and colleagues detected in the host star BD+05 4868 A, located in the constellation Pegasus. Although a transit occurred every 30.5 hours, the brightness took much longer to return to normal, suggesting the existence of a long trailing structure that continued to block the star's light. Even more intriguing, the depth of the dip changed with each orbit, suggesting that whatever passed in front of the star didn't always have the same shape or block the same amount of light.

“The shape of the transit is typical of a comet with a long tail,” Hon explains. “Except that this tail is unlikely to contain volatile gases and ice, as expected from a real comet.” However, mineral grains evaporated from the planetary surface may remain long enough to exhibit such a distinctive tail ,” he adds.

Given its proximity to its star, the team estimates the planet is roasting at about 1,600 degrees Celsius, or nearly 3,000 degrees Fahrenheit. As the star incinerates the planet, minerals on its surface likely evaporate and escape into space, where they cool into a long, dusty tail.

The dramatic disappearance of this planet is a consequence of its small mass, between that of Mercury and the Moon. The more massive terrestrial planets, like Earth, have a greater gravitational pull and are therefore able to retain their atmospheres.

In the case of BD+05 4868 Ab, researchers suspect there is too little gravity to hold the planet together.

“It's a very small object, with very weak gravity, so it easily loses a lot of mass, which weakens its gravity even further, causing it to lose even more mass,” Shporer explains. “It's an out-of-control process, and it's getting worse for the planet.”

Of the nearly 6,000 planets astronomers have discovered to date, scientists know of only three other disintegrating planets beyond our solar system. Each of these ruined worlds was detected more than 10 years ago using data from NASA's Kepler space telescope. All three planets featured comet-like tails. BD+05 4868 Ab has the longest tail and deepest transits of the four disintegrating planets known to date.

"This means that its evaporation is the most catastrophic and that it will disappear much faster than the other planets," Hon notes.

The planet's host star is relatively nearby and therefore brighter than the host stars of the other three disintegrating planets, making this system ideal for further observations with NASA's James Webb Space Telescope (JWST), which can help determine the mineral composition of the dust tail by identifying which colors of infrared light it absorbs.

This summer, Hon and graduate student Nicholas Tusay of Pennsylvania State University will lead observations of BD+05 4868 Ab with JWST. “This will be a unique opportunity to directly measure the interior composition of a rocky planet, which can tell us a lot about the diversity and potential habitability of terrestrial planets outside our solar system,” Hon says.

Researchers will also search TESS data for signs of other disintegrating worlds.

“Sometimes with food comes hunger, and now we're trying to start searching for exactly these kinds of planets,” Shporer says. “They're strange objects, and the shape of the signal changes over time, which is something we find difficult to find. But it's something we're actively working on.”

This work has been funded, in part, by NASA.