Disintegrating World Is Shedding Comet-like Tail – Sky & Telescope

A planet orbiting its star too closely may disintegrate in the intense heat. Now, astronomers have spotted this elusive sight: a planet being gobbled up by its host star.

Presented at the 245th American Astronomical Society (AAS) meeting, this discovery was made using the Transiting Exoplanet Satellite Survey (TESS), which searches for transits, or periodic dimmings in a star’s light as we see a planet cross in front of it. While TESS usually captures a brief dip that indicates a planet’s transit, the unique signature in one TESS observation instead indicates a long, comet-like tail trailing its disintegrating planet and partially encircling the star.

“We are really, really lucky to be witnessing the [figurative] final hours of this dying planet,” said Marc Hon (MIT), who led the study detailing the result. This crumbling world is only the fourth such planet discovered to date, and the first spotted by TESS. The target is a compelling study into the properties of a disintegrating world, providing a unique window into the end of a planet’s life.

A Different Kind of Transit

In its search for other worlds, TESS has identified dozens of small, rocky worlds that orbit their host stars much more closely than Earth orbits the Sun, their surfaces heating up to 2,000°C — hot enough to evaporate rock.

“These small rocky worlds are being roasted,” Hon said at a AAS press conference. The planets become molten, with “oceans of lava” and “atmospheres of rocky vapor.”  And sometimes, the planets experience catastrophic mass loss, disintegrating altogether.

The newly announced discovery, named BD+05 4868 Ab, orbits a a mere 0.02 astronomical unit from an orange K-class star only 140 light-years away from Earth. The team was able to determine key properties of this world from a light curve made from 29 of its transits.

The astronomers observed deep, periodic dimming events every 30.5 hours, with transits that came on suddenly but only gradually returned to full brightness. The team thinks large dust clouds are helping to block roughly 1% of the starlight every go-round.

“You don’t actually see the planet — it’s too small — but you have this large dust tail which obscures light from the star itself,” Hon theorized.“The shape of this dimming event is unlike any other that we’ve ever seen on a typical planet.”

The team analyzed the light curve’s shape, finding that the planet actually sports two tails — a longer trailing tail and a shorter leading tail. The trailing tail extends 9 kilometers (5.5 miles), covering roughly half of the planet’s tight orbit.

The team determined the dust tails’ size and composition by simulating how the star’s radiation would scatter the particles. Larger particles, around the size of desert sand, gather around the leading tail, while smaller grains about the size of soot particles accumulate along the trailing tail.

Understanding the dust tails allowed the team to calculate the planet’s mass loss, which amounts to a Moon’s worth of material every million years. “If we assume that this planet is really as small as Mercury or the Moon, then this mass loss, this rate of losing material, is catastrophic,” Hon said. “The planet will completely evaporate, or disintegrate, within the next 1 or 2 million years.” In the lifetime of a planet, that’s the blink of an eye.

Interiors of Fading Worlds

Planet disintegration is a “runaway process,” according to Hon — larger planets lose material more slowly to evaporation but will disintegrate faster as they become progressively smaller. The team therefore thinks the planet is small because of its large dust cloud, but its original size remains unknown.

That may change with observations of the planet’s dust, which could shed light on the planet’s interior. The team hopes to gather more clues by observing the planet’s transmission spectrum using the James Webb Space Telescope. As the star’s light is filtered, or transmitted, through the dust tails, material in those tails will absorb certain wavelengths of light. Webb has already observed the dust cloud spectrum of another disintegrating planet, K2-22b, finding hints of ice vapor, a result presented at the same AAS press conference.

Both K2-22b and BD+05 4868 Ab live in binary star systems, and the orbital dynamics between the two stars may have contributed to the planets’ inward spiral. The presence of ice in K2-22b hints that the planet may have formed far from its star, migrating closer from gravitational interactions. Nick Tusay (Penn State), who presented the K2-22b findings, described a potential scenario in which the binary stars are “hurling snowballs at each other.”

“We suspect that binary system dynamics might be at play for both K2-22b and BD+05 4868 Ab,” Hon agreed. However, further work is needed to understand how a binary system influences its planets’ orbits.

The extent of the dust tails, as well as the host star’s brightness and proximity to Earth make the system ideal for follow-up observations with JWST. The discovery of BD+05 4868 Ab is a unique opportunity to learn more about a planet’s fiery demise as it happens in real time.