This Distant Planet Has a 350,000-Mile-Long Cometlike Tail

Humanity has spied more than 5,500 worlds orbiting other stars, and some are truly exotic. One seems to have titanium clouds, while on another, storms of glass may rain down.

WASP-69b, a planet orbiting a star 160 light-years away, is the latest addition to the eccentric menagerie. As revealed this week at a meeting of the American Astronomical Society in New Orleans, this exoplanet has a 350,000-mile-long tail of helium gas that billows out behind it like a comet’s.

WASP-69b is slightly larger than Jupiter, although considerably less dense, and it is so close to its star that one full orbit takes just 3.9 Earth days. That makes it what astronomers call a Hot Jupiter, a common type of exoplanet.

Its flamboyant tail, however — which is 50 percent lengthier than the distance between Earth and the moon — is far from quotidian.

As the star’s intense radiation broils WASP-69b, the planet’s atmosphere heats to around 17,500 degrees Fahrenheit and puffs up. The outermost matter of the planet becomes ensnared by the stellar wind and is accelerated into space, eventually reaching speeds of 50,000 miles per hour.

“Most Hot Jupiters are losing mass in this way, but not all of them have tails,” said Dakotah Tyler, a doctoral candidate in astrophysics at University of California, Los Angeles, and an author of an accompanying study published this week in The Astrophysical Journal. “The only way to get the tail is if you have an excessive stellar wind that reshapes and sculpts it, basically like a comet.”

There had previously been hints that WASP-69b had a modestly sized helium tail, but scientists couldn’t resolve whether it was real.

Determined to find out, Mr. Tyler, Erik Petigura, an exoplanet researcher also at U.C.L.A., and their colleagues turned to the Keck Observatory atop Hawaii’s Mauna Kea volcano. They used its prolific starlight-scanning capabilities to take a detailed portrait of the exoplanet, confirm the tail’s existence and reveal its tremendous length.

WASP-69b’s planetary plumage is more than decorative, helping to address a question that exoplanet hunters have on their minds: Where are all the Hot Neptunes?

Conspicuously missing from the cornucopia of alien worlds are Neptune-size objects with tight orbits around their host stars. The dearth of Hot Neptunes may be explained by their inability to withstand savage bombardments of stellar radiation. Hot Jupiters have enough mass and gravity to hold on to much of their atmosphere over astronomical time scales. But it’s thought that the gaseous envelopes of the comparatively diminutive Hot Neptunes are effortlessly blown away, quickly turning them into tiny planetary husks.

WASP-69b may be shedding 200,000 tons of mass every second — but even at that rate, it will retain most of its atmosphere throughout the lifetime of its star. That makes it a persistent laboratory experiment for astronomers to monitor how planets lose mass. “WASP-69b helps us study it in real time,” Dr. Petigura said.

Although its cosmic caboose makes WASP-69b notable among its exoplanetary peers, “we have found other planets with tails,” said Jessie Christiansen, the project scientist at NASA’s Exoplanet Archive, who was not involved with the new study. Several other Hot Jupiters are known to have vaporous capes, and Kepler-10b, a rocky realm, is so close to its star that its surface is being evaporated into an iron and silicate streak.

“This process is going on, to some degree, with all planets,” Dr. Petigura said.

As atmospheric mass loss is a universal feature, using WASP-69b to better understand it will “let us predict how common planets like the Earth might be,” Dr. Christiansen said.

As ever, the saga of exoplanets is ultimately the story of our own cosmic isle.