This distant planet has a comet-like tail 350,000 miles long

Humanity has spied more than 5,500 worlds orbiting other stars, and some of them are truly exotic. One appears to have titanium clouds, while on the other, storms of glass can rain.

WASP-69b, a planet orbiting a star 160 light-years away, is the latest addition to the eccentric menagerie. As revealed this week on a meeting of the American Astronomical Society in New Orleans, this exoplanet has a 560,000 kilometer long tail of helium gas that waves behind it like a comet.

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

However, its flamboyant tail – which is 50 percent longer than the distance between the Earth and the moon – is far from ordinary.

As the star’s intense radiation hits WASP-69b, the planet’s atmosphere warms to about 17,500 degrees Fahrenheit and swells. The planet’s outer matter becomes entangled in the stellar wind and is accelerated into space, eventually reaching speeds of 50,000 miles per hour.

“Most Hot Jupiters lose mass this way, but not all of them have tails,” he says Dakota Tylera doctoral candidate in astrophysics at the University of California, Los Angeles, and author of a companion 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 reshapes it, basically like a comet.”

There was talk of it before indicates that WASP-69b had a modestly sized helium tail, but scientists couldn’t solve it whether it was real.

Determined to find out, Mr. Tyler… Erik Petigura, an exoplanet researcher also at UCLA, and their colleagues turned to the Keck Observatory atop the Mauna Kea volcano in Hawaii. They used its prolific starlight scanning capabilities to create a detailed portrait of the exoplanet, confirming the existence of the tail and revealing its enormous length.

WASP-69b’s planetary plumage is more than decorative and helps answer a question exoplanet hunters have on their minds: Where are all the hot Neptunes?

Conspicuously missing from the plethora of alien worlds are Neptune-sized objects with close orbits around their host stars. The lack of hot Neptunes can be explained by their inability to withstand ferocious bombardment of stellar radiation. Hot Jupiters have enough mass and gravity to retain much of their atmosphere on astronomical time scales. But it is thought that the gaseous envelopes of the relatively small Hot Neptunes are blown away effortlessly, quickly turning them into small planetary shells.

WASP-69b may be losing as much as 200,000 tons of mass per second, but even at that rate it will retain most of its atmosphere over 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,” said Dr. Petigura.

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

“This process occurs to some extent on all planets,” said Dr. Petigura.

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

As always, the saga of exoplanets is ultimately the story of our own cosmic island.