Astronomers spot strange exoplanet with a tail 350,000 miles long

By Georgina Torbet Published December 11, 2024

Astronomers using the W. M. Keck Observatory in Hawaii have made a startling finding: a distant exoplanet with a tail hundreds of thousands of miles long. Planet WASP-69 b is located 164 light-years away, and as it orbit, it is followed by a stream of escaping gas that forms a tail — making it look a little like a comet.

The planet is a type called a hot Jupiter, meaning it is a large gas giant that orbits very close to its star. So close, in fact, that a year there lasts less than four days and it has a blistering temperature of over 600 degrees Celsius.

This close proximity to the star also causes its most distinctive feature, the tail. Radiation from the star bombards the atmosphere of the planet, stripping away gases like hydrogen and helium. And as streams of particles from the star called stellar winds hit the planet, they pull these escaping gases into a tail shape. The tail has been observed to be more than 7.5 times the radius of the planet, meaning that it stretches for over 350,000 miles.

But it could be even longer as the researchers didn’t have enough telescope time to observe the full length of the tail. However, because it is formed by stellar winds, the tail could also shrink over time if the wind lessens.

“If the stellar wind were to taper down, then you could imagine that the planet is still losing some of its atmosphere, but it just isn’t getting shaped into the tail,” explained lead researcher Dakotah Tyler of the University of California, Los Angeles. “But if you crank up the stellar wind, that atmosphere then gets sculpted into a tail.”

The process of planets losing their atmospheres over time is a common one, and is thought to be comparable to what happened to planets like Mars in our solar system. But the shaping of a tail is unusual. However, even though WASP-69 b is losing a lot of gas, at 200,000 tons per second, because it is so large and massive, it will not be totally stripped any time soon — so the unusual tailed planet should continue to exist for thousands of years to come.

The research is published in The Astrophysical Journal.