Exoplanet WASP-69 b is followed by a staggeringly long tail

The planet’s atmosphere is escaping into space, creating a cometlike tail that stretches at least 350,000 miles.
By | Published: January 29, 2024

One hundred and sixty light-years from Earth, an exoplanet orbits the star WASP-69. Although it’s been previously studied, astronomers have just confirmed that the blazing-hot world is trailed by a 350,000-mile-long (563,270 kilometers) gaseous tail.

WASP-69, given the formal name Wouri by the International Astronomical Union in 2019, is a K-type star somewhat like our Sun but slightly smaller. The planet with the tail, however, is much different from anything in our own solar system.

That’s because WASP-69 b, named Makombé in 2019 to match its host star (Wouri and Makombé are both rivers in Cameroon), is a hot Jupiter. This class of planet is defined by two major factors: its size similarity to our own gas giant planets and its close-in orbit around its host star.

WASP-69 b certainly fits the bill: It is about 10 percent bigger than Jupiter — though only 30 percent its mass — and orbits its star at a distance just less than five percent the distance at which the Earth orbits the Sun. This means the exoplanet is constantly scorched by stellar radiation to a degree that would make even Mercury sweat.

WASP-69 b was the subject of considerable attention in the summer of 2022 when it became one of the first targets of the James Webb Space Telescope (JWST).

In new research published Jan. 9 in The Astrophysical Journal, a team led by astrophysicists at UCLA has revealed the planet’s atmosphere is escaping into space, creating a cometlike tail that stretches at least 350,000 miles (563,270 km).

Blowing in the wind

Much in the same way that the solar wind from our Sun creates the beautiful tails that sprout off comets when they swing into the inner solar system, WASP-69’s wind is heating the atmosphere of WASP-69 b and sending it flying off the planet in a direction opposite the host star.

“Work by previous groups showed that this planet was losing some of its atmosphere and suggested a subtle tail or perhaps none at all,” said Dakotah Tyler, a UCLA doctoral student and first author of the new study, in a press release. “However, we have now definitively detected this tail and shown it to be at least seven times longer than the planet itself is in diameter.”

The discovery of this long tail trailing behind WASP-69 b offers a crucial chance for scientists to study the ways in which planets evolve with relation to their host stars — particularly those planets with close orbits.

 “Over the last decade, we have learned that the majority of stars host a planet that orbits them closer than Mercury orbits our Sun and that the erosion of their atmospheres plays a key role in explaining the types of planets we see today,” said Erik Petigura, study co-author and UCLA professor of physics and astronomy. “However, for most known exoplanets, we suspect that the period of atmospheric loss concluded long ago. The WASP-69 b system is a gem because we have a rare opportunity to study atmospheric mass loss in real time and understand the critical physics that shape thousands of other planets.”

The team conducted their research using another NIRSPEC —  the high-resolution spectrograph on the 10-meter telescope at the W. M. Keck Observatory in Hawaii. Spectrographs split light into its component wavelengths, allowing researchers to identify atoms and molecules, as well as their behavior. NIRSPEC allowed the team to observe in enough detail to gauge the rate at which WASP-69 b’s atmosphere is evaporating. According to the study, the planet is losing atmospheric material at a rate of about one Earth mass every billion years.

Still, the scientists point out that, given its enormous mass and huge atmosphere, WASP-69 b will not fully lose its atmosphere within the lifetime of its host star. “At around 90 times the mass of Earth, WASP-69 b has such a large reservoir of material that even losing this enormous amount of mass won’t affect it much over the course of its life,” said Tyler.

So, while its tail could certainly create a breathtaking sight for cosmic wanderers, WASP-69b will be around for millennia to come.