A team of scientists led by Dakotah Tyler of the University of California, Los Angeles, recently reported about their discovery of an exoplanet that’s losing its atmosphere.
Named WASP-69 b, the exoplanet is a super-hot gas giant that orbits its host star closely. What captured the astronomer’s attention is its tail, through which it leaves a trail of gas in its wake.
When a star’s radiation heats up a planet’s outer atmosphere, it can cause photoevaporation. Light gases like hydrogen and helium get so hot that they escape into space. Over time, the star’s energy strips away the planet’s outer gases.
In addition, the star’s “stellar wind” — a stream of particles it sends out — can affect how this gas escapes. Instead of the gas leaving the planet in all directions, the strong stellar wind pushes the gas into a tail, much like a comet. So, on WASP-69 b, the wind from its star shapes the escaping gas into a tail behind the planet.
As the stellar wind has created the tail, it’s subject to change.
Lead author Dakotah Tyler, an astrophysicist at the University of California, Los Angeles, said, “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, adding that, without the stellar wind, that gas escaping on all sides of the planet would be spherical and symmetrical. But if you crank up the stellar wind, that atmosphere gets sculpted into a tail.”
Tyler compared the process to a windsock: when the wind blows, the sock takes on a more defined shape as it fills with air. Similarly, the stellar wind makes the tail of gas escaping from WASP-69 b more structured.
The team saw that the tail of gas stretched over 350,000 miles — more than 7.5 times the planet’s radius. However, this is just the minimum length they could measure. They had to stop observing before the tail’s signal faded so that it might be longer.
WASP-69 b is losing a lot of gas — about 200,000 tons per second. But it’s losing this gaseous atmosphere very slowly — so slowly that there is no danger of the planet being stripped or disappearing.
It’s also important to note that the tail can change over time. The stellar wind — the stream of particles the star sends out — affects the size and shape of the tail. Since the tail is visible when lit by the star’s light, changes in the star’s activity (like changes in the wind or brightness) can also change the tail’s observations.
WASP-69 b orbits a star in a solar system that’s about 7 billion years old. Over that time, the planet has been slowly losing gas from its atmosphere. Although the rate of this gas loss can change, it’s estimated that over 7 billion years, the planet has lost an amount of gas roughly equal to the mass of seven Earths. This shows how much material the planet has shed due to the effects of photoevaporation and the stellar wind.
Journal Reference:
- Dakotah Tyler, Erik A. Petigura, Antonija Oklopčić, and Trevor J. David. WASP-69 b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 Rp. The Astrophysical Journal. DOI 10.3847/1538-4357/ad11d0