Window into Hot Jupiter Reveals Multilayered Winds – Sky & Telescope

Astronomers have taken an unprecedented look inside the atmosphere of WASP-121b, a Jupiter-mass planet that’s circling its star at less than a tenth of Mercury’s distance from the Sun. The data reveal a 3D view of an atmosphere that’s not only unlike any in the solar system, it’s also unlike what theory predicts.

WASP-121b, also known as Tylos, orbits a yellow-white F-type star some 900 light-years away in the constellation Puppis. Its tight orbit, in which a full revolution takes only 1.3 days, causes the planet to be tidally locked, so that one side of the planet always faces the star’s scorching heat. Temperatures on the dayside approach 3,450°F (2150K) — almost as hot as the smallest stars.

While the planet is only slightly more massive than Jupiter, it’s almost twice its width. In our own solar system, Saturn’s so fluffy that it would float on water if we could find a tank big enough. Thanks to the inflating effects of the searing stellar heat, WASP-121b is 2.5 times fluffier than that.

To find out what happens to that kind of heat as it circulates in a planetary atmosphere, Julia V. Seidel (ESO) and colleagues turned the four 8-meter units of the Very Large Telescope in Chile toward WASP-121b to map out its atmospheric patterns in 3D. They combined data to capture a full transit of the planet across the face of its star, filtering the stellar light through the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO).

The result, published in Nature, is a collection of chemical fingerprints, each one probing the atmosphere at a different depth. Iron atoms probe the deepest layers (and highest pressures) that are still accessible, while sodium atoms trace the thinner atmosphere a bit higher up, in the mesosphere. Even higher than that, in the thermosphere, are ionized hydrogen atoms. These atoms all move around due to both winds and the planet’s spin.

“It is the first time that we have looked at a hot Jupiter with this amount of photon-collecting power,” Seidel says.

The results are surprising and difficult to explain. The iron atoms show expected flows from the planet’s hotter, star-facing side to the cooler, space-facing side. This iron wind heats up as it crosses the star-facing side, and it doubles in speed, too, from roughly 30,000 to 60,000 mph. So far, so expected.

Above the iron wind, though, the sodium and hydrogen atoms show evidence for a jet stream — one that moves even faster than the planet’s rotation.

“While Jupiter has a super-rotational jet, it sits much lower in the atmosphere (where the cloud bands are) and can be explained easily by current circulation theories,” Seidel explains. Those theories, however, cannot explain a jet stream at the high altitudes of the thermosphere.

“I truly didn’t expect the jet to reach so high up that it would even drag along hydrogen,” Seidel says. “This means the jet extends far upwards where the atmosphere ends and space begins, completely upending what we thought was the expected behavior of an atmosphere.”

The 3D data on WASP-121b challenge existing theories, and the team doesn’t plan to stop there. “We will push towards cooler and smaller planets, first for Neptune-like ones,” Seidel says. She adds that the 2028 expected start of operations for the Extremely Large Telescope — being built near the Very Large Telescope in Chile — opens the possibility of conducting the same kind of study for planets approaching Earth’s size.