An illustration of the exoplanet (extrasolar planet) Wasp-39b, also called a "hot Saturn."

The exoplanet WASP-39b (artist’s impression) is similar in composition to Saturn.Credit: Science Photo Library/Alamy

The James Webb Space Telescope (JWST) has produced the most detailed information ever on an exoplanet, making it the world we know most about after the eight major planets of our Solar System. Observations of the planet, called WASP-39b, reveal patchy clouds, an intriguing chemical reaction in its atmosphere, and provide hints about its formation.

“We’ve studied lots of planets before,” says Laura Kreidberg, director of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and part of the observation team. “But we’ve never seen a data set like this.” The team posted five papers1,2,3,4,5 on its findings on the arXiv preprint server on 22 November.

WASP-39b is a ‘hot Jupiter’ located 215 parsecs (700 light years) from Earth. The gas giant is about one-third the mass of Jupiter but is closer in composition to Saturn. It orbits its host star in just four Earth days and is eight times closer to it than Mercury is to the Sun, making it incredibly hot at nearly 900 ºC. This proximity, and its consequent brightness, makes the planet inhospitable to life as we know it. But it also made it the “perfect target” for the JWST to observe at an early stage in its operational life, enabling it to test its exoplanet capabilities, says Kreidberg.

The telescope, launched in December 2021, observed the planet for more than 40 hours in July. Initial results showed carbon dioxide in the planet’s atmosphere. It was the first time that the gas had been seen on an exoplanet.

Surprise chemistry

Using three of its instruments, the JWST was able to observe light from the planet’s star as it filtered through WASP-39b’s atmosphere, a process known as transmission spectroscopy. This allowed a team of more than 300 astronomers to detect water, carbon monoxide, sodium, potassium and more in the planet’s atmosphere, in addition to the carbon dioxide. The gives the planet a similar composition to Saturn’s, although it has no detectable rings.

The team was also surprised to detect sulfur dioxide, which had appeared as a mysterious bump in early observation data. Its presence suggests that a photochemical reaction is taking place in the atmosphere as light from the star hits it, similarly to how the Sun produces ozone in Earth’s atmosphere. In WASP-39b’s case, light from its star, which is slightly smaller than the Sun, splits water in its atmosphere into hydrogen and hydroxide, which reacts with hydrogen sulfide to produce sulfur dioxide.

“These spectra are just exquisite in their detail, and reveal an additional way that the star affects the planet’s atmospheric composition, through photochemistry,” says Victoria Meadows, an astronomer at the University of Washington in Seattle.

“Photochemistry, because it is such an important process here on Earth, is probably an important process on other potentially habitable planets,” says Jacob Bean, an astronomer at the University of Chicago in Illinois and the observation team’s co-leader. Until now, “we’ve only been able to test our understanding of photochemistry in our Solar System. But planets around other stars give us access to completely different physical conditions.”

Planetary migration

The results also showed a low ratio of carbon to oxygen on the planet. This suggests that WASP-39b had absorbed a high amount of water as ice, probably when it was in a different position, and that it formed much farther out in its solar system, perhaps comparable to “where Jupiter is” around our Sun, says Eva-Maria Ahrer, an astronomer at the University of Warwick, UK, and lead author on one of the papers3.

Such inward migration of hot Jupiters is expected, to explain their proximity to their stars. What remains unclear is whether this is a slow process, over perhaps tens of millions of years, or whether it results from a gravitational ‘shove’ from another planet or star. Knowing WASP-39b’s composition could help astronomers to determine which scenario occurred.

The planet is tidally locked to its star, with the same face always pointing towards it, because of the immense gravitational attraction between them. And the observations show that it has incomplete cloud cover — something that astronomers have never observed before on an exoplanet. At the boundary of night and day, the planet is “only about 60% covered by clouds”, says Bean — perhaps because clouds evaporate as they reach the hotter (day) side and condense as they reach the cooler side.

Benchmark planet

The JWST is observing some 70 exoplanets in its first year of science, which began last July. WASP-39b provides a “benchmark” for those studies, says Bean. Planets that the team hopes to study in the coming months include ones in the TRAPPIST-1 system, where a small red dwarf star is orbited by seven Earth-sized worlds — some of which could be revealed, through transmission spectroscopy, to be potentially habitable.

“This shows that, when it comes to delving into exoplanetary atmospheres, JWST is every bit as powerful as we hoped,” says Hugh Osborn, an exoplanet scientist at the University of Bern in Switzerland.

In December, the telescope will watch a planet called WASP-43b complete an entire orbit around its star, lasting one Earth day. This will reveal unprecedented details of the planet’s climate and chemistry. “We think this planet may have very thick clouds on its night side,” says Kreidberg. “We hope to determine what the clouds are made out of. It will be really spectacular.”