First active chemistry on an exoplanet revealed by Webb telescope

Illustration of an exoplanet (exoplanet) Wasp-39b, also called a "Saturn is hot."

Exoplanet WASP-39b (artist’s impression) is similar in composition to Saturn.Credit: Science/Scientific Picture Library

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

“We’ve studied a lot of planets before,” says Laura Kreidberg, director of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and part of the observation team, which has published five papers.1And the2And the3And the4And the5 for their feedback on the arXiv preprint server on November 22. “But we’ve never seen a data set like this.”

WASP-39b is a “hot Jupiter” planet located 215 parsecs (700 light-years) from Earth. The mass of the gas giant is about a third that of Jupiter and more similar in composition to Saturn. It orbits its host star in just four Earth days and is about eight times closer than Mercury is to our sun, making it incredibly hot at nearly 900 degrees Celsius. This proximity and consequent brightness make the planet inhospitable to life as we know it, but it made it the “ideal target” for JWST to monitor it early in its life and test its capabilities on exoplanets, Kreidberg says.

JWST, which launched in December 2021, observed the planet for more than 40 hours in July. Preliminary results show the presence of carbon dioxide in the planet’s atmosphere, marking the first time the gas has been seen on a planet outside the solar system.

Chemistry surprise

Using three of its instruments, 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 carbon dioxide. The planet gives a composition similar to Saturn, although it has no detectable rings.

The team was also surprised to find sulfur dioxide, which appeared as a mysterious bump in early observational data. Its presence indicates that a photochemical reaction takes place in the atmosphere when light from a star hits it, similar to the way our sun produces ozone in Earth’s atmosphere. In the case of WASP-39b, light from its star, which is slightly smaller than the sun, splits the water in its atmosphere into hydrogen and hydroxide, which reacts with hydrogen sulfide to produce sulfur dioxide.

“Photochemistry, because it is 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 co-leader of the observational team. So far, “we’ve only been able to test our understanding of the photochemistry in our solar system. But planets around other stars give us access to completely different physical conditions.”

planetary migration

The results also show a relatively low carbon to oxygen ratio on the planet. This indicates that WASP-39b previously absorbed a large amount of water as ice, perhaps when it was in a different position, and indicates that it formed much farther out in its solar system, possibly comparable to “where Jupiter is” around our sun, Eva Maria Arrier says. , an astronomer at the University of Warwick, UK, and lead author on a research paper.

This migration of hot Jupiters is expected to explain their proximity to their stars. What remains unclear is whether this is a slow process over tens of millions of years, or a faster process caused by gravitational ‘push’ from another planet or star. Knowing the formation of WASP-39b can help astronomers determine which scenario occurred.

The observations also show that the planet — gradually locked onto its star, so the same face always points toward it due to the massive pull of gravity — has patchy cloud cover, something astronomers have not observed before on an exoplanet. At the day-night boundary, the planet is “only about 60% covered by clouds,” Bean says, possibly caused by the clouds evaporating as they reach the hotter day side and condensing as they reach the cooler night.

standard planet

JWST is observing about 70 exoplanets in its first year of science, which began in July and will continue through next June. WASP-39b provides a “benchmark” for those studies, Bean says. This includes planets in the TRAPPIST-1 system, in which a small red dwarf star orbits seven Earth-size worlds, some of which can be detected as potentially habitable by transmission spectroscopy.

Meanwhile, in December, the telescope will watch a planet called WASP-43b complete a full orbit around its star, lasting one Earth day, revealing unprecedented details in the planet’s climate and chemistry. “We think this planet may have very dense clouds on its night side,” says Kreidberg. We hope to determine what clouds are made of. It would be really amazing.”

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