The James Webb Space Telescope uncovers chemical mysteries of a distant world and paves the way for exploration of Earth-like planets

Artist’s rendering of WASP b ​​and its staff. Image credits: NASA, ESA, CSA and J. Olmsted (STScI)

Since the discovery of the first planet orbiting a star other than the Sun in 1995, we have come to realize that planets and planetary systems are more diverse than we ever imagined. Such distant worlds – exoplanets – give us the opportunity to study how planets behave in different situations. And learning about their atmospheres is a crucial piece of the puzzle.

NASA’s James Webb Space Telescope (JWST) is the largest telescope in space. Introduced on Christmas Day 2021, it is the perfect tool for investigating these worlds. Now my colleagues and I have used the telescope for the first time to reveal the chemical composition of an exoplanet. And the data, released in preprint form (meaning it has yet to be published in a peer-reviewed journal), suggests some surprising findings.

Many exoplanets are too close to their host stars for even this powerful telescope to distinguish them. But we can use the trick by watching the planet pass (pass) in front of its star. During the transit, the planet blocks a small portion of starlight, and an even smaller portion of starlight is filtered through the outer layers of the planet’s atmosphere.

Gases in the atmosphere absorb some of the light and leave fingerprints on starlight in the form of a reduction in brightness at certain colors or wavelengths. The JWST is particularly suitable for studying the atmosphere of exoplanets because it is an infrared telescope. Most gases found in an atmosphere – such as water vapor and carbon dioxide – absorb infrared light rather than visible light.

I’m part of an international team of exoplanet scientists using JSTW to study a roughly Jupiter-sized planet called WASP-39b. Unlike Jupiter, however, this world only takes a few days to orbit its star, so it is being cooked, reaching temperatures in excess of 827°C. This gives us the perfect opportunity to study how a planetary atmosphere behaves under extreme temperature conditions.

We used JWST to recreate the most complete spectrum yet of this fascinating planet. In fact, our work represents the first chemical inventory of the planet’s atmosphere.

The James Webb Space Telescope uncovers chemical mysteries of a distant world and paves the way for the study of Earth-like planets

One of four separate measurements. Each bump corresponds to a different absorbing gas in the atmosphere. Photo credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

We already knew that most of this large planet’s atmosphere must be made up of a mixture of hydrogen and helium — the lightest and most abundant gases in the universe. And the Hubble telescope has already detected water vapour, sodium and potassium there.

Now we were able to confirm our evidence and carry out a measurement of the amount of water vapor. The data also suggests there are other gases, including carbon dioxide, carbon monoxide and, unexpectedly, sulfur dioxide.

Measurements of how much of each of these gases is present in the atmosphere means we can estimate the relative amounts of the elements that make up the gases – hydrogen, oxygen, carbon and sulphur. Planets form in a disc of dust and gas around a young star, and we expect different amounts of these elements to be available to a baby planet at different distances from the star.

WASP-39b appears to have relatively little carbon relative to oxygen, suggesting that it likely formed further away from the star, where it could easily have absorbed water ice from the disk (giving its increased oxygen). Orbit. If this planet migrated, it could help us develop our theories about planet formation, and would support the idea that the giant planets in our solar system also moved and shook quite a bit early on.

A sulphurous key

The amount of sulfur we found relative to oxygen is quite high for WASP-39b. We would expect sulfur to be more concentrated in rock fragments and debris than as atmospheric gas in a young planetary system. So this indicates that WASP-39b may have suffered an unusual number of collisions with sulphurous boulders. Some of this sulfur would be released as a gas.

The James Webb Space Telescope uncovers chemical mysteries of a distant world and paves the way for the study of Earth-like planets

Photochemistry on WASP-39b. Credit: NASA/JPL-Caltech/Robert Hurt; Center for Astrophysics-Harvard & Smithsonian/Melissa Weiss

In a planet’s atmosphere, different chemicals react with each other at different rates depending on how hot it is. Normally these settle into a state of equilibrium where the total amounts of each gas remain stable as the reactions balance each other out. We were able to predict what gases we would see in WASP-39b’s atmosphere for a number of starting points. But none of them came up with sulfur dioxide, expecting instead that sulfur would be trapped in another gas, hydrogen sulfide.

The missing piece of the chemical puzzle was a process called photochemistry. This is the case when the speed of certain chemical reactions is driven by the energy of photons – packets of light – coming from the star, rather than the temperature of the atmosphere. Because WASP-39b is so hot and reactions generally speed up at higher temperatures, we didn’t expect the photochemistry to be as important as it turned out to be.

The data indicate that water vapor in the atmosphere is split into oxygen and hydrogen by light. These products would then react with the hydrogen sulfide gas, eventually removing the hydrogen and replacing it with oxygen to form sulfur dioxide.

What’s next for JWST?

Photochemistry is even more important on cooler planets that are potentially habitable – the ozone layer on our own planet is formed by a photochemical process. JWST will observe the rocky worlds in the Trappist-1 system in its first year of operation. Some of these measurements have already been made – and all of these planets have temperatures more similar to Earth’s.

Some may even be the right temperature to have liquid water on the surface and possibly life. A good understanding of how photochemistry affects atmospheric composition will be crucial for interpreting the Webb telescope observations of the Trappist 1 system. This is particularly important because an apparent chemical imbalance in an atmosphere could indicate the presence of life, so we need to be aware of other possible explanations.

WASP-39b’s chemical inventory showed us just how powerful a tool JWST is. We are at the dawn of a very exciting era in exoplanet research, so stay tuned.

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