Since the first planet orbiting a star other than the Sun was discovered in 1995, we have realized 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 knowing their atmospheres is a crucial piece of the puzzle.
NASA’s James Webb Space Telescope (JWST) is the largest telescope in space. Launching on Christmas Day 2021, it’s the perfect tool to investigate these worlds. Now my colleagues and I have used the telescope for the first time to unravel the chemical makeup 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 parent stars for even this powerful telescope to distinguish them. But we can use the trick of watching as the planet passes in front of (transits) its star. During the transit, the planet blocks a small fraction of the starlight, and an even smaller fraction of the starlight is filtered through the outer layers of the planet’s atmosphere.
Gases within the atmosphere absorb some of the light, leaving fingerprints on starlight in the form of dimming at certain colors or wavelengths. JWST is particularly well suited to studies of exoplanet atmospheres 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 that used JWST to study a Jupiter-sized planet called WASP-39b. Unlike Jupiter, however, this world takes just a few days to orbit its star, then gets baked, reaching temperatures in excess of 827C. This provides us with the perfect opportunity to explore how a planetary atmosphere behaves under extreme temperature conditions.
We used JWST to recover the most complete spectrum yet of this fascinating planet. Our work represents the first chemical inventory of the planet’s atmosphere.
We already knew that most of this large planet’s atmosphere must be a mixture of hydrogen and helium, the lightest and most abundant gases in the universe. And the Hubble telescope has previously detected water vapor, sodium and potassium there.
Now, we’ve been able to confirm our detection and produce a measurement of the amount of water vapor. The data also suggests that there are other gases including carbon dioxide, carbon monoxide and, unexpectedly, sulfur dioxide.
Having measurements of how much of each of these gases is in the atmosphere means we can estimate the relative amounts of the elements that make up the gases: hydrogen, oxygen, carbon, and sulfur. Planets form in a disk 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 low carbon to oxygen, indicating that it probably formed further away from the star where it could have easily absorbed water ice from the disk (increasing its oxygen), than its very nearby stream orbit. If this planet migrated, it could help us develop our theories of planet formation and would support the idea that even the giant planets in our Solar System did quite a bit of moving and shaking to begin with.
A sulfur key
The amount of sulfur we detected relative to oxygen is quite high for WASP-39b. We would expect sulfur in a young planetary system to be more concentrated in rock fragments and rubble than as an atmospheric gas. So this indicates that WASP-39b may have experienced an unusual amount of collisions with sulfur-containing chunks of rock. Some of that sulfur would be released as a gas.
In a planet’s atmosphere, different chemicals react with each other at different rates depending on how hot it is. Usually, these settle into an equilibrium state, with the total quantities of each gas remaining 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 variety of starting points. But none of them found sulfur dioxide, expecting instead that the sulfur was locked up in a different gas, hydrogen sulfide.
The missing piece of the chemical puzzle was a process called photochemistry. This is when the rates of some chemical reactions are 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 accelerate at higher temperatures, we didn’t expect photochemistry to be as important as it turned out.
The data suggests that water vapor in the atmosphere is split by light into oxygen and hydrogen. These products would then react with the hydrogen sulfide gas, eventually stripping off the hydrogen and replacing it with oxygen to form sulfur dioxide.
What’s next for JWST?
Photochemistry is even more important on colder planets that may be habitable: the ozone layer on our planet is formed through a photochemical process. JWST will observe rocky worlds in the Trappist-1 system during its first year of operation. Some of these measurements have already been made, and all of these planets have temperatures more like Earth’s.
Some may even be just the right temperature to have liquid water on the surface and potentially life. Having a good understanding of how photochemistry affects atmospheric composition will be key to interpreting the Webb observations of the Trappist-1 system. This is especially important as an apparent chemical imbalance in an atmosphere could suggest the presence of life, so we need to be aware of other possible explanations for this.
Read more: Four ways to spot traces of alien life using the James Webb Space Telescope
The WASP-39b chemical inventory showed us how powerful a JWST tool it is. We are at the beginning of a very exciting era in exoplanet science, so stay tuned.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Joanna Barstow receives funding from the Science and Technology Facilities Council. She is a councilor and trustee of the Royal Astronomical Society.