Since the first planet orbiting a star other than the sun was discovered in 1995, we’ve 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 the behavior of planets 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 reveal the chemical composition of an exoplanet. And the data, published as a preprint (meaning it hasn’t yet been published in a peer-reviewed journal), suggests surprising results.
Many exoplanets are too close to their parent stars for even this powerful telescope to distinguish. But we can use the trick of watching the planet pass (transfer) its star. During transit, the planet blocks a small fraction of starlight, and an even smaller fraction of starlight is filtered through the outer layers of the planet’s atmosphere.
Gases in the atmosphere absorb some of the light, leaving fingerprints on starlight in the form of reduced brightness at certain colors or wavelengths. JWST is particularly suitable for exoplanet atmosphere studies because it is an infrared telescope. Most gases in an atmosphere, such as water vapor and carbon dioxide, absorb infrared rather than visible light.
I’m part of an international team of exoplanet scientists using JSTW to study a planet roughly the size of Jupiter called WASP-39b. Unlike Jupiter, however, this world only takes a few days to orbit its star, so it’s baking, reaching temperatures exceeding 827°C. This gives us the perfect opportunity to explore the behavior of a planetary atmosphere under extreme temperature conditions.
We used JWST to retrieve the most comprehensive spectrum yet of this fascinating planet. In fact, our work represents the first chemical inventory of the planet’s atmosphere.
We already knew that most of this great 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 have been able to confirm our detection and produce a measurement of the amount of water vapour. The data also suggests that there are other gases, including carbon dioxide, carbon monoxide and, unexpectedly, sulfur dioxide.
Having measurements of the amount of each of these gases present in the atmosphere means that we can estimate the relative amounts of the elements that make up the gases – hydrogen, oxygen, carbon and sulphur. 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 small planet at different distances from the star.
WASP-39b appears to have a relatively small amount of carbon compared to oxygen, indicating that it likely formed at a greater distance from the star where it could have easily absorbed water ice from the disk ( increasing its oxygen), relative to its current very close orbit. If this planet migrated, it could help us develop our theories about planet formation and support the idea that the giant planets in our solar system also moved and shook a lot early on.
A sulphurous 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 pieces of rock and rubble than as atmospheric gas. This therefore indicates that WASP-39b may have suffered an unusual number of collisions with pieces of rock containing sulfur. Some of this sulfur would be released as a gas.
In a planet’s atmosphere, different chemicals react with each other at different rates depending on the temperature. Usually these settle into a state of equilibrium, with the total amounts of each gas remaining stable as the reactions equilibrate. We successfully predicted what gases we would see in WASP-39b’s atmosphere for a range of starting points. But none of them came up with sulfur dioxide, instead expecting all sulfur to be 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 certain chemical reactions are determined 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 photochemistry to be as important as it turned out to be.
The data suggests that water vapor in the atmosphere is separated by light into oxygen and hydrogen. These products would then react with 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 colder planets that may be habitable – the ozone layer on our own planet forms via a photochemical process. JWST will observe the rocky worlds of 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 similar to those of Earth.
Some may even be the right temperature to have liquid water on the surface, and potentially life. Having a good understanding of how photochemistry influences atmospheric composition is going to be essential for interpreting Webb telescope observations of the Trappist-1 system. This is particularly important because 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.
The WASP-39b chemical inventory showed us how powerful a JWST tool is. We’re at the start of a very exciting era in exoplanet science, so stay tuned.
Provided by The Conversation
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