“Discovering Unprecedented Planetary Chemistry on Exoplanet WASP-39b”

Until a few decades ago, the only planets we knew of existed in our solar system, and they shaped how we thought about planet formation and planetary chemistry. Now, with so many identified exoplanets, we have lots of examples of things we’ve never seen before: tiny Neptune, super-Earth, hot Jupiter.

Figuring out what all this new stuff is telling us is a bit of a mixed bag. It is relatively easy to determine the density of a planet and how much energy it will receive from its host star. But a certain density usually corresponds to a variety of materials—dense rock is analogous to a large mineral core and a bloated atmosphere, for example. And a planet’s temperature will depend heavily on things like the composition of its atmosphere and how much light its surface reflects.

So knowing what we see when we look at data on exoplanets is difficult. But with the successful operation of the Webb Space Telescope, we started to go even further. In Wednesday’s issue of the journal Nature, scientists used data from a new telescope to deduce the chemistry of the hot gas giant and found that there are things going on that we would never see in our own solar system.

Big and hot

Investigative purposes Exoplanet WASP-39b, which is about 700 light years from Earth. It is a gas giant, but its mass is much less than that of Jupiter, two-thirds. Nonetheless, it is much larger than Jupiter, with 1.7 times the radius. The biggest contributor to this is the fact that the planet is hot. Its orbital radius is less than 5 percent of Earth’s, and it takes just over four Earth days to complete one orbit. The star it orbits is not a dim dwarf either; It is roughly the same size as the Sun and heats the planet up to nearly 900 degrees Celsius.

Therefore, WASP-39b is unlike any other planet in our solar system. Which makes it a great choice for seeing things we wouldn’t see close to home. It is also an attractive observation target because of its massive atmosphere. This means that as the planet passes between its host star and Earth, more light from that star will pass through WASP-39b’s atmosphere. When that happens, chemicals in the atmosphere absorb certain wavelengths, creating signatures we can read to learn more about planet formation.

For this reason, WASP-39b was one of the first planets targeted for observation by the Webb telescope. The data obtained showed that the planet’s atmosphere contains carbon dioxide and sulfur dioxide.

Both chemicals occur in Earth’s atmosphere, so their presence isn’t too surprising. But Earth’s atmosphere is an oxidizing environment, so oxidizing chemicals are key. In contrast, the gas giants are rich in hydrogen, which makes the atmosphere less dense. We should be looking at water, methane and hydrogen sulfide, not carbon dioxide and sulfur dioxide.

planetary chemistry

To find out what was going on, a large research team adapted some software that models chemical reactions to work with the conditions and precursors that might be present in WASP-39b’s atmosphere. The conditions were created using a general circulation model of the planet’s atmosphere, focusing on the morning and evening extremes — the locations where the planet’s day and night sides meet.

These models show that there is a pathway by which sulfur dioxide can form. But they started with the decomposition of water by ultraviolet light from a nearby star. UV light splits water into two reactive chemicals called radicals (H and OH radicals, in particular). At first, the hydrogen radicals release hydrogen, leaving behind sulfur. It then reacts with the OH radical, oxidizing it.

Models predict that sulfur dioxide will be more abundant in the morning, which is cooler than the night side of the planet. They also suggest that we are seeing precursors such as sulfur and sulfur dioxide, but these would not leave traces in starlight passing through the atmosphere.

One of the most interesting things about this is that there are several reasons why it doesn’t work well in our solar system. First, all the gas giants are very far away in the solar system and don’t receive much ultraviolet radiation. But the bigger problem is that the process is very sensitive to the ratio of heavy elements to hydrogen in planetary atmospheres (referred to by astronomers as planetary metallicity). Even in five times as much metal as our sun, you don’t make enough sulfur dioxide to produce signatures we can detect from Earth. You’d need about 10 times as much solar metal to produce a good match to the Webb data.

In contrast, SO2 production appears to be less sensitive to temperature. So a very hot WASP-39b is unlikely to play a role in its production. But in the solar system’s gas giants, the temperature is low enough that even if sulfur dioxide were formed, it would quickly condense into aerosol particles or undergo a chemical reaction in the presence of ammonia. Either of those two things would prevent the kind of spectral signature we see in light passing through WASP-39b’s atmosphere.

outside the solar system

So, for all these reasons, WASP-39b’s atmosphere appears to host a chemical environment that we shouldn’t encounter in our own solar system. As we begin to imagine additional planetary atmospheres, it is important to keep this in mind. Most of the atmospheres we observe are likely to have different chemical mixes, pressures, temperatures and radiation exposures, and so could host chemicals we don’t know about.

Alam, 2023. DOI: 10.1038/s41586-023-05902-2 (about DOIs).