Until a few decades ago, the only planets we knew about existed in our solar system, and that shaped the way we thought about planetary formation and planetary chemistry. Now, with so many exoplanets identified, we have plenty of examples of things we’ve never seen before: tiny Neptunes, super-Earths, hot Jupiters.
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 range of materials—solid rock can act out to be analogous to a large mineral core and bloated atmosphere, for example. And the planet’s temperature will depend greatly on things like the composition of its atmosphere and how much light its surface reflects.
So knowing what we’re looking at when we see data on an exoplanet is difficult. But with the successful operation of the Webb Space Telescope, we’re starting to move a little further. In the Wednesday issue of the journal Nature, scientists use data from the new telescope to infer the chemistry of the hot gas giant and find that there are things happening that we won’t see in our own solar system.
Big and hot
The aim of the investigation Exoplanet WASP-39b, which is about 700 light-years from Earth. It is a gas giant, but its mass is much smaller than that of Jupiter, by two-thirds. Despite this, it is much larger than Jupiter, with a radius of 1.7 times that. The biggest contributor to this is the fact that the planet is hot. Its orbital radius is less than 5 percent of Earth’s radius, and it takes a little more than four Earth days to complete an orbit. The star it orbits isn’t a faint dwarf either; It is roughly the same size as the Sun and heats the planet to nearly 900 degrees Celsius.
Therefore, WASP-39b is unlike any of the planets in our solar system. Which makes it a great choice for spotting things we wouldn’t see close to home. It is also an attractive target for observations because its atmosphere is so large. This means that as the planet passes between its host star and Earth, more light from the star will pass through WASP-39b’s atmosphere. When that happens, chemicals in the atmosphere will absorb specific wavelengths, creating a signature that we can read to learn more about the planet’s formation.
For these reasons, WASP-39b was one of the first planets targeted for observation by the Webb telescope. The data obtained indicate that the planet’s atmosphere contains carbon dioxide and sulfur dioxide.
Both chemicals appear in Earth’s atmosphere, so their presence isn’t much of a shock in that sense. But Earth’s atmosphere is an oxidizing environment, so oxidizing chemicals are the key. By contrast, the gas giants are rich in hydrogen, which makes for a reducing atmosphere. We should see water, methane and hydrogen sulfide, not carbon dioxide and sulfur dioxide.
To find out what was going on, a large research team adapted some software that models chemical reactions to work with conditions and precursors likely to 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 day and night sides of the planet meet.
These models showed that there are pathways in which sulfur dioxide can form. But they start with the breakdown of water by ultraviolet light from the nearby star. UV rays split water into two reactive chemicals called radicals (H and OH radicals, specifically). At first, hydrogen radicals strip hydrogen, leaving sulfur behind. Then that reacts with the OH radical, oxidizing it.
Models predict that sulfur dioxide will be more prevalent in the morning, which is cooler than the evening side of the planet. They also suggest that we should see precursors such as sulfur and sulfur dioxide, but these would not leave an imprint on starlight passing through the atmosphere.
One of the most interesting things about this is that there are several reasons why this doesn’t work well in our solar system. First, all of the gas giants are very far away in the solar system and don’t receive as much ultraviolet radiation. But the bigger problem is that the process is very sensitive to the ratio of heavy elements to hydrogen in a planet’s atmosphere (referred to by astronomers as the planet’s metallicity). Even at five times more metallic than our sun, you simply don’t make up enough sulfur dioxide to produce a signature that we can detect from Earth. You need approximately 10 times more solar metal to produce a good fit to Webb’s data.
By contrast, SO2 production does not appear to be very sensitive to temperature. So WASP-39b being extremely hot does not seem to play a role in its production. But in the gas giants of the solar system, the temperatures are low enough that even if sulfur dioxide were formed, it would quickly condense into aerosol particles or undergo chemical reactions with the presence of ammonia. Either of those two things would prevent the kind of spectral signature of its presence that we see in the light that has passed through WASP-39b’s atmosphere.
outside the solar system
So, for all of these reasons, WASP-39b’s atmosphere seems to host a chemical environment that we shouldn’t expect to encounter in our own solar system. As we begin to image the atmospheres of additional planets, it will be important to keep this in mind. Most of the atmospheres we’re looking at likely have a mixture of chemicals, pressures, temperatures, and radiation exposures that are different from each other, and so can play host to chemistry we don’t know about.
Nature, 2023. DOI: 10.1038 / s41586-023-05902-2 (about DOIs).
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