July 14, 2024
1 Solar System Way, Planet Earth, USA
Discovery

Sulfur makes a surprise appearance in this exoplanet's atmosphere

With our current level of knowledge, many exoplanet finds take us by surprise. The only atmospheric chemistry we can see clearly is that of Earth, and we still have many unanswered questions about how our planet and its atmosphere developed. With Earth as our main reference point, many things about exoplanet atmospheres seem baffling by comparison, raising excitement and deeper questions.

That's what happened to GJ-3470 b, a Neptune-like exoplanet about 96 light-years away.

Astronomers discovered the planet during a 2012 high-precision radial velocity planet finder (HARPS) Campaign. The campaign searched for short-period planets orbiting M dwarfs (red dwarfs). When it was discovered, it was called hot Uranus. You don't have to be an astrophysicist to figure out why that term has fallen out of favor and is now called the sub-Neptune planet.

GJ-3470 b is about 14 times more massive than Earth, takes 3.3 days to complete an orbit, and is about 0.0355 AU from its star.

New research presented at the 244th meeting of the American Astronomical Society and soon to be published in the Astrophysical Journal Letters shows that the planet's atmosphere contains more sulfur dioxide than expected. The principal investigator is Thomas Beatty, professor of Astronomy at the University of Wisconsin, Madison.

“We didn't think we'd see sulfur dioxide on such small planets, and it's exciting to see this new molecule in a place we didn't expect, as it gives us a new way to figure out how these planets formed.”

Thomas Beatty, University of Wisconsin, Madison

The atmosphere of GJ-3470 b is well characterized among exoplanets. The JWST has aimed its powerful spectroscopic eyes to the planet and revealed more details than ever. Spectroscopy examines the light from its star as it passes through the planet's atmosphere, revealing its chemical components.

Subneptunes like GJ-3470 b are the most common type of exoplanet detected. Astronomers have detected carbon and oxygen in two of them, TOI-270d and K2-18b, which represents important scientific results. But in the atmosphere of GJ-3470 b, astronomers also detected water, methane and, above all, sulfur dioxide (SO2).

“The thing is, everyone looks at these planets and often sees flat lines,” Beatty said. “But when we looked at this planet, we didn't really get a flat line.”

Finding SO2 was a surprise because GJ-3470 b is the smallest and coldest exoplanet to have the compound in its atmosphere.  Image credit: Beatty et al.  2024
Finding SO2 was a surprise because GJ-3470 b is the smallest and coldest exoplanet to have the compound in its atmosphere. Image credit: Beatty et al. 2024

This is the coldest and lightest exoplanet with sulfur dioxide in its atmosphere. The finding is significant in the effort to understand the different ways planets form and evolve. Sulfur dioxide probably comes from chemical reactions in the atmosphere, when radiation from the nearby star tears apart hydrogen sulfide molecules, releasing sulfur, which then binds to oxygen, forming sulfur dioxide.

The amount of sulfur dioxide is also surprising. There is about a million times more SO2 than expected.

“We didn't think we'd see sulfur dioxide on such small planets, and it's exciting to see this new molecule in a place we didn't expect, as it gives us a new way to figure out how these planets formed,” said Beatty, who worked as a scientist of instruments on the James Webb Space Telescope before joining the UW-Madison faculty. “And small planets are especially interesting because their composition really depends on how the planet formation process occurred.”

Astronomers found sulfur dioxide on WASP-39b, a hot Jupiter. But it is 100 times more massive and twice as hot as GJ-3470 b. It is formed in the same way on both planets.

This image shows what the powerful JWST found in WASP-39b's atmosphere.  It was the first exoplanet where carbon dioxide and sulfur dioxide were detected.  Image credit: NASA, ESA, CSA, J. Olmsted (STScI)
This image shows what the powerful JWST found in WASP-39b's atmosphere. It was the first exoplanet where carbon dioxide and sulfur dioxide were detected. Image credit: NASA, ESA, CSA, J. Olmsted (STScI)

“On both planets, SO2 is produced by photochemistry on the planetary daysides: light from the star hits the top of the atmosphere and breaks up sulfur-containing molecules, and then the remains of the sulfur atom from those collisions between photons. and molecules recombine with other molecules. in the atmosphere and forms into SO2,” Beatty told Universe Today.

Beatty and his co-investigators sought to identify pathways that could create SO2 through recombination. But the cold of the planet led to dead ends.

“Identifying the correct recombination pathways was an important part of understanding SO2 in WASP-39b, but they effectively predicted zero SO2 on a planet as cold as GJ 3470b,” Beatty told Universe Today. It turns out that atmospheric metallicity allows this to happen.

“As part of these observations, we determined that the high metallicity of GJ 3470b's atmosphere (it is about 100 times more metal-rich than WASP-39b) can drive SO2-producing reactions at much lower temperatures,” Beatty explained in an exchange of e-mail. “Put another way, we realized that all the ambient water and carbon dioxide in GJ-3470 b's atmosphere makes the recombination process to form sulfur dioxide much more efficient than on larger giant exoplanets like WASP-39b”.

Astronomers cannot reconstruct the history of a planet's formation without a complete description of its atmospheric constituents. With a complete list, they can begin to tell the story of their formation. “The discovery of sulfur dioxide on a planet as small as GJ 3470 b gives us one more important element on the list of planet-forming ingredients,” Beatty said.

But there's more to the planet's story than SO2 and other atmospheric chemicals. It follows a polar orbit, which is a strong clue that the planet has been knocked out of its original orbit. It is also extremely close to its star and has likely lost much of its atmosphere, blown into space by the star's powerful stellar wind. It may have lost 40% of its atmosphere.

“That migration history that led to this polar orbit and the loss of all this mass are things that we don't typically know about other exoplanet targets that we're looking at,” Beatty said. “Those are important steps in the recipe that created this particular planet and can help us understand how planets like this form.”

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