Revealing Cosmic Chemistry: Uncovering Surprising Exoplanetary Discoveries

Astronomers working with the James Webb Space Telescope have discovered that HD 189733 b, an exoplanet known for extreme weather, also hosts hydrogen sulfide gas in its atmosphere, a smell parallel to that of rotten eggs. This has implications for how scientists understand formation and atmospheric composition for gas giants outside of our solar system.

An exoplanet notorious for its killer weather has been concealing another weird feature—this thing stinks of rotten eggs. So says a new Johns Hopkins University study, based on data from the James Webb Space Telescope.

The atmosphere of the Jupiter-sized gas giant HD 189733 b contains trace amounts of hydrogen sulfide—a compound that not only smells like rotten eggs but also provides scientists with new clues on how sulfur could impact the core and atmosphere of gas worlds beyond our solar system.

The research is published today, July 8, in Nature.

The Importance of Sulphur to Planetary Formation
“Hydrogen sulfide is a major molecule that we didn’t know was there. We predicted it would be, and we know it’s in Jupiter, but we hadn’t really detected it outside the solar system,” said Guangwei Fu of Johns Hopkins, an astrophysicist who led the research. “We’re not looking for life on this planet because it’s way too hot, but finding hydrogen sulfide is a stepping stone for finding this molecule on other planets and gaining more understanding of how different types of planets form.”

Besides the detection of hydrogen sulfide and measurement of total sulfur in the atmosphere of HD 189733 b, Fu’s team precisely measured the major oxygen and carbon bearing species of the planet—water, carbon dioxide, and carbon monoxide.

“Sulfur is one of the essential elements to make more complex things, and—like carbon, nitrogen, oxygen, and phosphate—researchers really need to understand it better to understand how planets are made and what they’re made of,” Fu said.

HD 189733 b has been the benchmark planet for atmospheric characterization since its discovery in 2005. Credit: Roberto Molar Candanosa/Johns Hopkins University

Exoplanet HD 189733b
HD 189733 b has been the benchmark for atmospheric characterization since the time of its discovery in 2005. Credit: Roberto Molar Candanosa/Johns Hopkins University

Exoplanetary Atmosphere Studies
At a mere 64 light-years from Earth, HD 189733 b is the closest “hot Jupiter” astronomers can observe pass in front of its star. So it’s been the benchmark planet for detailed studies of exoplanetary atmospheres since its discovery in 2005, Fu said.

It orbits its star about 13 times closer than Mercury is to the Sun—in just about two Earth days. It is scorching, with temperatures of 1,700 degrees Fahrenheit, and notorious for vicious weather, such as raining glass that blows sideways on robust winds of 5,000 mph.

Observations of the Webb Telescope Enlighten
It now offers researchers on top of that a new means to track hydrogen sulfide and measure sulfur in gas exoplanet atmospheres, just like it did when it detected water, carbon dioxide, methane, and other critical molecules in other exoplanets.

“Say we do another 100 hot Jupiters and they’re all sulfur enhanced. What does that mean about how they were born and how they form differently compared to our own Jupiter?” Fu said.
The James Webb Space Telescope represents a new generation of space-based observatories. It replaces the Hubble Space Telescope, with advanced infrared capabilities that expand its mission to deeper and more distant views of the universe than have ever been seen before. Primary functions include exploring exoplanet atmospheres—a roadmap to discovering and characterizing distant worlds. This has offered new prospects for the understanding of composition, weather patterns, and habitability in exoplanets harbored across the galaxy. Credit: ESA/ATG medialab

The James Webb Space Telescope (JWST) represents the next frontier in space observatories. Designed to succeed the Hubble Space Telescope, the JWST is equipped with advanced infrared capabilities that allow it to look further back in time and in greater detail than ever before. One of its key functions is the analysis of exoplanet atmospheres, enabling scientists to detect and study the chemical makeup of distant worlds. This capability has opened up new possibilities for understanding the composition, weather patterns, and potential habitability of exoplanets across the galaxy. Credit: ESA/ATG medialab

New Discoveries and Future Research
The new data also ruled out methane with unprecedented precision at infrared wavelengths from the Webb telescope in HD 189733 b, countering previous claims about the abundance of that molecule in the atmosphere.

“We had been thinking this planet was too hot to have high concentrations of methane, and now we know that it doesn’t,” Fu said.
Implications for Exoplanet Metallicity
The level of heavy metals similar to those in Jupiter was also measured by the research team, a finding that could help scientists answer questions about how the metallicity of a planet is related to its mass—adding an important data point for comparison, said Fu.

Less massive giant icy planets, like Neptune and Uranus, are rich in metals compared to what is found in Gas Giants like Jupiter and Saturn. Higher metallicities for Neptune and Uranus suggest that these planets consumed higher fractions of ice, rock, and other heavyweight elements relative to the gases—hydrogen and helium—in the foreground of formation. Now scientists are testing whether or not that same correlation applies to exoplanets, said Fu.

“This Jupiter-mass planet is very close to Earth and has been very well studied. Now we have this new measurement to show that indeed the metal concentrations it has provide a very important anchor point to this study of how a planet’s composition varies with its mass and radius,” Fu said. Reporting their findings, “The results imply a picture in which the formation of planets from solid material enriching previous iron core formation is more plausible and self-natural enrichment with heavy metals.”
Conclusion and Future Directions
In the coming months, Fu’s team plans to track sulfur in more exoplanets and figure out how high levels of that compound might influence how far they form out from their parent stars.

“We want to know how those types of planets got there, and understanding their atmospheric composition will help us answer that question,” Fu said.

Reference: “Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet” 8 July 2024, Nature.
DOI: 10.1038/s41586-024-07760-y

This research was supported by NASA through the JWST GO program.

Other authors are Luis Welbanks, Dana R. Louie, and Michael Line of Arizona State University; Drake Deming, Jegug Ih, Arjun B. Savel, Eliza M.-R. Kempton, and Matt Nixon of University of Maryland; Julie Inglis and Heather A. Knutson of California Institute of Technology; Michael Zhang of University of Chicago; Joshua Lothringer of Utah Valley University; Julianne I. Moses and Gregory Henry of Tennessee State University; Everett Schlawin of University of Arizona; David K. Sing of Johns Hopkins; and Thomas Greene of NASA Ames Research Center.

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