Rotten Egg Revelations: Discovering Hidden Cosmic Chemistry on a Nearby Exoplanet

Exoplanet HD 189733b With Star

Concept art of HD 189733 b, the closest transiting hot Jupiter to Earth. Credit: Roberto Molar Candanosa/Johns Hopkins University

Researchers using the James Webb Space Telescope have discovered that the exoplanet HD 189733 b, known for its extreme weather, also contains hydrogen sulfide in its atmosphere, suggesting a sulfurous smell akin to rotten eggs. This finding provides new insights into the role of sulfur in the formation and atmospheric composition of gas giants beyond our solar system.

An exoplanet infamous for its deadly weather has been hiding another bizarre feature—it reeks of rotten eggs. This is according to a new Johns Hopkins University study of data from the James Webb Space Telescope.

The atmosphere of HD 189733 b, a Jupiter-sized gas giant, has trace amounts of hydrogen sulfide, a molecule that not only gives off a stench but also offers scientists new clues about how sulfur, a building block of planets, might influence the insides and atmospheres of gas worlds beyond the solar system.

The findings are published today (July 8) in Nature.

Significance of Sulfur in 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, an astrophysicist at Johns Hopkins 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.”

In addition to detecting hydrogen sulfide and measuring overall sulfur in HD 189733 b’s atmosphere, Fu’s team precisely measured the main sources of the planet’s oxygen and carbon—water, carbon dioxide, and carbon monoxide.

“Sulfur is a vital element for building more complex molecules, and—like carbon, nitrogen, oxygen, and phosphate—scientists need to study it more to fully understand how planets are made and what they’re made of,” Fu said.

Exoplanet HD 189733b

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

Exoplanetary Atmosphere Studies

At only 64 light-years from Earth, HD 189733 b is the nearest “hot Jupiter” astronomers can observe passing in front of its star, making it a benchmark planet for detailed studies of exoplanetary atmospheres since its discovery in 2005, Fu said.

The planet is about 13 times closer to its star than Mercury is to the sun and takes only about two Earth days to complete an orbit. It has scorching temperatures of 1,700 degrees Fahrenheit and is notorious for vicious weather, including raining glass that blows sideways on winds of 5,000 mph.

Insights From Webb Telescope Observations

As it did by detecting water, carbon dioxide, methane, and other critical molecules in other exoplanets, Webb gives scientists yet another new tool to track hydrogen sulfide and measure sulfur in gas planets outside the solar system.

“Say we study 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.

Webb Wallpaper

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 the presence of methane in HD 189733 b with unprecedented precision and infrared wavelength observations from the Webb telescope, countering previous claims about that molecule’s abundance 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 team also measured levels of heavy metals like those on Jupiter, a finding that could help scientists answer questions about how a planet’s metallicity correlates to its mass, Fu said.

Less-massive giant icy planets like Neptune and Uranus contain more metals than those found in gas giants like Jupiter and Saturn, the largest planets in the solar system. The higher metallicities suggest Neptune and Uranus accumulated more ice, rock, and other heavy elements relative to gases like hydrogen and helium during early periods of formation. Scientists are testing whether that correlation also holds true for exoplanets, Fu said.

“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. “The findings support our understanding of how planets form through creating more solid material after initial core formation and then are naturally enhanced 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 close they form near their parent stars.

“We want to know how these kinds 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” by Guangwei Fu, Luis Welbanks, Drake Deming, Julie Inglis, Michael Zhang, Joshua Lothringer, Jegug Ih, Julianne I. Moses, Everett Schlawin, Heather A. Knutson, Gregory Henry, Thomas Greene, David K. Sing, Arjun B. Savel, Eliza M.-R. Kempton, Dana R. Louie, Michael Line and Matt Nixon, 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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