Scientists find hydrogen sulfide outside our solar system for the first time on a planet with glass rain

Roberto Molar Candanosa/Johns Hopkins University

An illustration shows exoplanet HD 189733b, a gas giant 64 light-years away.

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A Jupiter-sized exoplanet has long intrigued astronomers for its scorching temperatures, howling winds, and sideways rain of glass. Now, data from the James Webb Space Telescope has revealed another intriguing feature of the planet known as HD 189733b: It smells like rotten eggs.

Researchers studying the atmosphere of HD 189733b used Webb’s observation to spot traces of hydrogen sulfide — a colorless gas that gives off a strong sulfurous odor and has never been seen outside our solar system. The discovery advances knowledge about the possible composition of exoplanets.

The findings, compiled by a team from multiple institutions, were published Monday in the journal Nature.

Scientists first discovered HD 189733b in 2005 and later identified the gas giant as a “hot Jupiter” — a planet with a similar chemical composition to Jupiter, the largest planet in our solar system, but with sizzling temperatures. Located just 64 light-years away from Earth, HD 189733b is the closest hot Jupiter that astronomers can study as the planet transits in front of its star. As a result, it is one of the best-studied exoplanets.

“HD 189733 b is not only a gas giant planet, but also a ‘giant’ in the exoplanet field, as it is one of the first transiting exoplanets ever discovered,” lead study author Guangwei Fu, an astrophysicist at Johns Hopkins University, said in an email. “It is the anchor for much of our knowledge of the atmospheric chemistry and physics of exoplanets.”

Roberto Molar Candanosa/Johns Hopkins University

The exoplanet orbits very close to its parent star, giving the planet a scorching surface temperature.

The planet is about 10 percent larger than Jupiter, but much hotter because it is 13 times closer to its star than Mercury is to our sun. HD 189733b takes only about two Earth days to complete a single orbit around its star, Fu said.

Due to its proximity to its star, the planet has a scorching average temperature of 926 degrees Celsius and a strong wind that rains glassy silicate particles from high clouds sideways around the planet at speeds of 8,046 kilometers per hour.

When astronomers decided to use the Webb telescope to study the planet and see what infrared light, which is invisible to the human eye, could reveal in HD 189733b’s atmosphere, they were in for a surprise.

Hydrogen sulfide is found on Jupiter and has been predicted on gas giants, but evidence for the molecule’s presence outside our solar system has been scarce until now, Fu said.

“Hydrogen sulfide is one of the most important sulfur reservoirs in planetary atmospheres,” Fu said. “The high precision and infrared capability of (the Webb telescope) allow us to detect hydrogen sulfide on exoplanets for the first time, which opens a new spectral window for studying the atmospheric sulfur chemistry of exoplanets. This helps us understand what exoplanets are made of and how they formed.”

In addition, the team found water, carbon dioxide and carbon monoxide in the planet’s atmosphere, Fu said. That means these molecules could be found on other gas giants (exoplanets) as well.

Astronomers don’t expect life to exist on HD 189733b because of the high temperatures, but detecting a building block like sulfur on an exoplanet does shed light on planet formation, Fu said.

“Sulfur is an essential 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 are made of,” Fu said.

Molecules with a specific smell, such as ammonia, have previously been found in the atmospheres of other exoplanets.

Webb’s capabilities allow scientists to identify specific chemicals in the atmospheres around exoplanets in greater detail than ever before.

In our solar system, ice giants like Neptune and Uranus contain less mass, but they contain more metals than the gas giants Jupiter and Saturn, the largest planets. This suggests that there might be a relationship between metal content and mass.

Astronomers think that more ice, rock and metals were involved in the formation of Neptune and Uranus than gases such as hydrogen and helium.

Webb’s data also showed that the abundance of heavy metals on HD 189733b is comparable to that on Jupiter.

“Now we have this new measurement to show that the metal concentrations (of the planet) are indeed a very important anchor point for 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 by creating more solid material after initial core formation and then naturally becoming reinforced with heavy metals.”

Now the team will look for traces of sulfur on other exoplanets and determine whether high concentrations of the compound affect the rate at which planets form relative to their parent stars.

“HD 189733b is a benchmark planet, but it represents just a single data point,” Fu said. “Just as individual humans exhibit unique characteristics, our collective behaviors follow clear trends and patterns. With more datasets to come from Webb, we aim to understand how planets form and whether our solar system is unique in the Milky Way.”

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