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An exoplanet the size of Jupiter has long fascinated astronomers because of its blistering temperatures, screaming winds and lateral rain made 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 HD 189733b’s atmosphere used Webb’s observations to detect trace amounts of hydrogen sulfide—a colorless gas that emits a strong sulfurous stench and has never been seen outside our solar system. The discovery advances what is known about the possible composition of exoplanets.
The results, compiled by a multi-institutional team, were published on Monday in the journal Nature.
An oddball planet with deadly weather
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 a temperature there. Located just 64 light-years from Earth, HD 189733b is the closest hot Jupiter astronomers can study as the planet passes in front of its star. For that reason, it is one of the most well-studied exoplanets.
“HD 189733 b is not only a gas giant, but also a ‘giant’ in the field of exoplanets because it is one of the first transiting exoplanets ever discovered,” said lead study author Guangwei Fu, an astrophysicist in Johns Hopkins University, in an email. “It is the anchor point for much of our understanding of the chemistry and physics of exoplanet atmospheres.”
The planet is about 10% larger than Jupiter, but is 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 one orbit around its star, Fu said.
That proximity to the star gives the planet an average temperature of 1,700 degrees Fahrenheit (926 degrees Celsius) and strong winds that send glass-like silicate particles raining apart from high clouds around the planet at 5,000 miles per hour (8,046 kilometers per hour).
A surprising stench
When astronomers decided to use the Webb telescope to study the planet to see what infrared light, which is invisible to the human eye, could reveal in the atmosphere of HD 189733b, they were surprised.
Hydrogen sulfide is present on Jupiter and has been predicted to exist on gas giant exoplanets, but there was no evidence of the molecule outside our solar system, Fu said.
“Hydrogen sulfide is one of the main reservoirs of sulfur within planetary atmospheres,” Fu said. “The high precision and infrared capability from (the Webb telescope) allows us to detect hydrogen sulfide on an exoplanet for the first time, opening a new spectral window for studying exoplanet atmospheric sulfur chemistry. This helps us understand what exoplanets are made of and how they came to be.
In addition, the team saw water, carbon dioxide and carbon monoxide in the planet’s atmosphere, Fu said — meaning these molecules may be common in other gas giant exoplanets.
Although astronomers don’t expect life to exist on HD 189733b because of its scorching temperature, finding a building block like sulfur on an exoplanet sheds light on the planet’s formation, Fu said.
“Sulfur is a critical element in 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.” they are done,” Fu said.
Molecules with a distinct odor, such as ammonia, have previously been detected within the atmosphere of other exoplanets.
But Webb’s capabilities allow scientists to identify specific chemicals within atmospheres around exoplanets in greater detail than before.
Planetary heavy metals
In our solar system, ice giants like Neptune and Uranus, although not as massive overall, contain more metal than the gas giants Jupiter and Saturn, which are the largest planets, suggesting that there may be a correlation between metal content and mass.
Astronomers believe that more ice, rock and metal – rather than gases such as hydrogen and helium – were involved in the formation of Neptune and Uranus.
Webb’s data also showed heavy metal levels on HD 189733b similar to those found on Jupiter.
“Now we have this new measurement to show that (the planet’s) metal concentrations have provided a very important anchor point for this study of how the composition of a planet varies with its mass and radius,” Fu said. “The results support our understanding of how planets form by creating more solid material after initial core formation and then naturally enhancing it with heavy metals.”
Now, the team will search for signatures of sulfur on other exoplanets and determine whether high concentrations of the compound affect how closely certain planets form compared to their host stars.
“HD 189733b is a benchmark planet, but it’s just one data point,” Fu said. “Just as individuals exhibit unique characteristics, our collective behaviors follow clear trends and patterns. With more datasets from Webb to come, we aim to understand how planets form and whether our solar system is unique in the galaxy.”
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