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A Jupiter-sized exoplanet has long intrigued astronomers for its scorching temperatures, screaming winds, and glassy sidereal rain. 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.
Using Webb’s observation, researchers studying the atmosphere of HD 189733b detected trace amounts of hydrogen sulfide, a colorless gas that emits a strong sulfuric odor and has never been seen beyond our solar system. The discovery advances what is known about the potential composition of exoplanets.
The findings, compiled by a multi-institutional team, were published Monday in the journal Nature.
A strange planet with deadly weather conditions
Scientists first discovered HD 189733b in 2005 and later described the gas giant as a “hot Jupiter” – a planet with a chemical composition similar to Jupiter, the largest planet in our solar system, but with scorching temperatures. Located just 64 light-years from Earth, HD 189733b is the closest hot Jupiter that astronomers can study, as the planet passes in front of its star. As such, 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 realm, as it is one of the first transiting exoplanets discovered,” lead study author Guangwei Fu, an astrophysicist at Johns Hopkins University, said in an email. “It underpins much of our understanding of exoplanet atmospheric chemistry and physics.”
The planet is about 10% larger than Jupiter, but much hotter because it is 13 times closer to our sun than Mercury. HD 189733b only takes about two Earth days to complete a single orbit around its star, Fu said.
This proximity to the star gives the planet a scorching average temperature of 926 degrees Celsius (1,700 degrees Fahrenheit) and powerful winds that rain glass-like silicate particles sideways from clouds high around the planet at speeds of 8,046 kilometers (5,000 miles) per hour.
A surprising smell
When astronomers decided to use the Webb telescope to study HD 189733b to see what effects infrared light, invisible to the human eye, might have on its atmosphere, they were in for a surprise.
Hydrogen sulfide has been found on Jupiter and is predicted to exist on the gas giant outer planets, but there is no evidence yet for the molecule to exist outside our Solar System, Fu said.
“Hydrogen sulfide is one of the main reservoirs of sulfur in planetary atmospheres,” Fu said. “(The Webb Telescope’s) high sensitivity and infrared capability enable us to detect hydrogen sulfide on exoplanets for the first time, opening a new spectral window to study exoplanet atmospheric sulfur chemistry. This helps us understand what exoplanets are made of and how they came to be.”
Fu also said the team detected water, carbon dioxide, and carbon monoxide in the planet’s atmosphere, meaning these molecules could be common on other gas giant exoplanets.
Astronomers don’t think life could exist on HD 189733b because of its scorching temperatures, but detecting a building block like sulfur in an exoplanet could shed light on planet formation, Fu said.
“Sulfur is an element that is vital to the formation of more complex molecules, such as carbon, nitrogen, oxygen and phosphate, so scientists need to study it further to fully understand how planets form and what they are made of,” Fu said.
Molecules with distinct odors like ammonia have been detected in other exoplanet atmospheres before.
But Webb’s capabilities allow scientists to detect specific chemicals in the atmospheres of exoplanets in greater detail than before.
Planetary heavy metals
In our solar system, ice giants such as Neptune and Uranus contain more metal than the largest planets, the gas giants Jupiter and Saturn, despite having less mass overall, suggesting a relationship between metal content and mass.
Astronomers believe that ice, rock and metals rather than gases such as hydrogen and helium played a role in the formation of Neptune and Uranus.
Webb’s data also showed that HD 189733b had levels of heavy metals similar to those found on Jupiter.
“We now have this new measurement that shows that metal concentrations (of the planet) really do provide a very important underpinning to this study of how a planet’s composition changes with its mass and radius,” Fu said. “The findings support our understanding of how planets form by forming more solid material after initial core formation and then naturally become enriched with heavy metals.”
Now the team will look for traces of sulfur on other exoplanets and determine whether high concentrations of the compound affect how close some planets form to their host stars.
“HD 189733b is a reference planet, but it only represents a single data point,” Fu said. “Just as individual humans exhibit unique characteristics, our collective behavior follows clear trends and patterns. With more data sets from Webb, we aim to understand how planets form and whether our solar system is unique in the galaxy.”
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