ATLANTA – A Jupiter-sized exoplanet has long attracted astronomers for its searing temperatures, howling winds and lateral shower of glass. Now data from the James Webb Space Telescope has revealed another interesting 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 gives off a strong sulfurous odor and has never been seen outside 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
Scientists first discovered HD 189733b in 2005 and later identified the gas giant as a “hot Jupiter” — a planet that has a similar chemical composition to Jupiter, the largest planet in our solar system, but with sizzling temperatures. 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 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,” said study leader Guangwei Fu, an astrophysicist at Johns Hopkins University. in email. “It is an anchor point for much of our understanding of exoplanet chemistry and atmospheric physics.”
The planet is about 10% 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 two Earth days to complete one orbit around its star, Fu said.
This proximity to the star gives the planet a scorching average temperature of 1,700 degrees Fahrenheit and strong winds that send glassy silicate particles raining down from high clouds around the planet at 5,000 miles per hour.
A surprising stench
When astronomers decided to use the Webb Telescope to study the planet to see what infrared light, invisible to the human eye, could reveal in HD 189733b’s atmosphere, they were in for a surprise.
Hydrogen sulfide is present on Jupiter and has been predicted to exist on gas giant exoplanets, but evidence of the molecule has been elusive outside our solar system, Fu said.
“Hydrogen sulfide is one of the major reservoirs of sulfur in planetary atmospheres,” Fu said. “(The Webb telescope’s) high precision and infrared capability allow us to detect hydrogen sulfide on exoplanets for the first time, opening a new spectral window for studying sulfur chemistry in exoplanet atmospheres. This helps us understand what exoplanets are made of and how they formed.”
In addition, the team detected water, carbon dioxide and carbon monoxide in the planet’s atmosphere, Fu said — meaning these molecules could be common on other gas giant exoplanets.
While astronomers don’t expect life to exist on HD 189733b because of its scorching temperatures, the detection of a building block like sulfur on the exoplanet sheds light on planet formation, Fu said.
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 are made of.
– Guangwei Fu, astrophysicist
“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 are made of,” Fu said.
Molecules with a distinct odor, such as ammonia, have previously been detected in other exoplanet atmospheres.
However, Webb’s capabilities allow scientists to identify specific chemicals in the atmospheres around exoplanets in greater detail than before.
Planetary heavy metals
In our solar system, ice giants such as Neptune and Uranus, although less massive overall, contain more metals than the gas giants Jupiter and Saturn, the largest planets, suggesting that there might be a correlation between metal content and mass.
Astronomers believe that Neptune and Uranus were made up of more ice, rock and metals – rather than gases like hydrogen and helium.
Webb’s data also showed heavy metal levels on HD 189733b that are similar to those on Jupiter.
“Now we have this new measurement showing that indeed the (planet’s) metal concentration provided a very important anchor point for this study of how the composition of a planet changes with its mass and radius,” Fu said. “The findings support our understanding of how planets form by building up solid material after initial core formation and then being naturally enriched in heavy metals.”
Now the team will look for signatures of sulfur on other exoplanets and determine whether high concentrations of the compound affect how close some planets form in relation to their host stars.
“HD 189733b is a comparison planet, but it only represents one data point,” Fu said. “Just as individual humans exhibit unique characteristics, our collective behavior follows clear trends and patterns. With the growing data sets from Webb, we are trying to understand how planets form and whether our solar system is unique in the galaxy.”