ATLANTA — Jupiter-sized exoplanets have long intrigued astronomers with their scorching temperatures, blustery winds and glassy 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.
Researchers studying HD 189733b’s atmosphere have used Webb’s observations to find traces of hydrogen sulfide, a colorless gas with a strong sulfurous smell that has never been found outside the solar system. The discovery advances our current knowledge of the potential composition of exoplanets.
The findings, compiled by a team from multiple research institutions, were published in the journal Neurology on Monday. Nature.
Strange 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 an extremely high temperature. Located just 64 light-years from Earth, HD 189733b is the closest hot Jupiter astronomers can study as it passes in front of its star, which makes it one of the most well-studied exoplanets.
“HD 189733 b “Not only is this planet a gas giant, it’s also a ‘giant’ in the exoplanet field because it’s one of the first transiting exoplanets ever discovered,” Guangwei Hu, an astrophysicist at Johns Hopkins University and lead author of the study, said in an email. “It anchors much of our understanding of the chemistry and physics of exoplanet atmospheres.”
The planet is about 10 percent larger than Jupiter, but 13 times closer to its star than Mercury is to the sun, making it much hotter. HD 189733b orbits its star in about two Earth days, Fu said.
Because of its close proximity to its star, the planet’s average temperature is extremely high – 1,700 degrees Fahrenheit – and it has strong winds and glass-like silicate particles that rain sideways from high clouds around the planet at speeds of up to 5,000 miles per hour.
Amazing stench
When astronomers decided to use the Webb Telescope to see what infrared light, invisible to the human eye, could reveal in HD 189733b’s atmosphere, they came across some surprising results.
Hydrogen sulfide is present on Jupiter and has been predicted to be present on gas giants, but there has been no evidence of the molecule’s presence outside the solar system, Hu said.
“Hydrogen sulfide is one of the main reservoirs of sulfur in planetary atmospheres,” said Hu. “Webb’s high resolution and infrared observation capabilities allowed us to detect hydrogen sulfide for the first time on an exoplanet, opening a new spectral window to study the chemical composition of sulfur in exoplanet atmospheres, which could help us understand what exoplanets are made of and how they formed.”
Additionally, the team found water, carbon dioxide, and carbon monoxide in the planet’s atmosphere, Hu said, meaning these molecules may be common on other gas giants as well.
Astronomers don’t think HD 189733b’s scorching temperatures would support life, but detecting components like sulfur on an exoplanet could shed light on planet formation, Hu said.
Sulfur is an essential element for building more complex molecules, and like carbon, nitrogen, oxygen and phosphate, scientists need to study sulfur further to fully understand how planets were created and what they’re made of.
–Astrophysicist Hu Guangwei
“Sulfur is an essential element for building more complex molecules, and like carbon, nitrogen, oxygen and phosphate, scientists need to study it further to fully understand how planets were created and what they’re made of,” Fu said.
Molecules with distinctive ammonia-like odors have been detected in the atmospheres of other exoplanets before.
But Webb’s capabilities allow scientists to pinpoint specific chemicals in exoplanet atmospheres in greater detail than ever before.
Planetary Heavy Metals
In our solar system, ice giants like Neptune and Uranus, despite their smaller overall mass, contain more metals than the largest planets, the gas giants Jupiter and Saturn, suggesting that there may be a correlation between metal content and mass.
Astronomers believe that Neptune and Uranus were formed with more ice, rock, and metals than with gases such as hydrogen and helium.
Webb’s data also showed that HD 189733b’s heavy metal levels are similar to those of Jupiter.
“Now, with these new measurements, we find that planetary metal concentrations provide a crucial anchor point for studies of how a planet’s composition varies with its mass and radius,” said Fu. “This discovery supports our understanding of how planets form by creating more solid material after the initial nucleation, which is then naturally enriched with heavy metals.”
The team now plans to look for sulfur signatures on other exoplanets to determine whether high concentrations of this compound influence how closely some planets form in relation to their host stars.
“HD 189733b is a benchmark planet, but it is only one data point,” says Fu. “Just as individual humans exhibit unique characteristics, our collective behavior follows clear trends and patterns. With more upcoming datasets from Webb, we aim to understand how planets form and whether our solar system is unique in the galaxy.”
