ATLANTA – A Jupiter-sized exoplanet has long intrigued astronomers with its high temperatures, strong winds and sideways rain of glass. Now, data from the James Webb Space Telescope has revealed another intriguing feature of this planet, known as HD 189733b: It smells like rotten eggs.
Researchers studying the atmosphere of exoplanet HD 189733b have used Webb observations to detect trace amounts of hydrogen sulfide—a colorless gas with a strong sulfurous odor that has never been seen outside our solar system. The discovery adds to what we know about the possible composition of exoplanets.
The results, compiled by a team from multiple institutions, were published Monday in the journal nature.
Weird planet with deadly weather
First discovered in 2005, scientists 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 much higher 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. For this reason, it is one of the most studied exoplanets.
“HD 189733 b is not only a gas giant, it is also a ‘giant’ in the exoplanet field because it is one of the first transiting exoplanets ever discovered,” said Guangwei Fu, an astrophysicist at Johns Hopkins University, in an email. “It is the fulcrum of much of our understanding of the chemistry and atmospheric physics of exoplanets.”
The planet is about 10 percent larger than Jupiter, but it is much hotter because it is about 13 times closer to its star than Mercury is to our sun. HD 189733b takes just 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 glass-like silicate particles raining sideways from clouds high around the planet at 5,000 miles per hour.
sudden bad smell
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 HD 189733b’s atmosphere, they were in for a surprise.
Hydrogen sulfide is present on Jupiter and was expected to be found on gas giant exoplanets, Fu said, but evidence of the molecule’s existence has remained elusive outside our solar system.
“Hydrogen sulfide is one of the most important reservoirs of sulfur in the atmosphere of planets,” Fu said. “The high resolution and infrared capability of the Webb telescope allows us to detect hydrogen sulfide for the first time on exoplanets, opening a new spectroscopic window to study the 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 may be common on other gas giant exoplanets.
Although astronomers don’t expect life to exist on HD 189733b because of its high temperatures, the discovery of a building block like sulfur on an 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 form and what they’re 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 form and what they are made of,” Fu said.
Molecules with distinctive odors, such as ammonia, have been detected in the atmospheres of other exoplanets.
But Webb’s capabilities enable scientists to pinpoint specific chemicals within the atmospheres around exoplanets in greater detail than ever before.
planetary heavy metals
In our solar system, the giant ice planets like Neptune and Uranus, although generally less massive, contain more metals than the gas giants Jupiter and Saturn, which are the largest planets, suggesting that there may be a link between metal content and mass.
Astronomers believe that a larger amount of ice, rock and metal – rather than gases like hydrogen and helium – participated in the formation of Neptune and Uranus.
Webb data also showed that levels of heavy metals on HD 189733b are similar to those on Jupiter.
“Now we have this new measurement that proves that the metal concentrations (in the planet) actually provided a very important anchor for this study of how the composition of a planet changes with its mass and radius,” Fu said. “The results support our understanding of how planets form by creating more solid material after the initial core forms and then being naturally 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 represents only one data point,” Fu said. “Just as humans as individuals exhibit unique characteristics, our collective behaviors follow clear trends and patterns. With more datasets from Webb, we aim to understand how planets form and whether our solar system is unique in the galaxy.”
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