Scientists discover exoplanet has “rotten egg smell”

An exoplanet the size of Jupiter has long intrigued astronomers because of its scorching temperatures, howling winds and glass-like sideways rain. Now, data from the James Webb Space Telescope has revealed another intriguing feature of the planet known as HD 189733b : he smells like rotten eggs.

Researchers studying the atmosphere of HD 189733b used Webb’s observation to detect small amounts of hydrogen sulfide — a colorless gas that gives off a strong sulfuric odor and has never before been detected outside our solar system. The discovery advances knowledge about the potential composition of exoplanets.

The findings, compiled by a team from several institutions, were published on Monday (8) in the journal Nature.

An eccentric 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 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. For this reason, it is one of the most studied exoplanets.

“HD 189733 b is not only a gas giant planet, but also a ‘giant’ in the exoplanet field because it is one of the first transiting exoplanets ever discovered,” said study lead author Guangwei Fu, an astrophysicist at Johns Hopkins University, in an email. “It is the anchor point for much of our understanding of the chemistry and atmospheric physics of exoplanets.”

The planet is about 10 percent 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 about two Earth days to complete a single orbit around its star, Fu says.

This proximity to the star gives the planet a scorching average temperature of 1,700 degrees Fahrenheit (926 degrees Celsius) and strong winds that cause glass-like silicate particles to rain sideways from high clouds around the planet at 5,000 miles per hour (8,046 kilometers per hour).

A surprising smell

When astronomers decided to use the Webb telescope to study the planet and see what infrared light, which is invisible to the human eye, could reveal in the atmosphere of HD 189733b, 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 says.

“Hydrogen sulfide is one of the main reservoirs of sulfur within planetary atmospheres,” he says. “The high precision and infrared capability of the[Webb telescope]allow us to detect hydrogen sulfide for the first time on exoplanets, which opens a new spectral window for studying the chemistry of sulfur in exoplanet atmospheres. This helps us understand what exoplanets are made of and how they formed.”

Additionally, 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 basic element 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 are made and what they are made of,” says Fu.

Molecules with distinct smells, such as ammonia, have previously been detected in other exoplanet atmospheres.

But Webb’s capabilities allow scientists to identify specific chemicals in atmospheres around exoplanets in greater detail than before.

Planetary heavy metals

In our solar system, ice giants like Neptune and Uranus, although less massive overall, contain more metals 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 metals — rather than gases like hydrogen and helium — were involved in the formation of Neptune and Uranus.

Webb’s data also showed levels of heavy metals in HD 189733b similar to those found on Jupiter.

“Now we have this new measurement to show that indeed the concentrations of metals[that the planet]has provide a very important anchor point for this study of how a planet’s composition varies with its mass and radius,” Fu said. “The findings support our understanding of how planets form by creating more solid material after the initial core forms and then are naturally enriched with heavy metals.”

Now, the team will look for signatures of sulfur on other exoplanets and determine whether high concentrations of the compound influence the proximity of some planets to their host stars.

“HD 189733b is a reference planet, but it represents just one data point,” says Fu. “Just as individual humans exhibit unique characteristics, our collective behaviors follow clear trends and patterns. With more Webb data sets to come, we hope to understand how planets form and whether our solar system is unique in the galaxy.”

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