Not tasty: Astronomers have detected sulfur hydrogen on an exoplanet for the first time — this gas is known for its pungent smell of rotten eggs. The James Webb Telescope has now shown the spectral signature of this molecule on the gas giant HD 189733b, only 63 light-years away, as the team reports in “Nature“. The nearest “hot Jupiter” to us is thus not only almost 1,000 degrees hot and has raging “glass storms” in its atmosphere – it probably also stinks.
The extrasolar gas giant HD 189733b is one of the best-studied exoplanets ever, as it is only 63 light-years away from us and visibly passes in front of its parent star about every two days. Therefore, astronomers have already determined some peculiarities of this nearly 1,000-degree hot, Jupiter-sized gas planet. For example, its hazy gas envelope contains carbon dioxide and carbon monoxide as well as water vapor and silicate particles. The latter are chased through the exoplanet’s atmosphere as tiny glass grains by storms of up to 7,000 kilometers per hour – not particularly pleasant.
Spectral Fingerprint of Hydrogen Sulfide
Now there are new insights into the atmosphere of this “hot Jupiter” closest to us: Astronomers led by Guangwei Fu from Johns Hopkins University in Baltimore have targeted HD 189733b with the high-resolution Near-Infrared Camera (NIRCam) of the James Webb Telescope. This allowed them to analyze the light spectrum and thus the composition of the planetary gas envelope more precisely than before.
The result: In addition to water vapor, CO2, and carbon monoxide, the astronomers discovered another spectral signature in the gas envelope of HD 189733b – the chemical fingerprint of hydrogen sulfide (H2S). This molecule is notorious on Earth for its pungent smell of rotten eggs. It is clearly perceptible even at low concentrations of H2S. Therefore, the hot Jupiter HD 189733b could also smell like rotten eggs.
First Extrasolar Detection of This Important Molecule
This first detection of hydrogen sulfide on the exoplanet is important primarily because it reveals more about the fundamental processes in the atmosphere of such gas giants. “Hydrogen sulfide is an important molecule that we didn’t know existed on this planet before,” says Fu. “While it’s present on Jupiter, we’ve never detected hydrogen sulfide outside our solar system before.”
As the astronomers explain, sulfur is an important building block for more complex molecules, including many biologically relevant compounds. “Of course, we don’t expect life on this planet because it’s far too hot,” Fu explains. But the detection of hydrogen sulfide on HD 189733b is a first step towards detecting this molecule on other, potentially more life-friendly exoplanets. “This also helps us understand how the different types of planets formed,” says the planetary researcher.
Too Hot for Methane and Many “Metals”
However, what the analyses did not find in the gas envelope of HD 189733b is also interesting: Unlike what earlier observations suggested, there seems to be no or extremely little methane on the hot Jupiter. “We already suspected that this planet is actually too hot for higher concentrations of methane. Now we know that’s true,” says Fu. According to this, the methane content on HD 189733b is probably below 0.1 parts per million (ppm).
The new measurements also provide new clues about where and how this extrasolar gas planet formed. From the spectral data, the astronomers determined that HD 189733b has a similar amount of heavier elements as our Jupiter. Its metallicity is about three to five times higher than that of its parent star, as the team determined. “This gives us an important clue as to how a planet’s composition varies with its mass and size,” Fu explains.
Accretion Instead of Gas Collapse?
According to current models, the proportion of heavier elements in a planet is higher the more solid components in the form of rock chunks and other planet building blocks it accumulated during its formation period. The values now measured for HD 189733b therefore suggest that the gas planet contains a solid core of rock, ice, and metals, and that this core was formed by the accumulation of water-ice-rich planetesimals, as the team reports.
This in turn allows conclusions to be drawn about how Jupiter-sized planets were generally formed – whether through local collapse of the star’s gas and dust cloud or through gradual accretion of dust and larger chunks, followed later by the thick gas envelope.
