The JWST has detected the presence of hydrogen sulfide (H2S) in the atmosphere of the planet HD 189733b, a nearby “hot Jupiter”. Already off the colonization list for being twice as hot as Venus and presumably having no solid surface to land on, HD 189733b can now add the smell of rotten eggs to its nope list. Nevertheless, the finding shows our models of planetary formation are getting good enough to make successful predictions.
Large planets, and those that orbit close to their stars, are the easiest to detect. Consequently, when astronomers first started discovering planets outside the Solar System, it looked as though the galaxy was filled with “hot Jupiters”, gas giants with masses close to Jupiter’s or greater orbiting near enough to their star to be scorched.
At 65 light-years away, HD 189733b is the closest of these known to transit in front of its star from our perspective, and therefore a natural priority for further investigation. At an estimated 920°C (1,700°F), it’s a truly extreme world, which models indicate has some of the fastest winds we know. However, the same things that make it so inhospitable, along with its relative closeness, also make it one of the easiest planets outside our own system to study, attracting the attention of the JWST.
A study of HD 189733b’s spectrum led by Dr Guangwei Fu of Johns Hopkins University described HD 189733b as “the benchmark planet for atmospheric characterization.”
The study collected light filtered through HD 189733b’s atmosphere during transits, and demonstrates the JWST’s capacity to detect molecules present in relatively small quantities, as well as vindicating astronomers’ models.
“Hydrogen sulfide is a major molecule that we didn’t know was there. We predicted it would be, and we know it’s in Jupiter, but we hadn’t really detected it outside the Solar System,” Fu said in a statement. “We’re not looking for life on this planet because it’s way too hot, but finding hydrogen sulfide is a stepping stone for finding this molecule on other planets and gaining more understanding of how different types of planets form.”
The team also detected water, carbon dioxide, and carbon monoxide in HD 189733b’s atmosphere, but these are expected to be much more abundant, and two of them had been found before with less powerful instruments. On the other hand, no sign of methane was found down to a concentration of one part in 10 million, contrary to some previous reports. This also confirms models suggesting methane would not survive on a planet that hot.
Besides demonstrating the capacity to detect hydrogen sulfide, the results increase confidence that sulfur is a common element on exoplanets. We might not like this particular sulfur compound, but Fu noted, “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’re made of.”
“Say we study another 100 hot Jupiters and they're all sulfur enhanced. What does that mean about how they were born and how they form differently compared to our own Jupiter?” Fu added. A hundred might be an ambitious target given the demands on the JWST’s time, but Fu is working on studying several.
Besides its association with eggs that have gone bad, hydrogen sulfide is common around volcanic vents, leading to the popular association of brimstone (an old word for sulfur) with hell.
HD 189733b has sometimes been described as having a temperature of 3,000°C (5,432°F), but the truth is a more modest 920°C (1,700°F) – still a bit uncomfortable though. Other features predicted by models are winds of 8,700 kilometers an hour (5,400 mph) and rains of glass.
The study is published in Nature.
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