ROBERTO MOLAR CANDANOSA/JOHNS HOPKINS UNIVERSITY
The atmosphere of HD 189733 b, a gas giant the size of Jupiter, has traces of hydrogen sulphide, a molecule that not only gives off a bad smell, but also offers scientists new clues about how sulphur, a building block of planets, can influence the interior and atmospheres of gaseous worlds beyond the solar system.
“Hydrogen sulphide is an important molecule that we didn’t know existed. We predicted it would exist and we know it’s on Jupiter, but we haven’t really detected it outside the solar system,” said Guangwei Fu, the Johns Hopkins astrophysicist who led the research. “We’re not looking for life on this planet because it’s too hot, but the discovery of hydrogen sulphide is a stepping stone to finding this molecule on other planets and gaining more understanding of how different types of planets form.”
As well as detecting hydrogen sulphide and measuring the total sulphur in HD 189733 b’s atmosphere, Fu’s team accurately measured the planet’s main sources of oxygen and carbon – water, carbon dioxide and carbon monoxide.
“Sulphur is a vital element for building more complex molecules and, just like carbon, nitrogen, oxygen and phosphate, scientists need to study it more to fully understand how planets are formed and what they are made of,” said Fu.
Just 64 light-years from Earth, HD 189733 b is the closest “hot Jupiter” that astronomers can observe passing in front of its star, making it a reference planet for detailed studies of exoplanetary atmospheres since its discovery in 2005, Fu said.
The planet is about 13 times closer to its star than Mercury is to the Sun and takes only about two Earth days to complete an orbit. It has scorching temperatures of 1,700 degrees Fahrenheit and is famous for its cruel weather, including glass rain that blows sideways with winds of 5,000 mph.
Just as it has done in detecting water, carbon dioxide, methane and other essential molecules on other exoplanets, Webb offers scientists another new tool for tracking hydrogen sulphide and measuring sulphur on gaseous planets outside the solar system.
“Let’s say we study another 100 hot Jupiters and they all have sulphur. What does that mean about how they were born and how they form differently compared to our own Jupiter?” said Fu.
The new data also ruled out the presence of methane on HD 189733 b with unprecedented precision and infrared wavelength observations from the Webb telescope, contradicting previous claims about the abundance of this molecule in the atmosphere.
“We thought this planet was too hot to have high concentrations of methane, and now we know that’s not the case,” said Fu.
The team also measured levels of heavy metals like those on Jupiter, a finding that could help scientists answer questions about how a planet’s metallicity correlates with its mass, Fu said.
Less massive icy giant planets, such as Neptune and Uranus, contain more metals than those found in gas giants like Jupiter and Saturn, the largest planets in the solar system. The higher metallicities suggest that Neptune and Uranus accumulated more ice, rock and other heavy elements in relation to gases such as hydrogen and helium during their initial periods of formation. Scientists are testing whether this correlation also applies to exoplanets, Fu said.
“This Jupiter-mass planet is very close to Earth and has been very well studied. Now we have this new measurement to show that, in fact, the metal concentrations it 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 is formed and then are naturally enhanced with heavy metals.”
In the coming months, Fu’s team plans to trace the sulphur in more exoplanets and find out how high levels of this compound can influence how close they form to their parent stars.
“We want to know how these types of planets got there, and understanding their atmospheric composition will help us answer that question,” said Fu.
This research was supported by NASA through the JWST GO program.
The findings were published in Nature.
