Astronomers using the James Webb Space Telescope (JWST) have discovered that one of the closest “hot Jupiter” planets to Earth smells like rotten eggs. The planet already has a bad reputation for deadly glass rain, extreme temperatures, and sideways winds of 5,000 mph (8,046 kph), but this discovery makes this world seem even less friendly.
JWST’s egg-smelling conclusion stems from the discovery of hydrogen sulfide, a molecule that emits a rotten-egg smell, in the atmosphere of this extrasolar planet, or “exoplanet.”
This could tell scientists how sulfur, considered a key element in planet formation, might affect the atmospheres of giant gas planets.
The exoplanet in question is HD 189733 b, which is about 1.13 times the size and mass of Jupiter. Discovered in 2005, the planet is located about 65 light-years from Earth in the constellation Vulpecula.
HD 189733 b orbits its star at a distance of about 3 million miles (4.8 million km), about 3% of the distance between Earth and the Sun. Its warm atmosphere and glassy clouds give it a deceptively calm, almost blue-white marbled appearance.
“Hydrogen sulfide is an important molecule that we didn’t know was there. We predicted it would be there, and we know it’s on Jupiter, but we hadn’t really detected it outside the solar system,” team leader Guangwei Fu, an astrophysicist at Johns Hopkins, said in a statement. “We’re not looking for life on this planet because it’s too hot, but finding hydrogen sulfide is a stepping stone to finding this molecule on other planets and gaining greater understanding of how different types of planets form.”
Fu and colleagues not only detected hydrogen sulfide in HD 189733 b’s atmosphere, they also measured the planet’s overall sulfur content. The team also found water, carbon dioxide, and carbon monoxide, determining the exoplanet’s oxygen and carbon sources.
“Sulfur is a vital element for building more complex molecules, and like carbon, nitrogen, oxygen and phosphate, scientists need to study it further to fully understand how planets form and what they are made of,” Fu continued. “Let’s say we examine 100 other hot Jupiters, and they are all enriched in sulfur. What does that mean about how they were born and how they formed differently compared to our Jupiter?”
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Is HD 189733 b more ‘metal’ than Jupiter?
Despite its fickle and foul-smelling nature, the star it orbits, HD 189733, is happy to keep the exoplanet close by. HD 189733 b orbits its star so tightly that its orbit matches the 13 orbits of Mercury, the Solar System’s first planet, around the Sun.
This proximity makes HD 189733 ba a “hot Jupiter” planet, a world as large as or larger than the solar system’s largest planet, orbiting its star much closer than the solar system’s gas giants do to the sun. HD 189733 b is, in fact, the closest hot Jupiter to Earth that we know of to cross, or “transit,” the face of its star.
This proximity to its star not only means that HD 189733 b completes its orbit in just 2.2 Earth days, but also means that the Jupiter-sized planet is about 1,700 degrees Fahrenheit (927 degrees Celsius) hot enough to melt lead.
HD 189733 b is “tidally locked” to its star. This means that one side of the planet, the “day side,” constantly faces HD 189733 and is exposed to its radiation, while the other side, the “night side,” always faces space. The heat of the planet’s day side is enough to vaporize the glass, and this world’s 5,000 mph (8,046 km/h) winds blow this glass vapor to the cooler night side, where it condenses and forms glass rain. But thanks to HD 189733 b’s extreme winds, this glass rain doesn’t just fall; it’s sheared sideways at high speeds.
The team found that this smelly and scary planet lacks methane. This molecule was previously suspected to be abundant in the atmosphere of HD 189733 b, but this JWST study ruled that out.
“We thought this planet was too hot to have high methane concentrations, and now we know it’s not,” Fu said.
This planet is certainly extreme, but how “metal” is it?
To astronomers, “metals” means any element heavier than hydrogen and helium, the two lightest elements in the universe. One thing this team is really interested in is the levels of metals in planets, which could help them determine how a world’s “metallicity” relates to its mass.
In the solar system, the smaller ice giants Neptune and Uranus are richer in metals than the larger gas giants Jupiter and Saturn. This suggests to scientists that the ice giants collected more ice, rock and other heavy elements than hydrogen and helium during their formation. Fu and his team are testing whether this relationship between mass and metals also holds true for planets outside the system.
“This Jupiter-mass planet is very close to Earth and has been very well studied. Now we have this new measurement that shows that its metal concentrations provide a very important baseline for this study of how a planet’s composition changes with its mass and radius,” Fu said. “The findings support our understanding of how planets form by forming more solid material after initial core formation and then naturally becoming enriched with heavy metals.”
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The team now wants to track the movement of sulfur in the atmospheres of other exoplanets, which will help them determine how concentrations of this element change with distance from the parent star.
“We want to know how such planets got there, and understanding their atmospheric structures will help us answer that question,” Fu concluded.
The team’s research was published in the journal Nature on Monday (July 8).