Heatwaves on Earth may be uncomfortable and even dangerous for some, but our planet has nothing in the scorching world of WASP-76 b.
Astronomers have taken a closer look at the exoplanet where temperatures soar to around 4,350 degrees Fahrenheit (2,400 degrees Celsius), hot enough to vaporize iron. In the process, the team identified 11 chemical elements in the planet’s atmosphere and measured their abundance.
Surprisingly, some of the rock-forming elements detected on this distant planet have not yet been measured in the solar system’s gas giants Saturn and Jupiter.
“Very rare are the moments when an exoplanet hundreds of light-years away can teach us something that otherwise would probably be impossible to know about our solar system,” said the team leader from the University of Montréal. Trottier Institute for Exoplanet Research PhD Stefano Pelletier said in a statement. “That’s the case with this study.”
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Located about 634 light-years away in the constellation Pisces, the strange planet of WASP-76b gets its incredible temperatures from its proximity to its parent star. Classified as “ultra-hot Jupiter,” which is a massive planet that exists incredibly close to its star, the exoplanet is one-twelfth the distance from its star, WASP-76, as Mercury is from the sun.
This gives WASP-76 b, which takes 1.8 Earth days to orbit its star, some other extraordinary properties. Although the planet is about 85% the mass of Jupiter, it is nearly twice as wide as the solar system’s gas giant and is about six times its volume. This is the result of the intense radiation from its star “swelling” the planet.
WASP-76 b has been the subject of intense study since it was found as part of the Wide Angle Search for Planets (WASP) program in 2013. This has led to the classification of several elements in its atmosphere. Most surprising was the discovery in 2020 that iron vaporized on the side of the tidally locked planet that permanently faces its stellar strikes to the relatively cooler “night side” that perpetually faces space and condenses, falling as iron rain.
Spurred by these previous investigations of WASP-76 b, Pelletier was inspired to obtain new observations of WASP-76 b with the MAROON-X high-resolution optical spectrograph on the Gemini North 8-meter Telescope in Hawaii, part of the International Gemini Observatory . This allowed the team to study the composition of ultra-hot Jupiter in unprecedented detail.
Due to WASP-76 b’s incredible temperatures, elements that would normally form rocks on terrestrial planets like Earth, such as magnesium and iron, are instead vaporized and hide as gases in the planet’s upper atmosphere.
This means that studying this world can provide astronomers with unprecedented insight into the presence and abundance of rock-forming elements in the atmospheres of giant planets. This isn’t possible for cooler giant planets like Jupiter as these elements reside lower in the atmosphere, making them impossible to detect.
What Pelletier and colleagues discovered during their investigations of WASP-76b was that the abundances of elements such as manganese, chromium, magnesium, vanadium, barium and calcium correspond closely, not just to the abundance of these elements in one’s star , but also to the quantities found in the sun.
The elementary abundances seen are not arbitrary; they are the result of hydrogen and helium being processed by successive generations of stars over billions of years. A star creates heavier elements until it runs out of fuel for nuclear fusion, dying in a supernova explosion. This explosion releases those elements into the cosmos and they become the building blocks of later stars, with the remaining material surrounding these infant stars as proto-planetary disks, which, as the name suggests, can spawn planets. This means that stars of similar ages have similar compositions with the same abundance of elements heavier than hydrogen and helium, which astronomers call “metals.”
However, since terrestrial planets like ours form through more complex processes, they have a different abundance of heavy elements than their stars. The fact that this new study shows that WASP-76 b has a similar composition to its star means that its composition is also similar to the protoplanetary disk of material that collapsed to give birth to it. And this may be true for all giant planets.
However, not everything that was discovered about the composition of WASP-76b was as expected. The team found that some elements in Wasp-76 b’s atmosphere appeared to be “depleted”.
“These elements that appear to be missing from WASP-76 b’s atmosphere are the very ones that require higher temperatures to vaporize, such as titanium and aluminum,” Pelletier said. “Meanwhile, those that match our predictions, such as manganese, vanadium or calcium, all vaporize at slightly lower temperatures.”
The team interpreted this depletion as indicating that the composition of the gas giant planets’ upper atmospheres is sensitive to temperature. Depending on the temperature at which an element condenses, it will either be present as a gas in the upper atmosphere or absent because it has condensed into a liquid and sunk to the lower layers. From low in the atmosphere, the element cannot absorb light, making its characteristic “fingerprint” absent from observations.
“If confirmed, this discovery would mean that two giant exoplanets that have slightly different temperatures from each other could have very different atmospheres,” Pelletier explained. “A bit like two pots of water, one at -1°C which is frozen, and one which is at +1°C which is liquid. For example, calcium is observed on WASP-76 b, but may not to be on a slightly colder planet.”
The team made another important discovery about WASP-76 b’s atmosphere; contains a chemical compound called vanadium oxide. The first time this compound has been seen in the atmosphere of a planet outside the solar system. The discovery will be of great interest to astronomers because vanadium oxide can have a large impact on hot giant planets.
“This molecule plays a similar role to ozone in Earth’s atmosphere: It is extremely efficient at heating the upper atmosphere,” Pelletier explained. “This causes temperatures to increase as a function of altitude, instead of decreasing as typically seen on colder planets.”
The team also found a greater-than-expected abundance of nickel around WASP-76 b, which could imply that at some point in its history, the gas giant planet gobbled up a smaller, Mercury-like terrestrial world that was rich of the item.
The astronomers behind these revelations will continue to study this exoplanet and other similar worlds, attempting to discover how temperatures affect the composition of their atmospheres. As they do so, the team said it hopes some of the things they learn could be applied to giant planets closer to home.
The research is described in an article published Wednesday (June 14) in the journal Nature.
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