Heatwaves on Earth may be uncomfortable and even dangerous for some, but our planet has nothing on the extremely hot world of WASP-76.
Astronomers have taken a deeper look at an 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.
It is remarkable that some of the rock-forming elements discovered on this distant planet have not yet been measured in the gas giants of the solar system Saturn and Jupiter.
“There are very few times when an exoplanet hundreds of light-years away can teach us something that is likely impossible to know about our own solar system,” team leader and University of Montreal Trottier Institute for Exoplanet Research Ph.D. Stephen Pelletier he said in a statement. “That is the case with this study.”
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Located about 634 light-years away in the constellation of Pisces, the strange planet WASP-76 b gets its incredible temperatures from its close proximity to its parent star. Classified as a “superheated Jupiter,” a massive planet found incredibly close to its star, the exoplanet is about a dozen the distance from its star, WASP-76, from Mercury to the Sun.
This gives WASP-76 b, which takes 1.8 Earth days to orbit its star, some other unusual properties. Although the planet contains about 85% of the mass of Jupiter, it is about twice the width of the solar system’s gas giant and about six times its size. This is the result of intense radiation from its star “puffing” the planet.
WASP-76 b has been the subject of intense study since it was found as part of Wide Angle Search for Planets Program (WASP) in 2013. This has led to the classification of several elements in its atmosphere. Most notable was the 2020 discovery that iron evaporated on the side of the planet that is tidal-locked and perpetually facing the strokes of its star to the relatively cooler “night side” that is always facing space and condensed, falling as iron rain.
Spurred on by these previous investigations of WASP-76 b, Pelletier was inspired to obtain new observations of WASP-76 b using the MAROON-X high-resolution optical spectrometer on the 8-meter Gemini North telescope in Hawaii, part of the International Gemini Observatory. This allowed the team to study the formation of superheated Jupiter in unprecedented detail.
Due to the impressive temperatures of WASP-76 b, elements that would normally be rocks on terrestrial planets like Earth, such as magnesium and iron, evaporate and evaporate as gases into the planet’s upper atmosphere.
This means that studying this world could give astronomers an unparalleled insight into the presence and abundance of rock-forming elements in the atmospheres of giant planets. This is not possible for cooler giant planets like Jupiter because these elements reside low in the atmosphere, making them impossible to detect.
What Pelletier and his colleagues discovered in their search for WASP-76 b is that the abundances of elements such as manganese, chromium, magnesium, vanadium, barium and calcium closely match not only the abundances of these elements in their respective star but also the quantities. Found in the sun.
The visible initial abundance is not random. It is the result of hydrogen and helium being processed by successive generations of stars over billions of years. The star creates heavier elements until it exhausts its fuel for nuclear fusion, and dies in a supernova explosion. This explosion releases these elements into the universe, and they become the building blocks of subsequent stars, with the remaining material surrounding these young stars as protoplanetary 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.”
Because terrestrial planets like ours are formed through more complex processes, they have different abundances 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 matter that collapsed into its birth. And this may be true of all the giant planets.
However, not everything that has been discovered about the formation of WASP-76 b has been so predictable. The team discovered that some elements in Wasp-76 b’s atmosphere appeared to be “depleted.”
“These elements that appear to be missing in WASP-76 b’s atmosphere are precisely those that require higher temperatures to vaporize, such as titanium and aluminum,” Pelletier said. “Meanwhile, those that matched our expectations, such as manganese, vanadium and calcium, all vaporize at slightly lower temperatures.”
The team interpreted this depletion as an indication that the composition of the gas giant’s upper atmosphere is sensitive to temperature. Depending on the temperature at which an element condenses, it will be present as a gas in the upper atmosphere or lost as it condenses into a liquid and sinks into the lower layers. From the lower part of the atmosphere, the element cannot absorb light which makes its “fingerprint” missing in the observations.
“If confirmed, this discovery means that two giant exoplanets that have very different temperatures from each other could have two completely different atmospheres,” Pelletier explained. “A kind of bowl of water, one at -1°C frozen, the other liquid at +1°C. For example, calcium has been observed on WASP-76 b, but it might not run on a slightly cooler planet.”
The team made another important discovery about WASP-76 b’s atmosphere; It contains a chemical compound called vanadium oxide. This is the first time this compound has been seen in the atmosphere of a planet outside the solar system. This discovery will be of great interest to astronomers because vanadium oxide can have a significant impact on hot giant planets.
“This molecule plays a similar role to ozone in Earth’s atmosphere: it is very effective at heating the upper atmosphere,” Pelletier explained. “This causes temperatures to increase as a function of altitude, rather than decreasing as is typical on cooler planets.”
The team also found a higher abundance of nickel than expected around WASP-76 b, which could mean that at some point in its history the gas giant planet swallowed a smaller, Mercury-like terrestrial world that was rich in the element.
The astronomers behind these discoveries will continue to study this exoplanet and other similar worlds, trying to discover how temperatures affect the composition of their atmospheres. As they do so, the team said they hope some of the things they learn can be applied to the giant planets closer to home.
The research is described in a research paper published Wednesday (June 14) in the journal nature.
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