In our system, the more massive the planet, the smaller the percentage of “heavy” elements (anything other than hydrogen and helium) in the planet’s atmosphere. But in the galaxy, the atmospheric compositions of the giant planets do not match the trend of the solar system, an international team of astronomers has discovered. Using the James Webb Space Telescope (JWST), researchers found that the atmosphere of exoplanet HD149026b, a “hot Jupiter” orbiting a star comparable to our sun, is super-abundant in the heavier elements, carbon and oxygen, well above what scientists would consider. expect for a planet of its mass. Furthermore, the diagnostic carbon-to-oxygen ratio of HD149026b, also known as “Smertrios”, is elevated relative to our solar system.
These findings, published in Nature, are an important first step in obtaining similar measurements for a large sample of exoplanets in order to look for statistical trends, according to the new research. They also provide information about the formation of planets. “It seems that every giant planet is different, and we’re starting to see those differences thanks to JWST,” said Jonathan Lunine, a professor of physical sciences at Cornell University and co-author of the study. “In this paper, we have determined how many molecules there are relative to the main component of the gas, which is hydrogen, the most common element in the universe. That tells us a lot about how this planet formed.”
The giant planets in our solar system exhibit a near-perfect correlation between overall composition and atmospheric composition and mass, Jacob Bean, a professor of astronomy and astrophysics at the University of Chicago and lead author of the paper, said in a statement. Extrasolar planets display a much greater diversity of overall compositions, but scientists did not know how varied their atmospheric compositions were until this analysis of HD149026b.
“We have definitively shown that the atmospheric compositions of giant extrasolar planets do not follow the same trend that is so clear for planets in the solar system,” said Bean. “Giant extrasolar planets display a wide diversity in atmospheric compositions in addition to their wide diversity in overall compositions.” Smertrios, for example, is super-enriched compared to its mass, Lunine said. “It’s the mass of Saturn, but its atmosphere appears to have up to 27 times the amount of heavy elements relative to its hydrogen and helium that we find on Saturn.”
This ratio, called “metallicity” (although it includes many elements that aren’t metals), is useful for comparing a planet to its parent star or to other planets in its system, Lunine said. Smertrios is the only known planet in this particular planetary system. Another key measurement is the ratio of carbon to oxygen in a planet’s atmosphere, which reveals the “recipe” for the original solids in a planetary system, Lunine said. For Smertrios, it’s about 0.84, higher than in our solar system. In our sun, it is a little more than one carbon for every two oxygen atoms (0.55).
“Together, these observations paint a picture of a planet-forming disk abundant in carbon-rich solids,” Lunine said. “HD149026b acquired large amounts of this material as it formed.” While an abundance of carbon may seem favorable for life’s chances, a high carbon-to-oxygen ratio actually means less water on a planet or in a planetary system, a problem for life as we know it.
Smertrios is an interesting first case of atmospheric composition for this particular study, said Lunine, who has plans to observe five more giant exoplanets next year using JWST. Many more observations are needed before astronomers can discover patterns between giant planets or in systems with multiple giant planets or terrestrial planets for compositional diversity astronomers are beginning to document.
“The origin of this diversity is a fundamental mystery in our understanding of planet formation,” said Bean. “Our hope is that more atmospheric observations of extrasolar planets with JWST will better quantify this diversity and produce constraints on the more complex trends that may exist.”