The James Webb Space Telescope Validates the Wide-ranging Differences in Giant Planet Atmospheres

Gas giants in our solar system exhibit a pattern where the more massive the planet, the lower the percentage of “heavy” elements in the planet’s atmosphere. However, an international team of astronomers has found that the atmospheric compositions of giant planets in the galaxy do not follow this trend.

Using NASA’s James Webb Space Telescope (JWST), the team discovered that the atmosphere of exoplanet HD149026b, a “hot Jupiter” orbiting a star similar to our sun, is super-abundant in heavier elements carbon and oxygen, far beyond what scientists expected for a planet of its mass. The diagnostic carbon-to-oxygen ratio of HD149026b, also known as “Smertrios,” is elevated relative to our solar system.

Published in “High atmospheric metal enrichment for a Saturn-mass planet” in Nature, these findings are an important first step towards obtaining similar measurements for a large sample of exoplanets in order to search for statistical trends. They also provide insight into planet formation.

The giant planets in our solar system exhibit a nearly perfect correlation between overall and atmospheric composition and mass. In contrast, extrasolar planets show a much greater diversity of overall compositions, but scientists didn’t know how varied their atmospheric compositions were until this analysis of HD149026b.

“We have shown definitively that the atmospheric compositions of giant extrasolar planets do not follow the same trend that is so clear in the solar system planets,” said lead author Jacob Bean, professor of astronomy and astrophysics at the University of Chicago. “Giant extrasolar planets show a wide diversity in atmospheric compositions in addition to their wide diversity of overall compositions.”

Smertrios is super-enriched compared to its mass. “It’s the mass of Saturn, but its atmosphere seems to have as much as 27 times the amount of heavy elements relative to its hydrogen and helium that we find in Saturn,” said co-author Jonathan Lunine, the David C. Duncan Professor in the Physical Sciences in the College of Arts and Sciences.

The ratio of carbon to oxygen in a planet’s atmosphere reveals the “recipe” of original solids in a planetary system. For Smertrios, it’s about 0.84—higher than in our solar system. Together, these observations paint a picture of a planet-forming disk with abundant solids that were carbon-rich. “HD149026b acquired large amounts of this material as it formed,” said Lunine.

Observations of more giant exoplanets are needed before astronomers can discover any patterns among giant planets or in systems with multiple giant planets or terrestrial planets to the 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 further atmospheric observations of extrasolar planets with JWST will quantify this diversity better and yield constraints on more complex trends that might exist.”