The atmospheres of gas giant planets orbiting the Sun show a clear pattern – the larger the planet, the lower the percentage of heavy elements. But an international team of astronomers has discovered that in our galaxy, the atmospheres of gas giants are variable and do not conform to the pattern of the Solar System.

Heavy metals and carbon-oxygen ratio

Using NASA‘s James Webb Space Telescope, the researchers have found that the atmosphere of the exoplanet HD149026b, a hot Jupiter in orbit around a Sun-like star, is abundant in elements heavier than carbon. The percentage is much higher than what scientists would expect for a planet of its mass.

In addition, the carbon-oxygen ratio of HD149026b is much higher than that of planets in the Solar System. These results, published in Nature on March 27th (ref.), are an important step towards obtaining a statistical sample for evaluating the exoplanets we discover, while providing information on planetary formation. “It seems that every giant planet is different and we are beginning to see these differences thanks to James Webb” said co-author Jonathan Lunine. “In this paper, we have determined how many molecules there are relative to the main component, hydrogen, the most common element in the universe. This tells us a lot about how this planet formed”.

The giant planets in our Solar System show an almost perfect correlation between overall composition, atmospheric composition, and mass, said lead author Jacob Bean. In contrast, extrasolar gas giants show much greater diversity and scientists did not know how variable their atmospheres were until this analysis of HD149026b.

Atmospheric compositions

“We have shown that the atmospheric compositions of extrasolar giant planets do not follow the same trend that is so clear in planets in the Solar System” said Bean. “Extrasolar giant planets show a wide diversity in atmospheric compositions in addition to their wide diversity of overall compositions”. HD149026b, for example, is much richer in heavy metals than its mass would suggest. “It has the mass of Saturn, but its atmosphere seems to have up to 27 times the amount of heavy elements compared to what we find on Saturn“.

This ratio, called “metallicity” is useful for comparing a planet with its parent star, or with other planets in its system, explains Lunine. Another key measure is the ratio of carbon to oxygen in a planet’s atmosphere, which reveals the “recipe” of the original solids in a planetary system.

For HD149026b, it is about 0.84, higher than in our Solar System. In our Sun, it is just over one carbon for every two oxygen atoms (0.55). “Together, these observations paint the picture of a planetary formation disk with abundant carbon-rich solids. HD149026b acquired large amounts of this material as it formed” Lunine explained. While an abundance of carbon may seem favorable for life, a high carbon-oxygen ratio actually means less water in a planetary system.

Future investigations

HD149026b is the first interesting case of atmospheric composition for this particular study. The scientists plan to observe five more giant exoplanets in the next year using the James Webb telescope. Many more observations are needed to discover any pattern in the formation of giant planets compared to the compositional diversity that astronomers are beginning to document.

“The origin of this diversity is a fundamental mystery in our understanding of planetary formation” said Bean. “Our hope is that further atmospheric observations of exoplanets with James Webb will better quantify this diversity and produce patterns of more complex realities that may exist in the universe”.

Stefano Gallotta
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