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NASA のジェームズ ウェッブ宇宙望遠鏡を使用している天文学者は、太陽系外惑星 HD149026b の大気が炭素と酸素の重元素が非常に豊富であることを発見しました。これは、科学者がその質量の惑星に期待するよりもはるかに高く、炭素と酸素の比率が上昇しています。これは、惑星形成に関する洞察を提供し、統計的傾向を検索するために系外惑星の大規模なサンプルについて同様の測定値を取得するための重要な最初のステップを提供する可能性があります。 クレジット: NASA/JPL-Caltech

私たちの太陽を周回する巨大ガス惑星は、明確なパターンを示しています。 惑星の質量が大きいほど、惑星の大気中の「重」元素 (水素とヘリウム以外のもの) の割合が低くなります。 しかし、銀河系外では、巨大惑星の大気組成が太陽系の傾向に適合しないことが、天文学者の国際チームによって発見されました。

使用する[{” attribute=””>NASA’s James Webb Space Telescope (JWST), the researchers discovered 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 – far above what scientists would expect for a planet of its mass. In addition, the diagnostic carbon-to-oxygen ratio of HD149026b, also known as “Smertrios,” is elevated relative to our solar system.

These findings, published in “High Atmospheric Metal Enrichment for a Saturn-mass Planet,” in the journal Nature on March 27 are an important first step toward obtaining similar measurements for a large sample of exoplanets in order to search for statistical trends, the researchers said. They also provide insight into planet formation.

James Webb Space Telescope Cold Side

This illustration shows the cold side of the Webb telescope, where the mirrors and instruments are positioned. Credit: Northrop Grumman

“It appears that every giant planet is different, and we’re starting to see those differences thanks to JWST,” said Jonathan Lunine, the David C. Duncan Professor in the Physical Sciences in the College of Arts and Sciences and a co-author of the study. “In this paper, we have determined how many molecules there are relative to the primary component of the gas, which is hydrogen, the most common element in the universe. That tells us quite a lot about how this planet formed.”

The giant planets of our solar system exhibit a nearly perfect correlation between both overall composition and atmospheric composition and mass, said Jacob Bean, professor of astronomy and astrophysics at the University of Chicago and lead author of the paper. 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.

HD 149026b Hot Jupiter

A ‘hot Jupiter’ called HD 149026b, is about 3 times hotter than the rocky surface of Venus, the hottest planet in our solar system. Credit: NASA/JPL-Caltech

“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,” Bean said. “Giant extrasolar planets show a wide diversity in atmospheric compositions in addition to their wide diversity of overall compositions.”

Smertrios, for one, is super-enriched compared to its mass, Lunine said: “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.”

This ratio, called “metallicity” (even though it includes many elements that are not metals) is useful for comparing a planet to its home star, or to other planets in its system, Lunine said. Smertrios is the only planet known in this particular planetary system.

Another key measurement is the ratio of carbon to oxygen in a planet’s atmosphere, which reveals the “recipe” of 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’s a bit more than one carbon for every two oxygen atoms (0.55).

“Together, these observations paint a picture of a planet-forming disk with abundant solids that were carbon-rich,” Lunine said. “HD149026b acquired large amounts of this material as it formed.”

While an abundance of carbon might seem favorable for chances of life, 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 in place to observe five more giant exoplanets in the coming year using JWST. Many more observations 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,” Bean said. “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.”

Reference: “High atmospheric metal enrichment for a Saturn-mass planet” by Jacob L. Bean, Qiao Xue, Prune C. August, Jonathan Lunine, Michael Zhang, Daniel Thorngren, Shang-Min Tsai, Keivan G. Stassun, Everett Schlawin, Eva-Maria Ahrer, Jegug Ih and Megan Mansfield, 27 March 2023, Nature.
DOI: 10.1038/s41586-023-05984-y

The study was supported by NASA and the University of Chicago.

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