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Giant planets farther out, circling on hundred-day schedules in cooler, quieter orbits, remain almost entirely unknown.<\/p>\n
Models had predictions for that unexplored category. One of them said that if you could peer inside, you\u2019d find methane. <\/p>\n
Whether the models were right was another question entirely \u2013 until now.<\/p>\n
A planet between extremes<\/p>\n
TOI-199b is the world in question. It\u2019s about the size of Saturn but only a fraction of its mass, and it orbits a sun-like star more than 330 light-years away. <\/p>\n
One full loop takes roughly a hundred days.<\/p>\n
The basic properties of TOI-199b had already been measured and documented in an earlier paper<\/a>.<\/p>\n The current study was led by Renyu Hu, an associate professor at Penn State (PSU)<\/a>.<\/p>\n At roughly 175 degrees Fahrenheit, the planet is much cooler than the thousand-degree worlds that dominate most exoplanet studies.<\/p>\n Only a handful of these temperate worlds are even known, and this is the first one to have its atmosphere pulled apart in detail.<\/p>\n Reading light through air<\/p>\n Working out what an exoplanet<\/a>\u2018s air is made of takes patience. <\/p>\n Astronomers wait for the planet to pass in front of its star, then watch as starlight filters through its outer layers.<\/p>\n Certain wavelengths get swallowed by whatever gases sit in the way, leaving a fingerprint embedded in the light that reaches the telescope. <\/p>\n The technique has a clinical name \u2013 transmission spectroscopy<\/a> \u2013 but the idea is simple.<\/p>\n Webb\u2019s instruments separate the incoming light into its component colors, one wavelength at a time. Subtle dips in the spectrum reveal which molecules are present.<\/p>\n A patient observation<\/p>\n The transit itself stretched for about seven hours, far longer than the breakneck passes of hot Jupiters that can wrap up in under an hour.<\/p>\n Around that, Webb sat on the star for close to 20 straight hours, building a clean baseline of what its light looked like with the planet out of the way. <\/p>\n Comparing those two records revealed exactly which colors the planet quietly absorbed.<\/p>\n Aaron Bello-Arufe, a postdoctoral researcher at NASA\u2019s Jet Propulsion Laboratory (JPL<\/a>), was first author of the study.<\/p>\n What the spectrum showed<\/p>\n Once the two records were aligned, one absorption pattern jumped out: methane<\/a>. <\/p>\n The atmosphere had soaked up the exact wavelengths that gas is known to absorb \u2013 a fingerprint no other common molecule mimics at those colors.<\/p>\n \u201cWhen we compared the spectra during the transit to the baseline, we saw that the atmosphere blocked the wavelengths of starlight absorbed by methane,\u201d Bello-Arufe said. <\/p>\n Models had long predicted as much for temperate gas giants.<\/p>\n Until this study, no one had observed that match directly in a planet of this kind. The confirmation gives modelers a real-world test in a temperature range they\u2019d never sampled.<\/p>\n Chemical clues beyond methane<\/p>\n The data also showed a smaller, less certain feature in a different part of the spectrum.<\/p>\n The team\u2019s models suggest it may indicate ammonia or possibly another nitrogen-bearing compound.<\/p>\n Both molecules interest planetary chemists, since their relative concentrations could reveal how much circulation occurs between the deep interior and the cooler upper atmosphere. <\/p>\n Pinning down that connection will take additional data. Faint hints of carbon dioxide appeared too.<\/p>\n None of these secondary signals carries the same weight as the methane detection, and sorting them out will take more observing time on Webb.<\/p>\n An echo of home<\/p>\n What turned up around TOI-199b looks distantly related to the atmospheres of Saturn and Jupiter, where methane and ammonia are familiar players.<\/p>\n Methane has also been spotted by Webb in a smaller, cooler world called K2-18b, in an earlier study<\/a> that drew wide attention because that planet sits in its star\u2019s habitable zone.<\/p>\n That pattern suggests methane shows up reliably in the atmospheres of temperate worlds with light, hydrogen-rich air.<\/p>\n This is something the field had suspected for years but couldn\u2019t confirm without instruments this sensitive.<\/p>\n Further observations are needed<\/p>\n The observation wasn\u2019t perfect. A pointing misalignment reduced the precision of the spectrum below what the team had originally planned. <\/p>\n Even so, the methane signal came through strongly. <\/p>\n Researchers also tested whether the atmosphere might contain hazes \u2013 suspended particles that could blur some absorption features \u2013 using models based on the chemistry of Saturn\u2019s moon Titan.<\/p>\n The preference for haze models over a clear atmosphere was weak. Whether TOI-199b actually has clouds or hazes will take a longer look to settle.<\/p>\n Future research directions <\/p>\n For decades, the chemistry of giant planets<\/a> that aren\u2019t blazing hot or solar-system-cold has been a blank space on the map. <\/p>\n Hu\u2019s team has filled in a single data point in that gap.<\/p>\n Methane sits in the atmosphere of a temperate gas giant in roughly the amounts the models predicted. <\/p>\n That tells researchers their underlying chemistry holds up when tested against a real planet in this temperature range.<\/p>\n With more Webb time, the team can pin down relative amounts of methane, ammonia, and carbon dioxide on TOI-199b and compare them against other temperate giants. <\/p>\n A sharper picture of how Earth\u2019s own atmosphere fits into the broader story of planetary chemistry should follow.<\/p>\n The study is published in The Astronomical Journal<\/a>.<\/p>\n Image Credit: NASA\/JPL-Caltech<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Most exoplanet atmospheres NASA\u2019s James Webb Space Telescope has analyzed belong to worlds where iron evaporates. 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