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H2O on exoplanet cloud tops could be found with new hi-res spectroscopy

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Sep 23, 2020, 9:25 PM EDT

With last week's stunning announcement of the life-indicating chemical compound phosphine discovered in the atmosphere of our neighboring planet of Venus, the scientific world is abuzz with questions about what other organic substances might be lurking in the clouds of remote exoplanets.

Now a new research paper delivered by astronomers at the University of Warwick and published in the online journal Monthly Notices of the Royal Astronomical Society has revealed that water vapor could possibly be detected in the upper atmospheres of exoplanets by peeking over the tops of their dense cloud cover using an array of high-tech, ground-situated instruments.

The methods by which this mission might be accomplished include employing next-generation, high-resolution spectroscopy fully capable of detecting H2O in these otherworldly clouds that are too thick for starlight to pass through. One of the prior obstacles in identifying atmospheric compositions of thick clouds on cooler planets using modern space telescopes is that their particular makeup can obscure and distort signals.

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However, as technology improves in the current decade, ground-based observatories are strong enough to peer straight into the wispy layer of atmosphere located at the upper reaches of exoplanet clouds. This new technique will produce fresh insights into planetary formation and can be harnessed in future exploratory endeavors to detect biomarkers for potential life signs on clouded-over planets.

While scientists have been successful in determining the atmospheres of numerous bigger and warmer "hot Jupiter" exoplanets that closely orbit their host stars, more diminutive exoplanets are currently being located at much cooler temperatures. These smaller, Neptune-sized planets are notorious for having thicker clouds that hinder investigations into their elemental makeup.

“We have been investigating whether ground-based high resolution spectroscopy can help us to constrain the altitude in the atmosphere where we have clouds, and constrain chemical abundances despite those clouds," explained lead study author Dr. Siddharth Gandhi of the Department of Physics at the University of Warwick. “What we are seeing is that a lot of these planets have got water vapour on them, and we’re starting to see other chemicals as well, but the clouds are preventing us from seeing these molecules clearly. We need a way to detect these species and high resolution spectroscopy is a potential way of doing that, even if there is a cloudy atmosphere.

“The chemical abundances can tell you quite a lot about how the planet may have formed because it leaves its chemical fingerprint on the molecules in the atmosphere. Because these are gas giants, detecting the molecules at the top of the atmosphere also offers a window into the internal structure as the gases mix with the deeper layers.”

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Currently, astronomers count on light emitted from an exoplanet’s home star to obtain data concerning its atmospheric composition. As the world transits in front of the star, scientists observe the transmission of stellar light as it penetrates the planet's upper atmosphere and shifts its spectrum.

This spectral analysis examines wavelengths that have spectral indicators for specific chemicals like water vapor, methane, and ammonia, which are only found in trace quantities amid hydrogen and helium rich planets.

But dense clouds block light from stabbing through the atmosphere, unfortunately leaving researchers empty-handed with a blank spectrum.

As most exoplanet observations have been enacted utilizing space-based telescopes like Spitzer or the Hubble, their resolutions are far too low to detect measurable signals from atop atmospheric clouds. High-resolution spectroscopy is the answer, with the significant advantage of it being capable of probing a wider range of altitudes.

“Quite a lot of these cooler planets are far too cloudy to get any meaningful constraints with the current generation of space telescopes," adds Dr. Gandhi. "Presumably as we find more and more planets there’s going to be more cloudy planets, so it’s becoming really important to detect what’s on them. Ground based high resolution spectroscopy as well as the next generation of space telescopes will be able to detect these trace species on cloudy planets, offering exciting potential for biosignatures in the future.”