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Simulating ‘haze’ may narrow search for alien life

Analyzing the murky haziness of simulated atmospheres whipped up in a lab is an important step toward using the James Webb Space Telescope to look for signs of life on planets far from our own, report researchers.

The simulations will help establish models of atmospheres that might exist on distant worlds orbiting stars in other solar systems, says Sarah Hörst, assistant professor of earth and planetary sciences at Johns Hopkins University.

“One of the reasons why we’re starting to do this work is to understand if having a haze layer on these planets would make them more or less habitable,” says Horst, lead author of the paper in Nature Astronomy.

Checking ‘fingerprints’

Planetary scientists and astronomers use today’s telescopes to learn what gases make up the atmospheres of exoplanets. “Each gas has a fingerprint that’s unique to it,” Hörst says. “If you measure a large enough spectral range, you can look at how all the fingerprints are superimposed on top of each other.”

Current telescopes, however, do not work as well with every type of exoplanet, falling short with those with hazy atmospheres. Haze consists of solid particles suspended in gas, altering the way light interacts with the gas. This muting of spectral fingerprints makes measuring gas composition more challenging.

“Part of what we’re trying to help people figure out is basically where you would want to look…”

Scientists have speculated that a primitive haze layer, akin to the ozone layer that now protects Earth from harmful radiation, may have shielded life here in the very beginning. This could be meaningful in the search for extraterrestrial life. Hörst believes her lab simulations can help exoplanet scientists determine which types of atmospheres are likely to be hazy.

Exoplanets are predominantly larger than Earth and smaller than Neptune. As this class of planets is not found in our solar system, our limited knowledge makes them more difficult to study.

With the launch of the Webb telescope expected next year, scientists hope to be able to examine the atmospheres of exoplanets in greater detail. The new infrared telescope will be capable of looking back even further in time than Hubble, with a light-collecting area around 6.25 times greater. Orbiting around the sun a million miles from Earth, the telescope will help researchers measure the composition of extrasolar planet atmospheres and even search for the building blocks of life.

“Part of what we’re trying to help people figure out is basically where you would want to look,” Hörst says.

Miniature atmospheres

Using computer models, Hörst’s team put together a series of atmospheric compositions that model super-Earths or mini-Neptunes. By varying levels of three dominant gases (carbon dioxide, hydrogen, and gaseous water), four other gases (helium, carbon monoxide, methane, and nitrogen) and three sets of temperatures, they assembled nine different “planets.”

The scientists then actually created the proposed atmospheres, mixing gases in a chamber and heating them. Over three days, the heated mixture flowed through a plasma discharge, a setup that initiated chemical reactions within the chamber.

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“The energy breaks up the gas molecules that we start with. They react with each other and make new things, and sometimes they’ll make a solid particle [forming haze] and sometimes they won’t,” Hörst says.

“The fundamental question for this paper was: Which of these gas mixtures—which of these atmospheres—will we expect to be hazy?” says Hörst.

All nine variants made at least some haze, the researchers found. The surprise lay in which combinations made more. The team found the most haze particles in two of the water-dominant atmospheres.

“We had this idea for a long time that methane chemistry was the one true path to make a haze, and we know that’s not true now,” says Hörst, referring to compounds abundant in both hydrogen and carbon.

For Hörst’s group, next steps involve analyzing the different hazes to see how the color and size of the particles affect how the particles interact with light. They also plan to try other compositions, temperatures, energy sources, and examine the composition of the haze produced.

“The production rates were the very, very first step of what’s going to be a long process in trying to figure out which atmospheres are hazy and what the impact of the haze particles is,” Hörst says.

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Additional coauthors of the study are from Johns Hopkins; the Space Telescope Science Institute; Grinnell College; the NASA Ames Research Center; Harvard University; the Space Science Institute; and Université Grenoble Alpes.

NASA and the Morton K. and Jane Blaustein Foundation supported the study.

Source: Sukanya Charuchandra for Johns Hopkins University

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