Wednesday, June 10, 2020

The Search for Habitable Worlds

From Site Tech Daily:
In a paper published today in the journal Nature Astronomy, she and her colleagues have observed in laboratory studies that microbes can survive and thrive in atmospheres that are dominated by hydrogen — an environment that is vastly different from Earth’s nitrogen- and oxygen-rich atmosphere. 
Hydrogen is a much lighter gas than either nitrogen or oxygen, and an atmosphere rich with hydrogen would extend much farther out from a rocky planet. It could therefore be more easily spotted and studied by powerful telescopes, compared to planets with more compact, Earth-like atmospheres. 
Seager’s results show that simple forms of life might inhabit planets with hydrogen-rich atmospheres, suggesting that once next-generation telescopes such as NASA’s James Webb Space Telescope begin operation, astronomers might want to search first for hydrogen-dominated exoplanets for signs of life. 
“There’s a diversity of habitable worlds out there, and we have confirmed that Earth-based life can survive in hydrogen-rich atmospheres,” Seager says. “We should definitely add those kinds of planets to the menu of options when thinking of life on other worlds, and actually trying to find it.” (Read more.)

From Scientific American:
In recent years, astronomers have announced numerous exciting discoveries regarding red dwarfs, such as Proxima Centauri b, a potentially habitable planet orbiting our sun’s nearest star, and the TRAPPIST-1 system, which contains a whopping seven Earth-sized worlds. Red dwarfs are not only abundant but are also the longest-living stars, with a lifetime that can span 10 trillion years—1,000-fold longer than that of our sun. Should a biosphere arise on a red dwarf world, it might stick around for an exceptionally long time.
Astronomers are therefore interested to know whether or not red dwarfs’ planets are good places to go looking for living creatures. “To have life as we know it, you need to have liquid water,” says Abraham Loeb, a co-author of the Nature study and an astrophysicist at the Center for Astrophysics at Harvard University and the Smithsonian Institution (CfA). “In order to have liquid water, you need an atmosphere.”
Kreidberg, who is also at the CfA, has been in the daily habit of checking for new results from NASA’s Transiting Exoplanet Survey Satellite (TESS), a space-based observatory hunting for nearby planets that “transit” their host stars—flitting across the faces of those stellar hosts and casting shadows toward our solar system. Among TESS’s first discoveries was the rocky world LHS 3844 b, located just under 49 light-years away, and Kreidberg quickly recognized that it was in an ideal position to test the atmospheric-retention capabilities of red dwarf exoplanets.
LHS 3844 b orbits incredibly close to its parent star, zipping around in a mere 11 hours. This orbit more or less guarantees that the star’s gravitational pull has tidally locked the planet, meaning one side of the world always faces the star. The exoplanet’s dayside is scorching, while its space-facing hemisphere sits out in the cold.
But while the exoplanet experiences 70 times more radiation than Earth, Kreidberg says it would not necessarily lose its atmosphere at this distance. For instance, an envelope of thick carbon dioxide could be heavy enough to endure the bombardment from the nearby star. Or the world might have once contained a vast ocean that was boiled off by the intense starlight, which also would have split the water into its constituent molecules. The lighter hydrogen could have drifted away, leaving an atmosphere of pure oxygen. (Read more.)

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