Simons Foundation Postdoctoral Fellow Alexandria Johnson is studying exoplanet clouds, not through a telescope, rather under a microscope in the Cziczo Lab.
Humans have long wondered whether life exists elsewhere in the universe. We are fortunate to live in an age where we not only know that there are planets orbiting other stars but where thousands of extra solar planets, or exoplanets, have been discovered. The identification and characterization of these planets gives context to Earth and the potential for analogs orbiting other stars. We have also learned that planet formation results in a great diversity of bodies, often creating exotic worlds with extreme physical conditions. To date the most well studied exoplanets are bright, tidally locked Hot Jupiters orbiting close to their host star, but the Kepler Mission has revealed that at least one in five sunlike stars has an Earth-like planet in the habitable zone, and most recently scientists have begun widening the range of exoplanets they can observe to include those circling ultracool dwarf stars. With such a high frequency of potentially habitable planets, we are bound to ask how many can sustain life as we know it and if we can detect the presence of such life from afar.
Life metabolizes and generates byproducts, and some of these metabolic byproducts dissipate into the atmosphere and can accumulate as biosignature gases. These represent our best opportunity for the detection of life on exoplanets by remote sensing techniques. However, if we are to observe biosignatures on other planets, we need to better understand the limitations of their detection. When planets pass in front of their host stars, or transit, a planet’s atmosphere can be probed using light from the host star. Past and current work in this area is focused on identifying the constituents of exoplanet atmospheres through spectro- scopic techniques and thermal variations with planet phase functions. Using this information the composition and physical conditions of exoplanet atmospheres can be determined, and the potential for habitable conditions and biosignature gases potentially identified. However, the spectral observations of some exoplanets have lead to a puzzling result—flat, featureless spectra when the exoplanet would otherwise be expected (through mass and radii measurements) to host an atmosphere. One such case is that of GJ1214b. The transmission spectra of this super-Earth lacks strong spectral features and scientists believe a likely cause for this is the presence of clouds. Cloud layers, like those proposed, would have the potential to greatly limit detection of biosignatures simply because they would obstruct our view of the atmospheres beneath them.
By observing the planets and moons in our solar system we’ve seen that clouds are not a uniquely terrestrial phenomenon. So it is reasonable to assume that clouds will likely be present in the atmospheres of many planets orbiting other stars.
Bridging laboratory experiments in Daniel Cziczo’s group investigating terrestrial clouds and Sara Seager’s exoplanetary atmospheric modeling on computers, EAPS Postdoctoral Fellow Alexandria Johnson is using the tools of terrestrial cloud research to investigate how exoplanet cloud particles interact with radiation from their host stars and how this might effect sampling of their atmospheres.
The projects aims are twofold: first, to better understand the role and properties of clouds in exoplanet atmospheres; second, to use this information to determine how clouds will limit our ability to detect biosignature gases.
“We believe this work is critical for recognizing biosignatures in the very different environments on exoplanets and that cutting edge laboratory research on exoplanet atmospheres and clouds is the best way to approach this problem,” Johnson says.
Using theory and lab-based experiments to determine how cloud particles interact with light across the visible spectrum and under a wide range of atmospheric composition, pressure, and temperature, Johnson’s work represents the first time that exoplanet clouds will be studied in the laboratory. How these particles scatter and modify light is key to understanding just how well biosignatures can be detected through cloud layers. The results of this work will guide the interpretation of observations of exoplanet atmospheres and biosignatures through direct imaging or transmission spectroscopy, as well as exoplanet atmospheric and radiative balance models which can give valuable information on the surface habitability of exoplanets.
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