Clouds and the Search for Life

Today we are going to talk about clouds—not the clouds above us, but far, far from us—clouds on exoplanets. Also known as an extrasolar planet, an exoplanet is a planet outside of our own solar system, orbiting a host star that is not our sun. There are thousands of known exoplanets, and thousands more telltale signatures in data that are waiting to be confirmed as exoplanets. These planets range in characteristics, much like planets in our own solar system; some are small and rocky, some are gas giants like Jupiter, and others are more like Neptune. Some exoplanets have no analog in our solar system. Many exoplanets have atmospheres, and therefore, clouds.
Some planets may also be in the habitable zone (HZ). The habitable zone is also occasionally called the Goldilocks Zone because it is the region around a star that is ‘juuuuust right’ for liquid water to exist on the planet’s surface. And liquid water is really interesting, because life as we know it needs water—so it is a logical place for beginning to look for extraterrestrial life.
The HZ is not as simple as ‘far enough from the host star to keep from being too warm but close enough to not be too cold’. There are other factors. For example, what kind of star is the host star? That will determine how hot the star is, and how much light and heat it is giving off. What is the planet’s albedo? Albedo, also known as reflectivity, is a measure of how much radiation (in this case, usually heat energy from the sun) a planet absorbs or reflects back into space. And does the planet have an atmosphere that could create a greenhouse effect?
Does that atmosphere have many clouds?
Today’s paper was published in 2013 in the Astrophysical Journal Letters . “Stabilizing cloud feedback dramatically expands the habitable zone of tidally locked planets”, written by Jun Yang, Nicolas Cowan, and Dorian S. Abbot, looks at that last question with respect to tidally locked planets. Tidal locking occurs under certain circumstances, where the effect of gravity slows a body so that the time it takes to spin around on its axis is the same time it takes to orbit. The Earth is not at all tidally locked; it rotates once in 24 hours but takes 365 days or orbit the sun. The moon is tidally locked to Earth: that is why we only see one side of it. The moon rotates once in the same time it takes to orbit the Earth once. With an exoplanet, the planet would rotate once in the course of one year; one side always faces its host star, in constant day, while the other faces away in constant night. Tidal locking especially occurs with exoplanets orbiting low mass stars in circular orbits.
Water clouds can either scatter radiation (cooling the planet) or absorb the radiation (warming the planet), depending on a variety of factors, such as the amount of clouds and how intense of a greenhouse effect they produce.
Clouds on Earth, with the Earth as well. An image credited to the NASA Earth Observatory, found on the NASA Visible Earth site.
Clouds on Earth, with the Earth as well. An image credited to the NASA Earth Observatory, found on the NASA Visible Earth site.
Most models created to look at the inner edge of the HZ have been one-dimensional models. One-dimensional models cannot look at the effects of clouds. In order to see the effect of clouds, you need three-dimensional models that can deal with the locations, heights, and coverage of the clouds. These are often called general circulation models, or GCMs.
Today’s paper used GCMs to look at the effect of clouds on tidally locked planets around M dwarf stars, or sometimes K class stars. Stars have a variety of classifications depending on their characteristics. M and K type stars are small (for stars) and have low temperatures in comparison to other stars (note: our sun is an entirely different type of star, known as G class). It appears that there averages about one planet in the habitable zone for every M class star, and these exoplanets are relatively easy to find. The authors worked with a variety of changing conditions on three idealized planetary surfaces: a world covered in a global ocean, a planet with one ridge interrupting the flow of the ocean, and a planet with two ridges interrupting the flow of the ocean.
In tidally locked planets, they found that the side facing the host star is mostly clouds, generally sixty to eighty percent. Thick clouds increase the albedo of the planet, which makes it cooler. The albedo actually went up as they modeled planets closer to their M or K class host star. The surface temperatures were in the right range for liquid water. Tidally locked planets also had smaller greenhouse effects, which combined with the high albedo, placed them in the habitable zone when they would otherwise be too close and therefore too hot for liquid water.
On the other hand, planets that were not tidally locked had lower albedos, closer to that of Earth’s (about 0.3), and they had less clouds. Albedo decreased as the heat and light from the host star increased.
As an example of the effect of clouds, the authors ran a simulation with and without clouds, but otherwise identical. With clouds, the modeled exoplanet had an albedo of 0.53 and an overall average surface temperature of 246 Kelvin (-27 C, or about -17 F). Without clouds, the albedo plummeted to 0.04 and the temperature rose to 319 K (about 46 C, or around 115 F). So yes, clouds make a very big difference.
The sensitivity of these results were tested along a variety of parameters, such as the cloud particle size, the amount of clouds, the surface pressure, the radius and gravity of the exoplanet, and more. Changing the amount of carbon dioxide, or CO2, in the atmosphere had little effect. The overall albedo of the planet was not sensitive to any of the parameters tested. The albedo from the clouds did change in some circumstances. If the clouds droplets were very large, albedo decreased. Particles of this size are not impossible, but are also not very likely. The cloud albedo also shifted for hypothetical exoplanets that were very large, but orbited their host star very quickly. Lastly, if heat was moved by an ocean to a large extent, then the contrast in the day and night sides was less drastic, making for less circulation in the atmosphere and less clouds.
Based off the results of these models, the habitable zone for tidally locked exoplanets is much greater than we had thought—it seems they can handle twice the heat and light that one dimensional models predicted. This means that some known exoplanets thought to be outside the habitable zone actually could have liquid water on the surface, and that the number and frequency of habitable planets is greater than expected. The authors note that the results can be further tested once the James Webb Space Telescope is running; its launch is currently scheduled for October of 2018.

 

Bibliography
Sources used to directly write this paper.
Bennett, Jeffrey et al. The Cosmic Perspective: The Solar System. 7th ed. San Francisco: Addison-Wesley. Print.
An astronomy textbook, used to refresh my memory about albedo.
Jet Propulsion Labratory. “PlanetQuest : The Search for Another Earth.” N.p., n.d. Web. http://planetquest.jpl.nasa.gov/
A fun website about the search for another Earth, which gave me the counts of confirmed exoplanets and exoplanet candidates. The site has several interactives. A more detailed breakdown of the exoplanets can be found along the right hand side of PlanetQuest’s interactive New Worlds Atlas.
National Aeronautics and Space Administration. “The Blue Marble: Land Surface, Ocean Color, Sea Ice and Clouds.” Visible Earth. NASA Visible Earth. 8 Feb. 2002. Web.
The source for the image used in this post.
National Aeronautics and Space Administration. “James Webb Space Telescope.” N.p., n.d. Web.. http://www.jwst.nasa.gov/
The homepage for the site dedicated to the James Webb Space Telescope. I used an answer under the FAQ section for the scheduled launch.
University of Nebraska-Lincoln. “Spectral Classification.” Spectral Classification of Stars. N.p., n.d. Web. http://astro.unl.edu/naap/hr/hr_background1.html
A resource I found on a page of my old astronomy notes, this page gives an explanation of stellar types. It has a few interactive features.
Yang, Jun, Nicolas B. Cowan, and Dorian S. Abbot. “Stabilizing cloud feedback dramatically expands the habitable zone of tidally locked planets.” The Astrophysical Journal Letters 771.2 (2013): L45.
Today’s main paper.
Further Links
Massachusetts Institute of Technology. “Cloudy, with a wisp of liquid rock: Clouds around exoplanets analyzed.” ScienceDaily. ScienceDaily, 3 March 2015. <www.sciencedaily.com/releases/2015/03/150303111734.htm>.
An incredibly relevant news article discussing the detection of clouds and atmospheres on exoplanets.
National Aeronautics and Space Administration. “Kepler.” N.p., n.d. Web. http://kepler.nasa.gov/
The website for Kepler, a telescope known for finding exoplanets.
National Aeronautics and Space Administration. “Eyes on Exoplanets.” N.p, n.d. Web. http://eyes.nasa.gov/exoplanets/index.html 
This is a really interesting interactive program that lets you explore the known exoplanets. Requires a one-time download.
SETI Institue. “SETI Institute.” N.p., n.d. Web.  http://www.seti.org/
SETI, the Search for Extraterrestrial Intelligence (more officially the SETI Institute), is an organization searching for signs of life on other planets.
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