This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, July 23, 2015

Kepler spots potentially habitable planet like our Earth

Temperature of the Earth under present
conditions with a solar flux of 341 W/m2,
and just before the runaway greenhouse is triggered
for a mean solar flux of 375 W/m2.
From the Leconte et al. article referenced in text.
NASA has just released an announcement this morning that the Kepler spacecraft has spotted a planet about 60% bigger than our Earth in a habitable zone of a star "similar to our sun."  The planet is dubbed Kepler-452b, and it's about 1400 light years from Earth in the constellation Cygnus. It's gravity would be about twice that of the Earth's. The star around which it orbits is a G2-type star like our sun, has the same temperature, is 20% brighter, and has a diameter about 10% larger.
Cover of Science in 2014
     Aside: Todays announcement is  a bit confusing because a year ago the Kepler team had the cover photo and a report in Science (v. 344, no. 6181, pp. 277-280, 2014) titled "An Earth-Sized Planet in the Habitable Zone of a Cool Star." This star has a radius of about 1/2 that of our sun, but the planet, Kepler 186f, is in the habitable zone and could support liquid water if it has an earth-like atmosphere and water at the surface. It appears that the difference between these two announcements is that the star around which Kepler-452b orbits is more similar to our sun than the Kepler 186f star.

      In 2013, Sid Perkins wrote a piece in Nature (December 11, 2013) discussing the habitable zone and summarizing the work of Jeremy Leconte at the Pierre Simon Laplace Institute in Paris (Leconte et al., Nature, 504, 268, 2013).  Leconte ran the first fully three dimensional model of hot, very moist planetary atmospheres (and thus the work only applies to planets that have abundant water like the Earth; there is no evidence yet whether the newly discovered planet in Cygnus has water). Previous models had been one-dimensional and considered only how the atmospheric conditions changed in the vertical direction, ignoring horizontal transport effects, whereas this model can take account of the Hadley circulation. Leconte et al. conclude that the runaway greenhouse will take effect at a mean solar insolation of about 375 W/m2. In this model, warming of the planet causes the formation of cirrus clouds at high altitudes. Such clouds trap heat, and the heating leads to more evaporation, which leads to more clouds and thus the feedback to a greenhouse effect. The model also suggests that the large-scale circulation (not possible in 1-D models) creates cloud-free areas in the mid-latitudes that allow heat to radiate back to space. The conclusion is that the inner edge of the Solar System's habitable zone is about 142 million kilometers from the sun.  Earth is at 149,600,000 kilometers so we are close to the inner edge of the habitable zone. Other authors, however, have concluded that the inner boundary could be considerably closer especially for planets that have much less water to feed the greenhouse effect (Petigura, et. al., Proc. Natl. Acad. Sci. USA 110, 19723, 2013).
     Wiki has a good summary of habitable zone thermodynamics here.

No comments: