April 15, 2016
Kepler-186 is one of the most interesting planetary systems. There, for the first time, an actually Earth-sized planet was found within the habitable zone of a star .
The star Kepler-186 is 490 light years away from Earth. It is a Red Dwarfs at the upper edge of the mass scale for this spectral type. It has about half the solar mass and half the luminosity of the Sun.
Four planets have been found on very narrow orbits: Kepler-186b with 1.07 Earth radii and one orbit within 3.8 days in 0.034 astronomical units Distance from the star, Kepler-186c (1.25 Earth radii, 7.27 days / orbit) and Kepler-186e (1.27 Earth radii, 22.4 days / orbit, 0.11 astronomical units). For planets crowd together in less then one third of the orbital distance of Mercury!
But the most interesting object is Kepler-186f. With only 1.11 Earth radii it is slightly larger than Earth. On its orbit at a distance of 0.43 astronomical units, it is near the outer edge of the habitable zone of its small star. At this distance, it receives about 32% of Earth's solar radiation - less than the planet Mars. Kepler-186f is thus a very cold world. If it were of the same composition and density as Earth, its mass would be 1.44 earth masses. Other compositions are also conceivable - 0.32 Earth mass for a pure body of water or 3.77 Earth masses if it consisted of pure iron. The actual situation should be somewhere in between.
For the scenario of a terrestrial world, Kepler-186f would be mostly icy. Huge ice caps and glaciers would cover the continents and bind most of the water supply. However, it is as well possible that the planet did not accumulate so much gases and water, due to the prolonged active phase known for Red Dwarf stars. In such a case, the surface of the planet would be similar to Mars - a cold and largely dusty desert.
Artistic view of Kepler-186f. Large ice caps in both hemispheres dominate the climate in this cold world. Most of the ice-free area is desert. There are lakes and small seas with very salty water. In the center of the image dust storms cover the sky.
This is only one of several possible representations which would be compatible with current data.
1. Quintana, E. V., Barclay, T., Raymond, S. N., Rowe, J. F., Bolmont, E., Caldwell, D. A., ... & Lissauer, J. J. (2014). An Earth-sized planet in the habitable zone of a cool star. Science, 344(6181), 277-280.
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