Monday, April 21, 2014

Habitability of Kepler-186f

The Kepler-186 system consists of 5 known planets circling a red dwarf star. The 5 planets have sizes ranging from 1.0 - 1.5 Earth-radius and orbital periods of 3.9 - 130 days. All 5 planets are probably rocky, since planets with large hydrogen-helium gas envelops tend to be larger than 1.5 - 2.0 Earth-radius. Of particular interest is Kepler-186f, the fifth planet in the system. Kepler-186f is the first confirmed Earth-sized planet in the habitable zone around another star. Its detection was reported by Quintana et al. (2014) in a paper published in the April 18 issue of the journal Science. Another paper by Bolmont et al. (2014) evaluates the habitability of the Kepler-186 system and, in particular, the habitability of Kepler-186f. Additionally, the paper also investigates the formation and tidal evolution of the Kepler-186 planetary system.

Figure 1: Artist’s impression of a habitable planet.

Figure 2: Orbital configuration of the Kepler-186 planetary system. The shaded regions denote the habitable zone. The bottom part of the plot shows a comparison between 4 different planetary systems that contain planets in the habitable zone: the Solar System, Kepler-62, Kepler-186 and GJ 581. Source: Bolmont et al. (2014).

Being situated in the habitable zone does not necessarily imply than Kepler-186f is habitable. Habitability also depends on the planet’s atmospheric characteristics. In the study, simple climate models are used to assess the habitability of Kepler-186f. The model atmospheres are assumed to be composed of carbon dioxide (CO2), nitrogen (N2) and water (H2O) only. A key criterion for habitability is the ability for a planet to sustain liquid water on its surface. In the case for Kepler-186f, to keep mean surface temperatures above 273 K - the freezing point of water, the models show that modest amount of CO2 are needed in most of the cases. For large amounts of atmospheric N2 (~10 bars), 200 - 500 mbar of CO2 is all that is required to keep surface temperatures above 273 K.

Figure 3: Surface temperature as a function of CO2 partial pressure, for different N2 partial pressures. Water triple point temperature of 273 K is indicated by the horizontal dashed line. Top to bottom rows: decreasing insolation - 0.32, 0.29 and 0.27 (Earth = 1.0). Left to right columns: increasing gravity. Source: Bolmont et al. (2014).

Given such favourable prospects for habitability, it is worth considering the possibility of photosynthesis occurring on Kepler-186f. The amount of insolation Kepler-186f receives from its host star is estimated to be 32 percent the intensity of insolation Earth receives from the Sun. An atmosphere containing 5 bar of CO2 and 1 bar of N2 would support a surface temperature of 285 K on Kepler-186f, close to Earth’s current mean surface temperature. Due to atmospheric absorption, such an atmosphere would further depress the amount of insolation that reaches the surface of Kepler-186f to a factor of 7 times less insolation than what the Earth’s surface gets. At wavelengths of 500 - 700 nm, corresponding to plant chlorophyll, the difference becomes even larger with Earth getting 10 - 20 times more flux than Kepler-186f. Although such a low level of insolation does not preclude photosynthesis, it does suggest that photosynthesis on Kepler-186f would occur at a much slower rate than on Earth.

Figure 4: Net stellar insolation received at the top of atmosphere (TOA) and at the surface for Kepler-186f, assuming an atmosphere containing 5 bar of CO2 and 1 bar of N2. This is shown in comparison to modern Earth. Source: Bolmont et al. (2014).

- Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science 18 April 2014: Vol. 344 no. 6181 pp. 277-280.
- Bolmont et al. (2014), “Formation, tidal evolution and habitability of the Kepler-186 system”, arXiv:1404.4368 [astro-ph.EP]