more on planetary atmospheres and habitability

Over the last two decades, more than 1000 planets outside our Solar System (exoplanets) have been confirmed and several thousands are classified as candidates. One of the current research questions in the field of exoplanets is how different orbital and planetary parameters affect the habitability of a planet, i.e. its ability to host life. Commonly, a planet is considered habitable if its surface conditions allow for the existence of liquid water. The estimation of habitability is then usually provided as an estimation of the habitable zone (HZ), i.e. the range of distances from the host star that would allow for habitable conditions. Consequently, two different processes define the inner and outer boundaries of the HZ: at the inner boundary, the run-away greenhouse effect leads to a complete evaporation of water at the surface; at the outer boundary, a completely frozen surface limits the habitability of a planet.

Climate models can provide insights to the investigation of exoplanets and their habitability. Large seasonal variability might indeed be a common feature of extra-solar planets as detected planets exhibit a large diversity of orbital eccentricities. Highly eccentric orbits feature dramatic intra- annual variability: while for e=0.2 a planet at periastron receives about twice the amount of energy at apoastron, this factor increases to around 9 for e=0.5 (Dressing et al., 2010). Also the polar obliquity of a planet θ plays an important role in determining seasonal variability. In particular the combination of a high value of e and a high value of θ can lead to extreme seasonal variability (Fig.1).

Fig. 1. Annual cycle of zonal mean incoming stellar radiation (contours, W m 2) and surface temperature (shading, K) as in Fig. 4 but showing the climates of planets on an eccentric orbit with e1⁄40.5; warm climate states at S=0.8 and (a) θ=0 deg, (b) θ=60 deg, (c) θ=90 deg; and snowball states at S=0.4 and (d) θ=0 deg, (e) θ=60 deg, (f) θ = 90 deg. From Linsenmeier, Pascale et al. (2015)

Using a GCM of intermediate complexity, we explored the atmospheric circulations and climates of idealised Earth-like planets for different astronomical parameters and examine the implications for habitability. Simulations with high obliquity and eccentricity show extremely variable climates, with almost half of the planet's surface experiencing freezing and complete melting in one year (Fig. 2). Surface temperatures show considerable oscillations with a maximal amplitude of the annual cycle of about 100 K at the equator. Apart from these unhabitable regions, however, high latitudes around the South Pole show rather moderate climatic conditions with a temperature oscillation of only about 20K.

Fig. 2. Seasonal cycle of surface temperature (K; colours, left), irradiation S (W/m2; lines, left), and sea ice thickness (hi/hi;max ; right) of the experiment with e=0.5, θ=90 deg at S=956 W /m2 ); note that the temporal evolution of S differs among the two hemispheres due to the eccentric orbit, leading to an asymmetric distribution of surface temperature and ice-thickness.

Seasonal variability can lead to a substantial extension of the habitable zone. While a planet with e=0 and θ=0 deg is habitable for S*>0.95 (warm initial state) or S*>1/3 (cold initial state), a planet with e=0.5 and θ=90 deg is habitable for S*=0.55 (Fig. 11). Moreover, also cold climate states can exhibit regions that are temporally and even continuously ice free. While the extent of the warm state is an appropriate measure of the HZ on planets without seasonal variations, it leads to a substantial underestimation of the HZ on planets with large seasonal variations.

Fig. 3. Degree of habitability for simulations with e=0 (left) and e=0.5 (right), and θ=0, 60, 90 (from top to bottom); the colour denotes the fraction of the planet's surface that is (partially) habitable (dark blue), temporally habitable (light blue) and unhabitable (white); the red vertical line marks the point of transition between the warm and cold state; the solid green vertical line shows the solar irradiation at 1 AU as reference.

References

Linsenmeier M., S. Pascale, V. Lucarini (2015), Climate of Earth-like planets with high obliquity and eccentric orbits: implications for habitability conditions, Planetary and Space Science, 105, 43-59