The habitable zone (HZ) is a region around a star where it is neither too hot nor too cold for a planet to sustain liquid water on its surface. It is marked by an inner edge (before it starts to get too hot) and an outer edge (before it starts to get too cold). For a given star, many factors influence where the inner and outer edges of the HZ may lie. When evaluating the habitability of a planet, the presence of water clouds can have a huge influence. Water clouds can either reflect incoming stellar radiation back to space (cooling the planet) or absorb and re-radiate thermal emission from the planet’s surface (warming the planet). In most cases, reflection of stellar radiation (cooling) dominates over absorption of planetary infrared radiation (warming), resulting in net cooling.
Figure 1: Artist’s impression of a habitable planet. Image credit: Fernando Rodrigues.
A study by Yang et al. (2013) used 3D general circulation models (GCMs) to understand the impact of water clouds on planets around red dwarf stars. In particular, the study focused on planets near the inner edge of the HZ. Red dwarf stars are by far the most abundant type of star in the galaxy, constituting ~75 percent of all stars. They are cooler, much less luminous and have smaller masses than Sun-like stars. Current estimates also suggest that there is on average ~1 Earth-size planet in the HZ around every red dwarf star.
Due to their low luminosities, the HZ around red dwarf stars is much closer-in compared to the HZ around Sun-like stars. This means planets in the HZ of red dwarf stars are subjected to stronger tidal forces. If these planets are in circular orbits, they are expected to be tidally-locked, with one side experiencing perpetual day and the other side experiencing perpetual night. The results by Yang et al. (2013) show that the presence of water clouds on tidally-locked planets can drive a stabilizing cloud feedback mechanism, allowing such planets to be habitable even at twice the amount of stellar flux previously thought to mark the inner edge of the HZ. It shows that the inner edge of the HZ can lie much closer-in to the star than previously thought. Consequently, planets that were once considered too hot for habitability can actually have clement surface conditions suitable for life.
On a tidally-locked planet, the substellar point is a spot on the planet’s surface where the planet’s host star is “directly overhead”, and it is where insolation is expected to be highest. Around the substellar point, near-surface convergence of air masses and the resulting convection can cause most of the planet’s dayside to be covered by water clouds. High-level and low-level water clouds can cover ~60 percent and ~80 percent of the dayside, respectively. These water clouds significantly increase the planet’s albedo (i.e. the planet’s reflectivity), allowing the planet to more efficiently reflect the incoming flux from its host star.
Figure 2: Cloud behaviour for a tidally-locked planet (left) and a non-tidally-locked planet (right). High-level cloud fraction (top) and low-level cloud fraction (bottom) are displayed in each case. The non-tidally locked case is in a 6:1 spin-orbit resonance (i.e. the planets makes 6 rotations per orbit). The stellar flux is 1400 W/m^2. The black dots in (a) and (c) denote the substellar point. Source: Yang et al. (2013).
At a higher stellar flux, like for a tidally-locked planet that is closer-in to its host star, the convection around the planet’s substellar point is stronger, producing greater water cloud coverage and further increases the planet’s albedo. The cooling effect associated with the higher reflectivity leads to much lower temperatures on the planet than if water clouds were not present, thereby creating a stabilizing cloud feedback mechanism where the planet’s albedo increases with stellar flux. The inner edge of the HZ around a red dwarf star is generally thought to be the distance from the star where the insolation reaches ~1200 W/m^2. However, with stabilizing cloud feedback, the inner edge of the HZ can shift much closer-in to the star. Even with an insolation of 2200 W/m^2, a tidally-locked planet around a red dwarf star can still have surface temperatures cool enough for the planet to remain habitable.
Figure 3: Climates of tidally locked (1:1) and non-tidally locked (2:1 and 6:1) terrestrial planets - (a) global-mean surface temperature (K), (b) stratospheric H2O volume mixing ratio at the substellar point, (c) planetary albedo and (d) global-mean greenhouse effect (K). 1:1 denotes a tidally-locked state, and 2:1 and 6:1 denote 2 or 6 rotations per orbit, respectively. The stellar spectrum is for an M-star (i.e. red dwarf star) or a K-star (i.e. stars that cover the ranged between red dwarf stars and Sun-like stars). Results for HD85512 b, a super-earth-size planet orbiting a K-star, are represented by a blue pentagram. The gray area denotes the HZ around an M-star with an inner edge of ~1200 W/m^2 and an outer edge of ~270 W/m^2 (not shown). Source: Yang et al. (2013).
The presence of substellar water clouds, indicative of a stabilizing cloud feedback mechanism, can significantly modify the thermal phase curves of tidally-locked planets. Basically, a planet’s thermal phase curve is the change in thermal radiation given off by the planet when different parts of the planet come into view as the planet circles its host star. Thermal phase curve features corresponding to the presence of a stabilizing cloud feedback mechanism would be detectable by the James Webb Space Telescope (JWST) in the near future, especially for super-Earth-size planets orbiting the nearest red dwarf stars. By allowing the inner edge of the HZ to be closer-in to the host star, the stabilizing cloud feedback mechanism enlarges the HZ around red dwarf stars. In turn, this can potentially increase the frequency of habitable, tidally-locked Earth-size planets around red dwarf stars by 50 to 100 percent.
Figure 4: Thermal phase curves of tidally-locked planets for different atmospheres with stellar flux fixed at 1200 W/m^2 - airless, dry-air, water vapour and water vapour plus clouds. Source: Yang et al. (2013).
Figure 5: Thermal phase curves of tidally-locked planets for a full atmosphere including water vapour and clouds for different stellar fluxes: 1400, 1600, 2000 and 2200 W/m^2. Source: Yang et al. (2013).
Recently, the first Earth-sized planet in the HZ of another star was discovered in the Kepler-186 planetary system. This planet, dubbed Kepler-186f, is the 5th planet in the system which consists of a total of 5 known planets circling a red dwarf star. Much of the attention goes to Kepler-186f, since the 4 inner planets are too close to the star and do not lie in the HZ. These 4 inner planets are also expected to be tidally-locked. Nevertheless, with the stabilizing cloud feedback mechanism proposed by Yang et al. (2013), the 4th planet, dubbed Kepler-186e, might be marginally habitable if it has a significant water cloud cover on its dayside.
- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]
- Bolmont et al. (2014), “Formation, tidal evolution and habitability of the Kepler-186 system”, arXiv:1404.4368 [astro-ph.EP]