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.
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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.
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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.
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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.
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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).
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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.
References:
- 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]