Tag Archives: Habitable Zones

Rotation Rates and Habitable Zones

New simulations using a 3D general circulation model (GCM) have expanded the habitable zone potentially pushing some already known exoplanets inside the region where liquid water could be present on the surface of rocky planets. The group at the University of Chicago have analysed the surface temperatures of rocky planets with varying rotation rates. The rotation rate effects the atmospheric dynamics and hence large scale cloud properties of planets. The distribution, thickness and altitude of clouds greatly effects the surface temperature of rocky planets.

The group found that slowly rotating planets have a lower surface temperature than an otherwise similar planet which rotates faster. In the case of rapidly rotating planets (like the Earth), the atmospheric dynamics is banded and split in to several regions. On Earth we get large cloud cover at the inter tropical convergence zone (ITCZ) which is important for reflecting solar radiation and keeping the planet cool. As solar radiation increases (move the planet closer to the star), the equator-pole dynamics (Hadley Cell) weakens causing a reduction in cloud cover in this region and therefore causing warming of the surface.

On the other hand, slowly rotating planets tend to have ‘planet-scale’ dynamics and exhibit large scale ascent on the dayside and descent on the nightside. This large scale ascent on the dayside atmosphere produces large amount of cloud. As the stellar radiation increases, the aerial extent of this ascent region increases, therefore producing a greater cloud coverage and a negative feedback process.

The authors of this study point out an interesting application of this relationship between rotation rate and the habitable zone. The results suggest that an Earth-like planet placed around a Sun-like star with the orbital distance and planetary rotation of Venus then the planet would be habitable despite a near doubling of the insolation.

The habitable zones of known exoplanets and some Solar System planets using the 3D GCM and 1D simulations. The 1D simulations lack the ability to reproduce the cloud-albedo stabilising feedback effect and therefore show a narrower habitable zone

The habitable zones of known exoplanets and some Solar System planets using the 3D GCM and 1D simulations. The 1D simulations lack the ability to reproduce the cloud-albedo stabilising feedback effect and therefore show a narrower habitable zone. Figure from Yang et al. 2014.

This provides interesting insights to the past evolution of the climate of Venus. There is evidence  of Venus being able to support an ocean which has subsequently been lost via a runaway greenhouse state. This means that if the oceans were lost early in its history, then the rotation period of Venus must have been a few weeks, or if it happened more recently a few months. This is a much more rapid rotation than the current rotation rate of 243 days. As an alternative, the water could have been lost from the atmosphere via hydrodynamic escape, negating the need of a slowing rotation.

In summary, this work pushes the inner edge of the habitable zone closer to the star for slowly rotating planets via a cloud-albedo feedback process. However, one must be careful, as it is not just the current rotation rate but also the past rotation rates which are important.

The paper by Yang et al. is available here: http://iopscience.iop.org/2041-8205/787/1/L2/pdf/2041-8205_787_1_L2.pdf


An Interesting Cloud Feedback Process

The determinations of the Habitable Zones (HZ) around stars, the orbital radii at which surface conditions are suitable for life, is becoming an increasingly important and useful concept as planet hunting missions continue to return new and updated censuses of planetary systems. One important aspect of determining the extent of the HZ around a star is the nature of clouds and hazes in the planetary atmosphere which effect the energy balance of the planetary climate. Many studies have utilised atmospheric models to estimate the extent of the HZ however they have typically been one-dimensional (1D) models which fail to accurately represent, or even completely neglect, important three-dimensional (3D) processes. Recently, 3D models have began to tackle this problem and shed new light on this interesting topic.

A recent paper by Jérémy Leconte et al. investigated the impact of increased solar forcing on the climate of Earth to place estimates for the onset of a runaway greenhouse state; the point at which thermal emission (cooling) of the atmosphere reaches a maximum and any further increase in incoming energy will lead to rapidly rising surface temperatures. Previous studies of hot climates with 1D models have suggested that the response of clouds to increased irradiation will help to resist warming and provide a negative radiative forcing. This was due to the assumption that clouds provide a net cooling of the climate and increased evaporation at the surface would lead to thicker and more reflective clouds. Cloud dynamics is, however, very much a 3D system and this new study suggests quite the opposite.

Leconte et al. found that the vertical extent of clouds is increased in their model when the solar radiation is increased. In other words the cloud tops are higher up in the atmosphere. This results in the thermal emission of clouds operating in a cooler environment and hence emit less radiation. This process enhances the warming impact of clouds. The reasoning of this increase in vertical extent is two-fold. Firstly, stronger irradiation of the atmosphere results in stronger dynamics and convection, extending the extent of the troposphere. Secondly, cloud drop formation requires the loss of latent heat during condensation. The increased infrared opacity of the atmosphere increases the altitude at which this energy can be radiated away, allowing the clouds to cool.

Storm Clouds over Brazil from the ISS. Image Credit: NASA

Storm Clouds over Brazil from the ISS. Image Credit: NASA

This is a major result in terms of understanding the important role of clouds in the energy balance in planetary atmospheres. Even though clouds were ruled out as a stabilising feature of the atmosphere, a new source of stabilisation was identified. As previously mentioned, the increased insolation strengthens the atmospheric dynamics, including the Hadley cell; a large-scale overturning of air masses in low latitudes resulting in the moist equator and the dry sub-tropics. The strengthened Hadley cell causes further drying of these sub-tropical regions and in effect forms a window of energy loss where the greenhouse effect is locally much less than the global average. This acts to cool the climate. This process could only be uncovered in 3D model and highlights the need to use models of varying complexities and abilities to study planetary climates.

The key result of this work is to extend the inner edge of the HZ around the Sun to ~0.95 AU due to the stabilising effect of these dry, sub-tropical atmospheric windows which strengthen as the solar radiation increases.

For more detailed information see the Nature paper by Leconte et al. 2013: http://www.nature.com/nature/journal/v504/n7479/full/nature12827.html