Habitability: An Atmospheric Consequence?

More than 3000 planets outside our Solar system have been discovered. Around 20 of them are “Goldilocks planets,” planets that orbit the region surrounding a star, where the probability of presenting conditions suitable for habitability increases. In this region, called the Habitable Zone, the distance between the planet and the star allows for temperatures where the planet can present a liquid ocean surface, the minimum condition of habitability.

Usually, planets outside the Habitable Zone are deemed uninhabitable. A planet close to its host star can suffer from the evaporation of its atmosphere due to the high temperatures. Meanwhile, a planet located far from its host star can be frozen. However, when thinking about future space exploration, will mild temperatures and the existence of a liquid ocean ensure mammals’ habitability?

One way to study whether planets with characteristics similar to Earth (Earth-like planets) are habitable is to consider habitability as an atmospheric consequence. 
The evidence provided by Earth’s current global warming shows that certain regions have become inhospitable due to high temperatures, and in some cases, can cause the death of its inhabitants by hyperthermia.

Mammals use evaporation as a cooling mechanism when air temperatures increase together with an increase in relative humidity. We stop dissipating internal heat, which can lead to hyperthermia or death due to heat shock. Sherwood et al., 2010 used the wet bulb temperature (Tw) as an atmospheric indicator of habitability in a future scenario of terrestrial global warming. Tw relates air temperature and relative humidity, indicating the limit where death by heat shock in mammals occurs, Tw > 35 ºC.

Figure 1 shows how Tw changes when humidity increases in the environment for certain values of air temperature. When the air temperature is lower than 30ºC, hyperthermia does not occur, even with a 100% relative humidity (green region). However, when air temperature increases above 40ºC, hyperthermia occurs even with low values of relative humidity (red region). On Earth, the highest air temperature recorded 56.7ºC occurred at Death Valley (USA) on July 10, 1913. At this air temperature, hyperthermia occurs with only 20% of relative humidity.

This value indicates how important it is to consider the atmosphere when describing the habitability of a planet.

The atmospheric activity of a planet depends mainly on the shortwave radiation from the host star. This radiation depends on planet-star distance and the type of star itself, since this determines the amount of radiation the planet receives at the top of its atmosphere. How the radiation is latitudinally distributed depends on the planet’s shape and the obliquity, the angle formed between its spin axis, and its orbital plane (see Fig. 2).

On Earth, with a current obliquity of 23.5º, the highest absorption of solar radiation is observed in the Equator, raising surface temperatures in the region, which decreases towards the Poles. This temperature difference between both Equator and Poles produces a flow of air masses balancing the temperatures between both regions.

How can obliquity affect the habitability of an Earth-like planet? 
In order to answer this question, by using a general circulation model, we simulate the global atmospheric activity by varying the obliquity between 30º and 90º of an Earth-like aquaplanet, a planet as Earth but completely covered by a liquid ocean, without continents, which maintains the terrestrial characteristics with respect to its size and distance to a Sun-type star.

The results showed significant differences in the atmospheric activity of the Earth-like aquaplanet, as the obliquity determines the latitudinal distribution of incoming radiation at the top of the atmosphere since the regions with the highest absorption of radiation are inverted with respect to that observed on Earth. As obliquity increases, higher temperatures occur in the Poles. Above 54º, the air masses move towards the Equator, the region with the lowest temperature, causing the change in the direction of the wind regime.

Finally, we studied the habitability of these planets using Tw. Figure 3 shows the longitudinal mean of Tw for each obliquity value, the months where each latitude is habitable appears between 0% and 100%.

Annually, planets with obliquities between 30º and 54º remain in the habitable range (Tw < 35ºC) throughout the year in all latitudes. On 54º of obliquity, there is an increase in the evaporation of the ocean, causing an increase in the relative humidity towards the poles, which decreases its habitability to 50% of the time. Only the region near the Equator has a 100% timeframe for habitability in planets with high obliquity.

If we apply the same study to our planet, we see that the region between +-40ºLat remains habitable throughout the year. However, habitability on Earth decreases towards the poles, due to low temperatures. Even when the risk of hypothermia is avoidable, our planet is less habitable than the modeled Earth-like aquaplanet.

Having described the habitability as an atmospheric consequence confirms the need to contemplate both air temperature and relative humidity in future space missions designed for human colonization.

These results are described in the article Atmospheric dynamics and habitability range in Earth-like aquaplanets obliquity simulations, recently published in the journal Icarus. This work is part of the doctoral thesis of Priscilla Nowajewski, in which she worked with Dra. Maisa Rojas, climatologist, and Dr. Patricio Rojo, astronomer, both from the University of Chile, and Dr. Stefan Kimeswenger from the Universidad Católica del Norte.

This is the first Chilean study investigating the atmospheric dynamics of exoplanets. Priscilla is currently seeking to promote the development of this area in Chile.