Editor’s Note: As a series at ENV, we are pleased to present “Exoplanets.” Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he critically examines recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos.

In the search for habitable planets, the circumstellar habitable zone is the spherical shell environment defined as habitable in a stellar system. Astrobiologists have used the term for many decades. This shell encompasses the distance at which a planet in a fairly circular orbit could maintain liquid water on its surface. We will start here for historical reasons, but broader environments also must be considered. The galactic neighborhood factors in, as does the galactic habitability zone. On a larger scale still, there is also a cosmic habitable age, needed to produce the necessary elements for life and life-supporting planets. These factors cannot be completely separated, and all of cosmic history also plays an important role.

The primary problem with liquid water on a planet’s surface is getting the planet in the right range for temperature and pressure.¹ We now know this concept needs to be modified to what is called the circumstellar continuously habitable zone. This zone allows a body to have liquid water not just at one point in time, but over several billion years.² After all, it has required nearly four billion years of stable conditions on Earth for intelligent technological life to appear. François Forget recognized the difficulty in his paper on this topic:

I will not discuss here the inherent difficulty of biological evolution and the fact that “a lot of luck” may have been required to make animals (Ward and Brownlee, 2000, Carter, 2008). Nevertheless, it is striking that when assessing the odds of having planets harboring complex or even intelligent life like in the Drake equation, one must estimate the frequency of planets which can remain continuously habitable for billions of years.³

This is much more restrictive. For a body to be continuously habitable for this length, many other factors come into play.

In order to understand the complexities of the CHZ, let’s look at it in more detail. The inner boundary of the CHZ can be defined in two different ways. It can be set by the “moist greenhouse” effect, where water is transported by energy from the host star into the upper atmosphere, where it is broken up by ultraviolet light into hydrogen and oxygen. The hydrogen has enough energy to be lost to outer space, slowly drying out the planet. The other defining inner limit of the CHZ is the runaway greenhouse effect, where water vapor acts as a greenhouse gas, and eventually heats the surface to the point it cannot support liquid water.⁴

The outer boundary can be defined by the maximum possible carbon dioxide-caused greenhouse effect that can allow water in the liquid state on the surface, without creating so many carbon dioxide clouds that reflectivity of the planet increases to the point where it cancels out the greenhouse effect.⁵ As far as the outer edge of our sun’s habitable zone, the classical model suggests the Earth is already on the outer edge to avoid runaway glaciation.⁶ But as Forget says, “In reality, on Earth there is a long-term stabilization of the surface temperature and CO₂ level due to the carbonate-silicate cycle.”⁷

To increase the size of the habitable zone, and thus the chances for finding habitable planets, it has been suggested that planets with masses between one and ten times that of Earth (called super-Earths in the literature)⁸ with a H₂-He (hydrogen-helium) thick atmosphere, could have greenhouse effects that would enable habitable zones out to ten astronomical units (AU).⁹ An AU is an astronomical unit, the average Earth-Sun distance. That is ten times further out than the Earth presently orbits the Sun. However, models show that the H₂-rich atmosphere will either rapidly escape to space after the planet forms, or remain thick, preventing liquid water with high surface pressure.¹⁰ We must also take account of the fact that the percentage of planets left with exactly the right atmospheric pressures to allow habitable temperatures after the early stages of atmospheric erosion is likely to be very small.¹¹ Although these systems could transition through an appropriate environment, the periods would be thousands to a few millions of years, too short for advanced life over the long haul.

Also to be considered is that animal lung systems can only efficiently work within a narrow range of pressure. The atmospheric pressure cannot vary much from Earth’s, which is dependent on the planet’s mass, or else the efficient mammalian lung system as we know it will not work.¹² Imagine a world where the atmospheric pressure limits intelligent life to the most basic activities of life, because it requires too much energy to do other productive work. The higher atmospheric pressure increases the viscosity of the air, and without energy to push above and beyond mere survival, any alien life’s ability to produce technology would be nil.

Venus and Mars are candidates in our solar system to test these models. The most restrictive CHZ models set the boundaries for our Sun at 0.95 AU for the inner and 1.37 AU for the outer one. The Earth couldn’t be much closer to the Sun than it is, but it enjoys, according to these models, the near-maximum energy from the Sun that life can utilize.¹³ A further study found that the inner boundary may be even more severe. It looks like the inner limit to avoid a moist greenhouse loss of water is 0.99 AU — almost exactly where the Earth sits in its orbit.¹⁴

The circumstellar habitable zone was historically calculated with constant circular orbits and constant energy flux, but newer studies include other variables that would certainly affect a planet’s habitability. Previous to 2011, most studies of habitable atmospheres were conducted with easy to model one-dimensional steady state radiative convective parameters that simulated average planet-wide conditions. These 1D models are too simple to gauge properly the habitability of a planet. Utilizing more sophisticated 3D models with time-based changing parameters, the actual habitability of a planet can be approximated much better. These also can give local habitability data by including day-night cycles, and seasons. They can also model the impact of clouds on habitability for the first time, which is important, since clouds can affect the boundaries of a star’s habitable zone.¹⁵

These more complex 3D models can also simulate with much better precision what can happen to carbon dioxide and water on a tidally locked planet, or one with high eccentricity or obliquity. Some planets could have liquid water that are further out than the classical models suggest if they have high levels of carbon dioxide.¹⁶ However, these high pressures would likely not enable efficient lungs such as exist on the earth, making it difficult to develop technological civilizations.

The other outcomes of these models seldom suggest habitable climates, and it seems that these complex simulations bring with them more reasons for pessimism in the search for habitable worlds. These studies include factors such as gravitational interactions, stellar winds, and particle radiation, UV radiation, and luminosity variations. Many other factors probably also come into play, but for the stellar habitable zone studies these are the most recently investigated.

Next up: The Right Kind of Star.

References Cited:

(1) Forget, “On the Probability of Habitable Planets,” 180.

(2) Gonzalez, “Setting the Stage for Habitable Planets,” 38.

(3) Forget, “On the Probability of Habitable Planets,” 180.

(4) Gonzalez, “Setting the Stage for Habitable Planets,” 37-39.

(5) Ibid., 37.

(6) Forget, “On the Probability of Habitable Planets,” 182.

(7) Ibid., 6.

(8) Dimitar Sasselov, The Life Of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet (New York: Basic Books, 2012), 64.

(9) Forget, “On the Probability of Habitable Planets,” 183.

(10) Ibid., 6.

(11) Robin Wordsworth, “Transient conditions for biogenesis on low-mass exoplanets with escaping hydrogen atmospheres,” Icarus 219 No. 1 (May 2012): 276-279.

(12) Michael J. Denton, Nature’s Destiny: How The Laws Of Biology Reveal Purpose In The Universe (New York: The Free Press, 1998), 128.

(13) Gonzalez, “Setting the Stage for Habitable Planets,” 37.

(14) Ibid., 37.

(15) Forget, “On the Probability of Habitable Planets,” 184.

(16) Ibid.

Image credit: ISS Expedition 7 Crew, EOL, NASA.