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Jaime Green

The Habitable Zone of Inhabited Planets

Posted by Jaime Green

07-07-2014 15:14 CDT

Topics: life, extrasolar planets

To at least some extent, the search for extrasolar planets is the search for extrasolar life. We don’t get excited about Earth-like planets because we’re worried our planet is lonely. We’re the ones looking to be less alone. Public interest piques at the mention of Earth-like discoveries: rocky worlds, roughly Earth’s size, in their star’s habitable zone. It seems simple. But of course, it’s not. Habitability is a very complicated thing, and if we want to search for real candidates for habitability, we have to delve more deeply into what habitability means—deeper than the headlines about the latest Earth-like find, for sure.

A team of Colombian researchers are arguing for a new refinement to the idea of the habitable zone that takes the presence of life itself into account. The habitable zone, or HZ, is the basic metric of habitability. It is the range of distances at which a planet can orbit its star and be at the right temperature to have liquid water on its surface.

Kepler-186f: A Second Earth

NASA / Ames / JPL-Caltech / T. Pyl

Kepler-186f: A Second Earth
The artist’s concept depicts Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. Kepler-186f resides in the Kepler-186 system about 500 light-years from Earth in the constellation Cygnus. The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth.

The simplest conception of the habitable zone is based purely on insolation, or energy coming in from the star and heating the planet's surface. In this bare-bones definition, habitability is a simple function of the star’s luminosity and the planet’s distance from that sun. Here, we’re looking for planets that are not too hot and not too cold—there’s a reason it's called the Goldilocks zone.

Water alone doesn’t make a planet habitable, but even the presence of liquid water, even just that question of temperature, depends on more than just incoming stellar heat. Earth’s surface temperatures are influenced by atmosphere, cloudiness, weather patterns, ground cover, geothermal heat, the eccentricity of our orbit, and more. An exoplanet’s habitability would be similarly complicated, since all of these conditions affect how stellar energy translates to a planet's surface temperature. 

In a study accepted for publication in the journal Biogeosciences, authors Jorge I. Zuluaga et al. write that yet another factor makes a strong enough impact on habitability that it should also be taken into account: life itself.

The authors make the case for splitting the standard idea of a habitable zone into two classifications. The first is the Abiotic Habitable Zone, or AHZ, which is basically the standard model I’ve previously discussed. The second is the Inhabited Habitable Zone, or InHZ, which takes life’s alterations on a planet into account. The idea is that life alters a planet’s habitability so much that the zone shifts closer or farther from a star; possibly both. 

While standard HZ models see habitability as a product of abiotic (non-living) factors; Zuluaga et al. argue that habitability may be an emergent property of “a very complex system involving the interaction among astrophysical, geophysical, and not less important biological factors.” This paper doesn't set the terms of the InHZ; rather, it argues for its validity and necessity as a concept. 

The logic here may seem paradoxical: Life makes a planet able to support life. But aside from the practical fact that a planet's habitability may change over time due to stellar or geophysical evolution, perhaps life can evolve under certain conditions and then contribute to climate stability. The authors propose a logical loophole: Abiotic habitability never asks how the water got there, just whether or not it is liquid. Likewise, inhabited habitability need not ask how the first life forms got there, just what their effects might be. 

A single paper can't unravel all of these riddles, so Zuluaga et al. focus on one question in particular: Does inhabitation affect habitability? Their answer is yes. Life is a powerful enough force for environmental change that it must be taken into account alongside abiotic factors like the planet’s distance from a star and its orbital eccentricity. Encouragingly, for those of us feeling lonely in the cosmos, this study says the presence of life seems to expand the habitable zone.

The authors use several strategies to make their case. First, they show how powerfully life on Earth affects its environment, and that the feedback cycle works in both directions. Life's impact on cloud cover is one of their strongest examples. Clouds—composed of either water or carbon dioxide—can have a dramatic impact on habitability through the greenhouse effect and albedo. (Albedo is a measure of a surface’s reflectivity. High albedo surfaces reflect back much of the light and heat that hits them, while low albedo surfaces absorb energy. High albedo clouds keep a planet cool by reflecting, rather than absorbing, stellar energy.)

Research indicates that on Earth, plant life leads to more cloud cover. Likewise, airborne microorganisms in an exoplanet’s atmospheric layers could serve as seeds that trigger more cloud formation. The effect can be either cooling due to albedo, or warming thanks to the greenhouse effect. In either case, clouds or a lack thereof may alter habitability, nudging a planet’s temperature in one direction or another.

Life also can affect the carbon cycle. Plants on Earth affect the amount of CO2 in the air; calcareous plankton have been a major factor in changes to Earth's carbon cycle. These systems impact the heat-trapping properties of our atmosphere and the chemistry of the atmosphere and oceans. 

There is also the dramatic impact that technologically advanced life—like us—can have on a planet’s livability. We may be in the process of nudging our own planet out of its habitable zone without altering its orbit an inch, thanks to carbon emissions. A planet-hunting alien might think Earth was prime habitable territory from what they could see from far away. However, we have the power to change that.

Examples from Earth’s biosphere aren’t the only way the authors make their case. After using our own planet to establish the premise, they use conceptual models to show how life could change the habitability of hypothetical planets.

Imagine a planet where life drives cloud formation during the day. This could be due to evapotranspiration—water vapor released by life—or tiny airborne lifeforms serving as nuclei, or seeds, around which clouds condense. It’s plausible that these cloud-generating mechanisms could be active during the day but not at night. In such a system, the skies would be cloudy during the day, and the clouds' high albedo could reflect more starlight out to space, keeping the planet cool. At night, the clouds clear, allowing heat to radiate into space and escape. Thus, the inhabited planet, with its atmospheric cycles driven by life, is overall kept much cooler than a planet with a steady cloud cover or one with clear skies. In this case, the cyclic nature of life (or rather life’s cyclical interactions with its environment) extends the HZ, in toward the star. 

Another example from the paper uses a simplified model of inhabitation called Daisyworld. This model has been used in many variations and permutations since 1983 to show the dynamical interactions of a simplified inhabited world. The basic Daisyworld reduces the interactions of life with its environment to two kinds of organisms: white daisies and black daisies. These daisies either grow or die off in response to changes in surface temperature and growing space. Patches of white or black daisies have different albedos—as does bare ground—and thus impact the surface temperature as they grow or die.

Daisyworld model results

Zuluaga et al.

Daisyworld model results
A typical result of solving the Daisyworld equations for three variations: dry uninhabited (red), wet uninhabited (blue), and wet inhabited (green). The inset box shows an equilibrium state within this solution characterized by temperature oscillations that, although dramatic, still keep the planet within the habitable range.

The addition of a water cycle driven by the daisies provides insight into life’s effect on the HZ. In this case, surface temperature impacts the formation of clouds, which contribute along with the daisies to changes in the planet’s albedo. Zuluaga et al. compare three Daisyworld scenarios: dry and lifeless, wet and lifeless, and wet with inhabitation. In both lifeless scenarios, surface temperature tracks roughly with how close the planet is to its star (closer is warmer, more distant is cooler, just like you'd expect). But when daisies are present, the habitable zone becomes wider. The interactions of life and the environment keep the planet habitable at both higher and lower levels of insolation than the planet would be without the effects of life. 

Of course, models are just models. Real biospheres are far more complex than a four-variable Daisyworld model. But currently, all of our definitions of habitability are based on models and speculation (at least until we touch down on alien soil—or alien barren rock—to make observations ourselves). In looking for Earth-like planets that might be home to life, we should be careful to keep our minds open to all possibilities, including that a planet might be habitable because life is there.

See other posts from July 2014


Or read more blog entries about: life, extrasolar planets


Stephen : 07/07/2014 10:48 CDT

"We may be in the process of nudging our own planet out of its habitable zone without altering its orbit an inch, thanks to carbon emissions." That statement appears to be insinuating that life on Earth is heading for extinction, presumably tipping the planet's climate towards some Venus-like hell-hole. Climate change--or rather man-made climate change (which is really what most people now mean by that term)--is a world-wide problem. If it continues there will undoubtedly be all kinds of problems for humanity to face, including those stemming from rising sea levels as the Greenland and Antarctic ice caps melt and shifting climate zones. But to go from there to life extinction seems a rather large jump when the Earth's climate has swung quite dramatically in the past (from Snowball Earths to eras when there were apparently no polar caps at all) without it causing the extinction of ALL life. In fact even during the great Permian extinction event, which was more severe than the better-known Cretaceous one, it managed to not only survive but to thrive afterwards. Such past examples suggest that the Earth will not be leaving its habitability zone any time soon, human efforts at "nudging" notwithstanding.

Ralph Lorenz: 07/08/2014 08:00 CDT

It seems odd, if not outright sloppy, not to observe that all of the ideas above (such as cloud feedbacks, CO2 uptake being modulated by biota, and indeed the Daisyworld toy model) were originated by James Lovelock in the 1970s in what were at the time heretical ideas, now more or less mainstream biogeochemistry, collectively referred to as the Gaia hypothesis. The new element in this paper is merely applying some of these concepts to refine the definition of habitable zone.

Bob Ware: 07/08/2014 04:32 CDT

I take this blog to mean that these scientists are not really redefining the HZ boundaries but instead they are creating multiple HZ classifications of niches that will define a specific stars HZ based fairly upon the specific planet in question. That is how I see this blog needs to be viewed. Look at Sol's (our star) HZ. To date we have planets 3 & 4. Planet 3 is life supporting. Planet 4 may have been or still is life supporting. MER-A & B along with MSL have found variations of water ranging across acidic to neutral which bodes well for life potential, present or past. As long as it voluntarily self replicates, consumes and expels it meets the basic definition of life. Exotic chemistry does not voluntarily do these things. Voluntarily is the key word. Sure exotic chemistry does mimic life but we need to know the difference between the two. Niches in the HZ Umbrella will be needed as we learn more over the explorations we are now embarking on. In time that will come and these scientists have foreseen that. They should proceed with their work in laying out various niches that they can use to proceed forward in getting the definition expanded upon.

Olivier De Goursac: 07/09/2014 08:04 CDT

"The logic here may seem paradoxical...". Well... this idea was submitted loooong ago, in 1967, at JPL by ...James Lovelock (Source : "Planetary Atmospheres: Compositional and other Changes Associated with the Presence of Life", James Lovelock, Contract No. NAS 7-100, 1967 (NASA), published in Advances in the Astronautical Sciences, 25, 1969), who is the father of the "Gaia theory" and who said again recently in 2012 about the sending of rovers to Mars : "The biologists are just one big tribe and in America the only thing that matters about Mars is to discover life. But we're almost certain there isn't. But they say, "if there's water, there must be life!" Why? It's the other way round. You have water there not because you don't get life without water, but because you don't get water without life. The biologists fight it like tooth and nail. There's a better chance there might be fossilised traces of former life, but it's taking an awful lot of time for even that degree of common sense to come through.". And his theories were comforted by a recent paper (“Nitrogen Isotopic Composition and Density of the Archean Atmosphere”, Bernard Marty, Laurent Zimmermann, Magali Pujol, Ray Burgess, Pascal Philippot, Science VOL 342, 2013) that proves that Earth would have been inhabitable very quickly into its youth without the emerging of life...

Bob Ware: 07/09/2014 01:53 CDT

Olivier - Regarding: "..."But they say, "if there's water, there must be life!" Why? It's the other way round. You have water there not because you don't get life without water, but because you don't get water without life. ..." -- Life does not generate water. Water exists in form as ice, liquid or gas. In all it's forms it is consumed, and digested by life. It also is a building block of life. I don't understand how they could come up with that statement/conclusion. My college biology class never taught that point.

Olivier De Goursac: 07/10/2014 04:23 CDT

Yes Bob, thanks : you are absolutely right about water itself. James Lovelock does not say that life creates water, BUT makes it liquid by helping raising (or lowering) the global average temperatures of the planet. Please, read those articles, they are self-explanatory.

Olivier De Goursac: 07/10/2014 01:56 CDT

Thanks also so much, Jaime, for all those good science articles of yours ! :=) :=) :=)

Bob Ware: 07/11/2014 11:46 CDT

Olivier - I will read them at some point. I have very little time right now and another rather lengthy S/C (spacecraft) article I need to finish reviewing first. Thanks for the links.

Torbj??rn Larsson: 07/13/2014 01:31 CDT

HZs are bound to be messy when we get into details. Moreover we have putative cases where a planet is habitable yet not inhabited for one reason or other. @Ralph Lorenz, Olivier de Goursac: That is complex I think. Some areas like geology think the ideas are worthwhile, others as in astrobiology here want to find evidence and find scant. "Nevertheless, the Gaia hypothesis continues to attract criticism, and today many scientists consider it to be only weakly supported by, or at odds with, the available evidence. In 2006, the Geological Society of London awarded Lovelock the Wollaston Medal largely for his work on the Gaia theory.[6]" [ ] Re the faint young sun, it seems going from 1D to 3D models solved the generic problem. (Without life.) "Our results indicate that a weak version of the faint young Sun paradox, requiring only that some portion of the planet's surface maintain liquid water, may be resolved with moderate greenhouse gas inventories." [ E.T. Wolf and O.B. Toon. Astrobiology. July 2013, 13(7): 656-673. doi:10.1089/ast.2012.0936. ] "There are many ways to solve the faint young Sun problem. According to our results, it is not so difficult a task,". [Especially increasing N2 pressure is not so effective in their model, since it combats the effects of larger water drops. Their model assumes 0.8 - 1 bar total pressure, quite consistent with the N2 partial pressure of 0.5 - 1.1 bar in Marty et al.] [Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM - B. Charnay et al]

Torbj??rn Larsson: 07/13/2014 01:33 CDT

@Bob Ware: The HZ measures are yet too crude, and without supporting observations of course, to be anything else than observational filters to direct interest. (So aiming for fastest progress.) We can make, in fact have made, progress in astrobiology without having a very constrained "definition of life". It is perhaps better to see it as a process that results in evolving populations (going from geophysical to biological individuals), where we have lamarckian (pre-genetic) and darwinian (genetic) evolution of traits.

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