Not long ago, Ramses Ramirez (Earth-Life Science Institute, Tokyo) described his latest work on habitable zones to Centauri Dreams readers. Our own Alex Tolley (University of California) now focuses on Dr. Ramirez’ quest for ‘a more comprehensive habitable zone,’ examining classical notions of worlds that could support life, how they have changed over time, and how we can broaden current models. We can see ways, for example, to extend the range of habitable zones at both their outer and inner edges. A look at our assumptions and the dangers implicit in the term ‘Earth-like’ should give us caution as we interpret the new exoplanet detections coming soon through space- and ground-based instruments.
by Alex Tolley
The Plains of Tartarus – Bruce Pennington
In 1993, before we had detected any exoplanets, James Kasting, Daniel Whitmire, and Ray Reynolds published a modeled estimate of the habitable zone in our solar system . They stated:
“A one-dimensional climate model is used to estimate the width of the habitable zone (HZ) around our sun and around other main sequence stars. Our basic premise is that we are dealing with Earth-like planets with CO2/H2O/N2 atmospheres and that habitability requires the presence of liquid water on the planet’s surface. The inner edge of the HZ is determined in our model by the loss of water via photolysis and hydrogen escape. The outer edge of the HZ is determined by the formation of CO2 clouds, which cool a planet’s surface by increasing its albedo and by lowering the convective lapse rate. Conservative estimates for these distances in our own Solar System are 0.95 and 1.37 AU, respectively; the actual width of the present HZ could be much greater. Between these two limits, climate stability is ensured by a feedback mechanism in which atmospheric CO2 concentrations vary inversely with planetary surface temperature. The width of the HZ is slightly greater for planets that are larger than Earth and for planets which have higher N2 partial pressures. The HZ evolves outward in time because the Sun increases in luminosity as it ages. A conservative estimate for the width of the 4.6-Gyr continuously habitable zone (CHZ) is 0.95 to 1.15 AU.”
Climate models have improved considerably over time, and now are capable of three dimensional (3-D) models as well as more advanced 1-D models. Parameter estimations are also being refined to account for different atmospheric features.
The 1993 Kasting et al paper set the bounds for a conservative HZ at 0.95 and 1.37 AU, the outer bound being inside the orbit of Mars. However, the authors noted that a maximal greenhouse with a dense, CO2 atmosphere would push the outer bound to 1.67 AU, that would include Mars, although this was considered optimistic. In 2013, Kopparapu, Ramirez, Kasting, et al, published new estimates on the HZs using a more advanced 1-D model . For the Solar System, the inner and outer edges were 0.99 and 1.67 AU respectively, with the now conservative outer limit at maximal greenhouse warming.
In 2018, Dr. Ramirez published a review of HZ research that included ideas that extended the HZ in time and space, as well as a wider range of stellar types . The conclusion of that paper included the decision tree concerning newly discovered planets and models of their environments. The decisions have identifying numbers added and are shown in figure 1.
In a recent Centauri Dreams essay, Revising the Classical ‘Habitable Zone’, Dr. Ramirez outlined his reasons for studying models of planetary habitable zones (HZ) and environments as part of the search for life. This post will try to tie the many ideas of the journal article to the decision points in figure 1 below.
Figure 1. Reproduced from the paper , annotated with numbers.
1. Assuming the planet is in the classic HZ, did planet have a runaway GHE during pre-main sequence?
The classic HZ calculates its inner limit as the point at which a runaway greenhouse effect (RGE) can occur. This inner edge is calculated for the period when the star is in the main sequence. For most stars, the main sequence starts quite quickly, within 0.1 Gyr of the nebula forming, allowing life to evolve after the main sequence had started and before the star in its potentially more luminous pre-main sequence state has had time to initiate the RGE. However, for the numerically numerous M-dwarfs, this pre-main sequence period may last for up to a 1 Gyr and have orders of magnitude more luminosity than during the main-sequence. Any planet currently in the classic HZ would have been subject to far higher luminosities and with a time period sufficient to desiccate the planet. The world might then be like Venus, a hot, dry world, with our without a dense CO2 atmosphere. Transit techniques will detect Earth-sized rocky worlds more easily around M-dwarfs, which have attracted a fair amount of discussion about the conditions for life on their surfaces. The issue of a high luminosity during a long pre-main sequence period may trump any favorable conditions during the current main sequence period. Therefore whether the exoplanet lost all of its water during this time needs to be addressed, which leads us to:
Figure 2. Evolution of stellar luminosity for F – M stars (F1, F5, Sun, K6, M1, M5, and M8) using Barrafe et al.  stellar evolutionary models. When the star reaches the main-sequence (red points) the luminosity curve flattens. 
2. Did the planet lose more than a few Earth oceans of water during the pre-main sequence period?
An Earth analog planet will desiccate like Venus if it loses more than a few Earth oceans of water. It is possible that the water loss was not severe when the planet entered the classic HZ. If the water loss was high, the world would be desiccated, like Venus.
If any of these conditions are not satisfied:
3. If water loss was not sufficient to desiccate the planet
If the period and intensity of water loss was insufficient to result in a complete RGE, or the starting volume of water was low, then the planet may be characterized as a desert world, with lakes of water, possibly at altitude.
One possibility is that the world is an ocean world with far higher quantities of water than an Earth analog. In this case, desiccation has been held off as a result. Density calculations for that world will indicate if the planet may be an ocean world. As far as life is concerned, we would return to the issue of whether such ocean worlds are suitable for sustaining any abiogenesis derived life.
If the planet has retained enough water it may join the candidates for the next decision: This allows the planet to be considered potentially habitable regardless of early heating.
The author then allows for a wider interpretation of the HZ:
4. Is the planet in another HZ that is outside the classic HZ?
The bulk of the paper deals with possible modifications of the HZ that extend its range at both the inner and outer edges. The paper documents a number of mechanisms that could widen the HZ and to what one might call an optimistic HZ.
Some examples are listed below:
A. Empirical vs model HZs
The author argues that empirical, rather than model limits for the classic HZ might be applied. These limits imply that the outer edge must have been further than the climate model as Mars has evidence that it once had running water on its surface when the sun’s luminosity was lower. For the inner edge, this puts the edge of the HZ closer to the sun at 0.75 AU, but still outside the orbit of Venus as the planet has no sign of surface water even 1 Gya when the sun was cooler [Figure 3].
B. Stellar spectral range
The author argues that the apparent rapid emergence of life on earth allows for hotter, shorter-lived stars to harbor life before they exit the main sequence. This extends the HZ to beyond the star types usually associated with life, but to large, hot stars.
C. Extensions of the HZ in space
Greenhouse gases like methane (CH4) and hydrogen (H2) can extend the HZ, particularly at the outer edge. Methane could extend the HZ beyond Mars, and as it is also produced biologically in greater quantities than geological processes, can maintain the gas in the atmosphere. It is often cited as a detectable gas out of equilibrium and possibly indicative of life. H2 may also contribute to warming and has be posited as a possible explanation for the warmer, ice-free Earth during the Archean.
Ocean worlds are often cited as having unregulatable temperatures due to the absence of exposed crust that prevent the carbon-silicate cycle to act as a carbon sink. One model of ocean worlds with ice caps suggests a way for this sink to operate with CO2 clathrates.
Binary stars would appear to be problematic for worlds to stay in their HZ. The climates are difficult to model, especially where the planet orbits one star, but the planet may create a temporary, benign temperature with periods of freezing as is transits into and out of an HZ.
3-D climate modeling is now increasingly able to compute the effects of rotation rates and different types of cloud cover and composition. The results vary regarding whether these parameters can extend the HZ.
This leads to a sub-question: about the outer edge of the extended HZ:
5. Is the planet near the outer edge of the classic HZ?
The classic HZ outer edge is set by the maximum warming effect of an atmosphere with CO2 and H2O. However, there are some possible ways to extend that outer edge using other greenhouse gases like CH4 and H2. Figure 3 shows the effect of CH4 and H2 on the outer limit of the HZ. For most worlds, retaining a light H2 component to the atmosphere is unlikely, unless it is maintained by some geologic or biotic process. Other possibilities include transient warming periods, possibly even limit cycles that create warm conditions on a periodic basis, perhaps with CH4 or H2 that can exist for short periods. Of course, life would have to survive in some form during the planet’s frozen period. Even on the hypothesized Snowball Earth, liquid water was probably present below the surface ice. For a solidly frozen world, the conditions for survival would be harsher.
For M dwarfs, with high luminosity, and long pre-main sequence periods, planets currently residing at the outer edge of the classic HZ may have been habitable during this pre-main sequence period. Life may have evolved during that period, then retreated to the subsurface as the planet cooled. This is analogous to the hopes of some that life on Mars may have evolved when the planet had surface water, but may still exist at depth as Mars surface cooled and dried.
Figure 3. The classical HZ (blue) with CO2-CH4 (green) and CO2-H2 (red) extensions for stars of stellar temperatures between 2,600 and 10,000 K (A – M-stars). Some solar system planets and exoplanets are also shown. 
6. Is the planet orbiting a hotter ( >~ 4500 K) star?
Because the star type influences the warming of the atmosphere and planet’s surface, only stars with a surface temperature greater than 4500K will extend the outer edge of the HZ with the greenhouse gas, Ch4, as part of a dense CO2 atmosphere. Unintuitively, for cooler stars, CH4 in the CO2 atmosphere brings the outer limit of the HZ closer to the star. This is clearly shown in Figure 3. [See also CD post The Habitable Zone: The Impact of Methane.] Hydrogen (H2) will quite considerably extend the outer limit of the HZ for all stellar types. Spectroscopy to determine the atmospheric mixing ratios will help in determining whether this is possibly the case.
The paper ignores the current vogue for life in icy moon subsurface oceans for good reason as any subsurface liquid ocean is undetectable by any telescopes that we have on the near horizon. Focusing on the HZ where surface water can exist makes operational sense.
This paper does us a service in not just offering routes to expanding the HZ, but also in the approach to characterizing planets, and ultimately to modeling them accurately once we have the empirical data to support these models.
In passing, the author gives credence to the issues of interpreting results. There is a criticism of the use of “HZ” and “Earth-like” and super-Earth” to imply that those exoplanets have Earth-like life in abundance on their surfaces. In many ways, the extending of HZs retains the concept of surface water possibly existing. In the original Kasting et al paper, their HZ definition required surface water as a necessary condition for life to evolve, as we currently believe. Nevertheless, these worlds may be sterile, as this condition may be insufficient.
As Moore states:
“Habitable planets, not habitable zones Similarly, the term habitable zone is misleading to both the public and the scientific community. On the face of it, habitable-zone planets should be, well, habitable, but in its now classic definition, this is the region in which the presence of a liquid water surface “is not impossible” with an atmosphere assumed to be Earth-like. It does not mean that a habitable zone planet would, in fact, have a wet surface or any other condition required for life. Furthermore, this definition ignores the potential for deep, chemosynthetic biospheres and biases our thinking toward only one of the many ways in which life manages to sustain itself on Earth. That the term habitable zone has such a disconnect with the concept of habitability is problematic for communicating ideas clearly and yet its use has become entrenched in discussions of new exoplanet results and it continues to inform the design of our exoplanet program.”
For life in the Earth’s Archean eon, prokaryotic methanogens create methane (CH4). This greenhouse gas must be present in sufficient quantities to be detected and should push out the inner bound of the HZ.
With these caveats in mind, the Ramirez paper suggests that we can use these expanded HZs to both focus the search for targets, and to validate the models so that targets can be more accurately determined as we extend our searches. This might be very timely as the Transiting Exoplanet Survey Satellite (TESS) all-sky survey is already turning up many potential targets.
Kasting, J.; Whitmire, D.; Raynolds, R. “Habitable Zones Around Main Sequence Stars.” Icarus 1993, 101, 108–128.
Kasting, J. How to Find a Habitable Planet. Princeton University Press, 2010.
Kopparapu, R. K.; Ramirez, R.; Kasting, J. F.; Eymet, V.; Robinson, T. D.; Mahadevan, S.; Terrien, R. C.; Domagal-Goldman, S.; Meadows, V.; Deshpande, R. “Habitable Zones Around Main-Sequence Stars: New Estimates.” Astrophys. J. 2013, 765, 131, doi:10.1088/0004-637X/765/2/131.
Ramirez, R M “A more comprehensive habitable zone for finding life on other planets” Geosciences 2018, 8(8), 280.
Moore et al “How habitable zones and super-Earths lead us astray” Nature Astronomy volume 1, Article number: 0043 (2017).