Defining the Habitable Zone
Since the first planet outside of our solar system was discovered almost 20 years ago, thousands of these exoplanets have been catalogued using devices such as NASA’s Kepler space telescope. This influx of data necessitates a system of categorization in order to prioritize studies of certain planets with regards to future research. The question of extraterrestrial life typically leads the discussion on these exoplanets, providing a useful focus for studies of the known universe. The “classic habitable zone,” defined by Kasting et al. as the range of orbital distances for which a planet can retain liquid water on its surface, provides a good basis for determining planets that could serve as hosts to other life forms (1993). However, the diversity of exoplanets has prompted research into broader forms of Kasting’s model, deviating from the more limited Earth-like concept of a habitable planet. A wider understanding of the ingredients necessary for a habitable planet is essential in the search for life in the universe.
The inner and outer edges of the habitable zone come directly from the effect of planetary surface temperatures on liquid water. Planets too close to their stars can experience the “runaway greenhouse effect”: hot temperatures lead to an increase in water evaporation to the point where the water vapor in the atmosphere acts as a greenhouse gas, warming the planet even more in a loop until all of the planet’s water has evaporated. In contrast, the outer boundary of the habitable zone marks the point at which the planet’s atmosphere can no longer sustain a warm enough temperature for liquid water (Forget 2013). Twenty years after Kasting used these definitions to calculate the habitable zone of our solar system, Kopparapu used recent data on common Earth greenhouse gases like carbon dioxide to update Kasting’s model, using a cloud-free climate model. Both models used “inverse climate modeling,” in which the surface temperature is kept constant while the models calculates the solar flux needed to maintain this temperature. The necessary orbital distance is then calculated from this flux. Kopparapu calculated the habitable zone of our solar system to be 0.99- 1.70 AU, where 1 AU is the average distance from the Earth to our Sun (Kopparapu et al. 2013). This is a fairly conservative estimate, as Kopparapu’s model operates on a variety of assumptions: a cloud-free atmosphere, a planet’s constant placement in the habitable zone over time, and Earth-like surface conditions.
Almost all experimental treatments of the habitable zone use climate models that do not consider the effects of clouds on a planet’s habitability. However, clouds can significantly affect a planet’s surface conditions by either blocking incoming solar radiation (cooling the planet) or absorbing and redirecting thermal emissions from the surface (warming the planet). Yang, Cowan, and Abbot provided the first treatment of the inner edge of the habitable zone that included complex cloud schemes within the climate models. The study found that the introduction of clouds into the calculation accounted for an extra 73 K in cooling, meaning planets could withstand up to two times as much solar flux to remain habitable as previously determined (Yang et al. 2013). Although the study focused solely on the inner boundary of planets around red dwarf stars, which are smaller than our Sun but common in the known universe, the implications of such a drastic increase potentially habitable planets should serve as fertile research ground for future study.
Kopparapu’s definition of the habitable zone also takes a limited view in regards to habitability over time. Although orbital distance is an important factor of liquid water retention, solar systems are inherently dynamic; a planet may not be in a star’s habitable zone for its entire existence. Rushby et al. propose that duration in the habitable zone (the “habitable zone lifetime” of a planet), in addition to placement, is an important factor in identifying habitable planets (2013). The authors provide the first model of habitability over time, specifically with regards to stellar evolution. A star’s luminosity varies over extended time intervals, changing the boundaries of the habitable zone accordingly to account for these shifts in stellar radiation. The study applied a simple model to Earth and seven other exoplanets previously deemed habitable in order to calculate each planet’s habitable zone lifetime (Rushby et al. 2013). Rushby and his colleagues calculated lifetimes for these planets as anything from 1 to 54.72×109 years. The astonishing range in these calculations demonstrates the importance of incorporating habitable zone lifetimes into any definition of a habitable planet, particularly when looking for planets with long enough lifetimes to host complex life forms similar to those found on Earth.
Both Rushby et al. and Kopparapu et al. assume the planets in their models are Earth-like in both atmospheric composition and amount of water on the surface (Rushby et al. 2013; Kopparapu et al. 2013). However, the multitude of exoplanets detected thus far varies greatly in these attributes. Assuming that all habitable exoplanets have an atmosphere composed of water and carbon dioxide like that of Earth does not account for any planets with other atmospheric makeups. The outer edge of the habitable zone marks the distance at which the carbon dioxide in the atmosphere condenses, ending the greenhouse effect on the planet therefore freezing any surface water. Pierrehumbert and Gaidos investigated the habitability of a planet with an atmosphere made up of hydrogen and helium using a cloud-free climate model similar to that used by Kopparapu et al. in 2013. The authors found that a planet with a mass at least three times bigger than that of Earth could maintain surface liquid water as far as 10 AU from a G-class star (a star in the same stellar class as our Sun). Hydrogen has a much lower condensation temperature than carbon dioxide, meaning that such an atmosphere could keep a planet warm at larger orbital distances (Pierrehumbert and Gaidos 2011). Although these results are limited to planets much larger than Earth, the authors’ results suggest a need for broader considerations of a planet’s individual characteristics in the discussion of planet habitability.
Another important geographical feature that affects habitability is the portion of the planet’s surface area that is covered by water. It is tempting to limit the candidates for habitable planets to only “aqua planets” that, like Earth, are mainly covered with liquid surface water. However, Abe et al. explored the idea of a habitable “dry planet,” on which only limited surface water is available (2011). Dry planets have advantages over aqua planets at both edges of the habitable zone. The relative lack of liquid water on a dry planet makes for a much lower inner edge of the habitable zone because it takes much more heat for the runaway greenhouse effect to heat up the planet as discussed earlier. On the outer edge of the habitable zone, dry planets are less likely to freezing completely due to lower amounts of water for ice and snow (Abe et al. 2011). The authors used a global climate model for both aqua and dry Earth-sized planets to calculate their respective habitable zones, finding that the dry planet’s habitable zone was up to three times larger than that of an aqua planet (Abe et al. 2011). Confirmation and further extensions of these results using such cloud schemes as proposed by Yang et al. could drastically affect the current definition of the habitable zone as evidenced by such a large increase in orbital distances for dry planets.
The habitable zone by definition assumes that a planet must be part of a solar system in the first place in order to qualify as habitable. This assumption is not surprising, as it is hard to imagine a planet being warm enough for liquid water without some form of stellar energy. Abbot and Switzer proposed the mechanism for such a planet, however, describing a “rogue planet” that had been ejected from its solar system by interactions with neighboring gas giants (2011). Such a planet would consist of a thick layer of ice on top of a subglacial ocean, kept warm through conduction of geothermal heat through the ice. Decay of radioactive elements in the planet’s interior as well as any residual heat from its formation would provide this geothermal heat (Abbot and Switzer 2011). The study found that a rogue planet with up to 3.5 times the mass of Earth could harbor such a liquid ocean. However, the authors acknowledged the necessary simplifications of their model, especially due to their assumption that the geothermal energy would only be carried through the water by conduction rather than convection, which is much less efficient (Abbot and Switzer 2011). No planets similar to the authors’ proposal had been identified by the time of the article, although they state that such an object could be identified using a powerful telescope if it came within 1000 AU of Earth (1 AU equals the distance from the Earth to the Sun). Abbot and Switzer present a purely theoretical treatment of this concept that remains to be confirmed through experimental data; however, the idea of a habitable rogue planet necessitates a wider look at habitable planets than is present in current research.
The importance of a planet’s individual characteristics on its habitability, such as amount of water and atmospheric composition, cannot be ignored in the categorization of habitable planets. Exoplanet research must move beyond the bias towards Earth-like planets that is present in the classic habitable zone. This diversity makes defining any sort of habitable zone tricky; in fact, there may never be a universal habitable zone. However, future research into the exact influences on habitability as well as further development of the climate models used to study these factors should greatly enhance our current understanding of habitable planets. Studies of the best candidates for habitability may hopefully lead to the first detections of life elsewhere in the universe.
Abbot, D. S., and E. R. Switzer. “The Steppenwolf: A Proposal For A Habitable Planet In Interstellar Space.” The Astrophysical Journal 735.2 (2011): L27. Web. 25 Jan. 2014.
Abe, Yutaka, et al. “Habitable zone limits for dry planets.” Astrobiology 11.5 (2011): 443+. Academic OneFile. Web. 26 Jan. 2014.
Forget, François. “On the Probability of Habitable Planets.” International Journal of Astrobiology 12.03 (2013): 177-85. ArXiv. Web. 22 Jan. 2014.
Kasting, James F., Daniel P. Whitmire, and Ray T. Reynolds. “Habitable Zones around Main Sequence Stars.” Icarus 101.1 (1993): 108-28. ScienceDirect. Web. 31 Jan. 2014.
Kopparapu, Ravi Kumar, et al. “Habitable Zones Around Main-Sequence Stars: New Estimates.” The Astrophysical Journal 765.2 (2013): 131. ArXiv. Web. 23 Jan. 2014.
Pierrehumbert, Raymond, and Eric Gaidos. “Hydrogen Greenhouse Planets Beyond The Habitable Zone.” The Astrophysical Journal 734.1 (2011): L13. ArXiv. Web. 25 Jan. 2014.
Rushby, Andrew J., et al. “Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars.” Astrobiology 13.9 (2013): 833-49. Web. 24 Jan. 2014.
Yang, Jun, Nicolas B. Cowan, and Dorian S. Abbot. “Stabilizing Cloud Feedback Dramatically Expands The Habitable Zone Of Tidally Locked Planets.” The Astrophysical Journal 771.2 (2013): L45. Web. 23 Jan. 2014.