Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars (PNAS, 2013)

• Chester Harman
• James Kasting
• Ravi Kopparapu

Full Citation:
Kasting, J. F., Kopparapu, R., Ramirez, R. M., & Harman, C. E. (2013). Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars. Proceedings of the National Academy of Sciences, 111(35), 12641–12646. https://doi.org/10.1073/pnas.1309107110

Abstract:
The habitable zone (HZ) around a star is typically defined as the region where a rocky planet can maintain liquid water on its surface. That definition is appropriate, because this allows for the possibility that carbon-based, photosynthetic life exists on the planet in sufficient abundance to modify the planet’s atmosphere in a way that might be remotely detected. Exactly what conditions are needed, however, to maintain liquid water remains a topic for debate. In the past, modelers have restricted themselves to water-rich planets with CO2 and H2O as the only important greenhouse gases. More recently, some researchers have suggested broadening the definition to include arid, “Dune” planets on the inner edge and planets with captured H2 atmospheres on the outer edge, thereby greatly increasing the HZ width. Such planets could exist, but we demonstrate that an inner edge limit of 0.59 AU or less is physically unrealistic. We further argue that conservative HZ definitions should be used for designing future space-based telescopes, but that optimistic definitions may be useful in interpreting the data from such missions. In terms of effective solar flux, Seff, the recently recalculated HZ boundaries are: recent Venus—1.78; runaway greenhouse—1.04; moist greenhouse—1.01; maximum greenhouse—0.35; and early Mars—0.32. Based on a combination of different HZ definitions, the frequency of potentially Earth-like planets around late K and M stars observed by Kepler is in the range of 0.4–0.5. As the appearance of this special issue of PNAS confirms, the search for exoplanets is by now well underway; indeed, it is one of the hottest research areas in all of astronomy. At the time of this writing, 428 exoplanets have been identified by ground-based radial velocity (RV) measurements and 279 planets have been discovered by the transit technique (1) and confirmed by various other methods, including RV. In addition, more than 3,000 “planet candidates” have been identified by the Kepler mission (2). Most of these planets are either too large or too close to their parent star to have any chance of harboring life. However, both astronomers and the general public are interested in identifying potentially habitable planets and in searching their atmospheres spectroscopically for evidence of life. This leads immediately to the question of how life can be recognized remotely, along with the related question of what are the conditions needed to support it. Liquid water is often mentioned as a prerequisite for life, and we will argue below that this restriction is appropriate for the astronomical search for life. However, some researchers have questioned this assumption (3), and so we begin by explaining why the presence of liquid water is so important in the search for life on planets around other stars.

URL:
https://www.pnas.org/content/111/35/12641

VPL Research Tasks:
Task C: The Habitable Planet