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

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 waterrich planets with CO 2 and H 2 O 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 H 2 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, S eff , 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.A s 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 groundbased 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.

Modelling the RV jitter of early-M dwarfs using tomographic imaging

“…Due to the low photospheric temperature of M-dwarfs, the planetary orbits that are located in the habitable zone (HZ) of the host star (i.e., within the proper range of orbital distances where liquid water may be stable at the planet surfaces) move closer in. For instance, for a M-dwarf with a typical mass of 0.5 M (like those studied in our paper), the HZ lies in a range 0.2 -0.45 AU (see Kasting et al 2014). It corresponds to orbital periods in the range 36 -157 d, i.e., to RV semi-amplitude of 1.5 -0.96 m s −1 for a planet mass of 5 M ⊕ (as opposed to 0.4 m s −1 for a planet of the same mass orbiting in the HZ of a Sun-like star).…”

A Statistical Comparative Planetology Approach to the Hunt for Habitable Exoplanets and Life Beyond the Solar System

“…One of the guiding principles in the search for other Earth-like planets is the concept of the liquid water habitable zone (Kasting et al 1993;Kasting et al 2014). However, the link between the canonical habitable zone (in terms of orbital distance) and the existence of surface liquid water has not yet been shown observationally, and therefore it does not currently provide a rigorous framework for interpreting the characterization of individual planets.…”

Exploring exoplanet populations with NASA’s Kepler Mission

“…Defined as the region where a rocky planet can maintain surface liquid water, the HZ is a useful starting point for identifying exoplanets that may have an atmospheric chemistry affected by carbon-based life (28). As we broaden our perspective, we stretch and prod the HZ limits.…”