Habitable Zones Around Main-Sequence Stars: New Estimates

Abstract

Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on 1-D, cloud-free, climate model calculations by Kasting et al. (1993). However, this model used band models which were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in Kasting et al. (1993) model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO 2 atmosphere. A conservative estimate for the width of the HZ from this model in our Solar system is 0.95-1.67 AU.Here, an updated 1-D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K and M stars. New H 2 O and CO 2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water loss (inner HZ) and maximum greenhouse -2 -(outer HZ) limits for our Solar System are at 0.99 AU and 1.70 AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 K and 7200 K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water loss limits for stars with T ef f 5000 K which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending upon the host star's spectral type. We suggest that conservative estimates of the HZ (water loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on η ⊕ , so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

Asteroid flux towards circumprimary habitable zones in binary star systems

“…The definition, modelling, and computation of the "classical HZ" (hereafter CHZ) is given in Kasting (1988Kasting ( , 1991, Kasting et al (1993), and Kopparapu et al (2013). All these studies are based on a 1D cloud-free climate model, where the inner edge of the HZ is computed by increasing the surface temperature and the outer edge by increasing the CO 2 partial pressure (maintaining a constant surface temperature at 273 K).…”

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…However, no study has yet investigated the influence of the rotation period on the outer edge of the habitable zone, which is defined by the maximum greenhouse limit (Kasting et al 1993;Kopparapu et al 2013). This limit corresponds to the orbital distance where the surface is completely frozen in a CO 2 -rich atmosphere.…”

Evaluating galactic habitability using high-resolution cosmological simulations of galaxy formation

“…Most commonly, this term is applied to individual star systems. Radiative transfer calculations of the atmospheric response of an Earthlike planet to insolation (Kasting, Whitmire & Reynolds, 1993;Kopparapu et al, 2013Kopparapu et al, , 2014 allow the identification of planetary orbits that permit surface liquid water. This is the principal definition of "the habitable zone", and we henceforth refer to it as the stellar habitable zone (SHZ).…”