Abstract
Abstract A planet’s climate can be strongly affected by its orbital eccentricity and obliquity. Here we use a one-dimensional energy balance model modified to include a simple runaway greenhouse (RGH) parameterization to explore the effects of these two parameters on the climate of Earth-like aqua planets—completely ocean-covered planets—orbiting F-, G-, K-, and M-dwarf stars. We find that the range of instellations for which planets exhibit habitable surface conditions throughout an orbit decreases with increasing eccentricity. However, the appearance of temporarily habitable conditions during an orbit creates an eccentric habitable zone (EHZ) that is sensitive to orbital eccentricity and obliquity, planetary latitude, and the spectral type of the host star. We find that the fraction of a planet’s orbit over which it exhibits habitable surface conditions is larger on eccentric planets orbiting M-dwarf stars, due to the lower broadband planetary albedos of these planets. Planets with larger obliquities have smaller EHZs, but exhibit warmer climates if they do not enter a snowball state during their orbits. We also find no transient RGH state on planets at all eccentricities. Rather, planets spend their entire orbits either in an RGH or not. For G-dwarf planets receiving 100% of the modern solar constant and with eccentricities above 0.55, an entire Earth ocean inventory can be lost in 3.6 Gyr. M-dwarf planets, due to their larger incident X-ray and extreme ultraviolet flux, can become desiccated in only 690 Myr with eccentricities above 0.38. This work has important implications for eccentric planets that may exhibit surface habitability despite technically departing from the traditional habitable zone as they orbit their host stars.
Highlights
With the rapidly expanding catalog of discovered exoplanets, much effort will be dedicated to characterizing these planets and identifying those that may be habitable—that is, possessing conditions conducive to the presence of liquid water (Kasting et al 1993; Kopparapu 2013; Seager 2013)
We find that in the case of a G-dwarf planet with an eccentricity e = 0, and an Earth-like atmosphere, the inner edge of eccentric habitable zone (EHZ) corresponds to a stellar flux of 119% S0, while the outer edge corresponds to 82.5% S0, with a significant ice cap
On the K, G, and F-dwarf planets, the warming effects of eccentricity have a stronger impact on the inner rather than outer edge of the EHZ, due to the extra energy required to thaw sea ice on an ice-covered planet, compared with the transition from a water world to a moist hot house
Summary
With the rapidly expanding catalog of discovered exoplanets, much effort will be dedicated to characterizing these planets and identifying those that may be habitable—that is, possessing conditions conducive to the presence of liquid water (Kasting et al 1993; Kopparapu 2013; Seager 2013). A first-order approach to identifying a potentiallyhabitable planet is to pinpoint one that orbits within the boundaries of its host star’s habitable zone—the region around a star where a planet with an Earth-like atmosphere may be warm enough for liquid water to flow on its surface (Kasting et al 1993). The inner edge of the habitable zone (IHZ) is determined by the onset of the runaway greenhouse, a climate state in which the atmosphere becomes opaque to outgoing thermal (longwave) radiation, inhibiting a planet’s ability to cool and desiccating the surface, leaving zero water content on the planet. The traditional boundaries of the habitable zone are based on the assumption of Earth-like planetary conditions and do not take into account the range of orbital eccentricities or obliquities possible in extrasolar planetary systems. Counter to the climatic state of present Earth, at an obliquity of 23.44◦, if a planet’s obliquity is 54◦ or
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
