Tidal Limits to Planetary Habitability
Rory Barnes, Brian Jackson, Richard Greenberg, Sean N. Raymond
aa r X i v : . [ a s t r o - ph . E P ] J un Tidal Limits to Planetary Habitability
Rory Barnes , , Brian Jackson , Richard Greenberg , Sean N. Raymond , ABSTRACT
The habitable zones of main sequence stars have traditionally been definedas the range of orbits that intercept the appropriate amount of stellar flux topermit surface water on a planet. Terrestrial exoplanets discovered to orbit Mstars in these zones, which are close-in due to decreased stellar luminosity, mayalso undergo significant tidal heating. Tidal heating may span a wide range forterrestrial exoplanets and may significantly affect conditions near the surface.For example, if heating rates on an exoplanet are near or greater than thaton Io (where tides drive volcanism that resurface the planet at least every 1Myr) and produce similar surface conditions, then the development of life seemsunlikely. On the other hand, if the tidal heating rate is less than the minimumto initiate plate tectonics, then CO may not be recycled through subduction,leading to a runaway greenhouse that sterilizes the planet. These two casesrepresent potential boundaries to habitability and are presented along with therange of the traditional habitable zone for main sequence, low-mass stars. Wepropose a revised habitable zone that incorporates both stellar insolation andtidal heating. We apply these criteria to GJ 581 d and find that it is in thetraditional habitable zone, but its tidal heating alone may be insufficient forplate tectonics. Subject headings: astrobiology — (stars:) planetary systems — stars: individual(GJ 581) — stars: low mass
1. Introduction
The discovery of extrasolar planets has made detecting and recognizing life-bearingplanets outside the solar system a real possibility. Considerations of stellar radiation and Department of Astronomy, University of Washington, Seattle, WA, 98195-1580 Virtual Planetary Laboratory Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 Center for Astrophysics and Space Astronomy, University of Colorado, UCB 389, Boulder CO 80309-0389 etal. et al. et al. ∼ . E ) (Pollack et al. etal. et al. E is a reasonable approximation for this critical mass, andhence, as is now standard, we assume that habitable planets must be less massive than thisvalue.Transit and radial velocity surveys are most sensitive to planets in small orbits ( < ∼ . etal. et al. ≥ . E ) (Mayor et al. e.g. Lammer et al. et al. et al. , in prep.). Here we focus on thethird issue and show that tides may place important constraints on planetary habitability.For example, the recently-announced 2 M E planet GJ 581 e (Mayor et al. et al. showed that this planet’s eccentricity occasion-ally reaches values of 0.1. Applying common models of tidal heating ( e.g. Peale et al. et al. et al. et al. e.g. plate tectonics) that maintain long-term climate stability,increasing potential habitability (Williams et al. et al. (2008a) showed thattidal heating in the center of the IHZ could be substantial for a few illustrative cases. Here weidentify the boundaries of this “tidal HZ” (THZ) for a range of stellar and planetary masses, M ∗ , and M p , respectively, as well as semi-major axis a and orbital eccentricity e . Thisdefinition excludes radiogenic sources of heat, which dominate the heating on the Earth.Estimating radiogenic heating rates on exoplanets seems a daunting task as it dependssensitively on the composition of the planet (itself a result of many different formationpathways [Raymond et al. and enough internal heat to drive plate tectonics, but not somuch as to cause Io-like volcanism. Combined with the other restriction, these additionalrequirements significantly reduce the range of detectable habitable environments around low-mass stars. In § § E planets. In §
2. Tidal and Atmospheric Models2.1. The Insolation Habitable Zone