2013 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2012 – AUG 2013

Stellar Effects on Planetary Habitability and the Limits of the Habitable Zone

Project Summary

In this task VPL Team members explore the interactions between a planet and its parent star and how these interactions affect whether or not the planet can support life. These interactions can be radiative, with light from the star affecting the planet’s climate, or UV from stellar flares affecting the radiation environment at the planet’s surface. Or they interactions can be gravitational, with the star periodically deforming planets on elliptical orbits and thereby transferring energy into the planet. Both radiative and gravitational effects can input too much heat into a planet’s environment and cause it to lose the ability to maintain liquid water at the surface. Research this year included looking at the limits of the habitable zone with new calculations, exploring how gravitational tidal energy could cause a planet to lose its ocean, and understanding the effects that tidal deformation and incoming stellar radiation would have on the habitability of exomoons.

4 Institutions
3 Teams
8 Publications
0 Field Sites
Field Sites

Project Progress

In this task, team members explored both gravitational and radiative effects of the host star on planetary habitability and the limits of the habitable zone. This was done for a diversity of host star types, including main sequence stars, and white and brown dwarfs.

Kopparapu, Ramirez, Kasting, Robinson, Domagal-Goldman, Meadows and collaborators updated the original Kasting et al., (1993) calculations of the habitable zone by using modern, more complete, molecular absorption line lists, and extended the model simulations down to planets orbiting much cooler M dwarfs (Kopparapu et al., 2013). Due to the use of stronger water and CO2 absorption coefficients, the resultant recalculations indicate that the inner edge of a planet’s habitable zone (HZ) moves further out from the star. For our Solar System, they found the inner edge of the HZ to be at 0.99 AU and the outer edge to be at 1.70 AU. An illustration of these HZ results is shown in the figure below:

Habitable Zone Limits
Habitable Zone limits in terms of stellar fluxes, with some of the confirmed terrestrial size/mass exoplanets (ref. i).

Using these results, Kopparapu recalculated work done by Dressing, Charbonneau and collaborators to conclude that the occurrence rate of Earth-size planets around Kepler M-stars is between 40%-50% (Kopparapu, 2013). Kasting, Kopparapu, Ramirez and Harman wrote a review on remote life detection criteria, habitable zone boundaries and the frequency of Earth-like planets around other M and late-K stars (Kasting et al., 2014, in press), and Kopparapu, Ramirez and Kasting are working on new calculations for the inner edge of the habitable zone for dry planets.

Walkowicz is continuing work to calculate the rate and energy distribution of flares on low mass stars, using data from NASA’s Kepler Mission. Walkowicz and undergraduate Nicole Loncke created code to detect and vet flares in the Kepler lightcurves. Stellar flares are important to habitability considerations for planets around low mass stars, as planets in the habitable zones of these stars orbit at close radii, making them particularly vulnerable to the effects of flares. Previous work with Segura and Meadows (Segura et al. 2010) showed that single flares were not necessarily detrimental to planetary habitability, but work still needs to be done on the effect of multiple flares with a range of energies. The flare rates and energy distributions determined from Kepler will be used to constrain future coupled photochemical-climate models of the response of a planetary atmosphere to repeated flares, as is expected to occur around low mass stars.

Crisp and colleagues continued to maintain the Spectral Mapping Atmospheric Radiative Transfer (SMART) model, which is widely used throughout the VPL project. Goldblatt, Robinson, Zahnle and Crisp used SMART to recalculate the radiation limit at which a runaway greenhouse state would occur for a terrestrial planet (Goldblatt et al., 2013). They found that Earth was much closer to a runaway greenhouse state than had previously been thought. Claire, his student Rushby and colleagues recast the habitable zone equations to more easily take into account stellar evolution, and used actual, current luminosities of stellar hosts to improve habitable zone estimates for potentially habitable Kepler planets. They found that several were now outside the habitable zone (Rushby et al., 2013).

We also explored the direct effect of the star’s gravity on planetary habitability for a number of different scenarios. In Barnes et al (2013), Barnes, Goldblatt, Meadows, Kasting and colleagues showed that tidal heating can trigger a runaway greenhouse on planets in the habitable zones of M dwarf stars, and especially for those that have planetary companions that can maintain the planet’s orbital eccentricity. A “tidal greenhouse” can be maintained for upwards of 1 Gyr prior to circularization and hence planets can evolve into the habitable zone after losing all their primordial water content. Barnes & Heller (2013) showed that a tidal-heating generated greenhouse is also possible on planets in the habitable zones of white dwarfs and brown dwarfs. For these planets, eccentricities as small as 10-6 can be sufficient to trigger the runaway greenhouse, severely limiting the chances that they may support life. Heller & Barnes (2013) showed that exomoons are also in danger of a tidal greenhouse. Mars-sized moons (or larger) that are located within ~10 planetary radii from a Jupiter-mass exoplanet can be tidally heated to the runaway greenhouse for more than 100 Myr with eccentricities of ~0.001. These exomoons may be detectable with Kepler through transit timing variations, and the magnitude of the signal can be used to constrain the level of tidal heating, and hence habitability. Luger and Barnes explored the evolution of atmospheric loss for small gas-dominated planets under the combined effects of stellar radiation and orbital evolution (Luger et al., in prep). Armstrong, Barnes, Domagal-Goldman and Meadows worked together on understanding the coupled effects of obliquity and orbital evolution on planetary climate and surface ice coverage (Armstrong et al., submitted).