2012 Annual Science Report
VPL at University of Washington Reporting | SEP 2011 – AUG 2012
Dynamical Effects on Planetary Habitability
The Earth’s orbit is near-circular and has changed little since its formation. The Earth is also far enough away from the Sun, that the Sun’s gravity doesn’t seriously affect the Earth’s shape. However, exoplanets have been found to have orbits that are elliptical, rather than circular, and that evolve over time, changing shape and/or moving closer or further to the parent star. Many exoplanets have also been found sufficiently close to the parent star that it can deform the planet’s shape and transfer energy to the planet in a process called tidal heating. In this VPL task we investigate how interactions between a planet’s orbit, spin axis, and tidal heating can influence our understanding of what makes a planet habitable. Scientific highlights include modeling of habitable planets around brown dwarfs, the first comprehensive analysis of exomoon habitability, the role of distant stellar companions on planetary system architecture, and an improved understanding of the origins of terrestrial planet composition.
This year VPL continued its work relating orbital properties of planetary systems to habitability. We expanded our studies to include more exotic worlds, which may be discovered soon. We conducted analyses of habitable planets around brown dwarfs, showing that tidal effects may push planets outward through the habitable zone (Bolmont et al. 2011), may be tidally heated to the point of triggering a runaway greenhouse (Barnes et al. 2012), and that the cooling of the host may lead to water loss prior to the arrival in the habitable zone (Barnes et al. 2012). We also showed these latter two effects are even more important for white dwarfs, and hence planetary habitability is very unlikely for these stellar hosts (Barnes et al. 2012).
We completed the first analysis of radiative and tidal effects on the potentially habitable exomoons which may be discovered by Kepler (Heller & Barnes 2012). Exomoons receive additional radiation from reflection by the host planet, and may have climates that are significantly impacted by the frequency of eclipses. Habitability may be further constrained by tidal heating, which can be strong enough to create a runaway greenhouse.
We also explored the potentially deleterious effects of distant stellar binaries on planetary orbits. We find that the binaries can eventually incite instabilities in the planetary system and eject planets on very long timescales, e.g. gigayears (Kaib et al 2011, 2012). This phenomenon may explain the orbits of known exoplanets, as well as represent a barrier for habitability. Although we also find that planets ejected from their planetary systems are unlikely to explain the observed population of free-floating planets (Veras & Raymond 2012).
Finally, we continue to model the compositional variations in the Earth and beyond due to planet formation processes, including review articles (Morbidelli et al. 2012, Raymond & Benz 2012). We followed-up on previous results regarding the inward-then-outward migration of Jupiter as a possible explanation of Mars’ low mass (Pierens & Raymond 2012), and the connection between debris disks and the presence of terrestrial planets (Raymond et al. 2012). We also expanded on our previous work on volatile delivery to terrestrial exoplanets by including the role of migration during the gaseous disk phase (Carter-Bond et al. 2012).
PROJECT INVESTIGATORS:Rory Barnes
Project InvestigatorThomas Quinn
Project InvestigatorSean Raymond
Project InvestigatorRavi Kopparapu
PROJECT MEMBERS:John Armstrong
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Indirect and direct astronomical observations of extrasolar habitable planets.
Sources of prebiotic materials and catalysts
Effects of extraterrestrial events upon the biosphere