2012 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2011 – AUG 2012

Dynamical Effects on Planetary Habitability

Project Summary

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.

4 Institutions
3 Teams
28 Publications
0 Field Sites
Field Sites

Project Progress

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).

A 1 Earth mass planet orbiting a 0.1 solar mass star could experience a wide range of tidal heating rates as well as insolation. This plot maps out qualitatively different types of planets, based on our experience in the Solar System. The black curves denote the classic habitable zone, the different line styles corresponding to different assumptions. The purple region represents Venus-analogues that are in a runaway greenhouse due to insolation, the green region supports Earth-analogs, and the grey region are snowball worlds experiencing global glaciation. The other colors represent planets experiencing significant tidal heating. The red region denotes planets that are in a runaway greenhouse due to tidal heating, and orange is in a runaway greenhouse due to both radiative and tidal heating. The yellow region experiences tidal heating between Jupiter’s volcanic moon Io and the runaway greenhouse limit. The blue regions correspond to planets with heating rates between the minimum for tectonic activity (as implied by Martian history) and Io’s heat flux: Dark blue is in the habitable zone, light blue is beyond. Tidal heating can eliminate habitability in the habitable zone (red and orange), be very dangerous for life (yellow), be similar to the Earth’s current (non-tidal) heating (dark blue), or be insignificant (green).