2011 Annual Science Report
VPL at University of Washington Reporting | SEP 2010 – AUG 2011
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 the star 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 the finding that tidal effects could be strong enough to cause a planet to overheat and ultimately lose its ocean, that large changes in the direction of the spin-axis of a planet could potentially increase the range of distances from the star in which the planet could remain habitable, and that the Sun may have moved significant distances outward through the Galaxy during its lifetime, changing the rate of at which large bodies have hit the Earth.
This year VPL continued its work relating orbital properties of planetary systems to habitability. We found a connection between the apparent brightness of debris disks around stars and the likelihood of terrestrial planets in or near the habitable zone (Raymond et al., 2011). This research could guide observers as they search for habitable planets.
We also explored tidal effects on planets as they orbit main sequence stars and brown dwarfs. For the former, we find that obliquities can be modified by tides very quickly around low-mass stars (Heller et al., 2011). We also determined that the spin properties of habitable planets can be modified by tides if the planets have large eccentricities, even for planets orbiting in the habitable zone of solar-mass stars where tidal forces are typically weaker. For brown dwarfs we find that planets have relatively short residence times in the habitable zone and hence have a decreased likelihood for habitability (Bolmont et al., 2011).
We have explored the role of orbital architecture on the obliquity evolution of exoplanets in systems with large mutual inclination (Barnes et al., 2011). These planets may undergo wild obliquity swings (Armstrong et al., 2010), which actually increase the width of the habitable zone by suppressing the ice-albedo feedback at the outer edge (Domagal-Goldman et al., 2011). Finally we began to explore the possibility that tidal heating may be strong enough to produce a runaway greenhouse on planets in the traditional habitable zone (Barnes et al., in prep). This possibility can only be realized for low luminosity objects, but could lead to sterilization of habitable planet candidates.
We also proposed that Jupiter performed and inward-then-outward migration which can explain the masses and orbits of the terrestrial planets (crucially explaining the mass of Mars) as well as the asteroid belt (Pierens and Raymond, 2011; Walsh et al., 2011). We also examined how orbital migration of our Sun through the galaxy may have impacted the structure of the Kuiper Belt. In particular we find that the orbit of Sedna has a 25% chance of resulting from this migration. (Kaib et al., 2011).
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