2013 Annual Science Report

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

Habitable Planet Formation and Orbital Dynamical Effects on Planetary Habitability

Project Summary

The VPL explores how variations in orbital properties affects the growth, evolution and habitability of planets. The formation process must deliver the appropriate ingredients for life to a planet in order for it to become habitable. After planets form, interactions between a habitable planet at its host star and/or other planets in the system can change planetary properties, possibly rendering the planet uninhabitable. The VPL models these processes through computer models in order to understand how the Earth became and remains habitable, as well as examining and predicting habitability on planets outside the Solar System.

4 Institutions
3 Teams
12 Publications
0 Field Sites
Field Sites

Project Progress

The VPL explores how orbital dynamics impact planetary habitability. There are two main thrusts of this effort: planet formation and the orbital evolution of habitable planets. Undergraduate and graduate students played vital roles in many of these research efforts.

Terrestrial planets form through pair-wise accretion of small bodies. Through this process, the ingredients for life must be endowed on habitable planets. The VPL examines how habitable planets can form both in the Solar System and among exoplanets. Team member Sean Raymond provided in-depth reviews of planet formation for the periodic international meeting Protostars and Planets 6 (Raymond et al., 2013a; Davies et al., 2013). He and his students also considered numerous accretional phenomena such as a final accumulation of volatiles (Raymond and Schlichting, 2013), the distribution of dust left over from formation (Bonsor et al., 2013), and planetary migration (Pierens, et al., 2013; Coussou et al., 2013). Additionally, Timpe and Barnes (2013) demonstrated that the observed orbital dynamics of multi-planet systems naturally arises from the ejection of fully formed planets at the very end of planet formation (see figure). Raymond also contributed to a chapter in Reviews in Mineralogy and Geochemistry which included a review of the timescale for volatile delivery to the growing Earth from different sources (Marty et al., 2013).

VPL modeled the environments at the planetary system-galaxy interface. These regions are far from the host star and hence features can be more easily separated from the starlight. We showed that systems that eject planets can have a closer cometary reservoir than the Solar System’s (Raymond & Armitage, 2013). We also showed wide binary star systems can be perturbed by passing stars violently enough to disrupt planetary systems, including their habitable zones (Kaib et al., 2013). Quinn and colleagues studied the influence of outer solar system architecture on the structural evolution of the Oort Cloud (OC) and the resultant flux of Earth-crossing comets. In particular, they worked to quantify the efficiency giant planets as “planetary protectors.” They find that either a Saturn or Jupiter mass planet can reduce the cometary flux through the inner Solar System by a factor of 2 or 3 compared to a system with only ice giants (Lewis et al., 2013).

Finally, the VPL has initiated several new investigations spanning formation to long-term orbital evolution. We are examining how large relative inclinations between orbital planes affects dynamical stability (Deitrick et al., in prep), and rotational dynamics and climates of potentially habitable planets (Armstrong et al., in prep.). We are exploring how tidal orbital evolution can differentiate between rocky and gaseous exoplanets (Barnes, in prep.), and couple with atmospheric mass loss to transform small gaseous exoplanets into large rocky exoplanets (Luger et al., in prep.). We continue to probe the planet formation process by simulating terrestrial planet accretion (Raymond et al., in prep.), and examining how planet-planet scattering can identify the relative frequency of Earth-mass exoplanets relative to the better sampled giants (Timpe et al., in prep.).

The orbits of planets in multi-planet systems evolve with time due to mutual gravitational interactions. This evolution can be parameterized by the precession of the planets' orbits. A wide range of behaviors is possible, and can be divided into two qualitatively distinct classifications: circulation or libration. For the former, the relative orientations of the elliptical orbits cycle through a full 360 degrees. For the latter, the orientations are confined to a narrow range of values. The parameter epsilon quantifies how close adjacent pairs of planets are to the boundary between these two types of motion. The VPL simulated the ejection of original planets from systems and found the resulting population (dashed line) is a near-perfect match to the observational data of massive exoplanets (thick grey line). Thus, the orbits of terrestrial planets are likely to be found to follow a similar trend and we can predict both their formation histories and their orbital properties, constraining their potential habitability.