2015 Annual Science Report

VPL at University of Washington Reporting  |  JAN 2015 – DEC 2015

Habitable Planet Formation and Orbital Dynamical Effects on Planetary Habitability

Project Summary

This task explores how habitable planets form and how their orbits evolve with time. Terrestrial planet formation involves colliding rocks in a thin gaseous disk surrounding a newborn star and VPL’s modeling efforts simulate the orbital and collisional evolution of a few to millions of small bodies to determine the composition, mass, and orbital parameters of planets that ultimately reach the habitable zone. After formation, gravitational interactions with the star and planet can induce short- and long-term changes in orbital properties that can change amount of energy available for the climate and to illuminate the planetary surface. The VPL simulates these effects in known and hypothetical planetary systems in order to determine the range of variations that permit planetary habitability.

4 Institutions
3 Teams
19 Publications
0 Field Sites
Field Sites

Project Progress

As part of VPL’s research in the area of planetary habitability, the team examines the roles of habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy.

In planet formation, Quinn and Backus ran supercomputer simulations of planet formation and migration around M dwarf stars, where planet-forming material can be relatively scarce near the star. As large planets are unlikely to form under these conditions, this work explores mechanisms for super-Earth migration and “parking” in the habitable zone. Raymond and colleagues explored the late stages of terrestrial planet formation, revealing: Jupiter and Saturn can migrate outward in multiple orbital configurations (Pierens et al. 2015), and gas giants provide a barrier to the inward migration of more distant planetary cores. We proposed that Jupiter’s presence prevented the ice giants from becoming super-Earths and identified observational tests of the model (Izidoro et al. 2015a). Furthermore, we proposed that the ice giants’ masses and orbits can be reproduced if they formed from a population of cores whose inward migration was blocked by Jupiter. Both of which imply a common origin for ice giants and hot super-Earths (Izidoro et al. 2015b). We show that the asteroid belt’s orbital structure is a key constraint on models of the formation of the terrestrial planets. The classical model appears to fail systematically in reproducing the inner Solar System (Izidoro et al. 2015c).

The VPL explored numerous aspects of the formation of the Moon and the subsequent coupled evolution of the Earth-Moon system. We showed that the compositional similarity between the Earth and Moon may have arisen naturally during the accretion process (Mastrobuono-Battisti et al. 2015). We showed that thermal blanketing by Earth’s water atmosphere limits the rate the Moon’s orbit can evolve in the first ten million years after the Moon-forming impact, provided the first good estimates of how quickly the surface of the Earth can freeze after the Moon-forming impact, and calculated the geothermal heat flow for the first hundred million years or so of the Hadean (Zahnle et al. 2015). We also extended satellite modeling to exomoons (natural satellites of exoplanets) to elucidate the role of planetary radiation on the exomoon’s habitability (Heller & Barnes, 2015).

We created a new code for calculating tidal and spin interactions in multiple planetary systems to show how planet-planet-tide interactions affect the long-term evolution of planetary systems (Bolmont et al. 2015) as shown in Fig. 1. Barnes (2015) showed that tidal circularization of exoplanet orbits proceeds at different rates and demonstrated how large samples of high quality transit data (e.g. from TESS) can be used to identify the boundary between rocky and gaseous exoplanets. Finally, Sleep examined the similarities and differences between tidally-induced cracks on Enceladus and the San Andreas Fault, which can be used to infer processes on tidally evolving exoplanets.

Figure 1. Rotational Evolution of the Kepler-62 Planets

Evolution of the rotational properties of the Kepler-62 planets assuming both planet-planet interactions and tidal effects are the only influences. Top: The time evolution of the obliquities, which eventually decay to a steady state value that is determined by the masses and orbital properties of the companion planets. Bottom: The evolution of the rotation periods are shown by the solid colored lines. The dashed lines correspond to the equilibrium rotation period predicted by tidal theory. The solid black line corresponds to the rotational period of the star. Innovative modeling efforts such as these provide valuable insight into the past and present state of potentially habitable planets' rotation period and obliquity, which are fundamental inputs to climate models.
Figure 1. Rotational Evolution of the Kepler-62 Planets Evolution of the rotational properties of the Kepler-62 planets assuming both planet-planet interactions and tidal effects are the only influences. Top: The time evolution of the obliquities, which eventually decay to a steady state value that is determined by the masses and orbital properties of the companion planets. Bottom: The evolution of the rotation periods are shown by the solid colored lines. The dashed lines correspond to the equilibrium rotation period predicted by tidal theory. The solid black line corresponds to the rotational period of the star. Innovative modeling efforts such as these provide valuable insight into the past and present state of potentially habitable planets' rotation period and obliquity, which are fundamental inputs to climate models.

To explore the star’s gravitational influence on planetary habitability, VPL scientists performed N-body simulations of real and hypothetical planetary systems. Deitrick, Barnes, Quinn, Luger and colleagues used orbital stability models to reveal the full three-dimensional orbital architecture of the Upsilon Andromedae system, the first system discovered with misaligned orbital planes (Deitrick et al. 2015). Barnes, Deitrick, Quinn, Raymond and colleagues discovered that terrestrial exoplanets in mean motion resonances with non-planar orbits can evolve chaotically for at least 10 Gyr (Barnes et al. 2015) as shown in Fig. 2. This chaotic motion can result in huge, rapid changes in eccentricity and inclination, which can severely impact a planet’s potential to support life.

Figure 2. Orbital Evolution of Exoplanets in an Inclined Orbital Resonance The orbital evolution of an Earth-like planet (black) and a Neptune-sized planet (red) with orbital periods that are in a ratio of 3 to 1. Left: The evolution over a million years shows that the Earth-like planet's orbit can reach very high eccentricities and inclinations. Right: Over 10 billion years, the system reaches consistently reaches large values and is strongly chaotic. Despite this chaos, the orbital resonance maintains stability and the system does not self-disrupt.