2010 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2009 – AUG 2010

Dynamical Effects on Planetary Habitability

Project Summary

VPL explored numerous features of orbits. We showed that comets are unlikely to have produced more than 1 mass extinction event in the past 500 million years. We catalogued the fractions of habitable zones of nearby stars that are capable of supporting a habitable planet. We also participated in the discovery of two planets whose orbital planes are offset by 30 degrees.

4 Institutions
3 Teams
32 Publications
0 Field Sites
Field Sites

Project Progress

Significant progress was made this year in assessing the role of orbital dynamics on planetary habitability. This year’s research focused on the frequency of cometary impacts on the Earth, the orbital evolution of exoplanets, and tidal effects on exoplanets. These investigations made fundamental breakthroughs in the role of orbital dynamics on planetary habitability.

Numerical simulations of the outer Solar System, Kuiper Belt and Oort Cloud demonstrated that comets are unlikely to have produced more than one mass extinction event on the Earth in the past 500 million years (Kaib & Quinn 2010). We determined this after careful calculations of comets interacting with the giant planets which identified a previously unknown pathway for Oort Cloud objects into the inner Solar System. This discovery leads to a new estimate for the mass of the Oort Cloud which reduced the number of possible comet showers.

We investigated the orbital stability boundaries of two-planet systems, in which one planet is terrestrial (Kopparapu & Barnes 2010). We performed tens of thousands of numerical integrations of hypothetical planetary systems on both sides of the stability boundary in order to precisely identify its location. We then fit the boundary to a function that permits a quick determination of the regions around a star which can support a terrestrial planet. We applied this model to stars known to host one planet and calculated the fraction of that star’s habitable zone which could support a terrestrial exoplanet (Figure 1). We find that most habitable zones are either fully stable or unstable. We now maintain a website that is continually updated that contains this information for known and newly discovered planetary systems.

Barnes, with external collaborators, helped discover that two known exoplanets, Upsilon Andromedae c and d, have orbital planes inclined by 30 degrees relative to each other (McArthur, Benedict, Barnes et al 2010). This is the first system of its kind, and it is markedly different from the nearly coplanar orbits of the Solar System. Barnes, Quinn and collaborators modeled this system and found that the orbits of both planets evolve rapidly, and the system lies very close to the stability boundary.

We continued to explore the role of tides raised by the gravitational interaction between star and planet on exoplanet properties. We mapped out configurations of the CoRoT-7 system which lead to the inner, and first-confirmed terrestrial, exoplanet to be tidally-heated at a rate as large or larger than on Io (Barnes et al. 2010). Barnes supervised research by European graduate students Rene Heller and Jeremy Leconte on the role of obliquity tides on terrestrial exoplanets, finding that tides quickly drive obliquities to zero, which may affect habitability. They also discovered that tides could even be important for Earth-like planets orbiting Sun-like stars, if the orbital eccentricity is large. Work led by undergraduate Kristina Mullins explored how tidal evolution can lead to planets that evolve through different stages due to the evolution of tidal heating, which can change by orders of magnitude over several billion years. We have also developed an internal structure model of tides that will permit more detailed modeling of this phenomenon.

Finally, building on the discovery of the relative inclinations of Upsilon Andromedae c and d, we began, with undergraduate Jonathon Breiner, a strongly interdisciplinary effort to explore how large inclinations drive large obliquity oscillations, that in turn drive climate change. We find that the obliquity of terrestrial planets in systems similar to Upsilon Andromedae may oscillate through 360 degrees in less than 10,000 years, leading to periodic ice and snow packs over large fractions of the planet’s surface.

Figure caption (FracHZ.jpg): Kopparapu & Barnes empirically identified
the orbital stability boundary for a 1 or 10 Earth mass planet with
one larger planet nearby. Planets were coplanar, but had
eccentricities up to 0.6. From this model they then computed the
fraction of each known system’s habitable zone that was stable. This
figure shows a histogram of those fractions. Most habitable zones are
either fully stable or fully unstable, but some intersect the
stability boundary. VPL now maintains a catalog of these fractions,
available at http://www.geosc.psu.edu/~ruk15/planets/index.shtml.