Exobiology and Evolutionary Biology

Formation, Evolution, and Detection of Ocean-Bearing Planets Close to Cool Stars

PI: Eric Gaidos

We propose to lay the theoretical foundation for a possible future discovery by exoplanet missions beginning with NASA’s Kepler: ocean-bearing (and potentially habitable) planets on short-period orbits around stars cooler than the Sun. Planets a few times the mass of the Earth, and on orbits with periods of weeks to months around K stars are arguably those habitable worlds most likely to be detected by Kepler and have their mass measured by ground-based observations. Our goals are to (i) investigate the mechanisms by which rocky and/or icy planets between the size of Earth and Neptune can form close to their parent stars, and make theoretical predictions of their water content; (ii) delineate the conditions under which water oceans are stable on such planets as a function of the mass, effective emitting temperature, and atmospheric surface pressure (if any); (iii) determine the effect of a thick ocean on the thermal evolution of the planet, its volcanic history, and the magnetic field created by a dynamo in its core; and (iv) ascertain whether oceans could be indirectly detected by their effect on the composition of an atmosphere. We will carry out these investigations using a suite of models, including (a) a model of planet-forming disk evolution that will produce the initial conditions for (b) an “off the shelf” dynamical model of planetary accretion and dynamics; (c) a thermodynamic analysis of the stability of liquid water oceans as a function of effective temperature, planetary water abundance, and surface gravity; (d) a 1-D model of a pure steam atmosphere coupled to a fully convecting ocean-ice shell model; (e) a widely-used 3-D spherical model of convection in a rocky mantle; (f) a single-point (“zero-D”) model of the thermal evolution of a metal core; and (g) a simple box model of ocean circulation and chemistry. This project will create a multidisciplinary collaboratory research network involving astrobiologists, geophysicists, and climatologists and several graduate students, two of whom will be supported by this research grant. The collaborators and students will meet annually at an “ocean worlds” workshop held each year at the PI’s institution. Undergraduate students will be involved through the NASA Space Grant program at the PI’s institution. Our research will assist in interpreting Kepler findings (and follow-up by the Hubble Space Telescope), and will make predictions for observations by the James Webb Space Telescope. Our work will better illuminate the circumstance under which oceans can appear and persist, and how prevalent are ocean-bearing planets on short-period orbits. Understanding their compositions and how and on what original orbits these objects formed is essential to arguments about their habitability, and the more general question of the frequency of habitable planets in the universe. It will also provide exciting opportunities for education and public outreach. Workshop participants will hold a public lecture and panel discussion, and the PI, who teaches a science documentary filmmaking course, will propose an affiliated E&PO proposal to fund the production of a short documentary on Earth’s oceans, with involvement by students.