2012 Annual Science Report

NASA Ames Research Center Reporting  |  SEP 2011 – AUG 2012

Disks and the Origins of Planetary Systems

Project Summary

This task is concerned with the formation and evolution of complex habitable environments. The planet formation process begins with fragmentation of large molecular clouds into flattened disks. This disk is, in many ways, an astrochemical “primeval soup” in which cosmically abundant elements are assembled into increasingly complex organic compounds and mixed in the dust and gas within the disk. Gravitational attraction among the myriad small bodies leads to planet formation. If the newly formed planet is a distance from its star that is suitable to support liquid water at the surface, it is in the so-called “habitable zone” (HZ). The formation process and identification of such life-supporting bodies is the goal of this project.

4 Institutions
3 Teams
12 Publications
0 Field Sites
Field Sites

Project Progress

Co-I S. Davis works on models of large-scale transport in protoplanetary disks. Davis has written a manuscript, now in review, on the location of water ice in the protoplanetary nebula using a new model that takes into account the evolution of the Sun (luminosity and temperature) over the million or so years that the disk is active.

Figure 1. Effect of Solar Luminosity on the Ice Line: The left panel indicates the ice line assuming an evolving sun. The right panel shows the ice line assuming current and constant solar luminosity.

He has also included for the first time the effects on disk opacity of grains as they evolve and grow from micron to millimeter sizes. These results show that the “ice line” is not a simple curve, but a two-branched line with a cusp that defines the innermost location of water ice (Fig. 1). The changing shape of the ice line over time gives some indication of the ultimate location of the HZ. Work is underway to extend these concepts to include predicting habitability zones in extra solar planetary systems. Preliminary results were presented at the 2012 Astrobiology Science Conference (AbSciCon2012). Co-I Davis also uses the opacity models to study tenuous atmospheres of the Moon and similar bodies.

Co-I J. Lissauer and collaborator E. Quintana continue their study of how planets beyond the ice line affect the accretion of volatiles by rocky planets within HZs. They find that giant planets are not required to provide HZ planets with several oceans worth of water. Results were presented at the June 2012 meeting of the American Astronomical Society (AAS Meeting #220, #505.07).

Co-Is Gorti and Hollenbach have been working on protoplanetary disk photoevaporation models with simultaneous dust grain growth and settling. The disk evolution models now include viscous accretion and photoevaporation by ultraviolet and X-ray photons from the central star, and they allow for the redistribution of angular momentum and disk surface density due to the influence of any embedded planets. Dust grains are considered to evolve under a coagulation/fragmentation equilibrium model, and settling and turbulence affect their vertical distribution at a given radius in the disk. Small dust grains (sub-micron sized) are coupled with the gas and are either accreted or carried away in the photoevaporative flow, whereas larger bodies are retained in the disk. Gorti and Hollenbach find that the inclusion of dust evolution does not significantly influence the timescale over which the disk disperses (Fig. 2); however it might have implications for the solid content of the disk that is left behind and contributes to planet assembly. Thermochemical modeling of Spitzer and Herschel line emission data from evolved transition disks around GM Aur and T Cha, both of which are suspected to harbor embedded planets, indicates the presence of significant amounts of gas in their inner dust holes and supports the early onset of planet formation. This past year, Gorti was invited to speak on this research at several venues, including HIA in Canada, SETI Public Colloquium Series, Leiden University, Netherlands, Kyoto University, Japan and the SOFIA Science Center. Gorti and Hollenbach are currently working on hydrodynamical models of photoevaporating disks to improve certain analytical aspects of our disk evolution theory and to continue our thermochemical modeling of gas line emissions from protoplanetary disks.

Figure 2. The mass evolution of a 0.1Mo disk around a solar mass star, under the effects of photoevaporation, viscosity and dust evolution (solid line). The dashed line shows a model with no dust evolution where the dust is assumed to be static and well mixed with the gas for comparison. There is little difference in the overall dispersal timescales, as the disk mass is mainly in the gas component and mostly unaffected by the ongoing dust evolution in the disk.

Co-I Zahnle is lead author of a hypothesis paper that argued that hydrogen escape to space is the fundamental reason why Earth’s atmosphere became oxygenated. There is of course little doubt that oxygenic photosynthesis is necessary for an oxygen-rich atmosphere like Earth’s. But available geological and geochemical evidence suggest that at least 200 Myr, and possibly more than 700 Myr, elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. This interregnum implies that at least one additional necessary condition for abundant free oxygen had to be met. Zahnle et al. argue that this condition was the oxidation of the surface and crust to the point where molecular oxygen became more stable than competing reduced gases such as methane, and that the cause of Earth’s surface oxidation is the same as that for other planets with oxidized surfaces, namely hydrogen escape to space. The duration of the interregnum would have been determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before molecular oxygen became favored. Finally, it is suggested that, because continents are more oxidized than the mantle, hydrogen escape may have determined the history of continental growth.

Figure 3. A cartoon history of the Great Oxidation Event. Hydrogen escape rates and the range of MIF-S values are indicative of general trends. Approximate tropospheric O2 levels are estimated but should not be taken too literally. Below 10-5 PAL (Present Atmospheric Levels), O2 levels are ill defined because the molecule would not in general be well mixed in the troposphere. For the illustration it is presumed that O2 is drawn down after ice ages by rapid weathering of glacial debris. Key dates are taken from Papineau et al (2007), Scott et al (2008), and Guo et al (2009).

Co-I Laughlin continues to spearhead the development of the publicly available Systemic Console software for the analysis of radial velocity and photometric data sets for extrasolar planets. The most up-to-date release of the software can be downloaded from www.oklo.org, and a new version, with a considerably faster computational back end written in C rather than Java, is currently being tested and is scheduled to be released within the next few months. During the past year, Laughlin has continued his NAI-supported research on the statistical nature of the galactic planetary census, and he and his students are studying the ramifications that the latest results from the Kepler Mission are having for the formation and evolution of planetary systems. Between September 2011 and October 2012, four papers appeared in the refereed literature, in addition to conference proceedings, a book review, and an abstract for an AAS poster talk.

In other activities, on October 19, 2011, Laughlin gave a large public talk, The H. C. Vernon Memorial Lecture, at the Mt. Cuba Observatory on the Campus of the University of Delaware in Dover, Delaware. The lecture entitled, “Searching for Other Habitable Worlds,” detailed the progress made to date on the discovery of extrasolar planets and was attended by over 200 people. Laughlin also consulted and presented on camera for a Science Channel television episode of the “Through The Wormhole” series hosted by Morgan Freeman. Laughlin’s episode, titled “Can we outlive the Sun?” is scheduled to air in the show’s fourth season in 2013. Laughlin’s work was featured in an extensive article in the Santa Cruz Sentinel on July 13, 2012.

    Sanford Davis
    Uma Gorti

    David Hollenbach

    Gregory Laughlin

    Jack Lissauer

    Kevin Zahnle

    Elisa Quintana

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 1.2
    Indirect and direct astronomical observations of extrasolar habitable planets.

    Objective 2.1
    Mars exploration.

    Objective 4.3
    Effects of extraterrestrial events upon the biosphere