2013 Annual Science Report
NASA Ames Research Center Reporting | SEP 2012 – AUG 2013
Disks and the Origins of Planetary Systems
This task is concerned with the 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 hydrocarbons 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 suitable distance from its star to support liquid water at the surface, it is in the so-called “habitable zone.” The formation process and identification of such life-supporting bodies is the goal of this project.
Co-I S. Davis works on models of large-scale transport in protoplanetary disks. Davis has contributed to a paper that treats the foundational problem of modelling opacity in protoplanetary disks and exoplanet atmospheres. This paper has been accepted in the ApJ Supplement series.
He has also included into a model for the first time the effect of grain evolution from micron to millimeter sizes and the effect on disk opacity. These concepts will enable models to include predicting habitability zones in extra-solar planetary systems. Preliminary results were presented at the 2012 AbSciCon meeting and at an Ames Symposium in March 2013. 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 Habitable-Zone rocky planets. They find that giant planets are not required to provide Habitable Zone planets with several oceans worth of water. Results were extended and a manuscript was submitted to the Astrophysical Journal in October 2013.
Co-I U. Gorti continued her work on understanding protoplanetary disk conditions that lead to planet formation. Specifically, she has been studying lifetimes of disks acting under the influence of photoevaporation from the central star, and modeling observations of disks with planets to determine their physical and chemical evolution. Early this year, she and her colleagues reported the first direct measurement of the gas mass of a protoplanetary disk using emission lines from HD, detected for the first time from a disk (Bergin et al. 2013). Surprisingly, although TW Hydrae is an old system (~ 10 Myrs), its disk is surprisingly massive at ~50 Jupiter masses. This study may indicate that gas in disks may survive long enough to form gas giant planets and circularize terrestrial planet orbits like those in our solar system. In other related work, Gorti and colleagues looked at emission line diagnostics of winds from disks (Hubble Space Telescope data), and concluded that disk winds due to photoevaporation may not be as vigorous as once believed (Rigliaco et al. 2013). They examined high-resolution data of optical forbidden lines from a small sample of protoplanetary disks, and found that more than half the emission—-previously attributed to winds—-actually arises from bound gas at the disk surface. These results indicate that mass loss rates from photoevaporating disks are low and suggest that gas disk lifetimes may typically be long enough for planet formation.
Co-I K. Zahnle contributed to a review paper addressing the effects of large body impacts on the climates of Earth, Titan, Mars, and Venus. The chapter is published in the book Comparative Climatology of the Terrestrial Planets for the University of Arizona Space Sciences Series. Zahnle also completed two manuscripts addressing the hypothesis that Earth was oxygenated by hydrogen escape. The argument is made that the advent of an O2-rich atmosphere depended both upon the advent of oxygenic photosynthesis and the preparation of an oxygen-friendly surface environment by a history of hydrogen escape from Earth. We contributed a related paper to a special issue of Chemical Geology dedicated to the life and work of Prof. Heinrich Holland. In addition, collaborating with C. Goldblatt, a former Ames team member, Zahnle used detailed radiative transfer models to address whether a runaway greenhouse is possible on Earth today.
Co-I G. 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 (http://oklo.org/downloadable-console/), and a new version of the package, with a considerably faster computational back end written in C rather than Java is now in beta release. During the past year, Laughlin has continued his NAI-supported research on the statistical nature of the galactic planetary census, and he (along with his students) are studying the ramifications that the latest results from the Kepler Mission are having for the formation and evolution of planetary systems. His research along these lines was summarized in a peer-reviewed paper that appeared in the Monthly Notices of the Royal Astronomical Society. The basic conclusion drawn from his work is that the dominant population of planets observed by the Kepler Mission (“super-Earth” type planets, often in multiple-planet systems, and with orbital periods P<100 days) formed in situ and did not require extensive planetary migration to arrive at their current locations. His work introduced and quantified the concept of a Minimum-Mass Extrasolar Nebula (MMEN), which describes the default mode of planet formation in the galaxy, and which we believe will have broad ramifications for the study of planetary origins.
PROJECT INVESTIGATORS:Sanford Davis
PROJECT MEMBERS:Uma Gorti
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Indirect and direct astronomical observations of extrasolar habitable planets.
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Earth's early biosphere.
Effects of extraterrestrial events upon the biosphere