2009 Annual Science Report
NASA Ames Research Center Reporting | JUL 2008 – AUG 2009
Disks and the Origins of Planetary Systems
This task is concerned with understanding the evolution of complexity as primitive planetary bodies form in habitable zones. The planet formation process begins with fragmentation of large molecular clouds into flattened protoplanetary 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 envelope 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 its surface, it lies within the so-called “habitable zone.” The goal of this project is to understand the formation process and identification of such life-supporting bodies.
We are studying the processes that determine the abundances of bio-important molecules such as water and O2 in star-forming molecular clouds via theoretical modeling. We extended previous models of the disk’s photodissociation region and included the freezing of species, simple grain surface chemistry and the desorption of ices (Hollenbach et al. 2009). Under certain conditions in the opaque cloud interiors, we find that gas-phase elemental oxygen freezes out as water ice and that the elemental C/O abundance ratio can exceed unity, leading to complex carbon chemistry. These results have important implications for the composition of protostellar disks that eventually form from the molecular cloud material.
We are are modeling photoevaporation processes in protoplanetary disks around young stars. Photoevaporation is caused by the heating of the protoplanetary disk surface by EUV (Lyman continuum), FUV, and X-rays from the central star. This photoevaporation clears the disk and helps determine the likelihood of habitable planet formation around a star of a given mass.
In the current reporting period we studied viscously accreting protoplanetary disks as they are irradiated by ultraviolet and X-ray photons from a solar-mass central star using static, thermo-chemical disk models (Gorti and Hollenbach 2009) and, more recently, time-dependent models (Gorti, Dullmond and Hollenbach 2009). We use results from our gas disk modeling studies (Gorti and Hollenbach 2008) to calculate mass loss rates due to photoevaporation and the lifetimes of protoplanetary disks. We calculate that disks around solar mass stars survive for 4 million years for typical stellar and disk properties. We find that Far-ultraviolet (FUV) and X-ray-induced photoevaporation and viscous accretion are both important mechanism for depleting disk mass.
Photoevaporation rates are most significant at ~ 1-10 AU where planets may be forming and in the outer disk beyond 30 AU. We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial. We followed disk evolution around stars of different masses and found that disk survival time is relatively independent of mass for stars with masses less than 3 solar masses and that disks around higher mass stars are short-lived (~105 years).
One of us worked with C. Pilcher (NASA Astrobiology Institute) to revise their article entitled “The Quest for Habitable Worlds and Life Beyond the Solar System”, which was published in September 2009. This article describes progress and prospects for detecting extrasolar planets and biosignatures thereon, and is geared towards a very well educated but diverse audience which includes philosophers and theologians in addition to scientists and engineers.
We continued our work on models of large-scale transport in protoplanetary disks. We have been developing numerical schemes to accurately describe turbulent diffusion and meridional advection along with chemical reactions. We addressed on aspects of the search for water on the Moon and has published a paper on the LCROSS impact event Davis (2009). We are developing models of optical scattering by astrophysical dust grains as reported in Astronomy & Astrophysics (Richard & Davis, 2008a) and presented at the 2008 Astrobiology conference (Richard & Davis 2008b). This work will further our understanding of scattering and radiative transport on the thermodynamical structure of disks as well as on the optical opacity. In collaboration with E. Young (UCLA), we reported new computations of the spatial distribution of oxygen isotopes in the protoplanetary nebula (Davis 2008). We recently extended this work to include the convection effect of chemically induced oxygen isotope distributions so that we can study the migration of these species into the planet-building zones and ultimately into the meteoritic record where these interesting istotopic anomalies are recorded. This work will be reported at the upcoming 2010 Astrobiology conference.
We continued several complementary projects during the past year, including: (1) the continued development of the Systemic Console, a flexible GUI-based computational tool for analyzing radial velocity and transit data for extrasolar planetary systems. The software code base is fully operational and available for download at www.oklo.org. Over the past year, we have made a number of specific improvements that enhance its functionality. (2) Investigation of the formation and delectability of the potentially habitable terrestrial planets in orbit around nearby red dwarf stars. In the following paragraphs, both projects are described in more detail. In the first instance, we implemented Markov Chain Monte Carlo optimization and error estimation into the code. Our user base exceeds 2000 users, drawn from a wide spectrum of professional astronomers, amateur astronomers, students, and the general public. Systemic users have made several important discoveries, including the first characterizations of the low-mass planets Gl 581c, and 55 Cnc e and f.
We published a paper (Meschiari et al. 2009, Proc. Astron. Soc. Pacific, in press) the detailed operation of the console, including the numerical algorithms that we employ for integration and optimization. One of us (Laughlin) has been working as a member of the Lick-Carnegie Planet Search team (Co-PI, along with Co-PI’s Steve Vogt and Paul Butler) and the systemic console is being used as the primary analysis tool for this new planet survey. The survey has had a number of recent successes. In particular, we have detected several super-Earth category planets and submitted a report for publication in the Astrophysical Journal.
We have also made progress in our study of planet formation and detectability, with our particular focus on the accretion of potentially habitable planets having terrestrial-mass and orbiting low-mass red dwarf stars. Our simulations (reported in Montgomery and Laughlin 2009) show that, for plausible ranges of protoplanetary disk densities, it is possible for large terrestrial planets to accrete in the inner 0.1 AU of the disks orbiting nascent red dwarf stars. The resulting class of planets is of considerable interest because its members induce readily detectable photometric dips (of order 1% depth) when the planets have orbits that transit the parent star. In our paper, we presented a detailed set of Monte-Carlo simulations of telescope detection networks, which indicate that the detection of such planets is readily feasible via a low-cost space mission such as the proposed TESS mission.
Regarding the early evolution of Earth’s environment, we documented a substantial decline in the quantity of Ni in the Proterozoic oceans compared to the Archean oceans, as measured by the amount of Ni precipitated in Banded Iron Formations. As a cause, we related the decline of oceanic Ni to the decline in the production of komatiitic lavas (which are Ni-rich), which in turn is caused by the cooling of the Earth. This trend had potentially significant implications for the biosphere. Ni is an essential nutrient for methanogens and sits at the heart of the key enzyme for fixing CO. Thus the first consequence of less Ni in oceans is that methanogens are starved and are put at a competitive disadvantage with respect to other fermenters such as sulfate reducers. Our previous work showed that if the biological methane source is throttled down, then a fundamentally oxidative photosynthetic global ecosystem could develop an atmosphere having substantial amounts of O2.
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.
Effects of extraterrestrial events upon the biosphere