NASA Astrobiology Institute (NAI)

1. Habitable Planets

   Project Investigators: Sanford Davis, David Hollenbach, Gregory Laughlin, Jack Lissauer, D Richard, Kevin Zahnle

   Other Project Members

   Uma Gorti (Collaborator)

   Summary

   This task is concerned with understanding planetary bodies as they 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 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.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of 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

   Project Progress

   Co-I Hollenbach working with collaborator Gorti is 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 examined the lifetime of the planet-forming disks as a function of the mass of the central star. It had been proposed by other researchers that the lifetime of disks may have a maximum for central star masses of order 1 to 3 solar masses. We tested this hypothesis in Gorti and Hollenbach (2008a) and found that there is no such maximum, but that stars with mass from about 0.3 to 3 solar masses all have disks with about the same lifetime (1-2 Myr) against photoevaporation. We did find, in accordance with earlier predictions, that stars more massive than 3 solar masses have short lifetimes. Our models also included stars of higher mass at 7 and 30 solar masses and we found that these have lifetimes of only ~0.1 Myr. Thus, habitable planets do have time to form around stars less massive than 3 solar masses, but are unlikely to form around more massive stars.

   
   

   Spatial distribution of mass-independent isotopic fractionation of oxygen by preferential dissociation of C18O by UV radiation. Maximum enhancement is in a thin band in the cooler, thinner regions of the protoplanetary disk. These isotopes appear in the meteoritic record which indicates large scale migration to the hotter, colder midplane planet forming region.

   Gorti and Hollenbach (2008b) report our final results on the infrared emission from planet-forming disks. We find that various infrared lines probe the density, temperature, and chemical abundances of gas at various distances from the central star. Some of the lines probing the terrestrial planet region around 1 AU include the pure rotational lines of H₂, H₂O, and OH, the ground state vibrational lines of CO and H₂O, and the fine structure lines of [FeI], [FeII], [SiII], [OI], [NeII], [NeIII], [ArII] and [SI]. Lines probing the 100 AU region of the outer disk include low J rotational transitions of CO, and the fine structure lines of [CII] and [OI]. Comparisons of [NeII] observational results from the Spitzer Space Telescope with these models have been made in Pascucci, Hollenbach et al (2007). One key conclusion is that this neon emission line is currently the most sensitive way to detect gas in the planet-forming region of a young star. More work is planned to exploit this sensitive probe of planet forming regions.

   Co-I Lissauer working with Pilcher (NASA Astrobiology Institute) wrote an article entitled “The Quest for Habitable Worlds and Life Beyond the Solar System”. This article describes progress and prospects for detection of 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.

   Co-Is Davis and Richard continue work on modeling large scale transport in protoplanetary disks and have been working on improving numerical schemes to describe accurately turbulent diffusion and meridional advection along with chemical reactions Davis (2007). Richard is also developing models of optical scattering by astrophysical dust grains as reported in Richard & Davis (2008) and presented at the 2008 Astrobiology conference (Richard & Davis 2008 to further our understanding of their influence on the thermodynamical structure of disks as well as on the optical opacity. These models should also provide additional tools to understand astronomical observations of disks, in particular to probe the structure and composition of the dust component. S. Davis, in collaboration with E. Young (UCLA) reported new findings concerning the spatial distribution of oxygen isotopes in the protoplanetary nebula (Davis 2008).

   
   

   Disk mass loss rates and derived disk lifetimes are shown for a range of stellar masses. Disk lifetimes are nearly independent of the mass of the central star for stellar masses less than 3 solar masses and sharply decrease for higher stellar masses. (Gorti and Hollenbach 2008).

   Co-I Laughlin has been working on several projects during the past year, including: (1) the 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 has been completed, and it is fully operational and available for download at www.oklo.org. (2) Investigation of the formation and delectability of the potentially habitable terrestrial planets in orbit around Alpha Centauri B. In the following paragraphs, both projects are described in more detail. In the first instance, we implemented dynamical transit fitting into the Systemic console software. Our user base now 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. The project continues to be the subject of a considerable amount of favorable media attention, stemming from an AP news service article (in early 2007) that appeared in newspapers worldwide. The after-effects of this attention continue to drive users to our website. (http://www.space.com/news/070116_ap_space_online.html/)

   We are finalizing a paper for publication that describes the detailed operation of the console, including the numerical algorithms that we employ for integration and optimization. Co-I Laughlin has recently joined the Lick-Carnegie Planet Search team (along with Co-Is Steve Vogt and Paul Butler) and the systemic console is being used as the primary analysis tool for this new planet survey.

   In area (2) we have also made major progress on our study of planet formation and detectability in the Alpha Centauri Binary system. (Guedes, et al 2008). In brief, we simulated the formation of planetary systems around Alpha Centauri B. The N-body accretionary evolution of a 1/r disk populated with 400-900 lunar-mass protoplanets was followed for 200 million years, and all simulations led to the formation of multiple-planet systems with at least one planet in the 1-2 Earth-mass range at 0.5-1.5 AU. We examined the detectability of our simulated planetary systems by generating synthetic radial velocity observations and including noise based on the radial velocity residuals to the recently published three planet fit to the nearby K0V star HD 69830. Using these synthetic observations, we found that we can reliably detect a 1.8 MEarth planet in the habitable zone of Alpha Centauri B after only three years of high cadence observations. We also found that the planet is detectable even if the radial velocity precision is 3 m/s, as long as the noise spectrum is white. Our results show that the greatest uncertainty in our ability to detect rocky planets in the Alpha Centauri system is the unknown magnitude of ultra-low frequency stellar noise. This paper also generated considerable media interest, including a story in National Geographic News: http://news.nationalgeographic.com/news/2008/03/080310-centauri-earth.html

   Mission Involvement

   Kepler

   Modeling from this module will be used to interpret observations of planets that Kepler discovers within binary star systems.

   Spitzer Space Telescope

   Co-I is investigating Spitzer basic theories and correlating data regarding disk dissipation under this module.

Publications

Davis, S.  (2007).  Ice Formation in Radiated Accretion Disks.  Astrophysical Journal, 660:1580-1587.

Davis, S. & Young, E.  (2008).  The Distribution of Oxygen Isotopes in a Protoplanetary Nebula [Abstract].  Astrobiology, 8.

Gorti, U. & Hollenbach, D.  (In Press, 2008).  Line Emission from Gas in Optically Thick Dust Disks around Young Stars.  Astrophysical Journal, 683:287.

Gorti, U. & Hollenbach, D.  (Submitted, 2008).  Photoevaporation of Circumstellar Disks by FUV, EUV and X-ray Radiation from the Central Star.  Astrophysical Journal.

Guedes, J., Rivera, E., Davis, E., Laughlin, G., Quintana, E. & Fischer, D.  (2008).  Formation and Detectability of Terrestrial Planets around Alpha Centauri B.  Astrophysical Journal, 679(2):1582-1587.

Nisbet, E., Zahnle, K., Gerasimov, M.V., Helbert, J., Jaumann, R., Hofmann, B.A., Benzerara, K. & Westall, F.  (2007).  Creating Habitable Zones, at all Scales, from Planets to Mud Micro-Habitats, on Earth and on Mars.  Space Science Reviews, 129(1-3):79-121.

Ozima, M., Yin, Q.-Z., Seki, H., Podosek, F. & Zahnle, K.  (2007).  Biotic Earth Wind as the Origin of Oxygen Isotope Anomalies in Contemporary Lunar Regolith.  Lunar and Planetary Science XXXVIII., 38(1338):1129.

Pascucci, L., Hollenbach, D., Najita, J., Muzerolle, J. & Gorti, U.  (2007).  Detection of [NeII] emission from Young Circumstellar Disks.  Astrophysical Journal, 663:383-398.

Pilcher, C.B. & Lissauer, J.J.  (In Press, 2008).  The Quest for Habitable Worlds and Life Beyond the Solar System..  Exploring the Origin, Extent and Future of Life.  Cambridge U. Press.

Richard, D.T. & Davis, S.  (2008).  Polarimetric Signature Of Dust and Aggregates Of Silicate [Abstract].  Astrobiology, 8.

Richard, D.T. & Davis, S.  (2008).  Lunar Dust Characterization by Polarimetric Signature: I Negative Polarization Branch of Sphere Aggregates of various Porosities.  Astronomy & Astrophysics, 483:643-649.

Segura, T.L., Toon, O.B., Colaprete, A. & Zahnle, K.  (2008).  A Sustained Greenhouse Climate and Erosion Period on Mars Following an Impact Event.  Lunar and Planetary Science XXXIX ., 39(1391):1793.

Zahnle, K., Arndt, N., Cockell, C., Halliday, A., Nisbet, E., Selsis, F. & Sleep, N.H.  (2007).  Emergence of a Habitable Planet.  Space Science Reviews 35-78, 129(1-3):35-78.

Zahnle, K. & Haberle, R.M.  (2008).  Atmospheric Photochemistry on Modern and Ancient Mars.  Lunar and Planetary Science XXXIX ., 39(1391):2220.