Background

Central to the context for life is the formation and evolution of habitable planets. Here we define habitable planet in the "classical" sense, meaning a planet with an atmosphere having liquid water on the surface. This is appropriate, because extrasolar planets on which liquid water and life are present at the surface should be observable spectroscopically in a search for evidence of life (Leger, 1993; Angel and Woolf, 1996), whereas subsurface biospheres may not be detectable.

The study of habitable planets directly addresses goals and objectives in the Astrobiology Roadmap and is especially timely now. From ground based observations over 100 extrasolar planets have been discovered to date. It is now known that extrasolar planetary systems exist, as opposed to single planets. Within a few years space based missions using transit photometry, such as the Kepler Mission to be launched in 2007, will show whether terrestrial type (predominantly rocky), in addition to jovian type (gas giant), planets are ubiquitous in extrasolar planetary systems. Furthermore, for all detected planets including terrestrial-sized planets, the transit observations will allow determination of planet radius, orbital semi-major axis, and under the most optimal circumstances, orbital eccentricity. An additional important scientific objective of the Kepler Mission is to determine whether a detected planet is within the habitable zone of the parent star, where habitable zone is defined as that region of space surrounding a star within which liquid water could exist at a planet's surface.

Well founded theoretical models now exist regarding planet formation, protoplanetary nebula evolution, and planetary system dynamics. There has also been progress in understanding the factors important for determining the habitable zone around a particular star. Thus, for the first time, there is now the opportunity to meaningfully combine observational and theoretical efforts in order to quantitatively evaluate the frequency and characteristics of extrasolar planets, and to evaluate whether or not they are habitable. A major objective of the proposed work is to conduct theoretical studies of terrestrial planets which will be directly relevant to the planning of, and scientific data interpretation for, missions such as Kepler and more advanced future missions, such as the planned Terrestrial Planet Finder Mission (TPF), which will be capable of obtaining spectral information of the atmospheres.

Habitable planets, in the sense we have defined habitable planets, must be terrestrial planets. We are undertaking a multifaceted, interconnected research program that addresses the formation, evolution, and climatology of terrestrial type planets, including terrestrial planets in our own Solar System, since they provide some guidance for understanding extrasolar terrestrial planets. Obviously we cannot resolve all the questions in their entirety. Instead, the goal is to identify particular key questions, and address those.

Figure 1 illustrates that the unifying theme of the proposed tasks is planet habitability. There is not necessarily a direct link between each of the processes we propose to address (e.g. protoplanetary disk processes are not directly linked to terrestrial planet climatology), however, each crucially affects planet habitability. As an example, from only a climatological perspective, solar type stars would be expected to be associated with habitable planets. However if the star formed in a stellar cluster, as many stars do, it is quite possible that the solar star's protoplanetary disk did not last long enough for planets to form at all. Since the density of suitable candidate stars in the Sun's vicinity is an important driver of the TPF mission design, considering just climatological criteria in a search strategy for habitable planets would not be prudent. Elements of the proposed research are relevant to several existing NASA and NSF R&A programs and would complement research being done under those programs. However, the research proposed here is beyond the scope of any single R&A program, and is beyond the interest of any group of R&A programs to coordinate. Hence, the research belongs within the purview of the Astrobiology Institute.

Research Objectives

The research objectives are to understand: how protoplanetary disks evolve and form terrestrial planets; what kinds of planetary systems are likely to harbor terrestrial planets; how volatiles are delivered to terrestrial planets by impacting planetesimals, and how impacts affect the climatology of terrestrial planets; the particular evolutionary paths of terrestrial planets that result in habitability; and how external characteristics, such as orbital eccentricity, and internal factors, such as atmospheric circulation, affect the habitability of terrestrial planets.

Relevance of Research

The proposed research is directly relevant to Goal 1 of the Astrobiology Roadmap, namely to understand habitable planets in the Universe. The proposed research addresses several of the issues described in the Roadmap, specifically, modeling the formation, evolution, and stability of planetary systems that might harbor habitable planets; the delivery of key volatiles, such as water, to potentially habitable planets; planetary processes which affect habitable conditions; the role of impacts on habitability. Portions of the proposed work dealing with the composition of the early atmospheres of Earth and an Earth-like Venus are also relevant for understanding spectra of extrasolar planets which will be obtained by advanced astrobiology missions such as Terrestrial Planet Finder (TPF).

The proposed research will benefit and enhance planned space based astronomical detection and observational astrobiology missions, such as Kepler and TPF, all of which have as goals the detection and characterization of terrestrial type extrasolar planets, and their classification as to habitability. Kepler is scheduled to be launched during the time period covered by this proposal, so the work proposed and its relevance to these missions is very timely.

Recent Progress

Co-Is Hollenbach and Laughlin, working with outside collaborators F. Adams and U. Gorti, have been modeling the photoevaporation of protoplanetary disks around young stars. During the past year, they have finished a calculation of the evaporation of disks around low mass (solar-type) stars caused by the external ultraviolet radiation from a nearby massive star in the birth cluster (Adams et. al. 2004). It is shown that photoevaporation can often affect the region where, at least in our solar system, the giant planets form.  The rapid photoevaporation of gas in as close as 10 to 20 AU also then means that gas disappears quite rapidly in the inner regions as well.  Hollenbach and Adams (2004 a, b) applied the results of the Adams et al paper to study whether photoevaporation could explain a sharp cutoff in the Kuiper Belt, the sharp drop of hydrogen content in the giant planets Neptune and Uranus compared to Saturn and Jupiter, and the deficit of planets seen in large clusters,  For conditions similar to the large cluster observed in the Trapezium of Orion, the conditions are so harsh --especially in the inner parts of the cluster--that all giant planet formation may be quenched, and even terrestrial planets may be strongly affected.

Co-I Greg Laughlin and collaborators at UC Santa Cruz have been investigating the dynamical viability of possible terrestrial planets orbiting in the habitable zones of known planet-bearing stars. Of particular interest are potentially habitable orbits in systems with several known planets (e.g. GJ 876, or 55 Cancri).  Because of the large amount of computing required, and because of intense public interest in both extrasolar planets and habitable worlds, they are building a distributed computing solution for this problem along the lines of the seti@home model. Specifically, they are designing a public user interface that leverages the existing www.transitsearch.org candidates site (see http://www.ucolick.org/~laugh/) and which uses the Berkeley Open Infrastructure for Network Computing Package (http://boinc.berkeley.edu/) to handle the back-end aspects of the distributed application.

Collaborators Segura and McKay, and Co-I Toon, investigated analytical solutions to the equations governing a runaway greenhouse to show that there is a region in flux and temperature space where there are actually two stable solutions.  Both are stable and correct.  The warm solution can only be reached if there is a significant temperature perturbation to the system, such as a large impact  They are investigating different regions of phase space and assess how large an impact would be required to produce a runaway greenhouse for a given planet such as Mars.

Co-I Hollingsworth has conducted both mechanistic modeling and fully coupled climate modeling studies using a variety of modeling tools to model the climate of the Earth and Earth-like planets under extreme but plausible environmental conditions. Adapting a mechanistic climate modeling approach helps identify important positive and negative feedbacks between various components of the climate system.  During this investigation, the focus has been on two main areas of research: (a) mechanistic fully-coupled climate simulations using the KNMI/ECBILT modeling system; and (b) realistic and extreme coupled climate simulations using the NCAR/CCM.

Co-I Davis and Collaborator Richard continue to investigate possible sources of turbulence in the protoplanetary nebula that will enhance mixing and inter radial transfer. They published a short contribution (Richard and Davis) on this subject in Astronomy and Astrophysics. They used this and related information to compute the migration of a condensation front (thought to be a factor in the rapid growth of gas giants such as Jupiter). A paper is submitted on this subject and a presentation was made at a recent conference on chemistry in the protoplanetary nebula.

Collaborators Rabbette, Pilewskie, and McKay, together with Co-I Young, submitted a paper for publication showing that the clear sky upward longwave flux as a function of sea surface temperature (SST) near SST = 300 K, observed over the tropical Pacific Ocean, exhibits the classic signature of the runaway greenhouse. The key role water vapor plays in the tropical clear sky greenhouse effect was investigated. The study highlights the fact that the water vapor greenhouse effect depends not only on the total column integrated amount, but more importantly on the vertical distribution of the tropospheric water vapor.

Ames Team Members Participating in this Investigation

NAME	ROLE	ORGANIZATION	EMAIL
Davis, Sanford	Lead Co-Investigator	NASA Ames Research Center	Sanford.S.Davis@nasa.gov
Hollenbach, David	Co-Investigator	NASA Ames Research Center	hollenbach@ism.arc.nasa.gov
Hollingsworth, Jeffrey	Co-Investigator	San Jose State University Foundation	jeffh@humbabe.arc.nasa.gov
Laughlin, Gregory	Co-Investigator	University of California, Santa Cruz	Laughlin@ucolick.org
Lissauer, Jack	Co-Investigator	NASA Ames Research Center	Jack.J.Lissauer@nasa.gov
McKay, Christopher	Collaborator	NASA Ames Research Center	cmckay@mail.arc.nasa.gov
Richard, Denis	Collaborator	San Jose State University	drichard@mail.arc.nasa.gov
Segura, Teresa	Collaborator	University of Colorado	segurat@colorado.edu
Sleep, Norman	Co-Investigator	Stanford University	norm@pangea.stanford.edu
Tolbert, M.	Collaborator	University of Colorado	tolbert@colorado.edu
Toon, Owen Brian	Co-Investigator	University of Colorado, Boulder	Toon@lasp.colorado.edu
Zahnle, Kevin	Co-Investigator	NASA Ames Research Center	kzahnle@mail.arc.nasa.gov

See the following Ames Team research pages:

Formation and Evolution of Habitable Planets
Prebiotic Organics from Space
Origin and Early Evolution of Proteins and Metabolism
Biosignatures in Chemosynthetic and Photosynthetic Systems
Modeling Ecosystems and Biospheres
Hind-Casting Past Environments
Interplanetary Pioneer