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1. GAUGING THE POTENTIAL FOR TERRESTRIAL PLANETS IN EXTRASOLAR PLANETARY SYSTEMS

1.1 Overview

	The search for terrestrial planets is an exciting component of extra solar planetary research. Artwork courtesy of JPL/NASA.

Observations and models for extrasolar planet formation can be compared with clues about planet-forming processes in the Solar System to answer the question: how typical are the processes that formed our Solar System? By addressing this question one gains insight into the likelihood of Earth-like planets elsewhere in our Galaxy. Collaborations between researchers studying conditions in nearby stellar systems (astronomers Becklin, Ghez, Hansen, Jura, Morris, Shuping, and Zuckerman) and those focused on the history of our Solar System as revealed through the study of meteorites (cosmochemists Lyons, McKeegan, Wasson, and Young) pave the way for new research opportunities related to terrestrial planet formation. These opportunities arise as a result of consultation between workers in fields that have been historically distinct in their approaches to elucidating how planets form.

The 2002 assessment of the NASA astrobiology program by the National Research Council's Committee on the Origins and Evolution of Life noted the weak level of interaction between research in the Astronomical Origins and the Astrobiology programs relative to analogous interactions between the astrobiology community and, for example, the geobiology community. The research outlined in Section 1 addresses this general shortcoming by strengthening, specifically, links between the Astronomical Origins and Astrobiology programs at UCLA.

One area of astrobiology where collaboration between UCLA astronomers and cosmochemists is proving fruitful is description of the first few million years of planet formation. Through studies of young stars surrounded by gas and dust, in the form of "protoplanetary" rings and disks, that could coalesce to form planets, Ghez, Jura, Morris, McKeegan, Shuping, and Young plan to evaluate the time scales over which such structures evolve, and perhaps infer time scales over which planets form.

It has been suggested that the presence of a Jupiter-like (giant) planet in orbit well outside the conventional habitable zone is a requisite for sustaining life on rocky planets similar to Earth. The gravitational field of a giant planet can relatively quickly cleanse a planetary system of the numerous planetesimals that must be part of the planetary formation process and can shield rocky planets from catastrophic impacts. Searches for giant planets orbiting many astronomical units (AU) from their star are therefore relevant to the problem of identifying planetary systems with favorable habitable zones.

The most successful technique used to detect extrasolar planets - measuring the wobble of a star due to the pull of an orbiting planet (Marcy et al. 2000) - has revealed mostly giant planets that reside far closer to their central star than does Jupiter. Notwithstanding an occasional exception, it is recognized that systems containing such proximal giant planets are, in general, unlikely to harbor planets that can sustain life. Members of the UCLA lead team are engaged in development and application of new techniques geared to image detection of giant planets located in orbits resembling those of the giant planets of our Solar System.

Advances in astronomy that utilize ground-based telescopes equipped with adaptive optics systems (Beckers 1993), as well as an infrared camera on the Hubble Space Telescope (HST), make it possible to image directly Jupiter mass planets (Macintosh et al. 2001). But a caveat is that such detections must be of thermal emission from young, warm planets rather than of reflected starlight from old, cold planets like Jupiter. For this reason, imaging of giant planets in systems resembling the Solar System requires finding stars within about 50 parsec of Earth and not older than a few tens of millions of years. Recent work on isotopes in the hafnium-tungsten system in chondritic meteorites and in terrestrial samples indicate that Earth’s core formed in <30 Myrs (Fitzgerald 2003).

Thus, to identify optimum stars at which to image cooling giant planets and to match the timescale for terrestrial planet formation, Zuckerman and Song will continue their compilation of very young stars close to Earth. Identification and cataloging of young, close, solar-like stars was a major part of the CAB activities over the past several years and will continue, with more of a focus on faint, low-mass stars, during the next five years.

Studies of the origins of meteorites can be used to deduce the processes by which Earth-like planets are formed. Although cosmochemistry is concerned with the origins of planets in the Solar System, the relevance of these studies to astrobiology is enhanced considerably if they are informed by astronomical evidence for analogous processes occurring around other stars. Cosmochemists Lyons, McKeegan, Young and Wasson continue their studies of how rocks and water coalesced to form terrestrial planets in our Solar System, but in a manner that makes better use of astronomical information. Their approach will depart from the more traditional research programs in cosmochemistry in that they will use astronomical measurements obtained by other team members to compare and contrast constraints on planet formation in our Solar System with those seen elsewhere. This cooperation has already resulted in new insights into the significance of some vexing features of meteorites in understanding planet-forming processes.

UCLA astronomers Morris and Shuping will establish a new observational program designed to test results from investigations of photochemistry in the early solar nebula (the Sun's protoplanetary disk, extant 4.6 Gyr ago) that may generally occur during rock formation in young circumstellar disks. Such observations are a direct manifestation of the synergy between the astronomical and cosmochemical communities being cultivated in the UCLA Center for Astrobiology.

Some details of the Center's research programs directed towards understanding the potential for terrestrial planets and Solar System-like planetary systems in our Galaxy are described below.