The search for habitable worlds: From the Terrestrial Planet Finder to SETI

Abstract

The primary topics of this dissertation are (1) target selection for searches for extrasolar life, especially for the Search for Extraterrestrial Intelligence (SETI) and the Terrestrial Planet Finder (TPF) and (2) remote detection of biosignatures, especially with regard to TPF. Chapter 1 gives a brief introduction to the field of astrobiology, and to the search for life on other planets. Chapters 2 and 3 ask, "What are the best places in the Universe to search for Earth-like life?" A class of stars, "habstars," is defined as stellar systems that are potentially habitable to Earth-like complex life. The physical properties of habstars are derived from the biological requirement of habitable zone stability, and these properties are translated into observable characteristics. In Chapter 2, the Catalog of Nearby Habitable Stellar Systems (HabCat), containing ∼17,000 "habstars" within 300 parsecs, is presented for use as a new target list for the Search for Extraterrestrial Intelligence with the Allen Telescope Array. In Chapter 3, HabCat is augmented with other targets of interest, including a list of ∼250,000 stars within 1000 parsecs from the Tycho-2 Catalog that are likely to be main-sequence (based on their proper motions) F, G, K and M stars (based on their B-V colors), old open clusters, and the nearest 100 stars. This work is refined in Chapter 4 for the Terrestrial Planet Finder (TPF), a mission to image and spectroscopically analyze extrasolar terrestrial planets. The TPF Target List Database is presented, and it contains all Hipparcos stars within 30 parsecs plus data that are relevant to planetary habitability and detectability. From this database, a sample of targets is selected and recommended for observation based on suitability for life. Chapter 5 asks, "What are the spectral signatures of a habitable, or inhabited, planet?" The Earthshine spectrum, from 0.3 to 2.5 microns, is presented and used to illustrate the spatially unresolved spectrum of a planet with abundant water and life. Water vapor, oxygen, ozone, methane, and carbon dioxide are unambiguously detected, while the vegetation signature is less certain. Chapter 6 explores possibilities for extending the earthshine work and submits recommendations for improving the TPF database content and usability.