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An observational signature of evolved oceans on extra-solar terrestrial planets.pdf

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An observational signature of evolved oceans on extra-solar terrestrial planets

a r X i v : a s t r o - p h / 0 4 0 2 5 8 4 v 1 2 5 F e b 2 0 0 4 AN OBSERVATIONAL SIGNATURE OF EVOLVED OCEANS ON EXTRA-SOLAR TERRESTRIAL PLANETS M. Jura Department of Physics and Astronomy, University of California, Los Angeles CA 90095-1562; jura@clotho.astro.ucla.edu ABSTRACT The increase in luminosity with time of a main sequence star eventually can lead to substantial evaporation of the oceans on an orbiting terrestrial planet. Subsequently, the gas-phase H2O in the planet’s upper atmosphere can be pho- todissociated by stellar ultraviolet and the resulting atomic hydrogen then may be lost in a wind. This gaseous envelope may pass in front of the host star and produce transient, detectable ultraviolet absorption in the Lyman lines in systems older than 1 Gyr. Subject headings: astrobiology – (stars): planetary systems 1. INTRODUCTION The possible existence of life on other planets is of central interest in modern astronomy. A standard working hypothesis is that liquid water is required for life, and here we describe an observational signature of evolved oceans on extra-solar terrestrial planets. The occultation of HD 209458 by a Jovian planet has demonstrated that it is possible to study the atmospheres of extra-solar planets with absorption line spectroscopy during a transit (Charbonneau et al. 2000, 2002). The Lyman α spectrum of HD 209458 observed during its companion planet’s transit can be explained as an outflow of hydrogen from this planet (Vidal-Madjar et al. 2003. Liang et al. 2003) that also has entrained other gases (Vidal-Madjar et al. 2004). Below, we describe an extension of this technique to study gaseous winds from terrestrial planets. To date, only Jovian mass extra-solar planets have been detected. The discovery of terrestrial planets by occultations of their host stars is the goal of future space missions such as Kepler (see, for example, Jenkins 2002). In addition to transits discovered by photometry, it may be possible to perform trans