An estimate of the age distribution of terrestrial planets in the universe: Quantifying metallicity as a selection effect

Planets such as the Earth cannot form unless elements heavier than helium a re available. These heavy elements, or "metals," were not produced in the B ig Bang. They result from fusion inside stars and have been gradually build ing up over the lifetime of the Universe. Recent observations indicate that the presence of giant extra-solar planets at small distances from their ho st stars is strongly correlated with high metallicity of the host stars. Th e presence of these close-orbiting giants is incompatible with the existenc e of Earth-like planets. Thus, there may be a Goldilocks selection effect: with too little metallicity, Earths are unable to form for lack of material ; with too much metallicity, giant planets destroy Earths. Here I quantify these effects and obtain the probability, as a function of metallicity, for a stellar system to harbor an Earth-like planet. I combine this probabilit y with current estimates of the star formation rate and of the gradual buil dup of metals in the Universe to obtain an estimate of the age distribution of Earth-like planets in the Universe. The analysis done here indicates th at three-quarters of the Earth-like planets in the Universe are older than the Earth and that their average age is 1.8 +/- 0.9 billion years older tha n the Earth. If life forms readily on Earth-like planets-as suggested by th e rapid appearance of life on Earth-this analysis gives us an age distribut ion for life on such planets and a rare clue about how we compare to other life which may inhabit the Universe. (C) 2001 Academic Press.