The prevalence of earth-like planets

The discovery of giant planets in orbit around nearby stars has increased g eneral interest in Earth-like planets. In the context of SETI, an Earth-lik e planet must have liquid water exposed on its surface, an appropriate abun dance of biogenic elements, an early history allowing chemical evolution th at leads to life, and subsequent climatic stability for at least 4.5 billio n years. In our solar system, only Earth satisfies these criteria. However, an examination of the atmospheres of Mars and Venus as we know them today suggests that all three inner planets began with very similar endowments of volatiles. Icy planetesimals (comets) are strongly implicated as the major carriers of these volatiles, suggesting that proper starting conditions sh ould be relatively common throughout the galaxy. Long-term stability requir es a planet of the right size at the right distance from its star. Mars off ers an enlightening example of an apparently Earth-like planet that is not Earth-like today. Satellites of these new giant planets also offer interest ing opportunities but must satisfy the same criteria. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.