The prevalence of Earth-like planets: (Pesek Lecture, Beijing, China: October 1996)

Abstract

The discovery of giant planets in orbit around nearby stars has increased general interest in Earth-like planets. In the context of SETI, an Earth-like planet must have liquid water exposed on its surface, an appropriate abundance of biogenic elements, an early history allowing chemical evolution that leads to life, and subsequent climatic stability for at least 4.5 billion 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 should be relatively common throughout the galaxy. Long-term stability requires a planet of the right size at the right distance from its star. Mars offers an enlightening example of an apparently Earth-like planet that is not Earth-like today. Satellites of these new giant planets also offer interesting opportunities but must satisfy the same criteria.