Persistent hazardous environments around stars older than the Sun

Abstract

Collisions amongst comets create belts of orbiting debris and, by using submillimetre wavelength observations, these collision zones can be imaged around nearby stars. An image of the closest Solar analogue, τ Ceti, shows that it possesses at least 20 times the content of the outer Solar System in cool debris particles. The inferred population of parent colliders is around 1 M_{[oplus ]}, also much larger than in the Sun's Kuiper Belt of comets. This system represents a different evolutionary outcome for a Sun-like star, with no Jupiter-like planet but many cometary bodies, and thus a potentially heavy and prolonged history of impacts on any inner terrestrial planets. Since τ Ceti is 10 Gyr old, life would have had to deal with massive bombardment over very long timescales. Furthermore, impactors in the 10 km-upwards class could arrive at intervals of 1 Myr or less, longer than recovery times on Earth, and so similar biology is unlikely. It is presently unknown whether nearby stars typically have comet belts similar to that of the Sun or of τ Ceti; extrapolations of existing data suggest many stars could be at least 2–5 times above the Solar debris level. Future large telescopes will be able to probe down to Solar System levels of cometary debris.