The robustness of planet formation

Abstract

The detection of two planets in an open star cluster demonstrates that planetary systems are able to survive disruptive events, such as supernova explosions, during the dense, early stages of the life of such clusters. See Letter p.55 Until now only four planets — with masses similar to Jupiter — have been found orbiting stars in old open clusters, compared with more than 800 — mostly Neptune-sized — orbiting 'field stars' outside clusters. Most stars and planets form in open clusters that break up within a few hundred million years as stars drift away to become field stars. Older open clusters survive because they were denser in stars when they formed, a stellar environment very different from that of other planet-hosting field stars. This paper, part of the Kepler Cluster Study, describes observations of the transits of two Sun-like stars by planets smaller than Neptune in the 1-billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in our Galaxy.