Most Sun-like stars in the Galaxy reside in gravitationally bound pairs of stars (binaries). Although long anticipated the existence of a 'circumbinary planet' orbiting such a pair of normal stars was not definitively established until the discovery of the planet transiting (that is, passing in front of) Kepler-16. Questions remained, however, about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we report two additional transiting circumbinary planets: Kepler-34(AB)b and Kepler-35(AB)b, referred to here as Kepler-34b and Kepler-35b, respectively. Each is a low-density gas-giant planet on an orbit closely aligned with that of its parent stars. Kepler-34b orbits two Sun-like stars every 289 days, whereas Kepler-35b orbits a pair of smaller stars (89% and 81% of the Sun's mass) every 131 days. The planets experience large multi-periodic variations in incident stellar radiation arising from the orbital motion of the stars. The observed rate of circumbinary planets in our sample implies that more than ∼1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.
The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a "fossil" record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles5. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System.
