Role of the galaxy in periodic impacts and mass extinctions on the Earth

Impacts of large comets and asteroids on the Earth are energetic enough to cause mass extinction of species. Several studies have concluded that large impact craters and extinctions show a correlation, and that both records display periods in the range of ∼31±5 m.y. This might be the result of periodic or quasiperiodic showers of Oort cloud comets with a similar cycle. One candidate for a pacemaker for such periodic comet showers is the Sun's vertical oscillation through the plane of the galaxy, with a half-period over the past 250 m.y. estimated in the range of ∼33 ± 7 m.y. Thus, major events in the history of life on the Earth may be partly related to the dynamics of the galaxy. This result could have significant astrobiological implications for other planetary systems with outer comet clouds.