Researchers combine constraints on galaxy formation histories with planet formation models, yielding the Earth-like and giant planet formation histories of the Milky Way and the Universe as a whole. In the Hubble Volume (10^13 Mpc3 ), we expect there to be ∼ 10^20 Earth-like and ∼ 10^20 giant planets; our own galaxy is expected to host ∼ 10^9 and ∼ 10^10 Earth-like and giant planets, respectively. Proposed metallicity thresholds for planet formation do not significantly affect these numbers. However, the metallicity dependence for giant planets results in later typical formation times and larger host galaxies than for Earth-like planets. The Solar System formed at the median age for existing giant planets in the Milky Way, and consistent with past estimates, formed after 80% of Earth-like planets. However, if existing gas within virialised dark matter haloes continues to collapse and form stars and planets, the Universe will form over 10 times more planets than currently exist. We show that this would imply at least a 92% chance that we are not the only civilisation the Universe will ever have, independent of arguments involving the Drake Equation.
Top-left panel: formation rate (in planets/yr) for Earth-like planets as a function of galaxy stellar mass and cosmic time. The dashed line indicates the median expected growth history of the Milky Way (Behroozi et al. 2013e). The dot-dashed line indicates [Fe/H]=−1.5, which has been suggested (Johnson and Li 2012) as the threshold metallicity for planet formation. Grey shaded areas indicate where galaxies are not expected to exist in the observable Universe. Top-right panel: same, for giant planets. Bottom-left panel: Earth-like planet formation rate multiplied by galaxy number density as a function of stellar mass and cosmic time, i.e., the volume density of planet formation (in planets/yr/comoving Mpc3 /dex). Contours indicate where 50% and 90% of all planet formation has taken place. The symbol indicates the Milky Way’s stellar mass and age at the formation of the Solar System. Bottom-right panel: same, for giant planets.