On The History and Future of Cosmic Planet Formation

TL;DR

This paper models the formation histories of Earth-like and giant planets in the Milky Way and the Universe, revealing their expected numbers, formation times, and implications for extraterrestrial civilizations.

Contribution

It combines galaxy formation constraints with planet formation models to estimate the number and timing of planet formation across the Universe and within our galaxy.

Findings

Expected ~10^20 Earth-like and giant planets in the Universe.

Our galaxy hosts ~10^9 Earth-like and ~10^10 giant planets.

High likelihood that we are not the only civilization the Universe will ever have.

Abstract

We 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 Mpc^3), 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.