On the history and future of cosmic planet formation

Peter Behroozi, Molly S. Peeples

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


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 (1013 Mpc3), we expect there to be ~1020 Earth-like and ~1020 giant planets; our own galaxy is expected to host ~109 and ~1010 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 per cent of Earth-like planets. However, if existing gas within virialized 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 per cent chance that we are not the only civilization the Universe will ever have, independent of arguments involving the Drake equation.

Original languageEnglish (US)
Pages (from-to)1811-1817
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Dec 1 2015
Externally publishedYes


  • Galaxies: formation
  • Planets and satellites: gaseous planets
  • Planets and satellites: terrestrial planets

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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