Planets in Other Universes: Habitability constraints on density fluctuations and galactic structure

TL;DR

This paper explores how galactic parameters and density fluctuations influence the potential for habitable planets in hypothetical universes, revealing that habitable zones could be more extensive and diverse than in our universe.

Contribution

It generalizes previous models by analyzing galactic structures, orbital stability, and radiation effects to assess habitability constraints across different universes.

Findings

Habitable zones exist in outer galaxy regions regardless of density fluctuation amplitude.

Higher density fluctuations reduce habitable star fractions due to orbital disruption and radiation.

Some galactic habitable zones could support more habitable planets than our universe.

Abstract

Motivated by the possibility that different versions of the laws of physics could be realized within other universes, this paper delineates the galactic parameters that allow for habitable planets and revisits constraints on the amplitude $Q$ of the primordial density fluctuations. Previous work indicates that large values of $Q$ lead to galaxies so dense that planetary orbits cannot survive long enough for life to develop. Small values of $Q$ lead to delayed star formation, loosely bound galaxies, and compromised heavy element retention. This work generalizes previous treatments: [A] We consider models for the internal structure of galaxies and find the fraction of galactic real estate that allows stable, long-lived planetary orbits. [B] We perform a large ensemble of numerical simulations to estimate cross sections for the disruption of planetary orbits due to interactions with passing stars. [C] We consider disruption due to the background radiation fields produced by the galaxies. [D] One consequence of intense galactic background radiation fields is that some portion of the galaxy, denoted as the Galactic Habitable Zone, will provide the right flux levels to support habitable planets for essentially any planetary orbit. As $Q$ increases, the fraction of stars in a galaxy that allow for habitable planets decreases due to both orbital disruption and the intense background radiation. However, the outer parts of the galaxy always allow for habitable planets, so that the value of $Q$ does not have a well-defined upper limit. Moreover, some Galactic Habitable Zones are large enough to support more potentially habitable planets than the galaxies found in our universe. These results suggest that the possibilities for habitability in other universes are somewhat more favorable and far more diverse than previously imagined.