Habitable zone predictions and how to test them

TL;DR

This paper reviews habitable zone predictions, challenges assumptions, and emphasizes the need for first principles approaches and future observations to better identify planets likely to support life.

Contribution

It critically examines classical habitable zone models and advocates for dynamic, first principles methods and future missions to improve habitability assessments.

Findings

Classical HZ assumptions may be oversimplified.

A-stars could be promising targets for habitability.

Future data will enable statistical analysis of habitable planets.

Abstract

The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-too-distant future, when possibly hundreds to thousands of HZ planets will yield the statistical data we need to go beyond just finding habitable zone planets to actually determining which ones are most likely to exhibit life.