Research Byte

Published in the RSAA Lunations
Vol1 Issue15 1–30 April 2021

Multi-Parameter Approach to Habitability

Forty years before Voyager 1 turned its lenses and snapped the famous image of the pale blue dot, 30-year-old biochemistry associate at Boston University School of Medicine by the name of Isaac Asimov published his first novel “Pebble in the Sky”, and in it, he wrote:

“To the rest of the Galaxy, if they are aware of us at all, Earth is but a pebble in the sky. To us it is home, and all the home we know.”

The same could be said about all the planets in the universe as they all, from afar, look just like a pebble in the sky or a pale dot. However, those looks could be deceiving. This goes to the heart of the problem we confront when searching for life. How do we use what we know based on the experience we have of life on Earth while acknowledging that life on other planets might be “something completely different”? How do we avoid searching just for a new Earth and instead keep looking for any life no matter how different?

If the past 26 years of exoplanet study have revealed anything, that is the diversity of planets in the universe. Using ground- and space-based telescopes, we have so far detected more than 4,000 planets, with more than 300 found in the habitable zones of their host stars, 60 of those potentially habitable and 24 of those more likely to have rocky composition and surface water.

However, within the next decade, upcoming surveys and telescopes working together will be able to remotely detect potential biosignatures in exo-Earth atmospheres and discover signs of life beyond our Solar System. In order to make the most of the near future observational opportunities, optimal target selection will be of the utmost importance. The selection of targets for this characterisation so far relies on the ambiguously defined concept of habitability, which is currently constrained by only the density of the planet and the distance from its host star. However, numerous planetary and astronomical factors influence an exoplanet's ability to maintain liquid water.

Continuing to expand to a multi-parameter approach to habitability by including factors such as magnetic field, plate tectonics, albedo, stellar type, orbit characteristics, tidal locking, and impact events will enable us to prioritise planets most likely to maintain liquid water. By analysing, modelling, and constraining how these factors interact on any given planetary body, we can generate a flexible framework for prioritisation involving multiple observable characteristics that influence continuous planetary habitability. Based on the results, we will be able to provide a revised model of planetary habitability and suggest a suitable strategy for future astrobiological and biosignature observations. This will help us determine optimal targets for near-future ground- and space-based spectroscopic observations of planetary atmospheres and the possible detection of life in space.

Sarah McIntyre

_{Image Credit: ESO/M. Kornmesser}