Habitability Models for Exoplanet Sciences

Habitability is positively correlated with the abundance and diversity of life on Earth. This correlation should also be true for other habitable worlds unless they operate under a different biological process than Earth. Future space telescopes, like the JWST, will search for biosignatures from the atmosphere of nearby transiting exoplanets, such as those around the TRAPPIST-1 system. Planets with higher habitability, i.e., with a potential for a larger biosphere, are expected to produce stronger biosignatures. Thus, estimates of habitability would be essential to interpret and complement biosignature detections. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them. However, procedures for measuring habitability, known as Habitat Suitability Models (HSMs), are well established in biology since the early 1980s. Ecologists have been using these models for more than four decades to study the habitability of Earth from local to global scales. These models are used to characterize the critical environmental factors that are responsible for the gradual transition from low to high habitability states. The models are extendable to global biospheres and hence suitable for exoplanet studies.

The main goal of this project is to develop a set of global habitability models for the exoplanet science community. Together with exoplanet climate models, these models could be used to explore the potential habitability states of known exoplanets. Our specific objectives are to (1) create and validate different global habitability models with terrestrial data, (2) determine the minimum set of measurable planetary parameters for global habitability assessments, and (3) apply these models to a few exoplanets of interest, such as the TRAPPIST-1 system. These models would improve the comparison and characterization of potentially habitable worlds, prioritize target selections, and help study correlations between habitability and biosignatures.

We are adapting and expanding the methods from the HSMs to global biospheres using a fundamental mass-energy habitability model. Our model is validated with terrestrial biomes using ground and satellite data from NASA’s Earth Science Data Systems (ESDS). Archived data from different global climate models (e.g., ROCKET-3D) and our runs of 1D and 2D climate models are used to explore habitability space as a function of measurable planetary parameters such as radius, mass, and stellar flux. We are using exoplanet data from NASA’s Exoplanet Archives and the PHL’s Habitable Exoplanets Catalog, for systems such as TRAPPIST-1, to determine the relative habitability potential of each planet. A positive correlation between habitability estimates and future biosignature detections might support any life-detection hypotheses.