Similarity Indexing: Application to potential habitability of rocky and water extremophiles on exoplanets

Searching life outside our solar system is a fascinating area of current research for astrobiologists. More than 4000 exoplanets were discovered and confirmed according to NASA exoplanet archive. Techniques like Earth Similarity Index and Mars Similarity Index were recently established to find Earth like and Mars like exoplanets (Kashyap et al. 2017). In present study we focused on water extremophiles and rocky extremophiles, which could potential survive on exoplanets. Part 1:For water extremophiles, we have established a metric tool for distinguishing the potential survivability of active and cryptobiotic tardigrades (known as water bears) on rocky-water and water-gas planets in our solar system as well as some exoplanets, taking into consideration the geometrical means of six physical parameters such as radius, density, escape velocity, revolution period, surface temperature, and surface pressure of the considered planets. From Planetary Habitable Laboratory - Exoplanet Catalog (PHL-EC), we have selected 57 exoplanets in our study including Earth and Mars, with rocky-water composition. The Active Tardigrade Index (ATI) and Cryptobiotic Tardigrade Index (CTI) are two metric indices with minimum value 0 (= tardigrades cannot survive) and maximum 1 (= tardigrades will survive in their respective state). Values between 0 and 1 indicate a percentage chance of the active or cryptobiotic tardigrades surviving on a given exoplanet. Among known planets some of the exoplanets are tabulated as ATI and CTI indices for sample representation like: Kepler-100d, Kepler-48d, Kepler-289b, TRAPPIST-1 f and Kepler-106e. The results with Mars as the threshold indicates that Mars could be the only rock-water composition planet that could be more suitable for tardigrades than other considered exoplanets (Kashyap et al. 2018). Part 2: For rocky extremophiles, we examined rocky exoplanets whose physical conditions are potentially suitable for the survival of rock-dependent extremophiles, such as the cyanobacteria Chroococcidiopsis and the lichen Acarospora. The Rock Similarity Index (RSI) is first introduced and then applied to 1659 rocky exoplanets. The RSI represents a measure for Earth-like planets on which physical conditions are potentially suitable for rocky extremophiles that can survive in Earth-like extreme habitats (i.e., hot deserts and cold, frozen lands) (Kashyap et al. 2020).