Exploring Habitability: From Earth’s Phanerozoic Eon to Exoplanets Orbiting K Dwarf Stars

Understanding an exoplanet in the context of its stellar and environmental parameters is essential for assessing its habitability drivers. Here we used a reconstruction of Earth’s environmental and biological properties of the Phanerozoic Eon from various published data sets and conducted a correlation analysis. We showed that environmental parameters such as oxygen, global average surface temperatures, runoff rates and carbon dioxide are interrelated and play a key role in the changes of biomass and biodiversity. We showed that there were several periods with a highly thriving biosphere, with one even surpassing present day biodiversity and biomass. Those periods were distinguished by elevated oxygen content and global runoff rates, coupled with moderate surface temperatures, provided there were no abrupt temperature fluctuations. Heightened oxygen levels serve as a telltale sign of prolific biomass production by terrestrial flora. We find that exceptionally high oxygen levels can at least in one instance compensate for decreased relative humidities, providing an even more habitable environment compared to today. These results will help us to understand how environmental parameters affect biospheres on extrasolar planets and guide us in our search for extraterrestrial life.

Beyond environmental parameters, stars with about 45 to 80% the mass of the Sun, so-called K dwarf stars, have previously been proposed as optimal host stars in the search for habitable extrasolar worlds. These stars are abundant, have stable luminosities over billions of years longer than Sun-like stars, and offer favorable space environmental conditions. So far, the theoretical and experimental focus on exoplanet habitability has been on even less massive, though potentially less hospitable red dwarf stars. Here we calculate for the first time the electromagnetic spectrum of a K dwarf star transmitted to the surface of a hypothetical habitable zone planet with an Earth-like atmosphere and present principal experimental data on the responses of photosynthetic organisms to this simulated radiation. We find that garden cress Lepidium sativum under K-dwarf radiation exhibits comparable growth and photosynthetic efficiency as under solar illumination on Earth. Notably, the cyanobacterium Chroococcidiopsis sp. CCMEE 029 exhibits significantly higher photosynthetic efficiency and culture growth under K dwarf radiation compared to solar conditions. Our findings of the affirmative responses of these two photosynthetic organisms to K dwarf radiation suggest that exoplanets in the habitable zones around such stars deserve high priority in the search for extrasolar life.

By synthesizing insights from the Phanerozoic Eon’s environmental parameters and experimental data on photosynthetic organisms’ adaptability to K dwarf radiation, we can discern potential life-sustaining environments beyond our solar system, particularly on exoplanets orbiting K dwarf stars.