Earth’s varying paleoenvironment and experimental tests provide insights into superhabitabitable conditions on exoplanets

In our search for life in the Universe, it would be anthropocentric to assume that there are no planets more suitable for life than Earth. We call such planets ‘superhabitable’¹. Some of the environmental conditions related to superhabitability are motivated astrophysically, while others are based on the varying habitability throughout Earth’s natural history². 

Here we present our initial results in which we investigate potentially superhabitable conditions which, among other parameters, include K-dwarfs as host stars. To simulate the astrophysical condition of superhabitability, we obtained an LED solar simulator with a variable spectrum module which we adapted for our needs. As part of the theoretical preparations for our physical setup of the experiment in the lab chamber, we modeled the emission spectrum of a 4300 Kelvin K-dwarf star using the PHOENIX spectral library. We calculated the emission spectrum at the top of the planetary atmosphere of the hypothetical planet in the center of the star’s habitable zone, at a distance of ~0.44 AU, where it receives 0.6 times the solar effective flux, i.e. ~820 W/m². Using Earth’s telluric spectrum we calculated for the first time the stellar spectrum of a K-dwarf star transmitted to the surface of a hypothetical habitable zone planet with an Earth-like atmosphere. We used this spectrum as a backbone for creating the LED spectral fit. As a test run, we built a small external-light-isolating chamber and are determining the responses of plant species (e.g. watercress) to the produced K-dwarf stellar spectrum in comparison to solar light.

At the same time we quantified the variations of habitability conditions during the natural history of Earth. Paleontological and geochemical records are key to the reconstruction of geological and atmospheric aspects that have affected Earth’s biosphere. We used these to understand how environmental tracers (e.g. surface temperatures, oxygen partial pressures and relative humidity) correlate with biological tracers (e.g. biomass production and biological diversity). Variations of the global average surface temperature in the Phanerozoic era have previously been demonstrated to be inversely correlated with biodiversity^3,4. Here we show for the first time that periods with elevated oxygen partial pressures in the atmosphere correlate with increased biomass production but not biological diversity, whereas humid periods correlate strongly with biodiversity and to some extent with biomass. These results extend the previously established effect of surface temperature on biological diversity to a similar correlation with biomass, and stress the impact of oxygen and relative humidity on the biosphere. Our results help us to better understand how the modern biosphere could change in the future, especially in light of the rapid anthropogenic changes. Beyond that, such tracers could soon be measured on extrasolar planets with space-based observatories^5–7, whose geo- or biological origin could be better interpreted using our results. 

Eventually we plan to apply theoretical climate-chemistry models to determine how life affects the biosignatures of a superhabitable planet around a K-dwarf host star by calculating synthetic transmission / emission spectra and atmospheric compositions, as it is vital to determine whether life that flourishes under such conditions can also be observable. These results will be used to provide a guide for space-based transit observations of extrasolar potentially habitable planets.

References

1. Heller, R. & Armstrong, J. Superhabitable Worlds. Astrobiology vol. 14 50–66 (2014).

2. Schulze-Makuch, D., Heller, R. & Guinan, E. In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World. Astrobiology 20, 1394–1404 (2020).

3. Mayhew, P. J., Jenkins, G. B. & Benton, T. G. A long-term association between global temperature and biodiversity, origination and extinction in the fossil record. Proc. Biol. Sci. 275, 47–53 (2008).

4. Song, H. et al. Thresholds of temperature change for mass extinctions. Nat. Commun. 12, 4694 (2021).

4. Krissansen-Totton, J., Garland, R. & Irwin, P. Detectability of biosignatures in anoxic atmospheres with the James Webb Space Telescope: A TRAPPIST-1e case study. Astron. J. (2018).

5. The LUVOIR Team. The LUVOIR Mission Concept Study Final Report. arXiv [astro-ph.IM] (2019).

7. Scott Gaudi, B. et al. The Habitable Exoplanet Observatory (HabEx) Mission Concept Study Final Report. arXiv [astro-ph.IM] (2020).