Calculating potential cumulative carbon fixed and evolutionary stage for Earthlike planets in our solar neighborhood

Abstract

We propose a novel method for estimating possible biological evolutionary stage on exoplanets based on the hypothesis that biological evolutionary state is a linear function of cumulative carbon fixed (photosynthesis) on an entire planet. We explore the implications of this hypothesis using spatially explicit climate simulations of TRAPPIST-1e, a tidally locked planet within the habitable zone of a red dwarf star ∼40 light years away. We estimate that Earth has cumulatively fixed ∼9.4 e25 g C carbon, and TRAPPIST-1e (T1e) as an ocean world with 400 ppm CO ₂ using photon energy of wavelengths 400–1100 nm would need 18 Gyr years to fix the same amount of carbon. Since T1e’s mean estimated age is 7.6 Gyr, we estimate it to be at a potential microbial, but not multicellular life stage. We then apply this technique to 29 nearby exoplanets that may have the conditions suitable for harboring life, using 400–1100 nm light and assuming a 30% continent ratio. We identify two planets that surpass Earth’s cumulative Net Primary Production (NPP) and which could have both multicellular and intelligent life and 6 planets at the potential multicellular stage. Planets most likely to have higher cumulative NPP than Earth are also most likely to be dominated (more than Earth) by precipitation limited ecosystems, like deserts or temperate ecosystems (versus boreal or tropical ecosystems). Planets GJ 1061c and K2-3d rank highest in cumulative productivity potential under a number of our scenarios because they are bigger, hotter, brighter and older than other planets in the solar neighborhood.