Swansong biospheres: refuges for life and novel microbial biospheres on terrestrial planets near the end of their habitable lifetimes

Abstract

The future biosphere on Earth (as with its past) will be made up predominantly of unicellular microorganisms. Unicellular life was probably present for at least 2.5 Gyr before multicellular life appeared and will likely be the only form of life capable of surviving on the planet in the far future, when the ageing Sun causes environmental conditions to become more hostile to more complex forms of life. Therefore, it is statistically more likely that habitable Earth-like exoplanets we discover will be at a stage in their habitable lifetime more conducive to supporting unicellular, rather than multicellular life. The end stage of habitability on Earth is the focus of this work. A simple, latitude-based climate model incorporating eccentricity and obliquity variations is used as a guide to the temperature evolution of the Earth over the next 3 Gyr. This allows inferences to be made about potential refuges for life, particularly in mountains and cold-trap (ice) caves and what forms of life could live in these environments. Results suggest that in high latitude regions, unicellular life could persist for up to 2.8 Gyr from present. This begins to answer the question of how the habitability of Earth will evolve at local scales alongside the Sun's main sequence evolution and, by extension, how the habitability of Earth-like planets would evolve over time with their own host stars.

Exoplanet Biosignatures: Future Directions

“…, where P(data|life) << P(data|abiotic) , even if we expect P(life) >0 ). In many cases, we may be unable to detect biosignatures from earlier organisms that are subsequently suppressed by later organisms and evolving chemical and climate conditions, those that may exist only in obscure niches such as deep hydrothermal vents, or are relicts in refugia toward the end of a planet's residence in the habitable zone (O'Malley-James et al , 2013 ). Yet, these marginal biospheres might explain extant life, being its precursor, or relict planetary chemistry.…”

Magnitudes and timescales of total solar irradiance variability

“…The early Sun was approximately 70 % as bright as at the present when it joined the main sequence about 4.6 billion years ago and has a current rate of increase in luminosity of 0.009 % per million years (Hecht 1994). At this rate, it will take ten-million years for the background solar-brightness to increase by the 0.1 % typical of a solar-cycle variation, and another 3.5 billion years for heating from the Sun to create Earthsurface conditions similar to those of the present-day Venus; although additional effects, such as feedback from enhanced ocean-evaporation, may accelerate this warming and make the Earth uninhabitable (at least to present-day complex lifeforms) in about one-billion years (O'Malley-James et al 2013). In 4.5 billion years or so, the Sun will leave the main sequence and transition to a red giant, increasing in radius by about 250 times and in luminosity by roughly 27 000 times but decreasing in temperature to 2600 K, spectrally shifting its radiant output toward longer wavelengths.…”

The UK Centre for Astrobiology: A Virtual Astrobiology Centre. Accomplishments and Lessons Learned, 2011–2016

“…2 ), the potential life they might sustain, and how we might detect it. UKCA has taken special interest in exoplanet research that directly links to microbiology and the use of known limits of life to inform ideas about the habitability of exoplanets (O'Malley-James et al, 2013 , 2014 , 2015 ; Brown et al, 2014; Cockell, 2014c ; Forgan et al, 2015 ; Schwieterman et al, 2015 , 2018 ; Yates et al, 2017 ), and it has applied its expertise to considering galactic habitability (Forgan et al, 2017 ).…”