A Limited Habitable Zone for Complex Life

Abstract

The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet's surface. Substantially more CO 2 than present in Earth's modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO 2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO 2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.

Implications of Abiotic Oxygen Buildup for Earth-like Complex Life

“…The hypothesis presented in this paper is complementary to that espoused in Schwieterman et al (2019) in two respects. First, Schwieterman et al (2019) showed that CO 2 toxicity may move the outer edge of the HZ inward, as higher CO 2 concentrations can prevail at the outer edge, whereas this work suggests that O 2 toxicity might shift the inner edge of the HZ outward due to the higher XUV fluxes and abiotic buildup of O 2 .…”

“…First, Schwieterman et al (2019) showed that CO 2 toxicity may move the outer edge of the HZ inward, as higher CO 2 concentrations can prevail at the outer edge, whereas this work suggests that O 2 toxicity might shift the inner edge of the HZ outward due to the higher XUV fluxes and abiotic buildup of O 2 . Second, the analysis by Schwieterman et al (2019) indicates that the accumulation of CO (which is toxic to many metazoans on Earth) in atmospheres of M-dwarf exoplanets may render them uninhabitable, while this paper implies that exoplanets around M-dwarfs could become inhospitable to Earth-like biota because of their high XUV fluxes, which aid in abiotic O 2 buildup and cause O 2 toxicity in the process. Therefore, in a nutshell, CO 2 , CO and O 2 toxicity might jointly conspire to preclude complex Earth-like life on many M-dwarf exoplanets.…”

“…Although parochial in nature, studies of this nature are nonetheless advantageous because they permit us to make concrete and testable predictions and could play a valuable role in the selection of target planets. It is for this reason that Earth-based limits are routinely employed in astrobiological studies, with the recent formulation of the habitable zone (HZ) for complex life based on CO 2 and CO constraints (Schwieterman et al 2019) constituting a compelling example of this approach.…”

The Grand Challenges of Exoplanets

“…What time period is required for life to form and evolve? The ancient Earth was clearly not a place we would enjoy today, so long-term planetary environmental changes are important to understand as well (Schwieterman et al, 2019b). Such questions as these illustrate the strong relation of biology, astrobiology, and planetary science to exoplanet research.…”

A Complex Life Habitable Zone Based On Lipid Solubility Theory

“…Recently, Schwieterman et al 14 used a 1-D radiative-convective climate model to argue that a HZ for complex life (e.g. animals and plants) would be significantly narrower than the classical main-sequence CO 2 -H 2 O HZ 1,3 .…”