CO2 Tolerance and CO Toxicity Narrow the Habitable Zone for Complex Life

To the point

Edward W. Schwieterman, Christopher T. Reinhard, Stephanie L. Olson, Chester E. Harman, and Timothy W. Lyons show that the zone around a star where complex aerobic life could exist is much narrower due to toxicity limits from CO2 and CO, based on climate and atmospheric models, which means fewer planets in the traditional habitable zone may host complex life and this affects how we look for biosignatures and technosignatures.

Imminent: Inside the Pentagon’s Hunt for UFOs

Luis Elizondo

Chariots of the Gods: 50th Anniversary Edition

Erich Von Daniken

UFO of GOD: The Extraordinary True Story of Chris Bledsoe

Chris Bledsoe

After the Flying Saucers Came: A Global History of the UFO Phenomenon

Greg Eghigian

Our recommendations from Amazon

harvard.edu

A Limited Habitable Zone for Complex Life

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 COSUB2/SUB 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 COSUB2/SUB 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 COSUB2/SUB 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.