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A Limited Habitable Zone for Complex LifeThe 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 CO2 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 CO2 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 CO2 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.
Document ID
20190026728
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
doi:10.3847/1538-4357/ab1d52
Authors
Schwieterman, Edward W. (California Univ. Riverside, CA, United States)
Reinhard, Christopher T. (NASA Ames Research Center Moffett Field, CA, United States)
Olson, Stephanie L. (NASA Ames Research Center Moffett Field, CA, United States)
Harman, Chester E. (NASA Ames Research Center Moffett Field, CA, United States)
Lyons, Timothy W. (California Univ. Riverside, CA, United States)
Date Acquired
June 27, 2019
Publication Date
June 10, 2019
Publication Information
Publication: The Astrophysical Journal
Publisher: IOP Science
Volume: 878
Issue: 1
ISSN: 0004-637X
e-ISSN: 1538-4357
Subject Category
Lunar And Planetary Science And Exploration
Report/Patent Number
GSFC-E-DAA-TN70116
Funding Number(s)
CONTRACT_GRANT: 80NSSC18M0133
CONTRACT_GRANT: NNA15BB03A
CONTRACT_GRANT: NNH13ZDA017C
CONTRACT_GRANT: NNA13AA93A
CONTRACT_GRANT: 80NSSC18K0829
Distribution Limits
Public
Copyright
Use by or on behalf of the US Gov. Permitted.

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