Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

Abstract

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708.

Dead zones of classical habitability in stellar binary systems

“…Ultimately, additional observational campaigns are desired. Those should also encompass the search for biosignatures, including molecules and molecular disequilibria that are apparently irreproducible through non-biological processes; see, e.g., Seager et al (2016), Meadows et al (2018), Olson et al (2018), Schwieterman et al (2018), and Arney (2019) for background information. Fig.…”

Constraints on Aquatic Photosynthesis for Terrestrial Planets around Other Stars

“…Perhaps most importantly, we assumed that the functional traits of putative photoautotrophs were akin to eukaryotic phytoplankton. The spectral diversity and flexibility of cyanobacteria analogs, especially their proven capacity for utilizing chlorophylls d and f at far-red and near-IR wavelengths (Nürnberg et al 2018;Schwieterman et al 2018), might render them increasingly important for Earth-analogs orbiting cool stars. As the predominant marine cyanobacteria species can grow at photon fluxes that are ∼ 10 times smaller than the compensation flux considered herein (Canfield et al 2005, Chapter 3.2.1), our results must be revised upward by a factor of 10 in accordance with Figs.…”

Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds

“…First, radial velocity, transit, and/or transit timing variation (TTV) methods can identify the mass, planet radius, and orbital radius that together can be used to classify an exoplanet as Earthlike and whether it is in its stars habitable zone. Second, visible and infrared transmission spectroscopy or reflectance spectroscopy can characterize atmospheric gases such as ozone, diatomic oxygen, carbon dioxide, water vapor, and possibly methane (Deming et al 2009;Domagal-Goldman et al 2016;Kaltenegger 2017;Kaltenegger & Traub 2009;Lovis et al 2017;Meadows 2017;Meadows et al 2018;Schwieterman et al 2018). The presence of oxygen (i.e., O 2 and O 3 ) in the atmosphere of a rocky planet is widely considered a robust biosignature (Meadows et al 2018).…”