Venus as a Laboratory for Exoplanetary Science

Abstract

The current goals of the astrobiology community are focused on developing a framework for the detection of biosignatures, or evidence thereof, on objects inside and outside of our solar system. A fundamental aspect of understanding the limits of habitable environments (surface liquid water) and detectable signatures thereof is the study of where the boundaries of such environments can occur. Such studies provide the basis for understanding how a once inhabitable planet might come to be uninhabitable. The archetype of such a planet is arguably Earth's sibling planet, Venus. Given the need to define the conditions that can rule out bio‐related signatures of exoplanets, Venus provides a unique opportunity to explore the processes that led to a completely uninhabitable environment by our current definition of the term. Here we review the current state of knowledge regarding Venus, particularly in the context of remote‐sensing techniques that are being or will be employed in the search for and characterization of exoplanets. We discuss candidate Venus analogs identified by the Kepler and TESS exoplanet missions and provide an update to exoplanet demographics that can be placed in the potential runaway greenhouse regime where Venus analogs are thought to reside. We list several major outstanding questions regarding the Venus environment and the relevance of those questions to understanding the atmospheres and interior structure of exoplanets. Finally, we outline the path toward a deeper analysis of our sibling planet and the synergy to exoplanetary science.

Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus‐Like Exoplanets

“…The stakes are high for answering this question, since many exoplanets have been discovered in the "Venus zone" just inside the traditional inner edge of the habitable zones of other stars (Kane et al, 2014). Efforts to simultaneously characterize the CO 2 concentrations and climates of a number of these exoplanets, combined with a focused observational strategy for unveiling the history of the "exoplanet next door" to Earth in our own Solar System (Kane et al, 2019), will be our best chance to understand whether the envelope for habitability and the emergence of life is much broader than usually assumed. Figure A1.…”

Transits of Known Planets Orbiting a Naked-eye Star

“…With the host star being similar to the Sun, the known HD136352 planets lie far interior to the inner boundaries of the Habitable Zone (Kasting et al 1993;Kopparapu et al 2013Kopparapu et al , 2014Kane et al 2016a), but they do lie within the Venus Zone . This is mostly relevant to planet b and other terrestrial planets that may be present within the system, because the exploration of planetary habitability and comparative planetology aims to study the major factors that drive the bifurcation of habitable versus uninhabitable environments (Hamano et al 2013;Kane et al 2019;Way & Del Genio 2020). Terrestrial planets orbiting close to a bright host star, such as those discussed here, provide the best opportunities to conduct the needed atmospheric studies to inform the diversification processes (Ostberg & Kane 2019).…”

Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets

“…The stakes are high for answering this question, since many exoplanets have been discovered in the "Venus zone" just inside the traditional inner edge of the habitable zones of other stars (Kane et al, 2014). Efforts to simultaneously characterize the CO 2 concentrations and climates of a number of these exoplanets, combined with a focused observational strategy for unveiling the history of the "exoplanet next door" to Earth in our own solar system (Kane et al, 2019), will be our best chance to understand whether the envelope for habitability and the emergence of life is much broader than usually assumed. a ID: The colored numbers correspond to those in Figure 2.…”