Presentation #500.02 in the session “Plenary Panel: Venus-Sized Planets”.
Venus is the most Earth-like planet in the solar system in terms of its size, mass, and bulk composition, yet the surface conditions of Earth and its sister world could not be more different. Venus has the hottest terrestrial surface in the solar system, yet it may once have hosted oceans in its deep past as indicated by its elevated D/H ratio. Recent 3-D modeling efforts (Way et al. 2016, 2020) indicate that cloud feedbacks driven by Venus’ slow rotation could enable habitable conditions for billions of years. Possibly, our solar system hosted two worlds with habitable surface environments for most of its history, and if planets receiving Venus-like levels of insolation can be habitable, this has important implications for the distribution of possible habitable environments elsewhere in the galaxy. Stellar tidal forces are likely to induce slow rotation for planets orbiting within and interior to the habitable zones of many low mass M dwarf stars (which represent 75% of all stars), and the same processes studied to enable habitable conditions on early Venus have also been applied to these planets (e.g. Yang et al. 2013; Kopparapu et al. 2016; Fujii et al. 2017). If hot, slowly rotating exoplanets are observed to indeed be habitable, this insight may shed light on the processes that operated on Venus in the past. Indeed, planets receiving Venus-like insolation levels are likely to represent the most observable class of terrestrial exoplanets for the upcoming James Webb Space Telescope (JWST; Kane et al. 2014). However, transit transmission observations that will be employed by JWST are particularly challenging for Venus-like planets, because they cannot probe below a global cloud layer. Recently, it has been shown that a planet with a thick atmosphere and a high-altitude cloud deck could appear spectrally similar to a planet with a thin, clear-sky atmosphere (Lustig-Yaeger et al. 2019). The challenges inherent to studying exo-Venus planets will require robust computer models validated and constrained using data from the Venusian environment, but there are still many unknowns about this mysterious planet’s past and present. No spacecraft has entered the Venus atmosphere since the 1980s, and the U.S. has not had a Venus mission since Magellan (launched 1989). In recognition of the vital role Venus can play in shaping our understanding of planetary habitability as a dynamic process that evolves over time, and given that Venus-like exoplanets are likely to be observed in the near future, it is crucial to better understand this “exoplanet in our backyard.”