The Resilience of Habitable Climates Around Circumbinary Stars

Abstract

Here we use a 3-D climate system model to study the habitability of Earth-like planets orbiting in circumbinary systems. In the most extreme cases, Earth-like planets in circumbinary systems could experience variations in the incident stellar flux of up to~50% on~100-day timescales. However, we find that Earth-like planets, having abundant surface liquid water, are generally effective at buffering against these time-dependent changes in the stellar irradiation due to the high thermal inertia of oceans compared with the relatively short periods of circumbinary-driven variations in the received stellar flux. Ocean surface temperatures exhibit little to no variation in time; however, land surfaces can experience modest changes in temperature, thus exhibiting an additional mode of climate variability driven by the circumbinary variations. Still, meaningful oscillations in surface temperatures are only found for circumbinary system architectures featuring the largest physically possible amplitudes in the stellar flux variation. In the most extreme cases, an Earth-like planet could experience circumbinary-driven variations in the global mean land surface temperature of up to~5 K, and variations of local daytime maximum temperatures of up to~12 K on seasonal timescales, while the global mean ocean temperatures vary by less than~2 K. Such seasonal temperature swings over land areas could potentially pose adaptability challenges for extant life. Still, habitable planets in circumbinary systems appear to be remarkably resilient against circumbinary-driven climate variations and can avoid any true climate catastrophes.Plain Language Summary A circumbinary system is one where a planet orbits around two stars simultaneously, resulting in large and rapid changes to both the stellar energy distribution and the total stellar energy received by the planet. Planets in circumbinary systems can experience significant variations in their received stellar energy over~10-to~100-day timescales. However, habitable planets in circumbinary systems are highly effective at buffering these changes in the received stellar energy due to the large thermal inertia of the oceans. Ocean surface temperatures exhibit little to no variation in time; however, land surfaces can experience modest changes in temperature. Even for the most extreme variations in stellar flux possible for circumbinary systems, Earth-like planets could avoid a permanent climate catastrophe, such as a runaway greenhouse or glaciation.

Stable Climates for Temperate Rocky Circumbinary Planets

“…The amplitude of surface temperature variability in the simulations of Wolf et al. (2020) depends strongly on the surface type, as surfaces with higher thermal inertia (e.g., oceans) show reduced variability. Figure 2 shows how the maximum temperature over the course of a day varies with latitude and time for four different longitudes in the case with maximum variability in instellation.…”

“…Interestingly, Wolf et al. (2020) find that even in the most extreme cases they considered, the variability in surface temperature was small, with excursions in the surface temperature of tropical land only ≈ 15 K above a single‐star control case. In the majority of simulations, the change in temperature was only a few degrees K due to the high thermal inertia of the oceans.…”

“…Wolf et al. (2020) instead considers Earth‐like circumbinary planets in the sense that they have the same continent distribution and atmospheric composition as modern Earth and receive the same time‐averaged instellation as Earth does. Additionally, Wolf et al.…”

“… Figure 6 of Wolf et al. (2020), which shows the time‐evolution of the maximum daily temperature at each latitude for four specific longitudes in their case with a maximal stellar flux amplitude due to variations in the binary instellation. The excursions in daily temperature are smallest over the ocean, and largest over tropical and subtropical land.…”

The Resilience of Habitable Climates Around Circumbinary Stars

“…For more information on exoplanet modeling with CESM1.2.1, see https://github.com/storyofthewolf/ExoCAM/ and https://github.com/storyofthewolf/ExoRT websites. The primary model outputs used in this study for analysis are multiyear high time cadence outputs and have been made publicly available on Zenodo at http://doi.org/10.5281/ zenodo.3964961 (Wolf et al, 2020a), http://doi.org/10.5281/zenodo.3972021 (Wolf et al, 2020b), and also available at the lead authors data repository through The Internet Archive (see https://archive.org/details/ Circumbinary_Climates).…”