Enhanced Habitability on High Obliquity Bodies near the Outer Edge of the Habitable Zone of Sun-like Stars

Abstract

High obliquity planets represent potentially extreme limits of terrestrial climate, as they exhibit large seasonality, a reversed annual-mean pole-to-equator gradient of stellar heating, and novel cryospheres. A suite of 3-D global climate model simulations with a dynamic ocean is performed with Earthlike atmospheres for low and high obliquity planets with various stellar fluxes, CO2 concentrations, and initial conditions to explore the propensity for high obliquity climates approaching the outer edge of the Habitable Zone to undergo global glaciation. We also simulate planets with thick CO2 or H2 atmospheres, such as those expected to develop near or beyond the outer edge of the Habitable Zone.We show that high obliquity planets are hotter than their low obliquity counterparts due to ice-albedo feedbacks for cold climates, and water vapor in warm climates. We suggest that the water vapor greenhouse trapping is greater on high obliquity bodies due to the different dynamical regimes that occur between the two states.While equatorial ice-belts are stable at high obliquity in some climate regimes, it is harder to achieve global glaciation than for a low obliquity planet. Temperate polar conditions can be present at high obliquity at forcings for which low obliquity planets would be in a hard snowball state. We suggest the conditions on high obliquity planets are likely to be more favorable for a robust biosphere to develop approaching the outer edge of the HZ. However, the influence of obliquity

Tides on Other Earths: Implications for Exoplanet and Palaeo‐Tidal Simulations

“…Little is known about these planets, which has led to the widespread use of numerical models of potential climatic conditions to determine a probability of a given body-hosting water (Seager, 2013), but there are large uncertainties in this approach. For example, varying host star irradiances and masses, and planetary atmospheric compositions, masses, rotation rates, eccentricities, and obliquities will drastically alter the conditions at the surface (Colose et al, 2019;Kasting et al, 1993;Way & Georgakarakos, 2017;Way et al, 2018Way et al, , 2016Williams & Pollard, 2002, 2003Yang et al, 2014). Few of these simulations, however, consider the effect of a water ocean tide, despite tidal friction being a key controller on orbital evolution, and hence habitability (Bills & Ray, 1999;Egbert et al, 2004;Green et al, 2019b;Lingam & Loeb, 2018).…”

“…There are links between a planet's potential habitability and its rotation rate, for example, full spin‐orbit synchronization at lower rates and the reduction of meridional atmospheric convection at higher rotation rates due to a stronger Coriolis force (Yang et al, 2014). To properly constrain a planet's climatology, and therefore habitability, a range of variables, including the rotation rate, topography and land/ocean mask, must be known (Colose et al, 2019; Way et al, 2016; Way & Del Genio, 2020; Yang et al, 2014). So a planet's past, present, and future total tidal dissipation rate must be quantified, to improve estimates of those other dependent properties.…”

The Fundamental Connections between the Solar System and Exoplanetary Science

“…There have been numerous studies regarding the role of the Moon in stabilizing the spin axis of the Earth (e.g., J. W. Barnes et al., 2016; Ćuk et al., 2016; Laskar et al., 1993; Lissauer et al., 2012), including suggestions that the such stabilization may have moderated the Earth's climatic variability (e.g., Waltham, 2004), and therefore habitability (Armstrong et al., 2014; Colose et al., 2019; Heller et al., 2011; Spiegel et al., 2009; Williams & Kasting, 1997). As such, the possible requirement of the presence of a substantial moon for long‐term habitability continues to be used as an argument toward the potential scarcity of habitable planets in the Universe.…”

Anthony Del Genio: Climates of Planets Near and Far

“…Led by my GISS colleague Mike Way and with contributions from many others in the GISS Earth GCM group (Way et al, ), a generalized planetary version of the GISS GCM has been developed. We have used it to explore the possibility that ancient Venus under the faint young Sun may have been habitable (Way et al, ); to understand the processes that put excessive water vapor into the stratosphere as incident stellar flux increases, a precursor to the eventual loss of a planet's oceans (Fujii et al, ); to determine how the thermal inertia and heat transport of a dynamic ocean might render a planet continuously habitable in the face of oscillations in planet eccentricity (Way & Georgakarakos, ); to examine scenarios for a possible habitable climate on the known exoplanet closest to Earth (Del Genio et al, ); to determine how the carbonate‐silicate cycle feedback that regulates CO 2 and allowed Earth to remain habitable over most of its history might vary as precipitation and runoff change with insolation and planet rotation (Jansen et al, ); to understand the transport of volatiles to permanently shadowed polar regions early in the Moon's history (Aleinov et al, ); to predict the planetary albedos and surface temperatures of exoplanets from sparse available information using Earth climate concepts (Del Genio, Kiang , et al, ); and to understand how high obliquity allows weakly illuminated planets to remain habitable (Colose et al, ). We have also tried to set a standard for data sharing by making the GCM output files and metadata for our published papers publicly available, as described in Way et al ().…”