Climate Instability on Tidally Locked Exoplanets

Abstract

Feedbacks that can destabilize the climates of synchronously rotating rocky planets may arise on planets with strong day-night surface temperature contrasts. Earth-like habitable planets maintain stable surface liquid water over geologic time. This requires equilibrium between the temperature-dependent rate of greenhouse-gas consumption by weathering, and greenhouse-gas resupply by other processes. Detected small-radius exoplanets, and anticipated M-dwarf habitable-zone rocky planets, are expected to be in synchronous rotation (tidally locked). In this paper, we investigate two hypothetical feedbacks that can destabilize climate on planets in synchronous rotation. (1) If small changes in pressure alter the temperature distribution across a planet's surface such that the weathering rate goes up when the pressure goes down, a runaway positive feedback occurs involving increasing weathering rate near the substellar point, decreasing pressure, and increasing substellar surface temperature. We call this feedback enhanced substellar weathering instability (ESWI). (2) When decreases in pressure increase the fraction of surface area above the melting point (through reduced advective cooling of the substellar point), and the corresponding increase in volume of liquid causes net dissolution of the atmosphere, a further decrease in pressure will occur. This substellar dissolution feedback can also cause a runaway climate shift. We use an idealized energy balance model to map out the conditions under which these instabilities may occur. In this simplified model, the weathering runaway can shrink the habitable zone and cause geologically rapid 10 3 -fold atmospheric pressure shifts within the habitable zone. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually a negative feedback or weak positive feedback on changes in atmospheric pressure. It can only cause runaway changes for small, deep oceans and highly soluble atmospheric gases.Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant for atmospheres that are thin, have low greenhouse-gas radiative efficiency, and have a principal greenhouse gas that is also the main constituent of the atmosphere. ESWI also requires land near the substellar point, and tectonic resurfacing (volcanism, mountain-building) is needed for large jumps in pressure. These results identify a new pathway by which habitable-zone planets can undergo rapid climate shifts and become uninhabitable.

Terrestrial Planets Across Space and Time

“…Sengupta (2015) argue that only a handful of the M-dwarfs with confirmed planets in the habitable zone meet the criteria for having Earth-like habitable conditions. Planets within the habitable zone of low-mass stars may also experience synchronous rotation and be tidally locked (Kasting et al 1993), leading to an unstable climate (Kite et al 2011).…”

A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved withHerschel

“…Also, GJ 581 is surrounded by at least four low mass planets with minimum masses of 1.9, 15.6, 5.4, and 7.1 M ⊕ , orbital radii of 0.03, 0.04, 0.07, and 0.22 AU, and eccentricities between 0.0 and 0.32, detected by radial velocity measurements (Bonfils et al 2005;Udry et al 2007;Mayor et al 2009;Forveille et al 2011). All these planets are within the tidal lock region of this M3 spectral type star (<0.25 AU) and hence are expected to be synchronously rotating and potentially undergoing atmospheric instabilities (Wordsworth et al 2011;Kite et al 2011). Planets GJ 581c and d are near and in the conventionally defined habitable zone (Selsis et al 2007), respectively.…”

WATER LOSS FROM TERRESTRIAL PLANETS WITH CO2-RICH ATMOSPHERES

“…Fascinatingly, some recent work has suggested that the abundance of water in the mantle may be more important to geodynamics than the planetary mass (Korenaga 2010;O'Rourke & Korenaga 2012). Finally, even in the absence of other variations, tidally locked planets around M-stars should have very different carbon cycles from Earth due to the concentration of all incoming stellar flux on the permanent day side (Kite et al 2011;Edson et al 2012).…”