Limit Cycles Can Reduce the Width of the Habitable Zone

Abstract

The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. Recent calculations have suggested that planets in the outer regions of the HZ cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to "Snowball Earth" episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we build on previous studies with an updated energy balance climate model to calculate this "limit cycle" region of the HZ where such cycling would occur. We argue that an abiotic Earth would have a greater CO partial pressure than today because plants and other biota help to enhance the storage of CO in soil. When we tune our abiotic model accordingly, we find that limit cycles can occur but that previous calculations have overestimated their importance. For G stars like the Sun, limit cycles occur only for planets with CO outgassing rates less than that on modern Earth. For K- and M-star planets, limit cycles should not occur; however, M-star planets may be inhospitable to life for other reasons. Planets orbiting late G-type and early K-type stars retain the greatest potential for maintaining warm, stable conditions. Our results suggest that host star type, planetary volcanic activity, and seafloor weathering are all important factors in determining whether planets will be prone to limit cycling.

No Snowball on Habitable Tidally Locked Planets

“…There has been a fair amount of recent work on the possibility of limit cycles between a snowball climate state and a warm climate state for planets in the habitable zone with a CO 2 outgassing rate that is too low to maintain the warm climate state (Kadoya & Tajika 2014;Menou 2015;Haqq-Misra et al 2016;Batalha et al 2016;Abbot 2016;Paradise & Menou 2017). If tidally locked planets do not exhibit a snowball bifurcation, then they will not be subject to this type of climate limit cycle.…”

“…Snowball Earth events are an acute stressor on life, but may actually increase the complexity of life through evolutionary pressure (Kirschvink 1992;Hoffman et al 1998) and by increasing the atmospheric oxygen concentration (Hoffman & Schrag 2002;Laakso & Schrag 2014, 2017 that might act as biosignatures to remote astronomers. Moreover, planets in the habitable zone with low levels of CO 2 outgassing may experience limit cycles between habitable and snowball climate states (Kadoya & Tajika 2014;Menou 2015;Haqq-Misra et al 2016;Batalha et al 2016;Abbot 2016;Paradise & Menou 2017). It is therefore important to understand the transitions into and out of snowball events for tidally locked planets.…”

Constraints on Climate and Habitability for Earth-like Exoplanets Determined from a General Circulation Model

“…Mars, for instance, currently resides within the habitable zone of Kopparapu et al (2013), but it was unable to retain sufficient CO 2 , to escape from its present frozen state. Kadoya and Tajika (2015) and Haqq-Misra et al (2016) argue for Earth that if the paleo-CO 2 outgassing rates were less than on Earth presently, carbon/silicate cycles may not have been able to prevent a snowball glaciation for most of Earth's history. Furthermore, planets entering a snowball phase may oscillate between frozen and thawed states, with a frequency dependent on the rate of outgassing (Tajika, 2007;Mills et al, 2011;Driscoll & Bercovici, 2013;Haqq-Misra et al, 2016).…”

Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs

“…It is also useful in determining the occurrence of potentially habitable planets in our galaxy, as was done with Kepler data. Several groups have studied the limits of the mainsequence HZ (Kasting et al 1993;Pierrehumbert & Gaidos 2011;Kopparapu et al 2013; Leconte et al 2013a;Yang et al 2013;Barnes et al 2013;Zsom et al 2013;Kopparapu et al 2014;Wolf & Toon 2014;Yang et al 2014a;Way et al 2015;Wolf & Toon 2015;Leconte et al 2015;Godolt et al 2015;Kopparapu et al 2016;Haqq-Misra et al 2016;Ramirez & Kaltenegger 2017;Salameh et al 2017) using both 1-D and 3-D climate models, and corresponding climate transitions that planets undergo at these limits. Many of these models assume water-rich (∼ 1 Earth ocean) planets, which is reasonable if one wants to study the surface habitability of a planet.…”