The effect of high nitrogen pressures on the habitable zone and an appraisal of greenhouse states

Abstract

The habitable zone (HZ) is the main tool that mission architectures utilize to select potentially habitable planets for follow-up spectroscopic observation. Given its importance, the precise size and location of the HZ remains a hot topic, as many studies, using a hierarchy of models, have assessed various factors including: atmospheric composition, time, and planetary mass. However, little work has assessed how the habitable zone changes with variations in background nitrogen pressure, which is directly connected to the habitability and life-bearing potential of planets. Here, I use an advanced energy balance model with clouds to show that our solar system's HZ is ~0.9 -1.7 AU, assuming a 5-bar nitrogen background pressure and a maximum 100% cloud cover at the inner edge. This width is ~20% wider than the conservative HZ estimate. Similar extensions are calculated for A -M stars. I also show that cooling clouds/hazes and high background pressures can decrease the runaway greenhouse threshold temperature to ~300 K (or less) for planets orbiting any star type. This is because the associated increase in planetary albedo enables stable climates closer to the star, where rapid destabilization can be triggered from a lower mean surface temperature. Enhanced longwave emission for planets with very high stratospheric temperatures also permits stable climates at smaller orbital distances. The model predicts a runaway greenhouse above ∼330 K for planets orbiting the Sun, which is consistent with previous work. However, moist greenhouses only occur for planets orbiting Astars.

The influence of surface $CO_{\mathrm{2}}$ condensation on the evolution of warm and cold rocky planets orbiting Sun-like stars

“…In our solar system, a conservative estimate spans from ~0.95 -1.67 AU (Kasting et al 1993;Ramirez 2018). Warm starts have traditionally been assumed in these and most other calculated habitable zone limits (Kasting et al 1993;Pierrehumbert & Gaidos 2011;Kopparapu et al 2013;Zsom et al 2013;Ramirez & Kaltenegger 2017;Ramirez 2020). In contrast, Kadoya & Tajika (2019) have shown that the habitable zone outer edge limit distance decreases for systems composed of cold start planets.…”

“…We make use of an advanced non-grey energy balance model (EBM), similar to that described in Ramirez & Levi (2018) and Ramirez (2020), which is itself based on other similar models (see also North & Coakley Jr (1979), North et al (1981), Kasting (1997), andVladilo et al (2013)) to determine the presence of 𝐶𝑂 2 ice condensation and its influence on the fate of bodies orbiting Sun-like stars. We study the evolution of Earth-like planets assuming cold (𝑇 surf =230 K) or warm (𝑇 surf =280 K) starts.…”

“…The EBM is coupled to a 1-D radiative-convective climate model that provides the radiative transfer calculations. The reader is redirected to Ramirez (2020) for a more detailed explanation of the model, but we reiterate some of the key details here.…”

How does the background atmosphere affect the onset of the runaway greenhouse?

“…Some studies (e.g. Goldblatt et al 2013;Kopparapu et al 2014;Ramirez 2020) have shown that the outgoing longwave radiation (OLR) may be strongly modified by radiatively inactive gases (e.g. N 2 or O 2 , as in the Earth's atmosphere).…”

An Energy Balance Model for Rapidly and Synchronously Rotating Terrestrial Planets

“…The discovery of exoplanets has also motivated numerous EBM studies that explore plausible climates of other terrestrial and giant planets to aid in ongoing attempts at identifying habitable planets. This has included investigations of: the extent to which of rotation rate (Spiegel et al 2008;Bahraminasr et al 2020), obliquity (Williams & Kasting 1997;Spiegel et al 2009;Armstrong et al 2014;Rose et al 2017;Bahraminasr et al 2020;Palubski et al 2020), eccentricity (Dressing et al 2010;Palubski et al 2020), and Milankovitch cycles (Spiegel et al 2010;Haqq-Misra 2014;Forgan 2016;Deitrick et al 2018a,b;Quarles et al 2021) can affect habitability; the effects of varying surface albedo (Shields et al 2013;Rushby et al 2019;Bahraminasr et al 2020) and surface pressure (Vladilo et al 2013;Ramirez 2020;Bahraminasr et al 2020) on climate; the role of mantle degassing and the carbonate-silicate cycle on climate evolution (Kadoya & Tajika 2014;Menou 2015;Kadoya & Tajika 2015;Haqq-Misra et al 2016;Kadoya & Tajika 2016, 2019; the climate stability of synchronously rotating planets around low-mass stars (Kite et al 2011;Checlair et al 2017); the possibility of habitable exomoons (Forgan & Yotov 2014;Forgan & Dobos 2016); and plausible climate for planets in binary star systems (Forgan 2014(Forgan , 2016May & Rauscher 2016;Haqq-Misra et al 2019;Okuya et al 2019;<...>…”