Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates

Abstract

The ideal exoplanets to search for life are those within a star’s habitable zone. However, even within the habitable zone, planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic (∼gigayear) timescales requires a planet to contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet’s energy budget is the heat produced by the decay of radioactive elements, especially 40K, 232Th, 235U, and 238U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here we estimate the probability distribution of the amount of these heat-producing elements that enter into rocky exoplanets through Galactic history by combining the system-to-system variation seen in stellar abundance data with the results from Galactic chemical evolution models. From this, we perform Monte Carlo thermal evolution models that maximize the mantle cooling rate, thus allowing us to create a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle through Galactic history. We apply this framework to a sample of 17 likely rocky exoplanets with measured ages, seven of which we predict are likely to be actively degassing today, despite our pessimistic assumptions. For the remaining planets, including those orbiting TRAPPIST-1, we cannot confidently assume that they currently contain sufficient internal heat to support mantle degassing at a rate sufficient to sustain a global carbon cycle or temperate climate without additional tidal heating or undergoing plate tectonics.

The Carbon-deficient Evolution of TRAPPIST-1c

“…Even as stellar phenomena strip material from planetary atmospheres, terrestrial planet atmospheres are replenished by geological processes, such as volcanic outgassing, as seen in recent geological history on Earth, Venus, and Mars. This outgassing may not last for the entire lifetime of the planet, as Unterborn et al (2022) suggest for the TRAPPIST-1 planets. Nevertheless, we can model both the escape and outgassing of terrestrial planet atmospheres as coupled time-dependent processes that evolve an atmosphere through time.…”

The Demographics of Terrestrial Planets in the Venus Zone

“…The planet spends all of its orbit within VZ and is only 0.03 au from the outer VZ boundary, which makes it an excellent case for testing runaway greenhouse conditions. In addition, Teegarden's Star was determined to have an age greater than 8 Gyr (Zechmeister et al 2019), allowing for the opportunity to study any correlations between planet age and the onset of the runaway greenhouse effect (Foley & Smye 2018; Foley 2019; Unterborn et al 2022).…”

Inference of Dynamic Geophysical Conditions and Probability of Advanced Life in Potentially Habitable Rocky Exo Planets

“…The possibility that these water bodies can disappear during the cessation and post cessation phases of volcanism cannot be ruled out. Major changes in atmospheric composition can be expected as an outcome of intense volcanic activity in these planets ( Unterborn et al, 2022) reference) So most likely the emergence of oxygen can be expected during the peak phases of volcanism. Emergence of ozone requires more time and hence it is likely to emerge during the declining phases of volcanism in the rocky planets.…”