Assessing the Habitability of the TRAPPIST-1 System Using a 3D Climate Model (1703.05815v2)

Abstract: The TRAPPIST-1 system provides an extraordinary opportunity to study multiple terrestrial extrasolar planets and their atmospheres. Here we use the National Center for Atmospheric Research Community Atmosphere Model version 4 to study the possible climate and habitability of the planets in the TRAPPIST-1 system. We assume ocean-covered worlds, with atmospheres comprised of N2, CO2, and H2O, and with orbital and geophysical properties defined from observation. Model results indicate that the inner three planets (b, c, and d) presently reside interior to the inner edge of the traditional liquid water habitable zone. Thus if water ever existed on the inner planets, they would have undergone a runaway greenhouse and lost their water to space, leaving them dry today. Conversely the outer 3 planets (f, g, and h) fall beyond the maximum CO2 greenhouse outer edge of the habitable zone. Model results indicate that the outer planets cannot be warmed despite as much as 30 bar CO2 atmospheres, instead entering a snowball state. The middle planet (e) represents the best chance for a presently habitable ocean-covered world in the TRAPPIST-1 system. Planet e can maintain at least some habitable surface area with 0 - 2 bar CO2, depending on the background N2 content. Near present day Earth surface temperatures can be maintained for an ocean-covered planet e with either 1 bar N2 and 0.4 bar CO2, or a 1.3 bar pure CO2 atmosphere.

• The paper employs a 3D climate model (CAM4) to evaluate the potential for liquid water on all seven TRAPPIST-1 planets based on atmospheric composition and stellar radiation.
• Results suggest inner planets b, c, and d likely underwent runaway greenhouse effects, while outer planets f, g, and h are likely frozen even with high CO2.
• TRAPPIST-1e is identified as the most promising candidate, capable of maintaining habitable surface conditions with Earth-like or pure CO2 atmospheres.

Assessing the Habitability of the TRAPPIST-1 System Using a 3D Climate Model

The research outlined in Eric T. Wolf's paper employs a sophisticated three-dimensional climate model to investigate the potential habitability of the seven terrestrial planets in the TRAPPIST-1 system. Utilizing the Community Atmosphere Model version 4 (CAM4), the paper explores the conditions necessary for sustaining liquid water, focusing on the interplay between atmospheric composition and stellar radiation from the ultracool dwarf star TRAPPIST-1.

The analysis systematically evaluates the likelihood of habitability for each planet, suggesting that planets b, c, and d lie within the inner edge of the traditional liquid water habitable zone. As a consequence, these planets have likely undergone runaway greenhouse effects, leading to desiccation and present-day uninhabitability. Conversely, the outer planets f, g, and h are situated beyond the maximum carbon dioxide greenhouse threshold, rendering them frozen with substantial ice cover even under high atmospheric CO$_2$ concentrations.

The central planet, TRAPPIST-1e, emerges as the most promising candidate for habitability. According to the model results, planet e can maintain habitable surface conditions with varying atmospheric compositions, including those similar to present-day Earth's climate. Specifically, with 1 bar of N$_2$ and CO$_2$ pressure ranging from 0 to 2 bars, or a pure CO$_2$ atmosphere between 0.25 to 1.3 bar, planet e can sustain environments suitable for liquid water. The paper underscores that with either 1 bar N$_2$ and 0.4 bar CO$_2$ or a 1.3 bar CO$_2$-only atmosphere, TRAPPIST-1e can maintain near-Earth-like surface temperatures.

The implications of this research are multifaceted. Theoretically, it challenges the simplistic boundary of traditional habitable zones by illustrating the influence of atmosphere and stellar type on habitability using complex climate models. Practically, it enhances our understanding of exoplanetary climates, guiding observational strategies for future missions aiming to characterize exoplanetary atmospheres.

Furthermore, the paper acknowledges potential challenges in initial water inventories due to the host star's super-luminous pre-main sequence phase, which may result in significant water loss. Future explorations will benefit from incorporating the effects of volatile cycling and planetary migration history to refine habitability predictions. Additionally, given the variations in model projections, the paper advocates for continued development and comparison among different climate models to validate conclusions about the TRAPPIST-1 system's capacity to support life.

The erratum addressed in the paper corrects errors in CO$_2$ absorption, affecting simulations with more than 0.2 bar CO$_2$. It notably adjusts findings for TRAPPIST-1f, suggesting habitable conditions with increased CO$_2$, while the conclusions regarding TRAPPIST-1d remain stable, highlighting the sensitivity of climate predictions to model parameters and assumptions.

Overall, Eric T. Wolf's research offers a detailed examination of the TRAPPIST-1 planets, providing critical insights and a solid foundation for subsequent studies assessing exoplanetary habitability. As 3D climate models continue to evolve, they promise to further unravel the complexities of extraterrestrial climates and life-supporting conditions.