Published in the Astrophysical Journal on January 10, 2024, their research illuminates the significance of carbon monoxide (CO) in the atmospheric conditions of Earth-like planets, potentially reshaping our approach to finding life-sustaining worlds among the stars.
The habitability of an exoplanet is traditionally gauged by its ability to sustain liquid water, necessitating a delicate balance of temperature and atmospheric conditions within the so-called habitable zone of its star. NASA's Kepler telescope has been instrumental in this search, revealing that a significant fraction of visible stars could host Earth-sized planets in such conducive orbits.
Yet, the presence of liquid water is not the sole criterion for habitability. On Earth, carbon compounds like carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) have been pivotal in crafting a climate that can support life. These compounds, through their intricate roles in the climate system and biogeochemical cycles, offer a blueprint for assessing the habitability of other worlds.
Ozaki and Watanabe's study delves into the atmospheric modeling of Earth-like planets circling sun-like stars, focusing on the conditions that could foster a CO-rich environment. This state, termed CO runaway, could have been prevalent in the early atmospheres of planets, laying a favorable groundwork for the emergence of life.
"The possibility of CO runaway is critical in resolving the fundamental problem regarding the origin of life on Earth because various organic compounds suitable for the prebiotic chemistry are more likely to form in a CO-rich atmosphere than in a CO2-rich atmosphere," Dr. Ozaki elucidates.
The research models the CO cycle, encompassing its production, transport mechanisms, and removal processes. Primary CO sources include the photolysis of CO2, atmospheric photochemical reactions, volcanic emissions, and the hydrothermal breakdown of formaldehyde (H2CO) in oceans. The primary removal mechanism involves reactions with hydroxyl (OH) radicals, with surface deposition playing a minor role.
A key finding is that CO runaway occurs when CO production outpaces its removal by OH radicals, a situation that can arise from elevated CO2 levels or the presence of reducing gases from volcanic activity. The researchers identified a specific range of CO2 partial pressures and temperatures under which CO runaway is triggered, highlighting how such conditions affect atmospheric CO, CO2, and CH4 levels.
This investigation has led to the identification of a "CO-runaway gap," a distinctive feature in the phase space defined by the ratios of these gases' partial pressures. "Our results suggest that this CO-runaway gap is a general feature of Earth-like lifeless planets orbiting Sun-like stars, providing insights into the characteristics and potential habitability of exoplanets," Dr. Ozaki states. This breakthrough offers a new lens through which astronomers can evaluate the habitability of exoplanets, enriching the criteria beyond the mere presence of liquid water.
Although the journey to unravel the conditions conducive to life's emergence continues, this study marks a significant advancement. By broadening the scope of habitable conditions to include atmospheric chemistry dynamics, researchers edge closer to identifying planets that might not only be home to liquid water but also possess the chemical precursors necessary for life's inception. As we expand our search across the nearly 40 billion Earth-size planets orbiting Sun-like stars in the Milky Way, discoveries like the CO-runaway gap equip us with valuable tools to refine our quest for worlds where life might flourish.
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