Newly discovered carbon monoxide gap may help identify habitable exoplanets

The search for habitable exoplanets involves searching for planets with Earth-like conditions, such as liquid water, suitable temperature range and atmospheric conditions. A crucial factor is the planet’s position in the habitable zone, the region around a star where liquid water could potentially exist on the planet’s surface.

NASA’s Kepler telescope, launched in 2009, revealed that 20 to 50 percent of visible stars could host such habitable, Earth-sized rocky planets. However, the presence of liquid water alone does not guarantee the habitability of a planet. On Earth, carbon compounds such as carbon dioxide (CO₂), methane (CH₄) and carbon monoxide (CO) play crucial roles in shaping climate and biogeochemistry and could have contributed to the emergence of life.

Given this, a recent study by Associate Professor Kazumi Ozaki of the Tokyo Institute of Technology and research associate Yasuto Watanabe of the University of Tokyo aims to expand the search for habitable planets. Published in The Astrophysics JournalResearchers used atmospheric modeling to identify conditions that could result in a CO-rich atmosphere on Earth-like planets that orbit sun-like stars (F, G, and K types).

Atmospheric models suggest that this phenomenon, known as runaway CO, may have occurred in early planetary atmospheres, potentially supporting the emergence of life.

“The possibility of runaway CO is essential to solving the fundamental problem regarding the origin of life on Earth, because various organic compounds suitable for prebiotic chemistry are more likely to form in a CO-rich atmosphere than in a CO atmosphere.₂-rich atmosphere,” says Dr. Ozaki.

The researchers modeled the CO cycle between the atmosphere and the oceans, considering the different sources of CO production, its transport mechanisms and the processes involved in its elimination. CO photolysis₂in which CO₂ decomposes to CO when exposed to light, was considered the main source of CO.

Other sources included photochemical reactions in the atmosphere, volcanic gas emissions, and hydrothermal decomposition of formaldehyde (H₂CO) in the ocean. The removal of CO from the atmosphere has occurred primarily through its reaction with hydroxyl (OH) radicals formed by the photolysis of water vapor and, to a lesser extent, through its deposition on the planet’s surface .

Researchers have discovered that a CO runaway occurs when CO production exceeds removal by OH radicals. This can occur due to an increase in CO₂ levels or the presence of reducing gases from volcanoes that compete for OH radicals. At a temperature of 277 K, the conditions for runaway CO are met when the partial pressure of CO₂ exceeds 0.2 bar.

However, at higher temperatures (300 K), runaway CO requires even higher CO emissions.₂ and levels of volcanic gases due to increased water vapor in the atmosphere, which is a major source of OH radicals. Once initiated, CO levels in the atmosphere are limited only by surface deposition, where CO is deposited on the planet’s surface.

In particular, changes in CO, CO₂ and CH₄ The levels before and after the runaway effect led to a difference reflected in the phase space defined by the ratios of their partial pressures (pCO/pCO₂ and PCH₄/pCO₂).

“Our results suggest that this runaway CO gap is a general feature of lifeless Earth-like planets orbiting Sun-like stars, thereby providing insight into the characteristics and potential habitability of exoplanets,” explains Dr. Ozaki.

Although the exact conditions that led to the emergence of life remain uncertain, discoveries such as the CO runaway fault provide valuable clues in our quest for habitable planets that could facilitate the origin of life among nearly of 40 billion Earth-sized planets orbiting a sun. stars of the Milky Way.