Tokyo Tech News

Newly Discovered Carbon Monoxide-Runaway Gap Can Help Identify Habitable Exoplanets


Published: February 8, 2024

A carbon monoxide (CO)-runaway gap identified in the atmospheres of Earth-like planets by researchers at Tokyo Tech can help expand the search for habitable planets. This gap, identified through atmospheric modeling, is an indicator of a CO-rich atmosphere on Earth-like planets orbiting Sun-like stars. CO is an important compound for the formation of prebiotic organic compounds, which are building blocks for more complex molecules for the formation of life.

Revealing Potential Habitable Planets from Atmospheric CO2, CO, and CH4 Levels

The search for habitable exoplanets involves looking for planets with similar conditions to the Earth, such as liquid water, a suitable temperature range and atmospheric conditions. One 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 20–50% of visible stars may host such habitable Earth-sized rocky planets. However, the presence of liquid water alone does not guarantee a planet's habitability. On Earth, carbon compounds such as carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) played a crucial role in shaping the climate and biogeochemistry and could have contributed to the emergence of life.

Taking this into consideration, a recent study by Associate Professor Kazumi Ozaki from Tokyo Institute of Technology, along with Associate Researcher Yasuto Watanabe from The University of Tokyo, aims to expand the search for habitable planets. Published in the Astrophysical Journal on 10 January 2024, the researchers used atmospheric modeling to identify conditions that could result in a CO-rich atmosphere on Earth-like planets that orbit sun-like (F-, G-, and K-type) stars. This phenomenon, known as CO runaway, is suggested by atmospheric models to have possibly occurred in early planetary atmospheres, potentially favoring 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," explains Dr. Ozaki.

The researchers modeled the CO cycle between the atmosphere and the oceans, considering the various sources of CO production, its transport mechanisms and the processes involved in its removal. The photolysis of CO2, in which CO2 breaks down into CO when exposed to light, was considered the primary source of CO. Additional sources included photochemical reactions in the atmosphere, emissions from volcanic gases, and the hydrothermal decomposition of formaldehyde (H2CO) in the ocean. The removal of CO from the atmosphere primarily occurred through its reaction with hydroxyl (OH) radicals formed due to the photolysis of water vapor, and to a lesser extent, by deposition to the planet's surface.

The researchers found that a CO runaway occurs when the CO production surpasses the removal by OH radicals. This can occur due to higher CO2 levels or the presence of reducing gases from volcanoes which compete for the OH radicals. At a temperature of 277 K, conditions for CO runaway are met when the partial pressure of CO2 exceeds 0.2 bar. However, at higher temperatures (300 K), a CO runaway needs even higher CO2 and volcanic gas levels due to increased water vapor in the atmosphere, which is a major source of OH radicals. Once initiated, the CO levels in the atmosphere are limited only by surface deposition, where CO is deposited onto the planet's surface.

Notably, the changes in the CO, CO2 and CH4 levels before and after the runaway effect led to a gap reflected in the phase space defined by the ratios of their partial pressures (pCO/pCO2 and pCH4/pCO2). "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," says Dr. Ozaki.

Although the exact conditions that lead to the emergence of life remain uncertain, discoveries like the CO-runaway gap provide valuable clues in our quest to find habitable planets that could facilitate the origin of life among nearly 40 billion Earth-size planets orbiting Sun-like stars in the Milky Way galaxy.


Authors :
Yasuto Watanabe1,2 and Kazumi Ozaki3,4,5,6,*
Title :
Relative Abundances of CO2, CO, and CH4 in Atmospheres of Earth-like Lifeless Planets
Journal :
The Astrophysical Journal
Affiliations :

1 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Japan

2 Meteorological Research Institute, Japan Meteorological Agency, Japan

3 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Japan

4 Earth-Life Science Institute, Tokyo Institute of Technology, Japan

5 Nexus for Exoplanet System Science (NExSS), National Aeronautics and Space Administration, USA

6 Alternative Earths Team, Interdisciplinary Consortia for Astrobiology Research, National Aeronautics and Space Administration, USA

* Corresponding author's email:

School of Science

School of Science —Exploring the Truth and Creating Knowledge—
Information on School of Science inaugurated in April 2016

School of Science

Schools, Departments, and Institute for Liberal Artsouter

Further Information

Assistant Professor Kazumi Ozaki

School of Science, Tokyo Institute of Technology



Public Relations Division, Tokyo Institute of Technology

Tel +81-3-5734-2975