Influence of CO versus CH₄ on organic haze formation in atmospheres of diverse terrestrial exoplanets

¹ National Key Laboratory of Deep Space Exploration/School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
² Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
³ Space Telescope Science Institute, Baltimore, MD, USA
⁴ NHFP Sagan Fellow, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 January 2026
Accepted: 3 April 2026

Abstract

Context. Terrestrial exoplanets are expected to host secondary, high-metallicity atmospheres derived from the outgassing of volatiles such as N₂, CO₂, H₂O, CH₄, and CO. Photochemical organic hazes are likely to form in such environments, significantly impacting both atmospheric observation and planetary habitability.

Aims. This study aims to investigate haze formation across representative terrestrial exoplanet atmospheres and assess how CH₄ versus CO as the primary carbon source differentially affects haze production rates, particle properties, and chemical complexity.

Methods. We conducted six laboratory simulations by exposing the initial gas mixture (a few millibars) to glow discharge at 300 K. Each simulated atmosphere comprises 75% of N₂, CO₂, or H₂O, 10% of each of the other two gases, and 5% of CH₄ or CO. We analyzed the gas-phase products using a residual gas analyzer. For solid products, we measured production rates and particle density, determined particle size distributions via atomic force microscopy, identified functional groups using Fourier-transform infrared spectroscopy, and characterized molecular composition with very high-resolution mass spectrometry.

Results. Experiments using CH₄ produce a wider diversity of gas-phase species and substantially higher haze yields compared to the corresponding CO-based experiments. CO-derived haze particles exhibit a restricted size range (10–80 nm), whereas CH₄-derived hazes form denser material with complex functional group signatures and thousands of unique molecular formulas. The pattern of the identified molecular formulas indicates molecular growth pathways linked to detected gaseous precursors such as HCN, CH₂O, and C₂H₄.

Conclusions. The atmospheric redox state critically controls haze formation in simulated terrestrial exoplanet atmospheres. CH₄ is significantly more effective than CO in initiating organic growth, leading to higher haze production rates and greater chemical complexity. These results provide crucial constraints for exoplanet atmospheric modeling and spectral interpretation, and further support the possibility that reducing atmospheres may facilitate prebiotic organic chemistry relevant to the emergence of life.