From Hycean Worlds to TRAPPIST-1e: Which Planets Are Most Likely to Reveal Prebiosignature Molecules?

When exploring life on exoplanets, the most intuitive approach is often to search for gases such as oxygen that are produced or consumed by living organisms. However, long before life itself emerges, a planet's atmosphere may already contain molecules essential for its origin. These substances are known as prebiosignature molecules. They are not products of life, but rather the chemical precursors that enable life to arise.

On early Earth, prebiotic chemistry relied on compounds such as water, methane, sulfur species, ammonia, and hydrogen cyanide. Today, astronomers aim to detect similar molecules in the atmospheres of distant exoplanets using space telescopes, thereby assessing whether those worlds possess the chemical conditions necessary for life to begin. This task is challenging, as these molecules are typically present in trace amounts, and their spectral signals can be obscured by other atmospheric constituents.

To address this, one study investigates the minimum atmospheric abundance required for a prebiosignature molecule to be confidently detected using the James Webb Space Telescope (JWST). The research team developed a comprehensive simulation pipeline that integrates atmospheric modeling, spectral calculations, noise simulation, and Bayesian statistical analysis to evaluate whether a molecule can be detected with sufficient confidence.

The molecules selected for analysis are central to prebiotic chemistry and widely involved in known reaction pathways. These include hydrogen cyanide (HCN), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), ammonia (NH₃), methane (CH₄), acetylene (C₂H₂), nitric oxide (NO), formaldehyde (CH₂O), carbon monoxide (CO), and cyanoacetylene (HC₃N). Hydrogen cyanide, for example, plays a key role in generating precursors of nucleotides and amino acids. Cyanoacetylene serves as a backbone molecule in several prebiotic pathways, while sulfur-bearing compounds and formaldehyde are involved in reactions leading to RNA, sugars, and amino acids. Detecting any of these molecules in an exoplanetary atmosphere would provide insight into how life-related chemistry might unfold beyond Earth.

One of the most critical factors influencing detectability is atmospheric thickness, or more precisely, the mean molecular weight of the atmosphere. Atmospheres dominated by light gases such as hydrogen or helium are more extended, producing stronger absorption features in transmission spectra. In contrast, atmospheres composed of heavier molecules like nitrogen or carbon dioxide are more compact, making spectral features weaker and harder to detect.

Based on these considerations, the study examines several representative planetary environments:

1. Hycean planets: modeled after K2-18b, these worlds possess thick hydrogen-dominated atmospheres, producing strong and easily identifiable infrared absorption features.

Simulated illustration of the Hycean planet K2-18b orbiting the red dwarf star K2-18（Image source：ESA/Hubble， CC BY 4.0 ）

2. Ultrareduced volcanic atmosphere planets: based on GJ 1132b, these planets experience continuous volcanic outgassing of reducing gases such as hydrogen and methane, resulting in highly reducing atmospheric conditions.

Simulated illustration of GJ 1132b（Image source：NASA/JPL-Caltech/Lizbeth B. De La Torre，CC0 1.0 ）

3. Post-impact atmosphere planets: these atmospheres arise after large planetary collisions. Such impacts vaporize surface water and release reduced materials from deeper layers, creating hot, hydrogen-rich environments that can rapidly generate prebiotic molecules. These conditions may resemble a chemically accelerated phase in early Earth history.

4. High mean molecular weight atmospheres, exemplified by TRAPPIST-1e: unlike hydrogen-rich cases, these atmospheres are dominated by nitrogen and carbon dioxide, similar to early Earth. Their compact structure leads to weaker spectral signals, making detection more difficult. Nevertheless, TRAPPIST-1e remains a prime observational target due to its proximity and Earth-like properties.

Simulated illustration of TRAPPIST-1e（Image source：NASA/JPL-Caltech，CC0 1.0 ）

These planetary environments represent the most promising scenarios for the accumulation of detectable prebiosignature molecules.

When JWST observes a transiting exoplanet, starlight passes through the planet's atmosphere, and molecules imprint characteristic absorption features onto the spectrum. The study uses the petitRADTRANS package to compute these spectra, and the PandExo tool to simulate realistic JWST noise. Bayesian statistical methods are then applied to determine whether a molecular signal remains detectable at a confidence level of 3σ after noise is included.

The results reveal that hydrogen-rich atmospheres are particularly favorable for detection. In Hycean planets, some molecules can be identified at concentrations as low as parts per million. For instance, cyanoacetylene may be detectable at approximately 0.06 ppm, sulfur dioxide at around 0.6 ppm, and acetylene at a few ppm. Other key molecules such as carbon monoxide, ammonia, and hydrogen cyanide are also detectable within realistic observational limits.

In contrast, ultrareduced volcanic atmospheres contain large amounts of strongly absorbing species such as methane and hydrogen cyanide, which can obscure the spectral signatures of other molecules. This increases detection thresholds. Nevertheless, molecules like cyanoacetylene, sulfur dioxide, and acetylene can still be detected at concentrations on the order of ~10 ppm.

Transmission spectra of different exoplanet models calculated using petitRADTRANS. (1) Hycean Planet: The spectrum exhibits large amplitude variations due to the highly extended hydrogen-dominated atmosphere, which produces strong absorption features. The overall spectral shape reflects a highly expanded, molecule-rich atmosphere. (2) Hydrogen-rich Super-Earth: The hydrogen atmosphere is thinner, resulting in smaller spectral variations compared to Hycean planets, but key molecular features remain detectable. (3) Ultrareduced Volcanic Planet: The atmosphere is dominated by volcanic outgassing, with high abundances of CH₄ and HCN producing strong absorption near 3 μm. The presence of thick clouds significantly reduces the detectability of prebiosignature molecules. (4) Post-Impact Planet: In the early stages following a large impact, the atmosphere is highly reducing and strongly expanded, enhancing spectral signals and making detection easier. As time progresses, atmospheric composition evolves, leading to changes in absorption features. (5) TRAPPIST-1e: The atmosphere is dominated by N₂ and CO₂, resulting in a high mean molecular weight. This produces very small spectral variations, weak signals, and shallow absorption features, making it the most challenging case for detecting prebiosignature molecules（Image source：Claringbold A et al. (2023)， CC BY 4.0 ）

Post-impact atmospheres provide some of the most favorable conditions for detection. Following a major impact, metallic iron reacts with water to produce large amounts of hydrogen (Fe⁰ + H₂O → FeO + H₂), resulting in highly expanded atmospheres with amplified spectral signals. In such environments, detection thresholds can drop to extremely low levels.

At approximately 100,000 years after impact, some molecules may be detectable at concentrations as low as 0.01 ppm or even lower. However, as the atmosphere evolves over time, cooling and compositional changes reduce the strength of spectral features. By around 10 million years after impact, higher abundances are required for detection.

Molecules such as hydrogen cyanide, methane, acetylene, cyanoacetylene, and formaldehyde are particularly detectable during the early stages. This suggests that if planets with conditions similar to early Earth exist elsewhere, JWST could directly observe the chemical ingredients involved in prebiotic processes.

In contrast, high mean molecular weight atmospheres like that of TRAPPIST-1e remain challenging. Even with extensive observations spanning 40 to 100 transits, only a limited set of molecules—such as methane, ammonia, acetylene, cyanoacetylene, hydrogen cyanide, and sulfur dioxide—are likely to reach detectable levels.

Overall, the study demonstrates that detecting prebiosignature molecules is within reach. It identifies which planetary environments are most promising, which molecules are most detectable, and how many observations are required to reveal the chemical foundations of life. More importantly, it extends the question of life's origin beyond Earth, suggesting that if such molecules are found on other worlds, life may be understood as a natural outcome of cosmic chemistry rather than a rare anomaly.

The James Webb Space Telescope may therefore open not only a new window onto distant planets, but also a pathway toward uncovering how life itself begins in the universe.

Author: Shui-Ye You

Reference:

Claringbold A et al. (2023). Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST. The Astronomical Journal.

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