Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
In this paper, Claringbold and colleagues present an analysis of the different thresholds in measuring prebiosignatures—chemical signatures that would have been part of prebiotic chemistries—for five classes of terrestrial planets using the James Webb Space Telescope (JWST).
The discussion begins with a presentation of the various chemical synthese experiments (Miller-Urey, cyanosulfidic scenario, etc.) and their associated precursor molecules. Given the fact that some of these molecules may be present in the atmospheres of exoplanets, these prebiosignatures could be detected by JWST.
Claringbold et al. first make the distinction between:
- primary prebiosignatures, that are direct products or feedstock of the prebiotic pathways themselves, and
- secondary prebiosignatures, that are created by abiotic processes (impacts, volcanism, stellar activity, lightning) and may be involved in the origin of life.
The various molecules that will be analyzed in the paper are shown on Figure 1:
Claringbold and colleagues then proceed to explain the process of detecting these signatures. Atmospheres are observable by transmission spectroscopy, secondary eclipse spectroscopy and direct imaging. Since what we’re after is the retrieval of abundances for these molecules, we use infrared transmission spectroscopy, which is particularly well-suited for the task, and thus focus on observations from JWST. Future missions such as the Large Interferometer for Exoplanets (LIFE) will eventually allow exploration via direct imaging of planets around Sun-like stars and non-transiting planets.
Targets considered here are planets around smaller M and K stars because of the stronger signal around small stars. This comes with both an advantage (habitable zone planets are abundant) and a disadvantage (there are considerable implications for habitability around these stars, such as stellar flares, etc.) Moreover, the most promising atmospheres for these observations are hydrogen-rich atmospheres, which have a low mean molecular weight and a “puffy” atmosphere.
To calculate the detection threshold of the various prebiosignatures, the authors have developed a novel detection tool called TriArc, and used the petitRADTRANS package to generate synthetic JWST noise. They then performed Bayesian inference analysis to determine the detection thresholds for each molecule.
The selected planets that were modeled have an atmosphere rich in hydrogen and helium, which means that the atmosphere has a reduced mean molecular weight and implies ideal conditions for detection of transmission spectrum signal.
They analyzed five physically-motivated background atmospheres:
- Hycean world: an ocean planet with a hydrogen atmosphere
- Ultrareduced volcanic world: an active volcanic planet with hydrogen and nitrogen-rich outgassing
- Post-impact world: a planet in the aftermath of a collision with another body, resulting in the evaporation of the oceans and reduction of atmospheric species
- Super-Earth: a rocky super-Earth with a thin hydrogen envelope
- TRAPPIST-1e: a high mean molecular weight atmosphere, such as the one Earth possessed for most of its history
All of these planets are modeled against an M4V star with a radius of 0.21 \(R_☉\).
For their analysis, Claringbold et al. considered three instruments from JWST:
- NIRISS SOSS (which covers from 0.8-2.9\(\mu\)m)
- NIRSpec G395M (2.9-5\(\mu\)m)
- MIRI LRS (5-10\(\mu\)).
They simulated 6 hours of observation per instrument, including 3 transits each.
The prebiosignatures evaluated, including the wavelength at which they’re detected and the instrument used to detect them, are shown on Table 3.
The different calculated thresholds are shown below (Figure 3), for each type of planet considered. The detectability of prebiosignature thus varies by several orders of magnitude, depending on which type of planet is considered, and which molecule we want to detect.
Claringbold and colleagues also simulated the impact of a cloud deck at various altitudes. They found that it increased the detection thresholds by a factor of 3-20, which varies for each atmosphere considered.
One first conclusion from this study is that all prebiosignatures considered are detectable in hydrogen-rich exoplanets with a modest observation time with JWST. Of these, HC\(_3\)N and CH\(_3\)O are the most readily detected primary signatures. Among secondary biosignatures, CH\(_4\) and C\(_2\)H\(_2\) are also particularly well-suited for detection. Finally, primary signatures SO\(_2\), HCN, CO and NH\(_3\) are detected in trace abundances in most cases. All these are detected at wavelengths explored by the G395M instrument.
Some considerations regarding the habitability of each type of planet:
- Sub-Neptunes and ocean planets: while they are particularly abundant in the population of planets discovered, the fact that they may not have a solid surface makes it incertain whether they can harbor life.
- Super-Earths and volcanic planets: volcanism is a potential source of prebiotically relevant molecules. However if we consider the surface temperature of these planets (e.g., planet GJ 1132b), it is likely above that suitable for prebiotic chemistry.
- Post-impact planets: the transience of an impact-derived atmosphere makes it less likely to be detected. Moreover, the exact nature of an atmosphere derived from an impact will be highly dependant on the size and composition of both the impacted planet and the impactor. Recently impacted planets way be chemically well-suited for prebiotic chemistry, but surface temperature will likely be a problem. As for post-impact planets in a later stage, they may be hostile due to the greenhouse effects of hydrogen.
- Early Earths: their high mean molecular weight atmospheres around red dwarfs will require significant observation time, and we know very few planetary systems amenable to such study. Of these, TRAPPIST-1e has accessible thresholds within 5-10 transits but other signatures may require a much longer observation program. This type of planet may thus be best suited for future observatories.
The key finding of this study are thus as follow:
- the observation of prebiosignatures in an exoplanetary context is within the capabilities of JWST, but mostly in the case of low mean molecular weight atmospheres or optimal target systems such as the planets orbiting TRAPPIST-1
- 10 prebiosignatures are thus detectable in H\(_2\)-rich atmospheres using a modest number of transits (<5)
- otherwise, high mean molecular weight atmospheres (such as that of Early Earth) are generally not suited for detection of these signatures
That being said, wavelengths considered by Claringbold and colleagues overlaps with that of the planned observatory LIFE (4-18\(\mu\)m), so detection of signatures through direct imaging of Earth-like planets may be possible in the (not so distant) future.