The First Remotely Detected Biosignature May Not Be the Most Common: Implications for JWST and HWO

Abstract

The first detected member of a new astronomical class is often not representative of the underlying population, but instead reflects the selection effects of the observing technique that found it. We apply this idea to the first remote detection of biosignatures with two leading near future strategies: JWST transmission spectroscopy and HWO reflected light direct imaging. Using the known signal scalings of the two methods together with a simple detectability model, we show how a rare but observationally favored planet class can dominate early detections even when it is intrinsically uncommon. For JWST, an early biosignature detection is most likely to arise from a detectability favored outlier, such as a sub-Neptune or other atmosphere rich planet around a nearby M dwarf, rather than from a true Earth analog. For HWO, the situation is subtler. Among accessible habitable-zone targets around FGK-type stars, differences in maximum observable distance and hence in effective survey volume may be smaller than in the JWST case, weakening the volume bias. At the same time, stellar-type-dependent photochemistry can alter biosignature abundances, so the first HWO biosignature may emerge from a balance between photochemical enhancement and geometric accessibility. Nevertheless, within the accessible sample, planets with stronger biosignature features and higher reflected light contrast may still be favored in early detections. A first HWO biosignature could be a selection favored outlier and should not be assumed to represent inhabited rocky planets in general. Crucially, the longest lived biosphere on a planet is not necessarily its most spectrally detectable one. If the first detection turns out to be an outlier, that may still suggest that a more broader range of habitable environments awaits discovery.