Biosignature verification on Earth-like exoplanets and top follow-up candidates

[John91]

I'm an amateur astronomer and I've been following the data from missions like TESS and JWST with fascination, especially the atmospheric spectroscopy of potentially habitable exoplanets. I'm trying to understand the practical challenges in confirming biosignatures like oxygen or methane, given the potential for false positives from geological processes. For those more deeply involved in the field, what are the current leading candidates for Earth-like exoplanets that warrant the most follow-up observation time, and what technological or methodological advancements on the horizon do you think will be most transformative for characterizing these distant worlds?

[RyanAP]

Right now the strongest Earth-like follow-ups are around nearby M-dwarfs with transiting, temperate planets: TRAPPIST-1e, TRAPPIST-1f, and TRAPPIST-1g are the poster children because JWST and the new generation of ground-based extremely large telescopes (ELTs) can pull atmospheric signals from multiple transits. LHS 1140 b and especially c are nearby rocky candidates in the habitable zone, which keeps them high on follow-up lists as spectra become accessible. TOI-700 d is another particularly promising transiting candidate from TESS that could yield atmospheric clues. Teegarden’s Star b and c are extremely nearby and Earth-sized, which makes them attractive targets if they host tenuous atmospheres. Proxima Centauri b remains a focal target due to its proximity, but atmospheric characterization is challenging because of the star’s activity and the faintness of a possible atmosphere; it nonetheless serves as a crucial testbed for future techniques. Note that some planets commonly discussed as “Earth-like” in the literature, like K2-18b, are more Neptune- or water-world type planets, so they’re less ideal as benchmarks for Earth-like atmospheres but still valuable for understanding atmospheric processes at small sizes around M dwarfs.