2025AJ....169..171H

Biosignature detection in the atmospheres of Earth-like exoplanets is one of the most significant and ambitious goals for astronomy, astrobiology, and humanity. Molecular oxygen is among the strongest indicators of life on Earth, but it will be extremely difficult to detect via transmission spectroscopy. We used the Bioverse statistical framework to assess the ability to probe Earth-like O₂ levels on hypothetical nearby habitable zone exo-Earth candidates (EECs) using direct imaging and high-resolution spectroscopy on the Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT). Assuming continued improvement in instruments and data processing, our analysis highlights the best-case scenarios. Earth-like O₂ levels could be probed on up to ∼7 and ∼19 EECs orbiting bright M dwarfs within 20 pc in a hypothetical 10 yr survey on the GMT and ELT, respectively. Four known super-Earth candidates, including Proxima Centauri b, could be probed for O₂ within about 1 week of observations on the ELT and a few months on the GMT. We also assessed the ability of the ELT to test the habitable zone oxygen hypothesis-that habitable zone Earth-sized planets are more likely to have O₂-within a 10 yr survey using Bioverse. Testing this hypothesis requires either ∼one-half of the EECs to have O₂ or ∼one-third if η_⊕ is large. A Northern Hemisphere large-aperture telescope, such as the Thirty Meter Telescope, would expand the target star pool by about 25%, reduce the time to probe biosignatures on individual targets, and provide an additional independent check on potential biosignature detections.