Finding Signs of Life on Transiting Earth-like Planets: High-Resolution Transmission Spectra of Earth through time around FGKM Host stars

Note: see version 1 for the high-resolution spectra figures and source data. See version 2 for P-T profiles and chemistry profiles.

The search for life in the universe mainly uses modern Earth as a template. However, we know that Earth’s atmospheric composition changed significantly through its geological evolution. Recent discoveries show that transiting, potentially Earth-like, exoplanets orbit a wide range of host stars, which strongly influence their atmospheric composition and remotely detectable spectra. A database for transiting terrestrial exoplanet around different host stars at different geological times, is a crucial missing ingredient to support observational searches for signs of life in exoplanet atmospheres.

Here, we present the first high-resolution transmission spectra database for Earth-like planets, orbiting a wide range of Sun-like host stars, throughout representative stages of Earth’s history. Our models cover four geological epochs, corresponding to a prebiotic high CO₂-world - about 3.9 billion years ago in Earth’s history - and three epochs through the rise of oxygen from 0.2% to modern atmospheric levels of 21%.

We demonstrate that the spectral biosignature pairs O₂+CH₄ and O₃+CH₄ in the atmosphere of a transiting Earth-like planet would show a remote observer that a biosphere exists for oxygen concentrations of 1% modern Earth’s, corresponding to about 1 to 2 billion years ago in Earth’s history for all host stars.

The high-resolution transmission spectra database, covering 0.4m to 20m, also provides the contrast ratio for absorption features for transiting Earth-like exoplanets orbiting a wide range of host stars – from young prebiotic worlds to modern Earths-analogs. Our full model database is available online and can be used as a tool to plan and optimize our observation strategy, train retrieval methods, and interpret upcoming observations with ground- and space-based telescopes.