  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. Thus, 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 host stars, throughout four
representative stages of Earth's history. These correspond 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 about 1% modern Earth's -
corresponding to about 1 to 2 billion years ago in Earth's history - for all
host stars. The full model and high-resolution transmission spectra database,
covering 0.4 to 20microns, for transiting exoplanets - from young prebiotic
worlds to modern Earths-analogs - orbiting a wide range of host stars is
available online. It 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.
