  Thousands of transiting exoplanets have already been detected orbiting a wide
range of host stars, including the first planets that could potentially be
similar to Earth. The upcoming Extremely Large Telescopes and the James Webb
Space Telescope will enable the first searches for signatures of life in
transiting exoplanet atmospheres. Here, we quantify the strength of spectral
features in transit that could indicate a biosphere similar to the modern Earth
on exoplanets orbiting a wide grid of host stars (F0 to M8) with effective
temperatures between 2,500 and 7,000K: transit depths vary between about
6,000ppm (M8 host) to 30 ppm (F0 host) due to the different sizes of the host
stars. CO2 possesses the strongest spectral features in transit between 0.4 and
20microns. The atmospheric biosignature pairs O2+CH4 and O3+CH4 - which
identify Earth as a living planet - are most prominent for Sun-like and cooler
host stars in transit spectra of modern Earth analogs. Assessing biosignatures
and water on such planets orbiting hotter stars than the Sun will be extremely
challenging even for high-resolution observations. All high-resolution transit
spectra and model profiles are available online: they provide a tool for
observers to prioritize exoplanets for transmission spectroscopy, test
atmospheric retrieval algorithms, and optimize observing strategies to find
life in the cosmos. In the search for life in the cosmos, transiting planets
provide the first opportunity to discover whether or not we are alone, with
this database as one of the keys to optimize the search strategies.
