  The characterization of the atmospheres of habitable-zone Earth-mass
exoplanets that transit across main-sequence stars, let alone the detection of
bio-markers in their atmospheres, will be challenging even with future
facilities. It has been noted that white dwarfs (WDs) have long-lived habitable
zones and that a large fraction of WDs may host planets. We point out that
during a transit of an Earth-mass planet across a WD, the planet's atmospheric
transmission spectrum obtains a much higher contrast over the stellar
background compared to a main-sequence host, because of the small surface area
of the WD. The most prominent bio-marker in the present-day terrestrial
atmosphere, molecular oxygen, is readily detectable in a WD transit via its
A-band absorption at ~0.76 micron. A potentially life-sustaining Earth-like
planet transiting a WD can be found by assembling a suitable sample of ~500 WDs
and then surveying them for transits using small telescopes. If and when such a
transiting case is found, the O_2 absorption in the planetary atmospheric
transmission spectrum would be detectable with the James Webb Space Telescope
(JWST) in about 5 hours of total exposure time, integrated over 160 2-minute
transits. Characterization of the planet atmosphere using other tracers such as
water vapour and CO_2 will be considerably easier. We demonstrate this future
discovery space by simulating a possible transmission spectrum that would be
obtained with JWST.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics