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

Detecting bio-markers in habitable-zone earths transiting white dwarfs