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

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.