A new study finds that researchers can detect oxygen in the atmosphere of a habitable planet orbiting a white dwarf.
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.
The best method for finding such planets is a transit search — looking for a star that dims as an orbiting planet crosses in front of it. Since a white dwarf is about the same size as Earth, an Earth-sized planet would block a large fraction of its light and create an obvious signal.
Researchers can study the atmospheres of transiting planets because as the white dwarf’s light shines through the ring of air that surrounds the planet’s silhouetted disk, the atmosphere absorbs some starlight. This leaves chemical fingerprints that indicate whether that air contains water vapor, or even signatures of life, such as oxygen.
Astronomers are particularly interested in finding oxygen because the oxygen in the Earth’s atmosphere is continuously replenished, through photosynthesis, by plant life.
Thus, the presence of large quantities of oxygen in the atmosphere of a distant planet would signal the likely presence of life there.
NASA’s James Webb Space Telescope (JWST), scheduled for launch by the end of this decade, promises to sniff out the gases of these alien worlds. Loeb and Maoz created a synthetic spectrum replicating what JWST would see if it examined a habitable planet orbiting a white dwarf. They found that both oxygen and water vapor would be detectable with only a few hours of total observation time.