The habitability and detection of Earth-like planets orbiting cool white dwarfs.

L. Fossati, S. Bagnulo, C.A. Haswell, M.R. Patel, R. Busuttil, P.M. Kowalski, D.V. Shulyak, M.F. Sterzik

Fig. 1 Comparison between white dwarf and solar synthetic emergent fluxes per unit area at the emitting photosphere. The thick line: 5800 K hydrogen white dwarf; dashed lines: hydrogen white dwarfs with T_eff = 4000, 5000, and 6000 K. With decreasing temperature the hydrogen lines weaken untill they almost disappear. Thin continuous red line: synthetic fluxes of the Sun calculated with MARCS models; dotted line: a 5777 K, black body, i.e. with the SunòÀÙs T_eff. The vertical black line at 3100 Å shows the limit for the DNA damaging fluxes. The shaded areas show the wavelengths playing a major role in the process of photosynthesis.

Abstract

Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the Continuous Habitable Zone (CHZ) for ∼8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf is 10² (10⁴) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow reveal the presence of a planet atmosphere, providing a first characterisation. Planets in the CHZ of a 0.6M_☉ white dwarf will be distorted by Roche geometry, and a Kepler-11d analogue would overfill its Roche lobe. With current facilities a Super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known cool white dwarf. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.

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Last Revised: 2012 August 1st