Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs

Abstract

Abstract. There are several ways planets can survive the giant phase of the host star, hence one can consider the case of Earth-like planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU from the star would remain in the continuous habitable zone (CHZ) for about 8 Gyr. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 10 2 (10 4 ) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a powerful tool to detect close-in planets around white dwarfs. Multi-band polarimetry would also allow one to reveal the presence of a planet atmosphere, even providing a first characterisation. With current facilities a superEarth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue. Preliminary habitability study show also that photosynthetic processes can be sustained on Earth-like planets orbiting CWDs and that the DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence white dwarfs are compatible with the persistence of complex life from the perspective of UV irradiation.

UV Surface Environments and Atmospheres of Earth-like Planets Orbiting White Dwarfs

“…Exoplanet searches are underway around white dwarfs (WDs) (e.g. Fulton et al (2014); Fossati et al (2015); Veras & Gänsicke (2015); Xu et al (2015)), as the similarity with Earth's size should make Earth-sized exoplanets transiting WDs easier to detect and characterize than such planets around much larger main sequence stars (e.g. via atmospheric profile measurements from transit observa-tions, "weather" modeling from orbital light curves, direct measurement of atmospheric constituents with spectroscopy, etc.)…”

The occurrence of planets and other substellar bodies around white dwarfs using K2

“…Therefore, an eclipsing Earth-sized planet around a WD has a ∼ 10 4 times larger transit depth than a solar-type star and would be an excellent candidate to observe faint atmospheric features or the first exomoons (Loeb & Maoz 2013). Furthermore, WDs host a stable (≥ 4 Gyrs) habitable zone (Agol 2011), have a similar photosynthesis-relevant-wavelength integrated flux and DNA weighted UV dose as the Sun (Mc-Cree 1971;Fossati et al 2014), thus making them excellent candidates to host habitable planets. Nevertheless, no intact planets around WDs have been found so far (Farihi et al 2005;Mullally et al 2006Mullally et al , 2008Kilic et al 2009;Drake et al 2010;Faedi et al 2011;Fulton et al 2014;Vanderburg et al 2015;Xu et al 2015;Sandhaus et al 2016).…”