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, 13 (3), 279-91

Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary

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Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary

Rory Barnes et al. Astrobiology.

Abstract

White and brown dwarfs are astrophysical objects that are bright enough to support an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, they cool and become less luminous with time; hence their IHZ moves in with time. The inner edge of the IHZ is defined as the orbital radius at which a planet may enter a moist or runaway greenhouse, phenomena that can remove a planet's surface water forever. Thus, as the IHZ moves in, planets that enter it may no longer have any water and are still uninhabitable. Additionally, the close proximity of the IHZ to the primary leads to concern that tidal heating may also be strong enough to trigger a runaway greenhouse, even for orbital eccentricities as small as 10(-6). Water loss occurs due to photolyzation by UV photons in the planetary stratosphere, followed by hydrogen escape. Young white dwarfs emit a large amount of these photons, as their surface temperatures are over 10(4) K. The situation is less clear for brown dwarfs, as observational data do not constrain their early activity and UV emission very well. Nonetheless, both types of planets are at risk of never achieving habitable conditions, but planets orbiting white dwarfs may be less likely to sustain life than those orbiting brown dwarfs. We consider the future habitability of the planet candidates KOI 55.01 and 55.02 in these terms and find they are unlikely to become habitable.

Figures

FIG. 1.
FIG. 1.
Luminosity as a function of cooling time for the WDs in the Bergeron et al. (2001) survey with masses 0.55≤M*≤0.65 MSun (stars), and the analytic fit of Eq. 1 (dashed curve).
FIG. 2.
FIG. 2.
Luminosity as a function of time for a 42 Jupiter-mass BD according to Burrows et al. (1997) (stars) and Baraffe et al. (2003) (diamonds).
FIG. 3.
FIG. 3.
Evolution of the locations of the IHZ for a 40 MJup BD (brown) and 0.06 MSun WD, assuming the Baraffe et al. (2003) and Bergeron et al. (2001) cooling models, respectively.
FIG. 4.
FIG. 4.
Evolution of the spectra of a 0.6 MSun WD and a 40 MJup BD. The XUV range, corresponding to those wavelengths that may photolyze water vapor, is shaded pink, while vertical lines denote the visible range used for photosynthesis on Earth. In the lower two panels we do not plot the corresponding WD spectra because such extremely low-temperature models are not available. We also show the Planck curve of a WD-sized and a BD-sized blackbody (BB) at 0.01 AU, respectively.
FIG. 5.
FIG. 5.
Desiccation timescale for an Earth-like planet orbiting a BD or WD at 0.01 AU. Contour lines represent the logarithm of the time for the Earth's inventory of hydrogen to be lost; that is, the planet's mass of hydrogen in water MH is equal to Earth's MH,Earth.
FIG. 6.
FIG. 6.
Planetary classifications for a tidally locked 1 MEarth planet with no obliquity in orbit about a 0.6 MSun WD. As WDs cool significantly with time, we show the phases and IHZ as a function of time, from 100 Myr (top left) to 10 Gyr (bottom right).
FIG. 7.
FIG. 7.
Planetary classifications for a tidally locked 1 MEarth planet with no obliquity, in orbit about a 0.04 MSun BD at 100 Myr (top left), 1 Gyr (top right), 5 Gyr (bottom left), and 10 Gyr (bottom right). White regions correspond to orbits that intersect the BD.

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