Interestingly hot desert worlds present similar observational challenges in

Interestingly hot desert worlds present similar

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Interestingly, hot desert worlds present similar observational challenges in transmission spectroscopy as do planets with Earth-like water inventories. The surfaces of hot desert worlds are still optically thick enough to hinder determinations of its surface properties (e.g., mixing ratio, temperature) [ 8 ]. 11. Binary Star Habitable Zones Several studies have assessed the habitable zones of binary stellar systems following the confirmation of their existence by Kepler (e.g., [ 221 230 ]). Binary star systems are very common and comprise ~40–50% of Sun-like systems [ 231 , 232 ] (revising the nearly 60% deduced in Duquennoy & Mayor [ 233 ], whom had overestimated their completeness correction [ 231 ]), which makes them a rich source of potentially habitable planets. There are two types of binary systems (Figure 13 ). In the P-type (planet-type) or circumbinary system, the planet orbits both host stars. In contrast, the planet only orbits one of the stars in S-type (satellite-type) systems [ 234 ]. Geosciences 2018 , 8 , x FOR PEER REVIEW 26 of 48 ( a ) ( b ) Figure 13. Schematic of P-type ( a ) and S-type ( b ) binary star systems. However, modeling binary star habitable zones presents additional challenges. One of these is that stellar fluxes will vary greatly (in intensity and possibly spectral energy distribution) along the planet’s orbit (e.g., [228,235,236]). The stellar fluxes received by the binary star planet are also determined via a weighted, not a direct, summation of the flux contributions of both stars [226–228]. This is because the heating response in atmospheres is greatly influenced by the spectral class (see Section 7.2). This has been addressed by using a representative blackbody effective temperature distribution for each of the stars [226] or via a spectral factor that explicitly weighs the stellar energy distribution (e.g., [227,228]). P-type systems can generally be modeled as single stars using the combined weighted fluxes of the individual hosts, assuming that the secondary star is sufficiently dim and considerably less massive than the primary [222,223,226]. However, S-type systems are more difficult to model because dynamical effects can modify the HZ boundaries as a function of the binary eccentricity, semi-major axis distance, and stellar mass ratio (e.g., [224,227]). Thus, not all solutions are dynamically stable for S-type binary systems (e.g., [224,227]). Even if orbital stability is maintained, both dynamical and flux effects may cause the planet to repeatedly enter and exit the HZ. Binary star HZ boundaries have been computed for multiple Kepler binary systems (e.g., [225–228]). Different criteria for assessing the habitability of binary star systems have been devised, which includes determining the amount of time that a planet remains within the HZ [224] to computing the fraction of habitable surface area over some time interval [229]. Subsequent work used a latitudinally- dependent energy balance model to evaluate the effect of Milankovitch cycles in binary star systems (both P- and S-type), finding that such cycles would be shorter (in some cases <1000 years), but of Figure 13. Schematic of P-type ( a ) and S-type ( b ) binary star systems.
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