Geoscience Reference
In-Depth Information
water content (such as on Earth) can host liquid water permanently on its solid
surface. This definition of the HZ assumes that the abundance of CO 2 and H 2 Oin
the atmosphere is regulated by a geophysical cycle similar to Earth's carbonate-
silicate cycle. The inner and outer boundaries of the HZ in this model are associated
with a H 2 O- and CO 2 -dominated atmosphere, respectively. Between those limits on
a geologically active planet, climate stability is provided by a feedback mechanism
in which atmospheric CO 2 concentration varies inversely with planetary surface
temperature.
The locations of the inner and outer boundaries of a single star's as well as a
binary's HZ depend also on the cloud fraction in the planet's atmosphere. That is
because the overall planetary albedo is a function of the chemical composition of the
clear atmosphere as well as the additional cooling or warming of the atmosphere due
to clouds. Since the model by Kopparapu et al. ( 2013a , b ) does not include clouds,
it is customary to define two types of HZ: the narrow HZ which is considered to
be the region between the limits of runaway and maximum greenhouse effect in the
model by Kopparapu et al. ( 2013a , b )andthe empirical HZ, as a proxy to the effect
of clouds, that is derived using the fluxes received by Mars and Venus at 3.5 and
1.0 Gyr, respectively (the region between recent Mars and early Venus). At these
times, the two planets do not show indications for liquid water on their surfaces (see
Kasting et al. 1993 ). In these definitions, the locations of the HZs are determined
based on the flux received by the planet (see, e.g. Kasting et al. 1993 ;Selsisetal.
2007 ; Kaltenegger and Sasselov 2011 ; and Kopparapu et al. 2013a ).
13.7.2
Effect of Star's Spectral Energy Distribution (SED)
The locations of the boundaries of the HZ depend on the flux of the star at the
orbit of the planet. In a binary star system where the planet is subject to radiation
from two stars, the flux of the secondary star has to be added to that of the primary
(planet-hosting star), and the total flux can then be used to calculate the boundaries
of the HZ. However, because the response of a planet's atmosphere to the radiation
from a star depends strongly on the star's SED, a simple summation of fluxes is
not applicable. The absorbed fraction of the absolute incident flux of each star
at the top of the planet's atmosphere will differ for different SEDs. Therefore, in
order to add the absorbed flux of two different stars and derive the limits of the HZ
for a binary system, one has to weigh the flux of each star according to the star's
SED. The relevant flux received by a planet in this case is the sum of the spectrally
weighted stellar flux, separately received from each star of the binary, as given by
(see Kaltenegger and Haghighipour 2013 for details),
F Pl .f;T Pr ;T Sec / D W Pr .f;T Pr / L Pr .T Pr /
r PlPr
C W Sec .f;T Sec / L Sec .T Sec /
r PlSec
: (13.1)
Search WWH ::




Custom Search