Habitability

It is widely accepted that a planet capable of developing life (as we know it), has to be able to continuously maintain liquid water on its surface and in its atmosphere. The capability of a planet in retaining water depends on its size and the processes involving its interior dynamics and atmospheric circulation. It also depends on its orbital parameters (i.e., its semimajor axis and orbital eccentricity) and the brightness of the central star at the location of the planet. While a dynamic interior and atmospheric circulation are necessary for a habitable planet to develop a CO2 cycle and generate greenhouse effect (which helps the planet to maintain a uniform temperature), a long-term stable orbit, at a right distance from the star, is essential to ensure that the planet will receive the amount of radiation that enables it to maintain liquid water on its surface. These seemingly unrelated characteristics of a potential habitable planet, have strong intrinsic correlations, and combined with the luminosity of the star, determine the system's habitable zone.

The inner and outer boundaries of a habitable zone vary with the star's luminosity and the planet's atmospheric circulation models [see Menou & Tabachnik (2003), Jones, Underwood, & Sleep (2005), and Jones, Sleep, & Underwood (2006) for a table of distances of the inner and outer boundaries of the habitable zones of exoplanetary systems]. A conservative estimate of the habitable zone of a star can be made by assuming that its inner edge is located at a distance closer than which water on the surface of the planet evaporates due to a runaway greenhouse effect, and its outer edge is at a distance where, in the absence of CO2 clouds, runaway glaciation will freeze the water and create permanent ice on the surface of the planet. As shown by Kasting, Whitmire, & Reynolds (1993), such a definition of a habitable zone results in a habitable region between 0.95 AU and 1.15 AU for the Sun (Fig. 9.23). This is a somewhat conservative estimate of the Sun's habitable zone and as noted by Jones, Underwood, & Sleep (2005), the outer edge of this region may, in fact, be farther away (Forget & Pierrehumbert, 1997; Williams & Kasting, 1997; Mischna et al., 2000).

Since the notion of habitability is based on life on Earth, it is possible to calculate the location of the boundaries of the habitable zone of a star by comparing its luminosity with that of the Sun. For a star with the surface temperature T and radius R, the luminosity L is given by where a is Boltzmann constant. Using Eq. (9.5) and the fact that Earth is in the habitable zone of the Sun, the radial distances of the inner and outer edges of the habitable zone of a star can be obtained from (Haghighipour 2006)

The quantities Tq and Rq in Eq. (9.6) are the surface temperature and radius of the Sun, respectively, and rG represent the radial distance of Earth from the Sun.

A

r S.

: K

Habitable zone--

/.-''SOLAR SYSTEM

M

*

1

—-Tidal lock radius :

t 1 111., it

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