The Search for Life on Exoplanets Exoplanets Habitable Zones

The habitable zone in a planetary system (see Sect. 7.3.2.1) is generally defined as the region in which a terrestrial-type planet may have liquid water at its surface dur ing part of its year (the period of revolution around its parent star). To calculate the location of this zone (a ring) as a function of the star's type, generally the following assumptions are made:

• the surface temperature of the planet is primarily caused by the heat flux from the star. As a result, all possible sources of internal energy (tidal forces, radioactivity, etc.) are ignored;

• the energy flux received by the planet corresponds to that emitted by the star, multiplied by a coefficient that lies between 0 and 1 and corresponding to the planet's albedo (reflection coefficient). The albedo has, in principle, a chromatic value, but it is considered to be constant over the whole range of radiation received from the star;

• thermodynamical equilibrium requires the incident flux to be equal to the flux emitted by the planet. From this we can deduce the effective temperature of the planet Tpl eff from the following equation:

where F is the energy flux received from the star at distance D, rpl is the planet's radius, A is its mean albedo, and a is the Stefan-Boltzmann constant.

The temperature estimated by the method just described most certainly does not take account of the effects caused by the existence (or lack of) an atmosphere, in particular, greenhouse gases. Assuming an albedo of 0.5 for Venus, for example, theoretically we obtain an effective temperature of 277 K, which neither corresponds to the surface temperature of Venus (more than 700 K), nor to the atmospheric temperature. The notion of the habitable zone should not be considered as a very rigid one.

Using the definition just given, the habitable zone in the Solar System extends approximately from 0.9 to 1.3 AU (see Sect. 7.3.2.1).

As just mentioned, in calculating the habitable zone, no account is taken of sources of internal heat. However, Europa (the second Galilean satellite of Jupiter) and, more recently, Enceladus (a satellite of Saturn) have shown indications that lead us to believe that there is liquid water beneath their frozen surfaces (see Sect. 9.5.2). In the case of Europa, the presence of the liquid water is ascribed to the dissipation internally of energy from tidal effects. Although located well outside the habitable zone, Europa and probably Enceladus do correspond to the definition of the habitable zone that we have given. Several definitions of the habitable zone have been advanced that take account of possible internal sources of heat, but our initial definition that takes just stellar radiation into account is by far the most commonly-used.

The notion of a 'continuously habitable zone' may also be defined. This is the zone within which the surface temperature of a planet allows the presence of liquid water over a specific period (for example 1000 million years). The zone thus defined is narrower than the habitable zone proper, and assumes several factors regarding the orbits of planets. It therefore needs to be used with even greater caution than the simple notion of a habitable zone.

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