Habitable Zones

The Earth has likely been habitable for an extremely long time, and could have become habitable as early as 10-20 Myr after the Moon-forming impact (Zahnle et al., 2007; Martin et al., 2006a). After the late heavy bombardment 3.7 Gya, the Earth was probably habitable continuously, at least in some environments (Catling and Kasting, 2007). Today, our Earth still maintains moderate temperatures and a liquid ocean, in stark contrast to the desert worlds that flank it, Mars and Venus. Earth's habitability, although governed by many factors, is dominated by its distance from the Sun and its near-circular orbit. The regions around a star in which a planet is habitable at a given time, and can be kept habitable over a continuous period of time are known as the 'instantaneous habitable zone' and the 'continuously habitable zone' respectively (Kasting et al., 1993; Kasting & Catling, 2003).

Within this distance range from its parent star, a planet is able to maintain liquid water on its surface. The boundaries of this zone are determined using climate models to calculate the inner star-planet distance at which the Earth's oceans would start to be lost in a process that would lead to a 'runaway greenhouse', and the outer distance at which it would suffer 'runaway glaciation', mediated by the formation of CO2 ice clouds. For our own Solar System the habitable zone (Fig. 10.1) is conservatively estimated to be between 0.95 and 1.37 AU (Kasting et al., 1993). Venus and Mars, both outside the classical habitable zone limits, have a mean solar distance of 0.7 AU and 1.52 AU respectively. The ellipticity of the planet's orbit can also affect its habitability, as highly eccentric orbits could take the planet in and out of its star's habitable zone. However, the presence of an atmosphere could help to buffer this effect (Williams and Pollard, 2002)

In addition to the instantaneous habitable zone, we are also interested in how long the planet may have been able to maintain habitability, so that life had an opportunity to originate and develop. While this does depend on many planetary environmental factors, a crude estimate can be obtained by estimating the evolution in luminosity of the parent star, and the migration of the habitable zone outward

Fig. 10.1. The Habitable Zone. This diagram, based on Fig 16. in (Kasting et al., 1993) shows the calculated habitable zone (diagonal strip) as a function of the distance from the parent star (x-axis) for parent stars of different mass and spectral type (the y-axis). Our Solar System, which orbits a G2V star, is shown for comparision. Note that the Earth sits squarely within the Habitable Zone, and Venus and Mars border it. Also shown is the "tidal lock radius". This is the distance from the parent star at which a planet is likely to become tidally locked to the star. For stars in the K and M classes, the habitable zone is within the tidal lock radius.

Fig. 10.1. The Habitable Zone. This diagram, based on Fig 16. in (Kasting et al., 1993) shows the calculated habitable zone (diagonal strip) as a function of the distance from the parent star (x-axis) for parent stars of different mass and spectral type (the y-axis). Our Solar System, which orbits a G2V star, is shown for comparision. Note that the Earth sits squarely within the Habitable Zone, and Venus and Mars border it. Also shown is the "tidal lock radius". This is the distance from the parent star at which a planet is likely to become tidally locked to the star. For stars in the K and M classes, the habitable zone is within the tidal lock radius.

over time to calculate a continuously habitable zone (CHZ) over a given time span. It is estimated that our Solar System has had a CHZ spanning 0.95-1.15AU in the past 4.6 Gy. However, in the next 1.1 Gy, the Sun is likely to become 10% brighter and the the Earth's surface temperature may increase to the point where we leave the CHZ in another 500-900 million years (Caldeira and Kasting, 1992).

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