Fig. 7.25 Boundaries of the Continuously Habitable Zone as a function of the mass of the central star. The oblique line indicates the orbital distance at which a planet with the Earth's mass would be trapped in synchronous rotation, and always turn the same side towards the star, in less than 109 years. (This relationship is valid for a circular orbit only.) The insert shows the relative populations of stars in the Galaxy as a function of their mass. [Diagrams after F. Selsis (limits of the habitable zone) and J.-M. Grießmeier (synchronization time)]
The habitable zone defined previously does not take account of anything other than the luminosity of the star. To be habitable, a planet must orbit within this zone, but this does not necessarily mean that a planet within the zone is inevitably habitable, nor that planets will actually be found there. We have already mentioned the planetary mass as a decisive factor, but other parameters must be taken into account.
The Sun is a G-type star and its lifetime is approximately 1010 years. Stars more massive than the Sun have shorter lifetimes: stars that are more than 2 M0 explode as supernovae in less than 109 years. Such stars are, however, very much in the minority. Most stars have a low, or even very low, mass. The habitability of possible planets orbiting these stars is therefore a very important point that we ought to discuss. Within the range 0.1-2 M0, 95 per cent of stars have a mass that is below 0.6 M0. These a M-type stars. Because of their low luminosity, the habitable zone is extremely close to the star, which results in the rotation of the planets being braked by tidal effects. So, if habitable planets exist around M-type stars, they are in synchronous rotation, that is, they always present the same face to the star (or approximately so, if their orbits are eccentric, as in the case of Mercury).
The climate of such planets in synchronous rotation have been studied by Joshi et al. (1997) and Joshi (2003). The results obtained show that the very significant greenhouse effect and the atmospheric circulation generated in a very dense (> 1.5 bar) CO2 atmosphere tend to reduce the temperature contrast between the day and night sides. Without this redistribution of the incident stellar energy, the atmosphere and the water would all completely condense on the dark side of the planet. However, in this case, it is more difficult to explain how the carbonate-silicate cycle could maintain the level of CO2 at its high, beneficial level. In the most recent studies, Joshi has shown that if the planet possessed an ocean, its thermal inertia and circulation would enable habitability to be maintained with distinctly lower levels of CO2 (a few tens of millibars). It remains to be seen, however, whether from an evolutionary point of view such a state could be attained and maintained.
Another problem calls into question the habitability around M stars. The extremely strong activity of these stars, which appears in the form of powerful X-ray and EUV emission, a significant mass loss (in a stellar wind), and violent coronal mass ejections, would result in considerable erosion of any atmosphere in a potentially habitable zone. If we were to place the present-day Earth in the habitable zone of an M-type star, the X-ray radiation would raise the upper atmosphere to such temperatures that it would escape violently. Atmospheric oxygen and nitrogen would be endangered. CO2, however, provides good protection against heating by X-rays and we have just seen that a habitable planet around an M-type star should anyway have an atmosphere that is much richer in CO2 than the Earth. The problem of X-rays is therefore solved by climatic aspects: either the atmosphere is condensed on the night side, in which case any escape is minor, or the partial pressure of CO2 is very high, and limits the temperature of the upper atmosphere and thus thermal escape. Nevertheless, the most formidable danger to the habitability of planets orbiting M stars is coronal mass ejections. The violent eruptions of material from the stellar corona occur frequently in low-mass stars and the atmospheric loss that they would cause on planets in the habitable zone might be considerable (Khodachenko et al., 2007). This erosion is made all the more effective by the slow rotation of the planets, which probably has the result in a weak magnetic field when compared with that of the Earth, thus weakening the protection against outbursts of the stellar wind.
f. A model for a habitable planet or Exo-Earth
The parameter space defining the atmosphere of a terrestrial planet is drastically reduced if we restrict ourselves to an Earth-like planet. The amount of atmospheric water vapour and CO2 are no longer free parameters, but depend on the orbital distance and the luminosity of the star, as shown in Fig. 7.25. We can then model the atmospheric structure of a habitable planet as a function of orbital distance, and produce the synthetic spectra shown in Fig. 7.26. One unknown remains the partial pressure of N2. It is generally accepted (although disputed by some) that practically all of the Earth's nitrogen was degassed in the form of N2 very early in the Earth's history (in the first 50 million years). By fixing nitrogen, life introduced a transfer of N from the atmosphere to the crust, but it seems that this flow involves only a tiny fraction of atmospheric nitrogen. The atmosphere of Venus contains about 4 times
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