6. Are invoked processes contingent to the solar system or generic?
The main argument in favour of the generic nature of the processes invoked is that they result from general physics and chemistry laws.
A basic assumption is that carbon is mainly present on telluric planets in its fully oxidised state (CO2 or carbonates) and that its greenhouse effect leads to a moderate climate on the planet. The first point results from the chemical equilibrium between CO2, CH4, H2O and H2 gases. Regarding the second point, telluric planets are searched for around neighbouring stars whose metallicity will be measured, and is expected to be similar to that of the Solar System within a factor of a few. CO2 or carbonates should be abundant on these planets, providing, if plate tectonics is present, the climate regulation process that has been described for planets in the CHZ. Indeed, the proposed IR missions will be able to check whether CO2 is actually present in the atmospheres thanks to the measurement of the 15 µm band.
Plate tectonics and volcanism also play a key role by providing an efficient sink for free O2. They are the result of internal planet heating by radioactive uranium, and of silicate fluidity enhanced by hydration and are expected to be generic whenever the mass of the planet is sufficient and liquid water is present on it. The presence of H2O will also be determined by the missions (absorption in the 6-8 µm range). The size of the (possibly) detected telluric exoplanets, and an estimate of their mass, can be determined as follows. The shape of the IR spectrum will yield the effective temperature of the emitting atmosphere and the absolute IR flux will yield its area, provided a sensible assumption is made of the planet's IR emissivity. Then, for planets of the size of, or larger than, the Earth, located in the CHZ and with spectroscopic evidence of H2O, the occurrence of plate tectonics and volcanism is expected as generic processes.
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998