"The question whether so-called exoplanets are habitable or not is difficult to answer, as we do not know all the necessary conditions a planet has to fulfils in order to be habitable," said Yann Alibert of the Center for Space and Habitability (CSH) at the University of Bern, Switzerland.

Based on the mass and radius of a planet, Alibert was able to determine criteria that exclude the possibility of life as we know it.
"This theoretical model will help astronomers concentrate on promising candidates in their search for Earth-like planets," said Alibert.

Two conditions, without which life is not possible, form the foundation of the theoretical models: Water in liquid form and a so-called Carbon cycle must be found on the exoplanet. The Carbon cycle is a geological process that regulates the CO2-level in the atmosphere and with that, the temperature of the planet's surface: In the ocean, CO2, in its dissolved form, undergoes a chemical reaction and is then transported into the Earth mantle. Because of the high temperature in the inner parts of the Earth mantle, the CO2 is released back into the atmosphere during volcanic eruptions.

If, however, a planet with a given mass has a very large radius, the density will be very low. Consequently there will be no Carbon cycle or liquid water on that planet.
The reason for this is that low density is an indicator for a lot of gas and/or water. If a planet consists of a lot of gas, the atmospheric pressure on the surface may be so high that water is not able to keep its liquid form.

If the planet is covered by an immense amount of water, the pressure at the bottom of the ocean will increase to such an extent that water occurs in the form of "Ice VII", which does not exist on Earth.

"Ice VII" has such a high density so that it settles on the ocean floor. There, it forms a barrier between the rocks on the ocean floor and the water above, preventing the Carbon cycle.


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