Observatoire de Paris
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Habitable zone around other stars
The "habitable zone" denotes the region around the star where we could in principle find liquid water, i.e. at a temperature between 0 and 100°C. It is also called the "Goldilocks Zone" (not too hot and not too cold). The figure shows the "habitable zone" in a star mass graph (in solar masses)./ semi-major axis (in astronomical units). Around more massive stars, the "habitable zone" is located in more distant regions.
Copyright : Paris Observatory / UFE

There are one hundred billion stars in our galaxy, and the observable universe contains at least one hundred billion galaxies. In addition, we know that there are some planetary systems around other stars, even if today we do not know their exact frequency of occurrence. It is hard to believe that the conditions and the processes which led to the emergence of life on Earth three billion years ago were not at work on other planets outside the Solar System.

  • These conditions are, certainly, quite strict. For instance, the necessary existence of liquid water requires that the temperature of the planet be neither too high nor too low.

  • Similarly, if its mass is too large, the planet will become a gaseous planet with an atmosphere made essentially of hydrogen and helium. In such conditions, the chemical reactions of life cannot occur.

  • We think that other conditions are necessary, as, for example, a not too frequent bombardment by asteroids. Indeed, such destructive impacts can make the life process start from scratch again.

    The presence of Jupiter probably helped to maintain life on Earth. Jupiter deflected a lot of meteorites from the Earth at the beginning of the formation of the Solar System.