The many constraints on life
Whilst life may exist even in the harshest of circumstances on other planets, possibly even in our solar system, what is it about our situation that allowed complex life and ultimately intelligence to evolve. First, the Sun is a single star rather than a multiple-star system, it is rich in metals but not too rich, and it is in a relatively sparse part of the galaxy rather that in the center where there is a greater threat of extinction from a supernova or a gamma-ray burster. As for Earth, it orbits the Sun in a circular path with a stable temperature that has allowed liquid water to exist for billions of years. Although a planet can have a stable orbit in a binary star system if the components are sufficiently separated for the planet to rotate about only one star, or if they are so close together for the planet to orbit the pair of them, even if that planet were to start off in a habitable zone, any complex life that developed may well be extinguished by the changing conditions if the stars were of different masses and evolved at different rates. The galaxy contains globular clusters which can host up to 100,000 stars in a volume only 100 light-years across. In contrast to the disk of our galaxy, in which the stars travel together in essentially circular orbits, the extremely high spatial density of the stars in a globular cluster and the fact that they travel in random orbits presents a severe risk of gravitational interactions that would disturb the orbits of planets and deny the long-term stability required for intelligent life to evolve. In contrast, there are only 23 stars within 13 light-years of the Sun, making interactions unlikely.
KeywordsNeutron Star Plate Tectonic Globular Cluster Star System Habitable Zone
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