Does our vast universe support life apart from Earth? According to Seth Shostak, an astronomer with SETI (Search for Extra-Terrestrial Intelligence), the following conditions must be present to support life:
1. The solar system's sun must not be a giant star, because those burn out before life can fully develop.
2. The system's star must not be a dwarf star, because such a star locks in the close planets, meaning one side of the planet forever faces its sun, resulting in horrific weather and unlikely venues for life.
3. The system's star cannot be a double star, because the unusual gravitational forces created by a double star sun might not allow stable planetary systems.
4. The system's star must not be a young star, because stars less than one billion years old have not had enough time for life to develop.
5. Ideally, the planet would have a large moon, which creates active tides.
6. The planet should have tectonic activity, which causes metals to be pushed up to the surface, since metals are valuable to a technological civilization.
7. The planet should have a large planet farther out in its solar system, which by its great gravitational pull cleans the inner solar system of deadly asteroids and comets.
8. The planet should not have a highly elliptical orbit, which is unsuitable for incubating life.
9. For life to live on the surface, the planet must have an atmosphere. Very small planets lose their air, and very large planets tend to sport poisonous atmosphere. Earth-sized planets are ideal.
Conclusion: it just so happens that Earth meets all these conditions.