1.2.7 Are we alone?
There are about 100bn galaxies in the visible universe, all of which contains about 100bn stars. However until now it seems that the sun is the only one that that creates the conditions necessary for life on one of its planets, the earth. The sun was formed about 4.6bn years ago; it is a large and bright star of the Milky Way.

The earth is the only planet of the sun that lies on an orbit in the Habitability Zone, that is where the average temperature is just right for liquid water one of the primary ingredient for life. All the other planets of the sun are either to hot or too cold.

Outside the Milky Way, scientists have discovered other planets orbiting other stars. There is no evidence yet that life, in any form, exists on any of them.