It bears mentioning that the Milky Way is only one of 150 billion galaxies visible to our telescopes - and each of these will have its own complement of planets.

Most of the things at the zoo don't look like us. We're one design that works. Our chimp pals sort of look like us, so that's a different take on the same basic design. But fish don't look like us, and giraffes don't. They look a little like us, but not too much. And insects certainly don't look like us, and they work just fine.

The usual metric for whether a planet is habitable or not is to ascertain whether liquid water could exist on its surface. Most worlds will either be too cold, too hot or of a type (like Jupiter) that may have no solid surface and be swaddled in noxious gases.

Seth Shostak Quotes