If there aren't any planets Quoting: Anonymous Coward 1600864
Well, about every thousand years we seem to have some (plague, nuclear weapons, etc.) that could have eliminated humanity.
Between the ability of an advanced civilization to shoot itself in the foot and those ever present space rocks, survival as an advanced civilization is a little precarious.
We have searched almost 800 solar systems. We haven't found a single rocky planet with water in the sweet spot like earth.
Gliese 581g is the closest we have come:
* Three times larger.
* Three times closer than Mercury (about 1/9 the distance of the earth from the sun).
* Surface temperatures from 160 to -26 (planet is almost tidally locked).
Since it does rotate - but very slowly - the surface gets periodically sterilized.
The odds of finding a planet like earth are 0.12% and getting smaller all the time.
There are 300 billion stars. Lets assume the average civilized time on a planet is 200,000 years. Lets assume the average period a planet is habitable is 3 billion years. The Galaxy is about 13B years old
* 300 billion * .12 = 380 million planets.
* 380 * 3/13 = 88 million
* 88 million * 200,000/3 billion = 5,800 civilizations that we would consider civilized.
If ET is out there there aren't many of them.