It’s distance from the sun, for one. The small difference a few hundred miles makes is huge. It separates the cold deathly chill from Alaska, from the warm and hot climate of California.

The rotation of the earth, helps keep the temperature from forming two extremes, in essence keeping the earth fairly well equal in temperature, compared to what it could be. (instead of Alaska being 10 degrees and California being 90, it could of very well ended up Alaska being -100 and California 230 degrees.)

However the rotation of the earth is not the only thing that keeps it “equalish”, We also have the moon. The moon keeps the oceans moving, it is responsible for the ocean keeping the range of temperatures that it does.

The atmosphere, if we breathed in an atmosphere that had just 3% more oxygen, we would become extremely flammable, 3% less and we would eventually suffocate. Of course this is based on our current bodies, however it’s an interesting point as to how lucky we are at the stability of our atmosphere.

Gravity- Gravity is something to keep in mind in seriousness.

If a planet has too little gravity, it will not have the atmosphere required to sustain life as we know it, not only because of breathing/life functions, but because the air is responsible for most of the temperatures on the planet. (Climbing a mountain, it gets colder only because the air is thinner and cannot retain the heat as well as thicker air.)

If it has to much gravity it would require so much energy to move and most movement would be neutralized so quickly that life would not even have a chance to form.

Water- Liquid water is pretty much the main ingredient in life as scientists have seen. So far every form of life on earth requires water in one way or another in order to survive.

Now with all these things considered, the chances of any one planet being able to support intelligent life is pretty slim, Being able to support simple (one cell organisms) is small but not unreasonably so.

Of course, the number of stars, planets and forms of matter in our universe make it so, while the chances of any one planet having life is small, the chances of there being only one planet in the universe having life is almost as, if not even smaller as the chance of anyone planet having life.

Chances are there is a planet that has life, at least of some kind.