SpaceTime - Colorado's Premier Science Edutainment

Nobody knows how many aliens there are out there, and if any of them are looking for us. BUT... we can take a guess. A really good guess. That guess is called the Drake Equation, which will calculate how many alien civilizations there are just in our one galaxy. And you can use it right now to see how many ETs might be looking for you!

First, you need to know how many stars there are in our Milky Way Galaxy.
Scientists are pretty sure the Milky Way has between 200 billion and 400 billion stars (that's 400,000,000,000!) But there may be even more, or perhaps you want to know how many aliens there are in a smaller or bigger galaxy. Choose whatever number seems fun!
How Many Stars Are There in The Milky Way?

Next, we need to know how many of those stars have planets around them. People used to think that planets were rare, but new telescopes like Kepler have told us that more than half of all planets have stars-at least!
It could be as high as 90%-that means that out of every 10 stars you see in the sky at night, 9 of them have planets!
How Many Stars Have Planets? (0.0 - 1.0) :

Most of those planets are going to be either balls of rock, like Mercury, or giant balls of gas, like Jupiter. Only a few of them will have solid ground, just the right gravity, and be just the right temperature (not too hot, not too cold) for water.
How Many Planets Per Star Are Just Right?

If the planet has water, chances are life will grow in it at some point. It may only be GREEN OOZE, but that's still some alien life!
But it's tough for that life to keep going. This is a tough question for scientists, because we haven't found any of it out there yet, but by looking in strange places on Earth, like cracks in the bottom of the sea floor, we now think that life will show up in most places with water.
On How Many of these Planets does Life Survive

But ooze isn't going to call us up on the radio, because it's just slime. That life has to get more and more complex, building things like worms, bugs, fish, dinosaurs, and monkeys.
Then, just maybe, some of the monkeys or dinosaurs (or bugs!) will get smart, and start using tools. Eventually, they might build radios and other things to talk to each other, and build telescopes to look out at the stars, just like we do!
Life that gets Smart(0.0 - 1.0) :

Just because our aliens have now built some radios, that doesn't mean they're trying to find us. Maybe they don't have smart people telling stories about people living on other stars. Maybe they don't have much imagination.
But let's hope not. Don't you think smart aliens would want to talk to the other aliens around them, and to us?
How many of the aliens want to say 'Hi!' to us?

Now here's a tricky question: How long do the aliens try to look for other life forms? Do they keep looking, and looking, for hundreds of thousands of years or do they give up if nobody's called them back in a few years?
Remember, humans have only had radios for 70 years now! Do you think we will still be using them in a hundred years? How about a thousand?
Maybe the aliens are trying to talk to us, but using a technology we haven't invented yet!
How long do the aliens communicate :

Even after removing soooo many planets in your previous steps, the calculations show that there are

Answer here

Alien Civlizations out there, trying to talk to us, right now!
Changing any one of your answers could lead to thousands more aliens out there, who might be looking for us, too!