The amount of Earth like planets in the galaxy...

I suck at math. I suck so bad at it, I had to become an artist. Or a bum. I choose the first. That's how much I suck at math.

Therefore I'm always in awe whenever someone like Phil Plait (the creator of Bad Astronomy, astronomer, lecturer, and author) solves questions like: How many Earth like planets are there in our galaxy?

Phil Plait came up with this surprising result: There's lots out there.We're not talking Earth mass planets, but the real deal. Planets roughly the same size as this pebble, inside the Goldilock Zone (where water will stay runny and wet, and not rock hard as mostly common out there) and orbiting the same type of sun as ours! The math behind it is quite amazing - nearly as amazing as the figure itself.

"The distance to the Gliese 581 system is what gets me excited: it’s 20 light years away. This planet is about 3 times the Earth’s mass, and it orbits its star in the right place. We don’t know what it’s made of, if it has an atmosphere, or really very much about it at all! But given its mass and temperature, it’s potentially habitable. Extrapolating from our one example, let’s say that habitable planets are roughly 20 light years apart in the galaxy (as we’ll see, that number can be a lot bigger or smaller, and the end result is still cool). That means there’s one star per cube 20 light years on a side:

In the drawing, each box is centered on a star, and the two stars are 20 light years apart. That means the cubes are 20 light years on a side, right? If we assume stars with livable planets are distributed throughout the galaxy like this, then there is one star per 20 x 20 x 20 = 8000 cubic light years. That’s the density of habitable planets in the galaxy.
So how many cubic light years are there in the galaxy?
A lot. Let’s say the Milky Way is a stubby cylinder 100,000 light years across, and 2500 light years thick. The equation of volume of a cylinder is
volume = π x radius of disk² x height of disk
so
volume = π x 50,000² x 2500 = 2 x 10¹³ cubic light years
Holy wow! That’s 20 trillion cubic light years!
Now we just divide the volume of the galaxy by the density of stars with planets to get
2 x 10¹³ / 8000 = 2,500,000,000 planets
Oh my. Yeah, let that sink in for a second. That’s 2.5 billion planets that are potentially habitable!

Even if my numbers are way off, there could be as few as hundreds of millions of planets, or as many as maybe hundreds of billions in our galaxy alone that we could live on!
Again, the point being that mathematically speaking, there may be a lot of habitable planets out there. And who knows; some may be marginally habitable and we can terraform them. And then there are moons of worlds, too… I don’t think I’m speaking too far out of school if I were to speculate that for every perfect Terra Nova out there, there might be three or four more planets we could live on with some work.

Of course, I’m ignoring how we’d get there! But that’s an engineering problem, and given enough time — oh, say, a century or two — I imagine we can overcome a lot of those issues." 

 Then there's all the other details - as one reader intelligently replies:

Remember that the Earth itself was incapable of supporting modern humans until almost 500 million years ago — that is, for 8/9ths of its existence. The rest of the time there was no oxygen in the atmosphere, or not enough, or there was no ozone layer, or carbon dioxide levels were so high that humans would be unable to breath.
Then there is the problem of composition. A world where sulfur dioxide or hydrogen sulfide are part of the planet’s respiration system would kill terrestrial life off pretty quickly. The planet’s crust might have a lot of toxic compounds that, if they didn’t kill terrestrial plants, would kill animals who ate those plants. Where there is only a vanishingly small chance that an exo-world would harbor a toxic virus or other pathogen, there is a much higher likelihood that the planet would have annoying — even fatal — allergens. And how many soaps, solvents and preservatives have we invented that mimic estrogen and other hormones? It is unlikely that other worlds would have naturally occuring compounds that pose a similar risk to human health?

 
Check it out here.