Medary.com

Next Big Future connects the dots--those dots being studies from Caltech:

Astronomers have discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy. These new results suggest that worlds with potential for life might be more common than we thought.

University of Arizona, Tucson, astronomer Michael Meyer and his colleagues used NASA's Spitzer Space Telescope to determine whether planetary systems like ours are common or rare in our Milky Way galaxy. They found that at least 20 percent, and possibly as many as 60 percent, of stars similar to the sun are candidates for forming rocky planets.

and from the Australian University of Sydney:

Astrophysicist Professor Bryan Gaensler led a team that has found that our galaxy - a flattened spiral about 100,000 light years across - is 12,000 light years thick, not the 6,000 light years that had been previously thought.

The Bad Astronomy blog does the math, using the pre-Sydney study total of 100 billion stars in the galaxy:

As it happens, about 10% of the stars in the Milky Way can be categorized as sun-like, which is about 10 billion stars. If 10% of them have rocky planets, as this study indicates, then there may be a billion Earths orbiting stars in our galaxy alone! And that’s only for stars like the Sun; lower mass stars also can form planetary systems, and there are far more of them then stars like the Sun. It is entirely possible that there are many billions of terrestrial planets in the galaxy… and there are hundreds of billions of galaxies in the Universe.

So, the Next Big Future folks took the billion Earth-like planets suggested by the Bad Astronomy people and multiplied by two.  Voila!  Two billion Earths!  Maybe more . . .