Scientists have recently extrapolated how many Earth-like planets, existing within their suns' habitable zones, there might realistically be in the Milky Way galaxy, and have come up with the figure 8.8 billion.

http://www.nbcnews.com/science/8-8-billion-habitable-earth-size-planets-...

This number is based on an actual calculation, not an estimation, using data gathered by the Kepler space telescope.