This particular estimate was given by NASA scientist Natalie Batalha, and one should also take a look at Kepler 452b with a raised eyebrow. After all, no one has seen it yet, only Kepler has detected it. It remains far too far from being detected with other instruments, and hence there is no way to tell of its physical composition and make up, and neither do we know whether there is water on the surface or not.
Astrophysicist Batalha estimated that if one were to take a conservative approach, then should 15% of stars have a planet between 1 and 1.6 times the size of Earth in what is called the “Habitable Zone”, then one can draw the conclusion that around 15% of 90% of 100 billion stars will feature such planets, resulting in 14 billion potentially habitable worlds. Mind blowing, yes?