The answer is 50 billion. But I find the number in the Goldilocks zone far more interesting.
At least 500 million of those planets are in the not-too-hot, not-too-cold zone where life could exist. The numbers were extrapolated from the early results of NASA’s planet-hunting Kepler telescope.
Hey look, every Facebook user could have one Goldilocks planet all his or her own.
These numbers could change drastically, but don’t expect to ever see any minuscule estimate by any measure. We have billions of stars in the Milky Way alone; we can predict the number of planets should reasonably be in the billions just by that fact. And if we venture our minds outside our little corner of the Universe, we realize there are more stars than grains of sand on Earth. The total number of planets in the Universe is undoubtedly in the trillions. And I’m probably being conservative. Earth is likely to be mind-blowingly mundane.