Based on observations by the Kepler space telescope, astronomers now estimate that the Milky Way may be stuffed with at least 50 billion exoplanets, and that 500 million of these planets could be located inside the habitable zones of their parent stars.
This announcement was made on Saturday by Kepler science chief William Borucki at the American Association for the Advancement of Science in Washington D.C. However, Kepler didn't actually count 50 billion exoplanets, this number comes from extrapolations of the data taken so far by the exoplanet-hunting space telescope.
For example, as Kepler has spotted 1,235 exoplanet candidates so far -- 53 of which orbit stars in their habitable zones -- knowing approximately how many stars there are in our galaxy (there are thought to be around 300 billion stars in the Milky Way), an estimate can be made of how many worlds are orbiting these stars.
Kepler has only studied 1/400th of the sky, and it can only detect exoplanets that pass in front of (or "transit") their parent stars. Also, it needs more time to detect exoplanets that orbit further away from their stars. Taking all these factors into account means that a lower estimate can be made. There's likely to be more than the 50 billion exoplanets Borucki describes.
One can only wonder how many planets there are in the universe, considering the Milky Way is just one of more than 170 billion galaxies in existence.