If you go back in time, a bit more than 15 years ago, there were only 9 known planets, the ones in our solar system! With the first definitive detection of an exoplanet orbiting another star in 1995 started the modern era of exoplanetary discovery.
Recently, the Kepler mission brought the constantly growing number of exoplanets up to 527 (these are confirmed exoplanets), with more than 1,200 planet candidates, including 54 in the habitable zones of their parent stars.
Considering the limitations of Kepler, the Kepler science team decided to give an estimate of the number of exoplanets in the Milky Way, our galaxy. And as a result, there would be at least 50 billion exoplanets in our galaxy! Among these, astronomers estimate that 500 million of these alien worlds are probably orbiting in the habitable zones of their 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.
So, how did astronomers come up with that number?
So far, Kepler has spotted 1,254 exoplanet candidates, studying only 1/400th of the sky. Also, it can only detect planets that transit in front of their parent stars. Considering there are approximately 300 billion stars in our galaxy, a lower estimate can be made: scientists took the frequency already observed and applied it to the number of stars in the Milky Way.
So there would be at least 50 billion alien worlds, including 500 million in the habitable zone, only in our galaxy. As the estimated number of galaxies in the Universe is 100 billion, that gives an astronomical (appropriate choice of word, right?) number of planets out there.
Of course, these are just estimates, but this raises the question of how many worlds could possibly harbor intelligent forms of life advanced enough to be wandering in space (which we cannot estimate, as so far we only know one such planet, Earth…).
Borucki asked: “The next question is why haven’t they visited us?”, before answering: “I don’t know”.
As I said, these are only estimates, there might be many more, or much less exoplanets in our galaxy, but for sure, there are quite a lot of other worlds out there, and the hunt has just begun.