Astronomers estimate that our galaxy has about one exoplanet for every star. Of course, some stars have many planets, such as our Sun, which has eight. And some stars have none. However, if a star lives long enough, planet formation appears to be the rule rather than the exception.

That doesn’t mean astronomers will be able to map all of those billions of stars. The number of exoplanets that have been measured or counted in some way is much smaller.

As of this writing, the total number of confirmed exoplanets stands at 5,108. Astronomers, on the other hand, are surprisingly good at figuring out what they can’t see. They are aware that their telescopes are insufficiently powerful or precise to detect the most elusive planets – those that are very small, very far from their stars, or orbit stars very far from Earth. And conversely, there are regions of space where astronomers are pretty confident they’ve found all the planets within a certain range.

By combining the knowledge of what they can see – the known exoplanets – with the knowledge of what they can’t see – the parts of space currently beyond our ability to investigate – astronomers end up at the approximation of one planet per star. 

READ MORE: How Many Galaxies Are There?

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