2  How many planets?

So, just how many planets are there in the Milky Way? Kepler found over 2600 planets orbiting around the 150 000 stars it studied. Some of those planets are in multi-planet systems; so Kepler has found planets around fewer than 2600 stars.

It seems therefore that only a small fraction of stars have planets – 2600 is less than 2 per cent of 150 000. Yet, if you followed the headlines about Kepler’s discoveries, you might have heard that there are more planets than stars in the Milky Way. So, what’s going on?