But these were just the worlds around our Sun, which houses (according to current definition) eight planets. Our Sun is just one of an estimated two-to-four hundred billion stars in our Milky Way galaxy, and looking up towards the night sky, one can’t help but wonder how many of those stars have planets of their own, and what those worlds are like.
There are a vast variety of stars out there in our galaxy. Our Sun is just one example — a G-class star — of seven different main types.
We may think of our Sun as being typical and on the relatively dim side, since a disproportionate number of stars visible to our eyes in the night sky are O, B, and A-class stars. But in reality, the Sun is more massive and intrinsically brighter than 95% of stars in our galaxy. The red dwarf stars — M-class stars — which are no more than 40% the mass of our Sun, make up 3 out of every 4 stars that are out there.
Stars can be clustered together in twos (binary stars), threes (trinaries), or groups/clusters containing anywhere from hundreds to many hundreds of thousands of stars.
My point is this: if you want to accurately estimate how many planets there are in our galaxy, you can’t just take the number of planets we find around our star and multiply it by the number of stars in our galaxy. That’s a naïve estimate that we’d make in the absence of evidence. But just for fun, that’d give us somewhere around two-to-three trillion planets in our galaxy. And as we know from our own Solar System, there’s a great variety of what the surfaces of those planets could look like.
But over the past two decades, we’ve been looking. We’ve been looking with a few different methods, in fact, and the two most prolific are the “stellar wobble” method, where you can infer the mass-and-radius of a planet (or set of planets) around a star by observing how it “wobbles” gravitationally over long periods of time:
And the transit method, where the light coming from a distant star is partially blocked by the disk of a planet in its solar system passing in front of it.
It’s important to recognize, when we do this, that we will not see the vast majority of planets that are out there. Take NASA’s Kepler Mission, for instance, which has discovered hundreds (if not thousands) of planets by looking at a field-of-view containing around 100,000 stars. But that does not mean that there are only a few planets-per-hundred-stars. Consider the following: if Kepler were looking at our Solar System, and our Solar System was oriented randomly with respect to our perspective, these are the odds that the alignment would be good enough to observe a transit of our star by one of our planets.
In other words, based on what we’ve seen so far, most stars are likely to have planets, and based on what we’ve seen in the inner solar systems of the ones that do, a large fraction of them are likely to have more rocky planets in their inner solar systems than even our own has, to say nothing of the outer solar system!
This doesn’t even include orphan planets (without a parent star), which we know exist, even if we don’t know their numbers yet. Over time, we’ll continue to learn more and refine our estimates, but right now, there are at least about as many planets as there are stars in our galaxy, and quite possibly many, many more than even eight times that number.
Our solar system may turn out to be average, slightly above average, or somewhat below average; we’re still not sure. But regardless of which way it goes, we’re talking about trillions of planets in our galaxy alone. And remember, our galaxy isn’t alone in the Universe.