Astronomers estimate that there is roughly one exoplanet per star in our galaxy. Of course, some stars have many planets – our own Sun has eight. And some stars have none. But if a star lives long enough, forming planets seems to be the rule, rather than the exception.
That doesn’t mean astronomers can map all of those billions of stars though. When it comes to exoplanets that have been measured or counted in some way, the numbers are much smaller.
The running counter of known exoplanets – as of this writing – stands at 4,108 confirmed worlds. But astronomers are surprisingly good at figuring out what they can’t see. They know that their telescopes aren’t powerful or precise enough to see the stealthiest planets – those that are very small, very far from their stars, or around 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.
There are 8 planets in our Solar System. These are (in order from the Sun):
The Sun is, of course, a star. It is one of a large number of stars in the galaxy that we are in, the Milky Way. More on this below.
There used to be nine planets in the Solar System, with Pluto being the additional one. However, in 2006 it was downgraded and taken off the list. This was because the definition of what makes a “planet” changed and Pluto was no longer considered one, essentially because it is too small:
When Pluto was first discovered in 1930 nobody knew how big it was. Later in the 20th century, it was discovered that it was in fact tiny in comparison to the other planets of the Solar System. It is just one-sixth the size of Earth and smaller than our moon.
The dwarf planet Eris was discovered by the astronomer Michael E. Brown in the same area of space (the Kuiper Belt). It is larger than Pluto, but not considered a planet itself.
Pluto was therefore downgraded to a dwarf planet when an official definition of a planet was agreed by the International Astronomical Union in 2006 (see further information below).
There are four dwarf planets in addition to Pluto in our Solar System. These are Ceres, Haumea, Makemake, and Eris.
How many stars and planets are in the Milky Way galaxy?
Given that we can’t be completely certain how many planets there are in the Solar System, it shouldn’t be too much of a surprise that once we look beyond that our estimates get much vaguer. But we are going to have a go anyway.
So, Earth and the planets of the solar system revolve around the Sun. Which is one star within our galaxy, the Milky Way.
Whilst our sun alone seems huge to us, the current best estimate is that the Milky Way contains between 100 billion and 400 billion stars (source).
Estimates vary considerably as it is extremely hard to calculate and so you can find different estimates out there (including this one estimating a trillion stars in the Milky Way).
If we then take the higher-end figure of 400 billion stars in the Milky Way, how many planets does this mean?
We know that our Sun has at least 8 planets, but the most recent analysis in Nature journal is that, on average, each star of the Milky Way hosts one planet.
Therefore we can take the estimate that there are 400 billion planets in the Milky Way.
This is some important information about how many planets are there in the Universe.