Space is mind-bogglingly big. Our galaxy alone has around 100 billion stars, and there could be trillions of galaxies in the universe. (And a trillion is almost definitely larger than you think it is!) But do we know how many planets are out there?
Astronomers have discovered 5,502 planets around other stars (known as exoplanets) in the Milky Way. Add in the eight in our solar system (not nine, sorry Pluto), and that gives us a total of 5,510 known planets, all located in our own galaxy. Counting planets is a hard task, though, and astronomers are certain there are many more out there we haven't found yet.
"Even though we only know of around 5,000 planets right now, we can estimate that there is roughly one planet for every star," Mark Popinchalk, an astronomer at New York City's American Museum of Natural History, told Live Science. "Our galaxy has 100 billion stars, and so likely has around that many planets. We can't give an exact number."
Popinchalk described determining exoplanet totals like trying to figure out how many people live in your city without an internet search. For an exact number, you could try to meet people one by one and count them up, but this is entirely impractical. It's a lot easier to get an estimate using data like the number of people who live in one home, and the number of homes in the city.
Astronomers estimate that every star has approximately one planet based on observations. In order to know what a typical stellar household looks like, astronomers look at our neighbors. Scientists have used a couple of different techniques to search for exoplanets, including the transit method used by the Kepler space telescope and the radial velocity method that led to the Nobel Prize-winning discovery of 51 Pegasi b. With both transits and radial velocities, astronomers look at the star instead of the planet, looking for little signs of the planet’s presence—dips in the amount of starlight when a planet orbits in front or wiggles in the star’s position from the gravitational tug of a planet, respectively.
All the planets discovered so far are well within the Milky Way, though; no one has yet for sure found a planet outside the galaxy (sometimes referred to as an extroplanet), simply because they're so far away and hard to see. One technique, called microlensing, has revealed a few possible extroplanets.
"In our own galaxy, microlensing planets are discovered when their host stars gravitationally bend the light of distant stars behind them, and the mass from the planet adds a little extra blip in the lensed light," Yoni Brande, an astronomer at the University of Kansas, told Live Science. "Lensing has long been a fixture of studies of distant galaxies, so it makes sense that we should be able to see faint planetary lensing signals in other galaxies as well, we just haven't confirmed any."
To continue Popinchalk's city analogy, by looking beyond the Milky Way we're asking how many people live in all the cities on Earth. "If our galaxy has around 100 billion planets, and there are one trillion other galaxies, and each of them probably has as many planets, we can multiply that together to get 100 sextillion planets in the universe," Popinchalk said. (That's a 1 followed by 23 zeroes.)
With such a humongous number of planets, people often argue that there must be at least one other planet with life somewhere in the universe. Astronomers still don't know, however, how rare life — and the conditions needed for it to arise — actually is. "We'll have to wait at least a few decades for the next generation of large exoplanet-focused space telescopes (like the Habitable Worlds Observatory) to really start to look for life elsewhere in the galaxy," Brande said.
