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How many planets are in the universe?
References
By Briley Lewis published 1 day ago
We currently know of 5,502 planets beyond the solar system, but we've
only found the tiniest fraction of the planets astronomers think lie
elsewhere in the universe.
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Kepler's exoplanets.
There are currently 5,502 known planets beyond the solar system. (Image
credit: NASA/JPL-Caltech/T. Pyle (SSC))
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."
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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.
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Briley Lewis
Briley Lewis
Freelance science writer
Briley Lewis (she/her) is a freelance science writer and Ph.D.
Candidate/NSF Fellow at the University of California, Los Angeles
studying Astronomy & Astrophysics. Follow her on Twitter @briles_34 or
visit her website www.briley-lewis.com.
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