How Many Planets Are in the Universe?

The numbers are staggering when you start contemplating how many planets are in the universe. For example, Earth resides in the Milky Way Galaxy. It’s a spiral-type galaxy with four arms full of stars. We’re (earthlings) located on one of the arms.

Scientists think most of our galaxy’s stars host their own planets, at least one, but likely more than one planet. We call them exoplanets when they’re outside our solar system. And there are currently thousands of exoplanet discoveries.

Take a moment to think about this. The Milky Way Galaxy is one among billions of galaxies across the universe. And each galaxy has millions to billions of stars. So if each of the stars has a planet group, there are quadrillions of planets in the universe.

What Is a Planet?

Like in many solar systems, there are more planets than stars in our solar system. Eight (planets) to one (star) in our solar system alone. The definition of planets has changed over time and discovery, but in 2006, the International Astronomical Union (IAU) said three things make a planet.

  • It has to orbit a star (Earth orbits the Sun.)
  • It has to be large enough for its gravity to keep it spherical shaped.
  • Its size must be significant enough for its gravity to clear away similar-size objects from its orbit around the star.

What Is an Exoplanet?

NASA defines an exoplanet as any planet that lies beyond our solar system. Exoplanets usually orbit other stars, but some gain the name of a “rogue planet” since they’re free-floating. These rogue exoplanets don’t remain tethered to a star; instead, they orbit the galaxy’s center.

Astronomers discovered the first exoplanets in the 1990s, and since then, they’ve located thousands more. Most come from a small region of our Milky Way galaxy that NASA’s Kepler Space Telescope explores. But scientists know that many more planets than stars exist. They expect the count to rise into the tens of thousands within the next decade as our Earth- and space-based telescopes continue gaining technology and observing power.

How Many Planets Are in the Universe?
Image: W. Stenzel and NASA

How Are Exoplanets Found?

Astronomers find exoplanets primarily through indirect methods. For example, scientists measure a star’s variability to tell if dimming happens when a planet passes in front of it. The transit method occurs when a world pulls on a star and causes its light to shift. Observers and space telescopes see it as the planet crossing between the star and the telescope to block some of the starlight.

The wobble method of finding exoplanets comes from gravitational tugs. Scientists measure the radial velocity changes within a star. Very simply, it means that orbital planets tug the star’s light wavelengths. The light becomes stretched or squeezed as the star moves closer or further from its world. So scientists essentially measure the wobble.

Gravitational microlensing is another exoplanet-finding method that aids scientists in finding untethered exoplanets. Planets that are far from their stars aren’t quite as easy to discover, but scientists believe that, as a rule, rather than the exception, stars have a minimum of one planet orbiting them.

There are other methods used to find exoplanets, and a combination of techniques led to the finding of the TRAPPIST-1 planets about forty light-years from Earth. You can read more about them below.

How Many Solar System Planets Are There?

Our solar system contains eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) plus a dwarf planet (Pluto.) It’s easy to remember their names with this easy memory trick method but remember it came about during Pluto’s previous planet classification.

My Very Easy Method: Just Set Up Nine Planets

Mercury, Venus, Earth, Mars (terrestrial): Jupiter, Saturn, Uranus, Neptune (Gas and ice giants), and Pluto (dwarf planet). The first letter of each word “My Very Easy…” represents the name of a planet. 

How Many Planets Are in the Universe?

Scientists estimate there are trillions and trillions of planets in our universe. And here’s how they come up with the numbers.

There are approximately 200,000,000,000 galaxies.

They get multiplied by 100,000,000,000 estimated planets in each galaxy.

So there are approximately 20,000,000,000,000,000,000,000 planets in the universe.

Additionally, scientists estimate that one in five planets reside in a star’s habitable (or Goldilocks) zone. So that means approximately 4,000,000,000,000,000,000,000 planets could support life of some type.

Those are crazy huge numbers! That means about 200 quintillion planets are in the universe. And a fifth of them might have some life form. With those odds, we’re probably not alone.

How Do We Know How Many Planets There Are?

NASA’s Keppler Space Telescope mission explored planetary systems’ structure and diversity. It surveyed an enormous star sample to determine how many terrestrial or more giant planets are in stars’ habitable zones. It also helped scientists estimate how many worlds exist in multiple-star systems. It also looked at the stars themselves to understand their planet-harboring properties.

Because of Keppler, scientists know more planets than stars exist in the Milky Way Galaxy.

The TRAPPIST-1 Planets

Astronomers examine the seven TRAPPIST-1 planets with space and ground-based telescopes to learn their diameters and the planets’ gravitational influence on each other. Those two items let scientists determine the mass of each world.

Then the scientists measure how much starlight makes it to the planets’ surfaces as a way to determine their temperatures. And while there is still much the researchers have to learn about these exoplanets, it helps them extrapolate data to gain knowledge about how many planets are in the rest of the universe.

The TRAPPIST-1 planets are the most studied solar system planets outside our own.

TRAPPIST-1 Planets
Trappist Planets, Artist Concept: NASA and JPL-Caltech

WASP-18 b

Astronomers found WASP-18 b in 2009 and have studied it ever since. The exoplanet lies 400 light-years from Earth, and it fascinates scientists since there’s nothing like it in our solar system.

WASP-18 b is an ultra-hot gas giant, ten times Jupiter’s mass. And since Jupiter is enormous, that’s saying something! The planet orbits its star in 23 hours. So one year there is shorter than an Earth day.

The James Webb Space Telescope recently mapped the exoplanet’s brightness, which allowed scientists to determine temperatures within the planet’s atmosphere. And the instrumentation is so precise they can even see water vapor molecules. Astronomers can use that data to detect water vapor on other planets.

WASP-18 b
Artist Depiction of WASP-18 b: NASA and JPL-Caltech, K. Miller and IPAC

Final Thoughts: How Many Planets Are in the Universe?

Learning about individual exoplanets helps scientists make statistical inferences about how many planets are in the universe. And studying large samples of planets gives an idea of the distribution of the different types of planets, such as ice giants or terrestrials. So astronomists can extrapolate what they learn to estimate the total number of universe planets.

Of course, there’s no way to observe all the planets in the universe directly. But there are currently more than 4,000 confirmed exoplanets in the galaxy. And scientists expect the number to grow into the tens of thousands within the next ten years.

And then there’s the extrapolated number of planets in the universe. Scientists multiply an estimate of the number of galaxies by the number of planets in them for a whopping estimate of 200 quintillion planets in the universe.

It’s a vast number and hard to wrap your head around. But as telescope technology continues improving, scientists expect to find more and more planets.