The planets of our Solar System have captured the human imagination for centuries. Long ago, the ancients gazed up and gave them names. But now astronomers are learning much more about how many planets there are. And there are endless planets spread throughout the universe. Likely billions, maybe even trillions, of planets in the Milky Way alone. According to NASA’s latest data, there are 5,502 exoplanets. And another 9,820 await confirmation!
But as human understanding of the infinite skies evolves, so does the definition of a planet. Long ago, people thought planets were distant points of light. But now we know they are diverse worlds. And each has unique characteristics and histories. Some may even have the potential to harbor life.
Astronomers continue trying to take an accurate count of planets. They’re studying the worlds within our Solar System. But they’re also examining exoplanets (those outside our system.) So let’s try to answer how many planets are there.
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The Hunt for Exoplanets Beyond Our Solar System
Keep reading as we jump into the question that challenges astronomers and space geeks alike. How many planets are there? First, we’ll explore the worlds in our solar system. And then we’ll stretch to the exoplanet realm. That’s where we’ll examine planets orbiting distant stars.
Planets are fascinating. There are familiar gas giants like Jupiter, intriguing dwarf planets, and exotic exoplanets. They stretch our knowledge boundaries as we peer into future space explorations.
Who knows? Scientists may even chance upon an exoplanet that can help unlock the secrets of life beyond Earth. For example, research shows that 10-15% of planets fall within a star’s habitable zone. And that means tens of billions of exoplanets could potentially host some form of life.
So let’s see if we can answer the age-old question of how many planets exist.
Our Solar System: Eight Familiar Planets
Our solar system is home to eight planets. And each is captivating in its own way. There’s the blazing intensity of Mercury. But the distant blue serenity of Neptune captures our imaginations too. Our familiar planets have a remarkable diversity of sizes, compositions, and orbital paths.
Their stories and histories offer a fascinating glimpse into the cosmic forces that have shaped our portion of the universe. And studying our familiar planets helps scientists know what to look for in the exoplanet hunt!
Mercury: The Scorched Rock Closest to the Sun
This tiny planet is only about the size of Earth’s Moon. And it boasts a surface temperature with dramatic mood swings. It has scorching daytime highs of around 800 degrees Fahrenheit (427 degrees Celsius.) But the night temperatures drop to frigid lows of about -290 degrees Fahrenheit (-179 degrees Celsius.)
Because Mercury is so close to the Sun, intense solar winds strip it down to a thin, fragile atmosphere. And that means it can’t retain heat. So as soon as the Sun sets, temperatures plummet.
Mercury is the solar system’s fastest planet. It zooms through the cosmos at almost 29 miles (47 kilometers) per second. And that’s also due to how close it is to the Sun. It orbits our central star in only 88 Earth days. So the years are short, but Mercury’s days are long! The small rocky planet takes 59 Earth days to make one complete rotation on its axis.
Here’s a photomosaic of Mercury’s southern hemisphere. Mariner 10 took images during its planet flyby. Then scientists compiled them into this stunning mosaic.
Venus: Earth’s “Sister Planet” With a Runaway Greenhouse Effect
Venus is the second planet from the Sun. And it is similar in structure and size to Earth. But that’s about where the similarities of this sister planet end. And even though Mercury is closer to the Sun, Venus is much hotter.
This rocky planet has a toxic atmosphere that keeps it continually covered in yellow sulfuric acid clouds. The surface air pressure is 90 times what we experience on Earth. So if you were standing on Venus’ surface (which you can’t), it would feel like you’re a mile under the ocean.
So, Venus is the most scorching hot of all the planets. It has a poisonous atmosphere that traps heat, creating a runaway greenhouse effect. And you could melt lead on its surface! With surface temperatures reaching 900 degrees Fahrenheit (475 degrees Celsius), you could melt metal.
But that’s not all the exciting facts setting Venus apart in the count of how many planets are there. It’s also one of two solar system planets that rotates backward on its axis. So it’s unlike here on Earth where the Sun rises in the east. On Venus, the Sun shines first from the west and then sets in the east.
Check out these Mariner 10 images of Venus.
Earth: The Cradle of Life and Our Only Home
Here, we are on the third rock from the Sun, our home planet, Earth. It’s the largest terrestrial world with a radius of 3,959 miles (6,371 kilometers.) And three things set Earth apart from the other solar system planets. We have liquid water oceans on our surface. We have varying life forms. And our world isn’t named for Greek or Roman gods. Instead, it comes from the German word for “the ground.”
Earth’s atmosphere contains 78% nitrogen, 21% oxygen, and one percent of miscellaneous ingredients. So it creates ideal conditions for us to live and breathe. Our atmosphere further protects the planet by breaking up meteoroids headed our way. And that keeps us from taking too huge an impact from them.
Space distance measurements come in astronomical units (AU) from the Sun. Earth provides the guidelines, so one AU is 93 million miles (150 million kilometers.) An astronomical unit gives scientists a simple way to compare how far other planets are from the Sun.
For our planet, a distance of 93 million miles means it takes sunlight about eight minutes to reach us. It becomes mind-blowing to think about. But the sunshine we feel is already eight minutes old. So by the time it gets to us, the Sun’s intense radiation spreads out and decreases enough to keep from burning out all life on our planet.
Look at this incredible DSCOVR (Deep Space Climate Observatory) image animation of the Moon passing by Earth. This view is awe-inspiring because it shows us the Moon’s side facing the Sun. So we never see that side of the Moon from Earth.
Doesn’t North America look small from this viewpoint about a million miles away? This image helps put into perspective the vastness of the skies. So you can better understand how difficult it is to answer the question of how many planets are there in the universe.
Mars: The Red Planet and Its Potential for Colonization
Today, Mars is a cold desert where red dust swirls. And the planet’s atmosphere is very thin compared to Earth’s. But scientists believe that things were different billions of years ago. Evidence shows that the atmosphere was thicker. And Mars’ surface was warmer and wetter. So what does that mean for potential life on the fourth planet in our solar system? How do the changes help astronomers look for more planets in space?
Mars intrigues scientists so much that it is the first planet we’ve sent rovers to explore. Current surface explorations include
- Curiosity – Rover
- Perseverance – Rover
- InSight – Lander
- Ingenuity – Helicopter that rode to Mars on Perseverance.
Perseverance is the most advanced and the largest NASA rover on Mars. It landed on the red planet’s surface on February 18, 2021, after its 203-day journey.
Mars has a wild topography. There are higher mountains than Earth’s and deeper canyons. And even though the days are similar in length, the two planets have vastly different temperatures.
|Hours in Day
|Year in Earth Days
|Grand Canyon 1.1 miles deep
|Valles Marineris 4.35 miles deep
|Mauna Kea ~6.34 miles high
|Olympus Mons ~16 miles high
So what does all that mean for potential future colonization? The Mars Exploration Program is NASA’s study of the planetary system to understand the planet better. As a result, humans could potentially explore the planet in the future. The program’s goals provide the infrastructure for more exploration.
For example, NASA scientists hope to learn answers about specific items.
- How did Mars form and evolve?
- What climate and geological processes shaped Mars throughout its history?
- Did Mars have the potential to have previously hosted life? Does it have biological potential?
- How best can humans explore the planet in the future?
- And how do Mars and Earth compare and contrast?
Mars is the most accessible place to humans in our solar system and the universe. It has a complex geology with a climate that changes over time. Mars’ similarities to Earth mean scientists can learn what may happen to Earth in the future. And exploring the planet helps answer broader solar system questions.
So whether or not humans can live on Earth is yet to be seen. But current and future exploration on Mars provides fascinating details in the meantime.
Here’s a selfie from the Perseverance rover. It collects samples in tubes, then deposits them in a depot for future collection. The WATSON (Wide Angle Topographic Sensor for Operations and eNgineering) camera on the rover’s robotic arm snapped the photo in January 2023.
Jupiter: The Gas Giant With Its Mesmerizing Great Red Spot
Jupiter is a gigantic planet, so massive that all the other solar system planets combined are still smaller. It’s twice the mass of all the other planets! And this gas giant even has a raging, twisting storm called the Great Red Spot that’s bigger than Earth.
Think of Jupiter’s enormous size like this. If Earth were a grape, Jupiter would be a basketball. Now that’s quite a size disparity! But even though Jupiter is so humongous, it’s light. Its atmosphere holds mostly helium and hydrogen. And it doesn’t have a solid surface. Instead, swirling gases may surround a small solid inner core. However, scientists are not yet positive if Jupiter does indeed have a core.
Life as we know it can not exist on Jupiter. But some of its 95+ moons may have the potential for life forms. The four largest moons on Jupiter are called the Galilean satellites. Their names come from Galileo Galilei, an Italian astronomer who found the moons in 1610.
These four moons provide the dust and particles that make up the gas giant’s rings. Scientists think they are captured asteroids. The remaining moons orbit further away from the planet. They are likely collision fragments the planet captured over time.
In addition, to the centuries-old Great Red Spot, Jupiter has multiple cyclones at each pole. These storms remain trapped in polygonal arrangements. There are eight storms in an octagon shape at the north pole. And five hurricanes in a pentagon shape in the south. Cyclones directly over the poles push the other storms back, holding their forms.
Saturn: The Ringed Wonder
Although all four gas and ice giants have rings, none are as spectacular as Saturn’s. This is the sixth planet from the Sun. It’s also the second most vast, after Jupiter. But like its larger counterpart, Saturn is also made mostly of helium and hydrogen. So it, too, is enormous but light, with no solid surface.
Saturn has a short day at only 10.7 hours to rotate once on its axis. But it has very long years. It takes about 29 Earth years for this ringed gas giant to make its way around the Sun.
Additionally, Saturn is 9.5 AU from the Sun. So it takes only eight minutes for sunlight to reach Earth. But for the same light to reach Saturn takes a whopping 80 minutes. So by the time it gets to the gas giant, the Sun’s warmth is relatively diluted.
But the most intriguing part of Saturn is its rings. Scientists think they come from asteroids, shattered moons, or comets. Saturn’s immense gravity tears the objects apart. And that leaves millions and billions of tiny ice chunks and dust-covered rocks.
Most ring particles are small ice grains the size of dust. But other chunks are house-sized. And incredibly, there are mountain-sized particles within the rings too. The rings would appear white if you could stand on Saturn’s cloudy non-surface.
One fact we find the most interesting is how the rings orbit the planet. They each do so at different speeds! Saturn’s seven rings extend into space by 175,000 miles (282,000 kilometers) above the planet. And each of the main three is about 30 feet (10 meters) high.
The rings were named alphabetically based on their discovery dates. So closest to Saturn is Ring D, then C, B, the Cassini Division, Rings A, F, G, and finally Ring E.
NASA’s Cassini spacecraft spent 20 years collecting space data. Thirteen of its years were Saturn explorations, dipping between the icy rings. But when it exhausted the fuel supply, Cassini made its final dive into Saturn’s atmosphere. In September 2017, the mission ended, but the spacecraft continued sending data right up to the end.
With missions like Cassini’s, scientists hope to learn how many planets are there. The Hubble Space Telescope image below comes from the Outer Planets Atmospheres Legacy (OPAL) project. This program helps scientists study the atmospheres and evolutions of the gas giants. They specifically track Saturn’s weather patterns to identify trends.
Uranus: The Sideways Planet and Its Icy Mysteries
The solar system’s seventh planet from the Sun appears to lay on its side as it spins around space. Uranus rotates almost 90 degrees from its orbit plane, so this ice giant looks like it’s lying over to nap.
The planet has a small rocky core surrounded by water, ammonia, and methane. But the planet’s extreme tilt creates most of its extreme weather conditions. The angle likely came from an Earth-sized object colliding with the planet many years ago.
It takes about 84 Earth years for Uranus to orbit the Sun. But the tilt causes long dark winters, 21 years long. As the Sun shines over the pole, the other half of the planet plunges into darkness. So the ice giant freezes at one end while it has perpetual daylight at the other.
In some places on the planet, Uranus is colder than Neptune! Minimum temperatures range as low as 49 Kelvin (-371℉, -224.2 ℃.) And the wind chill factor drops it even further! Especially when you consider that wind speeds burst up to 560 miles (900 kilometers) per hour. We’d call that extreme weather!
As scientists study our familiar solar system planets, it gives them a better idea of what to look for throughout the universe. That helps provide more details on how many planets there actually are.
Here’s an infrared view of Uranus’ two hemispheres. Astronomers captured it with the Keck Telescope.
Neptune: The Farthest Known Planet and Its Striking Blue Hue
The eighth and furthest planet in our solar system is Neptune. The ice giant is dark and cold. And if you think Uranus had speedy winds, it pales compared to Neptune’s supersonic gales.
The planet’s wind reaches top speeds higher than 1,200 miles (2,000 kilometers) per hour. Remember that Earth’s strongest hurricanes and cyclones produce winds up to 250 miles (400 kilometers) per hour. The difference is unbelievable! The winds force frozen methane clouds across the planet. And that’s what makes Neptune look blue. Methane absorbs the Sun’s red light while reflecting blue waves into space.
Like the other ice and gas giants, Neptune has no solid surface. Instead, it boasts gases like helium, methane, and hydrogen. The gases extend deeply into the planet before turning into melted ice forms and water. Scientists think the world has a solid core around Earth’s mass.
In August 1989, Voyager 2 used a narrow-angle camera to photograph Neptune over two days. Voyager 2 captured two and a half planet rotations during the almost continuous imaging. And the images are stunning! Check this one out.
Dwarf Planets and Other Celestial Objects
When accurately counting how many planets are there, dwarf planets come into play. The International Astronomical Union (IAU) sets planetary science definitions. And it defines a dwarf planet this way:
- It is a celestial body orbiting the Sun.
- And it has enough mass to assume a nearly round shape.
- And it has not cleared the neighborhood around its orbit.
- And finally, it is not a moon.
So, let’s look briefly at a few of the most well-known.
Pluto: The Demoted Planet and Its Complex History
Pluto got demoted from planet to dwarf planet in 2006. And public outrage was a thing! The internet saw many memes about what Pluto must be feeling. Textbooks had to change. The news media went wild. It generated quite a change in public thinking.
Pluto is a tiny dwarf, only about the width of half the United States. And its largest moon is still only about half its size.
Eris: The Largest Dwarf Planet in the Kuiper Belt
Scientists discovered Eris (nicknamed Xena) in the Kuiper Belt in 2005. At the time, they thought it was bigger than Pluto, which sparked the debate about what defined a planet.
After months of discussion, the IAU decided to change the definition. That immediately reclassified Pluto and lowered the number of planets within our solar system to eight. Eris got a name change as a dwarf planet. Now named for the Greek goddess of lawlessness, discord, and strife, Eris is fitting due to the uproar of Pluto’s demotion.
Haumea: A Fast-Spinning Elongated Dwarf Planet
Haumea is a fast-spinning dwarf planet about a third of Earth’s size. With one of the fastest rotations of any solar system object, Haumea’s shape distorts to a football-looking dwarf.
Discovered in 2003, one team gave it a preliminary nickname of Santa. Haumea is also within the Kuiper Belt, which orbits beyond Neptune. The Belt is a donut-shaped region of space filled with icy bodies.
Makemake: The Icy World With a Reddish Hue
Makemake also lies in the Kuiper Belt. Its 2005 discovery entered it into the planet-definition discussion along with Eris. This tiny dwarf planet is only about one-ninth of Earth’s size but is still the second brightest object in its surroundings. Pluto is brighter when seen from Earth.
It lies about 45.8 astronomical units from the Sun, taking about 305 Earth years to make one trip around it. From that far away, sunlight takes about six hours and 20 minutes to travel to Makemake.
Scientists think the dwarf planet has a reddish-brown surface. And while they can’t see many details, they believe Makemake has frozen methane pellets on its cold surface.
Ceres: The Asteroid Belt’s Largest Object
This dwarf planet is the only one found in the inner solar system. Ceres lies between Jupiter and Mars in the asteroid belt. Astronomers discovered it in 1801 and called it an asteroid until its reclassification to dwarf planet in 2006.
NASA’s Dawn spacecraft visited it in 2015, making it the first dwarf planet with a flyby. Ceres makes up about a quarter of the asteroid belt’s overall mass. But Pluto still outranks it at 14 times its mass!
Other Trans-Neptunian Objects (TNOs): Exploring the Kuiper Belt
The Kuiper Belt is a space region of leftover objects from the solar system’s early days. And even though the name is “belt,” this region is more donut or thick disk shape than a thin belt. Objects within this region are Trans-Neptunian Objects (TNOs) or, more simply, Kuiper Belt objects.
The Kuiper Belt lies past Neptune, where objects orbit the Sun from about 30 to 55 AU away from the Sun. In addition, TNOs are smaller and icier than Earth’s Moon. Scientists think hundreds of thousands of icy bodies are within the Kuiper Belt.
Arrokoth is one such Trans-Neptunian Object. NASA New Horizons flew by it on January 1, 2019. Astronomers were surprised by its flattened snowman appearance. And its red coloring even surpasses Pluto’s redness.
Scientists say Arrokoth and the other TNOs are well-preserved, creating frozen outer solar system samples. They represent the condition of space objects after our solar system formed. It’s like a 4.5 billion years old window in space. And that information helps predict how many planets are there in the universe.
Exoplanets: How Many Planets Are There?
Large worlds in our solar system are planets. But astronomers find many worlds outside our system. And they call them exoplanets, planets that orbit a star other than the Sun. We can’t discuss how many planets are there without considering exoplanets.
The Search for Exoplanets and the Kepler Space Telescope
NASA’s Kepler Space Telescope mission aims to survey our space neighborhood within the Milky Way. Scientists hope to find hundreds of Earth-size terrestrial exoplanets. Some worlds may be half Earth’s size, while others could be twice as massive.
But scientists specifically hope to find exoplanets within the habitable zone of their stars. They’re looking for planets that might hold liquid water and the building blocks for life. With the potential for hundreds of billions of stars in the Milky Way, scientists hope to determine which ones host planets.
By 2019, Kepler had discovered more than 3,700 exoplanets. They range from rocky super-Earths to gas giants and mini-Neptunes. And after the Kepler mission ended, its spacecraft ferried the K2 exoplanet mission across space. It cataloged data and photographed even more sky regions than the original Kepler mission.
TESS: How Many Planets Are There?
TESS is the Transiting Exoplanet Survey Satellite) and it launched in mid-April 2018. As part of NASA’s Exoplanet Exploration Program, TESS data led researchers to an Earth-sized world. It orbits TOI 700. And planet “e” lies within the habitable zone. TOI 700 e is about 95% of Earth’s size. And scientists think it is less rocky than our home planet. The exoplanet’s central star is small in size. It’s a cool-temperature M dwarf star in the southern Dorado constellation about 100 light-years away.
TESS monitors huge sectors of space for about 27 days at a time. Since it is looking for an extended period, the satellite can track changes in stellar brightness when a planet crosses (or transits) between it and Earth. The satellite started capturing data in the southern skies. Then it moved north before returning to the southern sky in 2020. Because it gathered more data, the research team amended some initial calculations. They realized some exoplanets were about ten percent smaller than initially thought.
Now TESS is in its second year back in the northern hemisphere skies. Scientists learn more about our solar system’s early history when they learn more about these older Earth-size worlds. And that also gives a better idea of the number of planets there are.
The James Webb Space Telescope: Exoplanet Studies
TESS found a likely exoplanet. And the James Webb Space Telescope confirmed its existence earlier in 2023. That makes LHS 475 b the first official find from Webb’s NIRSpec (Near-Infrared Spectrograph.)
The research team chose this target after reviewing interesting areas found by TESS. It is an Earth-size, rocky planet. The team’s excitement about this confirmed exoplanet extends to Webb’s ability to help gain more insight into Earth-like worlds. It opens the possibilities of what the telescope can find in the future.
Webb is the only telescope capable of characterizing exoplanet atmospheres. And while the research team doesn’t yet know what LHS 475 b’s atmosphere contains, they can tell what it doesn’t have. They know it does not have a thick atmosphere filled with methane. And it’s also possible the exoplanet has no atmosphere at all.
Researchers are at the forefront of rocky exoplanet studies. But the Webb telescope is helping them scratch the surface. And that opens the door to answering how many planets are there.
Types of Exoplanets: Hot Jupiters, Super-Earths, and More
The potential for exoplanets is almost unlimited because of the vast number of stars. So there could be hundreds of millions of planets about just our galaxy’s hundreds of billions of stars. But when you think of it in terms of our solar system, our Sun supports eight planets and multiple exoplanets.
So if the billions of stars each host several exoplanets, the actual number of how many planets there are explodes.
Researchers have clear evidence of three exoplanet types, with many known examples. There are gas giants, ice giants, and hot-super-Earth planets that have short-period orbits.
Proxima Centauri B: Earth’s Closest Known Exoplanetary Neighbor
This super-Earth exoplanet orbits an M-type star at only 0.04856 AU away. Its years are only 11.2 Earth-days long since that’s how long Proxima Centauri B takes to orbit its star. This world is our closest known exoplanet neighbor at only four light-years from Earth.
It resides within its star’s habitable zone, but that doesn’t mean the exoplanet is habitable. Only that its distance means it could potentially hold liquid water. However, Proxima b receives extreme radiation from its star. And that radiation has enough energy to strip away hydrogen, oxygen, and nitrogen.
So even though water could potentially exist, the atmosphere might not let anything survive. And the atmosphere is essential for life. It helps with climate regulation and maintains a surface pressure that is water-friendly. Atmospheres also shield planets from dangerous space objects and weather. And finally, it holds the chemical building blocks that support life.
TRAPPIST-1 System: Seven Earth-Sized Planets and Potential Habitability
Astronomers found the giant star TRAPPIST-1 in 1999. But it didn’t get this name until May 2016 after scientists found three planet candidates nearby. The researchers in Chile used the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in the discovery. And the star got its name from the telescope.
By 2017, scientists found a total of seven exoplanets around TRAPPIST-1. And three are even in the habitable zone. That’s the region around a star where rocky planets can potentially have liquid water and the possibility for life.
These exoplanets lie so close together. So if you could stand on one world’s surface, you’d see the other exoplanets like we see the Moon from Earth. We don’t have actual images of the exoplanets. But this artist’s illustration stems from collected data about the planet’s masses, diameters, and distances from TRAPPIST-1.
Counting the Stars: Estimating Exoplanet Numbers
The Kepler Space Telescope changed the exoplanet-hunting game! When scientists calculate how many planets there are, they start with how many stars there are. But the problem is that star counts are also elusive. Researchers say there are at least 100 billion stars in the galaxy. But they also say the number could reach as high as 400 billion Milky Way stars.
And because of Kepler’s trailblazing explorations, we now know that many of those stars have planets orbiting them. If you estimate three worlds for every star, that’s a minimum of 300 billion planets!
But if there are up to 400 billion stars, the number of planets reaches the trillions!
Details from NASA’s exoplanet archive examine the number of confirmed and yet-to-be-confirmed exoplanets. Let’s look at actual numbers to answer how many planets are there. Remember that this is a small representative of the number of exoplanets scientists think are in the universe.
|NASA’s Exoplanet Counts as of August 22, 2023
|All Confirmed Exoplanets
|Kepler Exoplanet Candidates
|K2 Exoplanet Candidates
|TESS Exoplanet Candidates
|TESS Transit-like Events Proved False
The number of confirmed and candidate exoplanets is relatively small. But scientists think there could be as many planets throughout the universe as there are grains of sand on Earth’s beaches. And that number is hard to wrap your head around!
Scientists say there are approximately 10^11 to 10^12 stars in the Milky Way. Then you realize there are 10^11 to 10^12 galaxies beyond ours. Now remember that each of those stars likely has three (or more) exoplanets. And you start to really understand just how many planets there are in the universe and beyond!
Final Thoughts: How Many Planets Are There?
Our solar system holds beautiful and diverse planets. And even though Earth is the only world we know about that hosts life, some solar system moons hold potential. Scientists continue with their ongoing quest to understand our place in the universe. So through studying our eight familiar planets, researchers have a better idea of what to look for in space.
We embrace new planet definitions that include dwarf planets. And scientists use space telescopes to explore exoplanets. Through it all, they continue to acknowledge new ideas dredged from the data of Kepler, TESS, James Webb, and other telescopes and satellites.
So, how many planets are there? There are likely billions and maybe even trillions of planets sprinkled throughout the universe. Some are smaller-sized, like Pluto. But others are larger Earth-sized exoplanets.
And with all those terrestrial worlds orbiting stars, it seems ever more likely that some lie within the habitable zone. If the star is young and hot enough and the planet orbits close enough to warm up without scorching, then who knows?! Maybe, scientists can one day prove we are not alone. And what an astounding day that will be!