What If? Lab
What if there were a planet between Mars and Jupiter?
Between Mars and Jupiter there are lots and lots of space rocks. There is no planet there. What if there were?
Between Mars and Jupiter lies the asteroid belt: millions of rocky leftovers that never built a planet. NASA explains that young Jupiter's gravity stopped them. Explore the belt, Jupiter's tug, the dwarf planet Ceres, and what kind of planet could have formed there.
The main asteroid belt holds an estimated 1.1 to 1.9 million bodies larger than 1 km, yet their combined mass is less than the Moon's. NASA attributes the lack of a planet to the gravity of newly formed Jupiter, which halted accretion and drove destructive collisions. Ceres, once counted as a planet, is the clearest sign of what was interrupted.
The big question
The planets go around the Sun in order. Mercury, Venus, Earth, Mars… then a big gap with lots of rocks… then Jupiter.
What if there were a planet in that gap?
Count the planets outward from the Sun: Mercury, Venus, Earth, Mars, and then a wide region filled with millions of rocky asteroids before you reach Jupiter. There is no planet there.
Why not? And what if there were one? This is a thought experiment. We can't build a planet, but we can use what scientists know about the asteroid belt, Jupiter and the dwarf planet Ceres to imagine it carefully.
The region between Mars (about 1.5 AU) and Jupiter (about 5.2 AU) hosts the main asteroid belt rather than a planet. That raises two questions: why did no planet form there, and what would one have been like?
This thought experiment works from NASA's account of the belt's formation and from Ceres, the largest object in the belt. Each step separates established facts from reasoned ideas.
Explore the idea
Make a prediction
If all the asteroids squeezed into one ball, how big would it be?
If every asteroid in the belt joined into one ball, how would its mass compare?
The combined mass of all asteroids is…
- Bigger than Earth
- About the same as Mars
- Less than our Moon
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Established fact
Jupiter's strong pull stopped the belt from becoming a planet.
The gravity of the newly formed Jupiter ended planet-building in the asteroid belt (NASA).
Newly formed Jupiter's gravity halted accretion in the belt and drove fragmenting collisions (NASA).
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An idea scientists discuss
This is a pretend journey. A planet there is imagined.
A planet between Mars and Jupiter is a thought experiment: its size and effects are reasoned from today's belt, not known.
The counterfactual planet's mass and influence are inferences, not predictions.
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Established fact
There are millions of asteroids in the belt.
The belt is estimated to hold between 1.1 and 1.9 million asteroids larger than 1 kilometer across, and millions of smaller ones.
Estimated 1.1–1.9 million main-belt asteroids above 1 km in diameter, plus millions smaller, mostly on modestly eccentric orbits.
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Established fact
All the asteroids together weigh less than our Moon.
The total mass of all the asteroids combined is less than that of Earth's Moon.
The combined mass of all asteroids is less than the Moon's (NASA).
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Established fact
Jupiter's pull stopped a planet from growing there.
Early in the solar system's history, the gravity of the newly formed Jupiter brought an end to planet-building in the belt and made the small bodies collide and break apart.
Newly formed Jupiter's gravity ended planetary formation in the belt and caused fragmenting collisions (NASA).
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Established fact
Jupiter still moves some asteroids around.
Jupiter's gravity, and occasional close passes by Mars, can change asteroids' orbits and knock them out of the belt.
Jupiter's gravity and encounters with Mars alter asteroid orbits and can eject bodies from the main belt (NASA).
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An idea scientists discuss
Without Jupiter so close, maybe a planet could have grown there.
If Jupiter's pull had not stirred up the rocks, perhaps they could have kept sticking together. That is an idea; no one can rerun the early solar system.
A belt without Jupiter's stirring might have continued accreting; this counterfactual cannot be tested directly.
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Established fact
Ceres was called a planet when it was found.
Ceres was discovered between Mars and Jupiter by Giuseppe Piazzi in 1801 and was first classified as a planet. As more objects were found nearby, it became the first known asteroid.
Piazzi found Ceres in 1801; first counted as a planet, it became the first of the class later called asteroids or minor planets (NASA).
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Established fact
Ceres started to become a planet but did not finish.
NASA calls Ceres an embryonic planet: it started to form but didn't finish, because nearby Jupiter's strong gravity prevented it.
Ceres holds about 25% of the belt's mass and is described by NASA as an embryonic planet stunted by Jupiter's gravity.
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Established fact
A spacecraft visited Ceres in 2015.
NASA's Dawn spacecraft reached Ceres in 2015, making it the first dwarf planet visited by a spacecraft. Ceres may be as much as 25 percent water.
Dawn arrived in 2015; Ceres likely has a rocky core and icy mantle and could be up to 25% water (NASA).
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Established fact
Ceres takes more than four Earth years to go around the Sun.
Ceres orbits about 2.8 astronomical units from the Sun and takes 1,682 Earth days, or 4.6 Earth years, to go around once.
Ceres: 2.8 AU, orbital period 1,682 days (4.6 years), rotation about 9 hours (NASA).
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An idea scientists discuss
All the belt rocks together would make a world lighter than our Moon.
Since all the asteroids together have less mass than the Moon, joining them would make a world lighter than our Moon, not a planet like Earth.
Merging today's belt yields less than a lunar mass; a terrestrial planet would require a far more massive early belt.
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An idea scientists discuss
A bigger planet there might change how other things move.
A large planet there would pull on Mars, the asteroids and maybe Earth. Exactly how is something no one can say for sure.
A massive planet at ~2.8 AU would perturb neighboring orbits; the outcome is speculative without detailed modeling.
The asteroid belt today
The asteroid belt has millions of space rocks. Some are big. Most are small.
If you put them all together, they would still weigh less than our Moon!
NASA estimates that the belt holds between 1.1 and 1.9 million asteroids larger than 1 kilometer across, plus millions of smaller ones. Most circle the Sun on paths that are not very stretched out.
That sounds like a lot of rock, but it isn't. All the asteroids put together have less mass than Earth's Moon. The biggest single object, the dwarf planet Ceres, holds about a quarter of the belt's mass by itself.
NASA's asteroid facts give 1.1 to 1.9 million main-belt asteroids above 1 km in diameter and millions more below that, mostly on modestly eccentric orbits. Vesta, at about 530 km, is the largest asteroid; Ceres, about 476 km in radius, is classified as a dwarf planet.
The total mass of all asteroids is less than that of the Moon, and Ceres alone accounts for about 25 percent of the belt's mass. Whatever might once have built a planet here is largely gone.
A planet in the gap
Imagine all the belt rocks squeezed together. They would make a world lighter than our Moon.
So a planet there would be small. A big planet needs much more rock.
Let's imagine gathering every asteroid into one ball. Because their total mass is less than the Moon's, the result would be lighter than our Moon, not a planet like Earth or Mars. That is an idea worked out from NASA's numbers.
A bigger planet would have needed much more material to stay in the belt instead of being broken up and scattered. If one had formed at Ceres's distance, it would take about 4.6 Earth years to circle the Sun, as Ceres does.
Combining today's belt would give a body of less than one lunar mass: a small world, not a terrestrial planet. A Mars- or Earth-sized planet would require far more material than now remains, so the counterfactual implies a very different early solar system.
A planet at Ceres's distance, 2.8 AU, would share its 1,682-day (4.6-year) orbital period. How such a planet would affect Mars, Earth or the asteroids is an idea, not something the evidence can settle.
Jupiter's strong tug
Jupiter is the biggest planet. Its pull is very strong.
Long ago, Jupiter's pull stirred up the rocks. They smashed into each other instead of sticking together.
Planets grew as small pieces bumped together and stuck. But NASA explains that early in the solar system's history, the gravity of the newly formed Jupiter put an end to planet-building in the belt. Instead of gently sticking, the rocks were stirred up and smashed into one another, breaking into the asteroids we see today.
Jupiter is still at work: its gravity, and close passes by Mars, can knock asteroids out of the belt.
Planets accreted from colliding, sticking planetesimals. NASA's account is that newly formed Jupiter's gravity ended the formation of planetary bodies in the belt and caused the small bodies to collide and fragment rather than merge. NASA calls Ceres an 'embryonic planet' whose growth Jupiter's gravity prevented.
Jupiter's influence continues: its gravity, together with occasional encounters with Mars, alters asteroid orbits and can eject bodies from the main belt.
The planet that was
Long ago, in 1801, people found a round world between Mars and Jupiter. They called it a planet!
Today it is called a dwarf planet. Its name is Ceres.
In a way, people once believed there was a planet between Mars and Jupiter. When Giuseppe Piazzi discovered Ceres in 1801, it was classified as a planet. As more objects were found in the same region, Ceres became the first of a new group: the asteroids.
In 2006 scientists classified Ceres as a dwarf planet. NASA's Dawn spacecraft visited it in 2015, the first spacecraft to visit a dwarf planet. Ceres may be as much as 25 percent water.
Ceres, found by Giuseppe Piazzi in 1801, was first classified as a planet; as similar objects accumulated it became the first asteroid, or minor planet. In 2006 it was reclassified as a dwarf planet, and in 2015 Dawn made it the first dwarf planet visited by a spacecraft.
Ceres is 1/13 of Earth's radius, has a layered interior with a probable rocky core and icy mantle, and could be up to 25 percent water. NASA describes it as a planet that started to form but didn't finish.
Amazing facts
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Some asteroids have their own little moons.
More than 150 asteroids are known to have small moons, and some even have two.
Over 150 asteroids have known companion moons; binary and triple asteroid systems also exist (NASA).
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A day on Ceres is only about 9 hours long.
Ceres spins once every 9 hours, one of the shortest days in the solar system.
Ceres rotates in about 9 hours and is tilted only 4 degrees, so it has no real seasons (NASA).
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Ceres might have more water than Earth!
Ceres could be as much as 25 percent water. If that's right, it has more water than Earth does.
If Ceres is up to 25% water, its total water would exceed Earth's (NASA).
Words to know
- asteroid
- a rocky object, smaller than a planet, that orbits the Sun
- asteroid belt
- the region between Mars and Jupiter where most asteroids orbit
- dwarf planet
- a round world orbiting the Sun that has not cleared other objects from its path
- accretion
- the slow growth of a body as smaller pieces bump into it and stick
- gravity
- the pull that every object with mass has on every other object
Discovery challenge
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Where is the asteroid belt?
Where is the main asteroid belt?
The main asteroid belt lies between the orbits of…
- Between Earth and Mars
- Between Mars and Jupiter
- Beyond Neptune
Show the answer
Between Mars and Jupiter
Between Mars and Jupiter.
Between Mars and Jupiter, where most asteroids orbit.
…Mars and Jupiter.
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What stopped the belt rocks from making a planet?
According to NASA, what stopped a planet from forming in the asteroid belt?
NASA attributes the failure of planet formation in the belt to…
- Jupiter's gravity
- The Sun's heat
- Earth's Moon
Show the answer
Jupiter's gravity
Jupiter's strong pull.
Young Jupiter's gravity stirred the rocks so they smashed and broke instead of sticking together.
…the gravity of newly formed Jupiter, which halted accretion and drove fragmenting collisions.
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What was Ceres called when it was found in 1801?
When Ceres was discovered in 1801, how was it first classified?
Ceres was originally classified as…
- A planet
- A comet
- A moon
Show the answer
A planet
A planet! Now it is a dwarf planet.
A planet. Later it became the first asteroid, and in 2006 a dwarf planet.
…a planet; later the first asteroid, and a dwarf planet since 2006.
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All the asteroids together weigh…
All the asteroids put together have…
The total mass of all asteroids is…
- More than Earth
- Less than our Moon
- The same as Jupiter
Show the answer
Less than our Moon
Less than our Moon!
Less mass than our Moon, NASA says.
…less than the Moon's mass.
Experiments to try
- Watch What If? Lab What if an asteroid struck the Moon? The Moon's craters are a record of space rocks landing over billions of years, and small ones still land there every day. Scientists watch for tiny flashes of light on the Moon and later find the brand-new craters. Explore what happens when a rock hits the Moon, and why the Moon keeps going around Earth as always.
- Watch What If? Lab What if you visited an icy moon? Jupiter's moon Europa and Saturn's moon Enceladus are covered in ice, but scientists think salty oceans lie underneath. The squeezing pull of their giant planets keeps the insides warm. Imagine walking on the ice, peeking at the ocean below and joining the search for places where life could exist.
- Watch What If? Lab What if Saturn’s rings vanished? Saturn's rings are billions of icy chunks, and some of that ice is slowly raining down onto the planet. NASA scientists estimate the rings could be gone in less than 300 million years. Explore what the rings are, how they fade, how they already seem to vanish from Earth's view, and what Saturn would be like without them.
- Watch What If? Lab What if Pluto were still called a planet? Pluto was called our ninth planet for 76 years. Then astronomers found many similar icy worlds beyond Neptune, and in 2006 the International Astronomical Union wrote a definition of 'planet' that made Pluto a dwarf planet. What if Pluto had kept its old title? Explore how a planet is defined, the other dwarf planets, and why science sometimes changes its words.
- Watch What If? Lab What if Earth were the only planet? Imagine a solar system with just the Sun, Earth and our Moon. The bright 'wandering stars' would vanish from our night sky, the paths of asteroids and comets would change, and scientists would lose the other worlds that teach us how planets work. Explore what Earth would miss.
- Watch What If? Lab What if planets were not round? Big worlds are round because gravity pulls everything toward the center from all sides. Small asteroids don't have enough gravity to do that, so most stay lumpy. Imagine a cube-shaped planet, meet the lumpy asteroids, and compare worlds to see how big something must be before gravity rounds it off.
Explore next
- Watch What If? Lab What if an asteroid struck the Moon? The Moon's craters are a record of space rocks landing over billions of years, and small ones still land there every day. Scientists watch for tiny flashes of light on the Moon and later find the brand-new craters. Explore what happens when a rock hits the Moon, and why the Moon keeps going around Earth as always.
- Watch What If? Lab What if you visited an icy moon? Jupiter's moon Europa and Saturn's moon Enceladus are covered in ice, but scientists think salty oceans lie underneath. The squeezing pull of their giant planets keeps the insides warm. Imagine walking on the ice, peeking at the ocean below and joining the search for places where life could exist.
- Watch What If? Lab What if Saturn’s rings vanished? Saturn's rings are billions of icy chunks, and some of that ice is slowly raining down onto the planet. NASA scientists estimate the rings could be gone in less than 300 million years. Explore what the rings are, how they fade, how they already seem to vanish from Earth's view, and what Saturn would be like without them.
- Watch What If? Lab What if Pluto were still called a planet? Pluto was called our ninth planet for 76 years. Then astronomers found many similar icy worlds beyond Neptune, and in 2006 the International Astronomical Union wrote a definition of 'planet' that made Pluto a dwarf planet. What if Pluto had kept its old title? Explore how a planet is defined, the other dwarf planets, and why science sometimes changes its words.
- Watch What If? Lab What if Earth were the only planet? Imagine a solar system with just the Sun, Earth and our Moon. The bright 'wandering stars' would vanish from our night sky, the paths of asteroids and comets would change, and scientists would lose the other worlds that teach us how planets work. Explore what Earth would miss.
- Watch What If? Lab What if planets were not round? Big worlds are round because gravity pulls everything toward the center from all sides. Small asteroids don't have enough gravity to do that, so most stay lumpy. Imagine a cube-shaped planet, meet the lumpy asteroids, and compare worlds to see how big something must be before gravity rounds it off.
- Watch Try it What If? Lab What if Earth had no Moon? The Moon's pull stretches Earth's oceans and gives us most of our tides. Take the Moon away in our tide model and see what is left: the Sun still makes tides, but they are less than half as big as the Moon's.
- Watch Try it What If? Lab What if Earth had two moons? Our Moon goes round Earth in a neat, repeating loop. Add a second moon in our orbit model and watch what changes: the two moons tug on each other as well as on Earth, so their paths start to wobble.
For parents & teachers
What children will learn
- The asteroid belt between Mars and Jupiter holds millions of rocky bodies with less total mass than the Moon.
- Planets grow by accretion, and Jupiter's gravity stopped that process in the belt.
- Ceres was once called a planet and is now classified as a dwarf planet.
- A thought experiment separates known facts from reasoned ideas.
Recommended ages: 11 to 17 · About 15 minutes
Talk about it
- Why do you think scientists once called Ceres a planet, and then changed their minds?
- How can something as far away as Jupiter affect rocks in the asteroid belt?
- If a planet had formed there, what might be different about our night sky?
Try at home or in class
- Roll small balls of modeling clay and press them gently together: they stick and grow. Now shake them hard in a closed container. What happens to the bigger lumps?
- Make a scale line of the planets with the asteroid belt using a long strip of paper: Mars at 1.5 units, the belt around 2.8 units, Jupiter at 5.2 units.
Key vocabulary: asteroid, asteroid belt, dwarf planet, accretion, gravity
Where these facts come from
- Asteroids: Facts , NASA Science
- Ceres: Facts , NASA Science
- What Is a Planet? , NASA Science
- Planetary Fact Sheet (Metric) , NASA Goddard Space Flight Center
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.