What Is a Planet?
Quick Answer
A planet is a large, round object in space that orbits, or travels around, a star like our Sun. Our solar system has eight planets, including the Earth we live on. Planets do not make their own light like stars do. Instead, they shine because they reflect light from the star they orbit.
Explaining By Age Group
Ages 3-5 Simple Explanation
You know how you live in a house, and your house is on a street? Well, Earth is like our house, and the solar system is like our street! Earth is a planet, and there are seven other planets in our neighborhood that all go around the Sun together.
A planet is a really big ball that floats in space and travels around a star. Our star is the Sun. Earth goes around the Sun once every year, and that trip is what gives us birthdays!
Some planets are rocky like Earth, and you could stand on them if you visited. Other planets are made of gas, like really big puffy clouds, and you could not stand on them at all because there is no ground!
The planets in our solar system have cool names like Mars, Jupiter, and Saturn. Saturn is the one with the beautiful rings around it! Each planet is different and special in its own way.
Ages 6-8 More Detail
A planet is a big, round object in space that travels around a star. In our solar system, all eight planets travel around the Sun. Starting from the closest to the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
To be called a planet, a space object has to do three things. It has to orbit a star. It has to be big enough that its own gravity pulls it into a round shape. And it has to be the biggest thing in its area of space, meaning it has cleared away most of the smaller rocks and debris around its path.
The first four planets, Mercury, Venus, Earth, and Mars, are called the rocky planets because they have solid, hard surfaces you could walk on. The next four, Jupiter, Saturn, Uranus, and Neptune, are called gas giants or ice giants because they are mostly made of gases and liquids with no real ground to stand on.
Jupiter is the biggest planet in our solar system. It is so huge that you could fit all the other planets inside it and still have room left over! The smallest planet is Mercury, which is only a little bigger than our Moon.
You might be wondering about Pluto. Pluto used to be called the ninth planet, but in 2006, scientists decided it was too small and had not cleared its neighborhood of other objects. So they reclassified it as a dwarf planet. Pluto is still out there and still interesting, it just has a different label now.
Ages 9-12 Full Explanation
A planet is a large object in space that orbits a star, has enough gravity to pull itself into a roughly round shape, and has cleared the neighborhood around its orbit of other debris. That third rule is actually why Pluto got reclassified as a dwarf planet back in 2006. It orbits the Sun and it is round, but it shares its orbital neighborhood with lots of other icy objects in the Kuiper Belt.
Our solar system has eight planets divided into two main groups. The inner planets, Mercury, Venus, Earth, and Mars, are smaller and made of rock and metal. They have solid surfaces with features like mountains, craters, and valleys. The outer planets, Jupiter, Saturn, Uranus, and Neptune, are much larger and made mostly of gases and ices. Jupiter and Saturn are called gas giants, while Uranus and Neptune are called ice giants because they contain more frozen water, ammonia, and methane.
Each planet in our solar system is wildly different. Venus has surface temperatures hot enough to melt lead, thanks to a runaway greenhouse effect. Mars has the largest volcano in the solar system, Olympus Mons, which is nearly three times the height of Mount Everest. Saturn's rings are made of billions of chunks of ice and rock, some as small as grains of sand and some as big as houses.
Scientists have also found thousands of planets orbiting other stars, called exoplanets. As of now, we have confirmed over 5,000 exoplanets, and there could be billions more in our galaxy alone. Some of these are in the habitable zone of their star, the range of distances where liquid water could exist on the surface, making them interesting candidates in the search for life.
One thing that blows people's minds is the sheer size difference between planets. Jupiter's diameter is about eleven times Earth's, and you could fit roughly 1,300 Earths inside Jupiter. Yet Jupiter is tiny compared to the Sun. The variety in our own solar system, from tiny rocky Mercury to giant gaseous Jupiter, shows just how many different ways a planet can form.
Studying planets helps us understand how solar systems form and evolve, and whether Earth-like conditions might exist elsewhere. Every new exoplanet discovery teaches us something about how common or rare our own world might be in the universe.
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Tips for Parents
A planet can be a challenging topic to discuss with your child. Here are some practical tips to help guide the conversation:
DO: Follow your child's lead. Let them ask questions at their own pace rather than overwhelming them with information they haven't asked for yet. If they seem satisfied with a simple answer, that's okay — they'll come back with more questions when they're ready.
DO: Use honest, age-appropriate language. You don't need to share every detail, but avoid making up stories or deflecting. Kids can sense when you're being evasive, and honesty builds trust.
DO: Validate their feelings. Whatever emotion your child has in response to learning about a planet, acknowledge it. Say things like 'It makes sense that you'd feel that way' or 'That's a really good question.'
DON'T: Don't dismiss their curiosity. Responses like 'You're too young for that' or 'Don't worry about it' can make children feel like their questions are wrong or shameful. If you're not ready to answer, say 'That's an important question. Let me think about the best way to explain it, and we'll talk about it tonight.'
DO: Create an ongoing dialogue. One conversation usually isn't enough. Let your child know that they can always come back to you with more questions about a planet. This makes them more likely to come to you rather than seeking potentially unreliable sources.
Common Follow-Up Questions Kids Ask
After discussing a planet, your child might also ask:
Why is Pluto not a planet anymore?
In 2006, scientists created a strict definition of what counts as a planet. To qualify, an object must orbit the Sun, be round due to its own gravity, and have cleared its orbital neighborhood of other debris. Pluto meets the first two rules but not the third, since it shares its area with many other icy objects. So it was reclassified as a dwarf planet.
Could humans live on another planet?
Not easily with today's technology. Mars is the most likely candidate because it has a solid surface and some ice water. But it has no breathable air, extreme cold, and dangerous radiation. Scientists are working on ideas for how humans might one day live there with special habitats, but it would be very challenging.
What is the hottest planet?
Venus is the hottest planet in our solar system, even though Mercury is closer to the Sun. Venus has a super thick atmosphere that traps heat like a blanket, pushing surface temperatures to about 900 degrees Fahrenheit. That is hot enough to melt lead.
How many planets are there in the universe?
Scientists estimate there could be trillions of planets in the universe. In our galaxy alone, there may be more planets than stars, which would mean at least 100 billion. We have confirmed over 5,000 so far, but new ones are being discovered all the time.
Do all planets have moons?
No. Mercury and Venus have no moons at all. Earth has one, Mars has two small ones, and the giant outer planets have dozens each. Jupiter has over 90 known moons, and Saturn has more than 140. Some moons are even bigger than the planet Mercury.
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