Explore Planets, Moons, and Asteroids in Our Solar System

A celestial body is any natural object found in space. You can find many different types of celestial bodies in our solar system, including the Sun, planets, moons, asteroids, and comets. Each one has its own special features that make it different from the others.

Our solar system is made up of the Sun at the center and everything that orbits around it. You will learn about Orbital Patterns and Planet and Moon Movements as you explore how each object travels through space.

The Sun is the star at the center of our solar system. It is a giant ball of hot gas made mostly of hydrogen and helium. The Sun produces its own light and heat through a process called nuclear fusion, where hydrogen atoms are pressed together to form helium and release huge amounts of energy.

The Sun's powerful gravity holds all eight planets, moons, asteroids, and other objects in orbit around it. You will explore gravity more deeply when you study Gravity Effects and Gravitational Forces in Space.

There are eight official planets in our solar system. In order from closest to farthest from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. You can remember this order using the phrase: My Very Excited Mother Just Served Us Nachos.

The first four planets Mercury, Venus, Earth, and Mars are called the inner planets. They are smaller and made mostly of rock and metal. Jupiter, Saturn, Uranus, and Neptune are the outer planets and are much larger, made mostly of gas or ice. You will learn more about how planets move in Planetary Motion, Orbits and Rotations.

Mercury is the smallest planet and the closest to the Sun. It gets extremely hot during the day and very cold at night because it has almost no atmosphere. Venus is the second planet and is the hottest planet in the solar system because its thick atmosphere traps heat in a greenhouse effect, reaching about 465°C.

Earth is the third planet and the only known planet with liquid water on its surface and life. Mars, the fourth planet, is called the Red Planet because its surface is covered in reddish iron oxide, which is basically rust. Jupiter is the largest planet more than 1,300 Earths could fit inside it and it has a famous feature called the Great Red Spot, a giant storm that has lasted for hundreds of years.

Saturn is the sixth planet and is famous for its spectacular ring system made of billions of pieces of ice and rocky debris. Uranus is unique because it spins on its side with an axial tilt of about 98 degrees. Neptune is the eighth and farthest planet from the Sun.

A moon is a natural object that orbits around a planet, held in place by the planet's gravity. Earth has one Moon, which is its only natural satellite. Other planets have many moons Saturn has the most confirmed moons of any planet, and Jupiter has dozens as well.

One of Jupiter's moons, Europa, is of great scientific interest because scientists think a liquid water ocean may exist beneath its frozen surface. You always see the same side of Earth's Moon because the Moon takes the same amount of time to spin as it does to orbit Earth about 27 days. This is called synchronous rotation. The Moon's changing position as it orbits Earth also causes the phases of the Moon that you can observe from Earth, which connects to what you learned in Rotation Effects and the Day/Night Cycle.

Asteroids are rocky, irregularly shaped objects that orbit the Sun. Most asteroids are found in the Asteroid Belt, a region of space located between the orbits of Mars and Jupiter. Scientists believe asteroids are leftover material from the formation of our solar system.

When a space rock enters Earth's atmosphere and burns up, creating a streak of light, it is called a meteor. If the rock survives and lands on Earth's surface, it is called a meteorite. A rock still in space is called a meteoroid. A meteor shower happens when Earth passes through a trail of debris left by a comet, and many particles burn up in the atmosphere at once.

Comets are often called "dirty snowballs" because they are made of ice, dust, and rocky material frozen together. When a comet travels close to the Sun, the ice heats up and releases gases that form a glowing tail pointing away from the Sun.

A dwarf planet is a round object that orbits the Sun but has not cleared other objects out of its orbital path, unlike the eight main planets. Pluto is the most famous dwarf planet. It was once called the ninth planet but was reclassified as a dwarf planet in 2006. Pluto orbits the Sun in a distant region called the Kuiper Belt.

Celestial Body: Any natural object found in space, such as a planet, moon, star, asteroid, or comet.

Planet: A large, round object that orbits the Sun and has cleared other objects out of its orbital path. Examples include Earth and Mars.

Moon: A natural satellite that orbits a planet, held in place by the planet's gravity. Earth has one Moon, and Jupiter has many.

Asteroid: A rocky, irregularly shaped object that orbits the Sun, mostly found in the Asteroid Belt between Mars and Jupiter.

Comet: An icy body made of ice, dust, and rocky material that grows a glowing tail when it passes close to the Sun.

Star: A massive ball of hot gas, like our Sun, that generates its own energy through nuclear fusion and produces light and heat.

Dwarf Planet: A round object that orbits the Sun but has not cleared other objects out of its orbital path. Pluto is the most famous example.

Asteroid Belt: The region of space between the orbits of Mars and Jupiter where millions of asteroids orbit the Sun.

Orbit: The curved path that one object travels around another object due to gravity. Earth orbits the Sun, and the Moon orbits Earth.

Gravity: An invisible pulling force between all objects with mass. The Sun's gravity keeps all the planets in their orbits.

Nuclear Fusion: The process inside the Sun where hydrogen atoms are pressed together to form helium, releasing huge amounts of light and heat energy.

Inner Planets: The four rocky planets closest to the Sun Mercury, Venus, Earth, and Mars.

Outer Planets: The four large planets farther from the Sun Jupiter, Saturn, Uranus, and Neptune made mostly of gas or ice.

Gas Giant: A very large planet made mostly of gas, such as Jupiter and Saturn.

Great Red Spot: A giant storm on Jupiter that has been raging for over 350 years and is wider than the entire planet Earth.

Meteor: A space rock that enters Earth's atmosphere and burns up, creating a streak of light sometimes called a shooting star.

Meteorite: A space rock that survives passing through Earth's atmosphere and lands on Earth's surface.

Meteoroid: A rocky object in space that has not yet entered Earth's atmosphere.

Meteor Shower: An event when many meteors burn up in Earth's atmosphere at once, caused by Earth passing through a trail of comet debris.

Synchronous Rotation: When the Moon takes the same amount of time to spin as it does to orbit Earth, causing you to always see the same side of the Moon.

Phases of the Moon: The different shapes of the Moon you see from Earth, caused by the Moon's changing position as it orbits Earth.

Kuiper Belt: A distant region of the solar system beyond Neptune where Pluto and other dwarf planets orbit the Sun.

Europa: One of Jupiter's moons that scientists find interesting because a liquid water ocean may exist beneath its frozen surface.

You can test your understanding by naming the eight planets in order from the Sun. Try using the mnemonic My Very Excited Mother Just Served Us Nachos to help you remember. You can also practice sorting objects into groups: planets, moons, asteroids, comets, and dwarf planets.

As you study Space Technology, Exploration and Observation Tools, you will discover how scientists use telescopes and spacecraft to learn about these celestial bodies. You can also explore how Scientific Models help scientists represent and study the solar system.

You have already learned about Earth's Movement, Rotation and Revolution, which explains how Earth spins on its axis and travels around the Sun. You also studied Day and Night and Earth's Rotation Effects, which connects to how planets spin and how moons create phases. Your knowledge of Seasonal Changes, Earth's Tilt and Orbit also helps you understand how Earth's position in the solar system affects life on our planet.

This topic connects to many exciting areas of science. You already explored Rotation Effects and the Day/Night Cycle and Revolution Effects and Seasonal Changes, which show you how Earth's movements in the solar system affect everyday life. Understanding the Internal Structure and Layers of the Earth and Surface Features like Mountains, Valleys, and Oceans helps you appreciate what makes Earth a rocky inner planet.

Next, you will build on this knowledge by studying Planetary Motion, Orbits and Rotations and Gravity Effects and Gravitational Forces in Space to understand why planets stay in their paths around the Sun. You will also explore Exploration Tools, Satellites and Space Probes and Space Research and Current Space Exploration to learn how scientists continue to discover new things about our solar system.