Solar System

The solar system is our Sun and everything that travels around it, held together by gravity.

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Definition

The solar system is made up of the Sun at the center, eight planets that orbit it, their moons, and many smaller objects like asteroids, comets, and dwarf planets. The Sun's gravity keeps all of these objects moving in curved paths called orbits around it.

Remember the rule

My Very Educated Mother Just Served Us Nachos — the first letter of each word matches the planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Key words

Solar System: The Sun and all the objects that orbit around it, including planets, moons, asteroids, and comets.
Orbit: The curved path an object takes as it travels around another object in space, like Earth traveling around the Sun.
Gravity: An invisible pulling force that attracts objects toward each other; the Sun's gravity keeps planets in orbit.
Planet: A large, round object that orbits the Sun and has cleared other objects out of its path.
Moon: A natural object that orbits a planet; Earth has one moon, but other planets can have many.
Asteroid: A rocky object, smaller than a planet, that orbits the Sun — most are found in the asteroid belt between Mars and Jupiter.
Comet: A small, icy object that orbits the Sun and develops a glowing tail of gas and dust when it gets close to the Sun.
Dwarf Planet: A round object that orbits the Sun but has NOT cleared other objects from its path; Pluto is the most famous dwarf planet.

Worked examples

List the eight planets of the solar system in order from closest to farthest from the Sun.

→ Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. · Use the mnemonic 'My Very Educated Mother Just Served Us Nachos' to remember the order.

Which planet is largest in our solar system?

→ Jupiter is the largest planet. It is so big that more than 1,300 Earths could fit inside it. · Jupiter is a gas giant, meaning it is made mostly of gas, not rock.

Why does Earth have seasons?

→ Earth is tilted on its axis at about 23.5 degrees. As Earth orbits the Sun over one year, different parts of Earth receive more direct sunlight at different times, causing summer and winter. · Seasons are caused by Earth's tilt, NOT by how close Earth is to the Sun.

How long does it take Earth to complete one orbit around the Sun?

→ Earth takes about 365.25 days, which is one year, to travel all the way around the Sun once. · That extra 0.25 day is why we add a leap day (February 29) every four years.

What is the difference between a planet and a dwarf planet?

→ A planet is large enough to have cleared other objects from its orbital path. A dwarf planet, like Pluto, orbits the Sun and is round, but has NOT cleared its path of other objects. · Pluto was reclassified as a dwarf planet in 2006 because it shares its orbit region with many other icy objects.

How far is Earth from the Sun?

→ Earth is about 93 million miles (150 million kilometers) from the Sun on average. Scientists call this distance one Astronomical Unit, or 1 AU. · All other planet distances are often measured by comparing them to this Earth-Sun distance.

Common mistakes

Thinking Pluto is still the ninth planet — Pluto was reclassified as a dwarf planet in 2006, so the solar system officially has eight planets.
Thinking seasons are caused by Earth being closer to the Sun in summer — seasons are caused by Earth's tilt, not its distance from the Sun.
Confusing 'rotation' and 'revolution' — rotation is a planet spinning on its own axis (making day and night), while revolution is a planet orbiting the Sun (making a year).
Thinking the Sun is just a star far away — the Sun IS a star, and it is the center of our solar system; it just looks bigger and brighter because it is much closer than other stars.
Thinking all planets have the same length year — each planet takes a different amount of time to orbit the Sun; Mercury's year is only 88 Earth days, while Neptune's year is about 165 Earth years.

FAQs

Why doesn't Earth fly off into space or crash into the Sun?

Gravity pulls Earth toward the Sun, but Earth is also moving forward very fast. These two things balance perfectly — the pull of gravity curves Earth's path into a stable orbit instead of letting it fly away or fall in.

How many moons does Earth have, and do other planets have moons too?

Earth has 1 moon. Other planets have many more — Mars has 2, Jupiter has at least 95, and Saturn has over 140 known moons. Mercury and Venus have no moons at all.

What is the asteroid belt?

The asteroid belt is a region between Mars and Jupiter filled with millions of rocky objects called asteroids. It is not a solid ring — there is actually a lot of empty space between the asteroids.

Is the Sun moving, or does everything orbit a perfectly still Sun?

The Sun is actually moving too! The entire solar system slowly orbits around the center of our galaxy, the Milky Way. But within our solar system, we say the Sun is at the center because everything else orbits it.

Could humans live on other planets in the solar system?

Not easily. Most planets are too hot, too cold, have poisonous atmospheres, or have no liquid water. Scientists are studying Mars as a possible future destination because it is the most Earth-like, but it would still require enormous technology to survive there.

What keeps the solar system together?

Gravity is the key force. The Sun has so much mass that its gravitational pull reaches across billions of miles, keeping all the planets, moons, asteroids, and comets traveling in their orbits around it.

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