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Gravity Holds the
Solar System Together

Discover how gravity acts as the invisible force that keeps planets in orbit, moons circling their planets, and our entire solar system bound together.

Earth & Space Science

What Is Gravity?

Gravity is an attractive force that exists between any two objects that have mass. The more mass an object has, the stronger its gravitational pull. You experience gravity every time you jump and land back on the ground: Earth's mass pulls you toward its center.

Key Vocabulary

Mass
The amount of matter in an object, measured in kilograms (kg). More mass means stronger gravity.
Orbit
The curved path one object follows around another due to gravity, like the Earth orbiting the Sun.
Inertia
An object's tendency to keep moving in a straight line. Gravity curves this straight path into an orbit.

Why Does Gravity Matter?

Without gravity, the planets would fly off in straight lines into deep space. The Moon would drift away from Earth. Earth's atmosphere would scatter into the void. Gravity is the glue of the cosmos: it holds moons to planets, planets to stars, and stars to galaxies.

Sun Mercury Earth Jupiter Dashed arrows show gravity pulling each planet toward the Sun

Newton's Law of Universal Gravitation

Isaac Newton discovered that the gravitational force between two objects depends on their masses and the distance between them. The mathematical relationship is:

How Distance Affects Gravity

Distance \( r \) Force \( F \) Close: strong force Far away: weak force

How Orbits Work

Sun Planet Inertia (straight line) Gravity (pulls inward) Curved path Gravity + Inertia = Orbit

Gravity vs. Inertia

Gravity Inertia
What is it? An attractive force A tendency of motion
Direction Pulls objects toward each other Keeps objects moving straight ahead
Depends on Mass and distance Mass and velocity
Role in orbits Curves the path inward Prevents the object from falling in
Without it? Planet flies off in a straight line Planet crashes into the Sun

The Sun: 99.8% of the Mass

The Sun contains about 99.8% of all the mass in the entire solar system. This enormous mass is why the Sun sits at the center, gravitationally anchoring all eight planets, dwarf planets, asteroids, and comets.

Sun: 99.8% of total mass Everything else: 0.2% (All planets, moons, asteroids, comets combined)

How to Model Gravity's Role

1
Identify the massive object at the center of the system (usually the Sun for planets, or a planet for its moons).
2
Draw the orbiting object on a curved path around the central body.
3
Add a gravity arrow pointing from the orbiting object toward the central body.
4
Add an inertia arrow pointing tangent to the orbit (perpendicular to gravity), showing forward motion.
5
Explain the balance: gravity curves the straight-line motion into a closed orbit.

Example: The Earth-Moon System

Earth Moon Gravity Inertia
\( m = 5.97 \times 10^{24} \) kg
\( m = 7.35 \times 10^{22} \) kg
 384,400 km

Example: Jupiter and Its Moons

Jupiter Io Europa Ganymede Callisto Jupiter's mass is 318 times Earth's: it holds 95 known moons

What If Gravity Vanished?

Sun Flies off in a straight line! No Gravity Scenario

Gravity at Every Scale

You + Earth
Gravity pulls you to the ground. Your weight is the measure of this force.
Moon + Earth
Gravity keeps the Moon orbiting Earth and causes ocean tides.
Planets + Sun
The Sun's gravity holds all eight planets in stable orbits.
Stars + Galaxy
Gravity binds billions of stars into the Milky Way galaxy.

Common Mistakes

✕ "Only large objects have gravity"
Every object with mass has gravity: you, your pencil, a grain of sand. Small objects just have very weak gravity.
✕ "Gravity pushes things down"
Gravity pulls objects toward each other. It works between any two masses, in all directions, not just "downward."
✕ "There's no gravity in space"
Astronauts float because they're in free-fall (orbiting), not because gravity is absent. Gravity reaches across the entire universe.

Summary: Gravity in the Solar System

Concept Key Idea Example
Gravity Force that attracts masses together Apple falling from a tree
Mass effect More mass = stronger gravity Sun holds all planets
Distance effect Greater distance = weaker gravity Neptune feels less pull than Mercury
Orbits Gravity + inertia = curved path Moon orbiting Earth
Universal Works at every scale Stars held in galaxies

Question 1 of 5

What is the primary force that keeps planets in orbit around the Sun?

Question 2 of 5

According to Newton's law of gravitation, what happens to the gravitational force if the distance between two objects is doubled?

Question 3 of 5

An astronaut is floating inside the International Space Station. Does this mean there is no gravity acting on the astronaut?

Question 4 of 5

If the Sun's gravity suddenly disappeared, what would the planets do?

Question 5 of 5

Why does the Sun have the strongest gravitational pull in the solar system?