MS-ESS1-2: How Gravity Shapes the Solar System and Galaxy
Also called NGSS MS-ESS1-2 · Topic: Space Systems · Earth and Space Science
The Official Wording
Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.
Clarification statement: Emphasis for the model is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students' school or state).
Assessment boundary: Assessment does not include Kepler’s Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-ESS1-2 Means in Plain English
Middle schoolers use a model to show how gravity holds the solar system and the Milky Way together. They explain why planets circle the sun, why moons circle planets, and why the sun itself travels around the center of our galaxy.
The model explains orbits in words and pictures, without Kepler's laws or why planets sometimes seem to drift backward in our sky. It draws on gravity pulling toward Earth's center (5-PS2-1) and leads to Newton's law of gravitation (HS-PS2-4).
MS-ESS1-2 I Can Statements
- I can explain how gravity keeps planets in orbit around the sun.
- I can describe how mass and distance affect the pull of gravity.
- I can explain why the sun orbits the center of the Milky Way.
MS-ESS1-2 Worked Example
A gravity model built from a stretchy sheet, a heavy ball and a handful of marbles.
How does a heavy object in the middle change the path of a rolling marble?
Set It Up: A sheet of spandex stretched tight over a hula hoop, a heavy ball about the size of a softball, and several marbles.
- Roll a marble across the flat sheet with nothing in the middle.Straight path across
- Set the heavy ball in the center and roll the marble past it.Path bends toward the ball
- Roll a marble sideways at a moderate speed near the ball.Circles several times, then spirals in
- Roll the marble much faster along the same line.Curves, then escapes off the sheet
- Compare marbles circling close to the ball with ones circling far out.Close marbles circle faster
The central ball bent every marble's path, and a marble with the right sideways speed circled it several times.
Why it works: A massive object pulls on everything near it, and an orbit is sideways motion constantly bent into a curve by that pull.
Common Mistakes with MS-ESS1-2
Thinking space has no gravity
“Astronauts float because there is no gravity up there.”
Try this: Gravity on the space station is still about 90 percent as strong as on the ground. Astronauts float because they are falling around Earth.
Thinking orbits need a push to keep going
“Planets need an engine or the sun pushes them along.”
Try this: Planets keep moving because nothing in space slows them. Gravity only bends that motion toward the sun, turning a straight path into an orbit.
Taking the sheet model literally
“Space is a giant stretchy sheet and planets roll on it.”
Try this: The sheet only shows how the pull grows near a large mass. Real orbits do not slow down from friction the way marbles do.
Where MS-ESS1-2 Fits
Practice MS-ESS1-2
Quick Check
- What would the moon do if Earth's gravity suddenly vanished?
- Why can the sun hold onto planets much farther away than Earth holds the moon?
- What holds the hundreds of billions of stars in the Milky Way together?