Objects in the Solar System and the Role of Gravity
On a clear night in Texas, you might see a bright streak cross the sky for one second and vanish. Was it a star? A tiny piece of rock? Something much bigger? To answer, you need a map of what is out there, and an idea of what holds it all together.
The Sun and the planets
Our Solar System is the Sun and every object that travels around it. The Sun is a star, a huge ball of hot gas that makes its own light. It holds more than 99% of all the mass in the Solar System. Its light and thermal energy reach every planet.
A planet is a large, round object that orbits a star. There are eight planets, and they fall into two groups.
| Inner planets | Outer planets | |
|---|---|---|
| Members | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Size | Small | Much larger |
| Made of | Rock and metal (solid surface) | Mostly gas and liquid (Uranus and Neptune also have lots of ice) |
| Moons | Few (0 to 2) | Many (dozens each) |
Between Mars and Jupiter lies the asteroid belt, which separates the two groups.
A moon is a natural object that orbits a planet, not the Sun. Earth has one Moon. Jupiter has dozens, including Europa. A moon is not a "small planet": it circles a planet, while a planet circles a star.
Name the object at the center of our Solar System. Then state whether it is a planet, a moon, or a star, and tell what it provides to the planets that orbit it.
Name the object at the center of our Solar System. Then state whether it is a planet, a moon, or a star, and tell what it provides to the planets that orbit it.
A student says, "A moon is a small planet that orbits the Sun." Correct this statement by defining what a moon is and giving one example, such as Earth's Moon or…
A student says, "A moon is a small planet that orbits the Sun." Correct this statement by defining what a moon is and giving one example, such as Earth's Moon or Jupiter's moon Europa.
Emma makes a model of the Solar System. She places Mercury, Venus, Earth, and Mars on one side of the asteroid belt and Jupiter, Saturn, Uranus, and Neptune on the other…
Emma makes a model of the Solar System. She places Mercury, Venus, Earth, and Mars on one side of the asteroid belt and Jupiter, Saturn, Uranus, and Neptune on the other side. (a) Which group is made of the inner planets, and which of the outer planets? (b) Describe one physical difference between the two groups, such as size, composition, or number of moons.
Smaller objects: asteroids, comets, and meteors
Not everything in the Solar System is a planet or a moon.
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An asteroid is a rocky or metallic body, much smaller than a planet. Most are in the asteroid belt between Mars and Jupiter.
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A comet is a ball of ice, dust, and rock. Near the Sun, the ice turns to gas and a glowing tail forms.
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A meteoroid is a small piece of rock or metal traveling through space. When it enters Earth's atmosphere and burns up, the streak of light is a meteor (a "shooting star"). If a piece reaches the ground, it is a meteorite.
| Object | Made of | Where or when |
|---|---|---|
| Asteroid | Rock, metal | Mostly between Mars and Jupiter |
| Comet | Ice, dust, rock | Grows a tail near the Sun |
| Meteor | A burning meteoroid | Streak of light in Earth's atmosphere |
Classify each object as a comet, an asteroid, or a meteor: (a) a streak of light in the night sky as a small piece of rock burns up in Earth's atmosphere, (b) a rocky…
Classify each object as a comet, an asteroid, or a meteor: (a) a streak of light in the night sky as a small piece of rock burns up in Earth's atmosphere, (b) a rocky body found mostly between Mars and Jupiter, (c) an icy body that grows a glowing tail when it gets close to the Sun.
The Kuiper belt and the Oort cloud
Beyond Neptune, the Solar System keeps going.
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The Kuiper belt is a flat, doughnut-shaped region just beyond Neptune's orbit. It holds many icy objects, including dwarf planets such as Pluto.
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The Oort cloud is a huge, spherical shell of icy objects surrounding the whole Solar System, far beyond the Kuiper belt, about a thousand times farther than Neptune. We have not seen it directly. It is thought to be the source of comets that take thousands of years to orbit.
Jayden reads that the Kuiper belt and the Oort cloud both contain icy objects. Compare the two regions by describing (a) where each is located relative to Neptune and…
Jayden reads that the Kuiper belt and the Oort cloud both contain icy objects. Compare the two regions by describing (a) where each is located relative to Neptune and the Sun and (b) one kind of object that is found there.
Gravity keeps everything in orbit
Gravity is the force of attraction between objects that have mass. The more mass, the stronger the pull. The closer the objects, the stronger the pull. The Sun is so massive that its gravity reaches every object in the Solar System.
Why doesn't Earth fall straight into the Sun? Because it is also moving sideways. Think of swinging a ball on a string: the string pulls the ball inward, but the ball keeps trying to move straight ahead. The result is a curved path. For Earth, gravity is the "string."
Gravity also explains orbit times and speeds:
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Farther away means a longer trip. Neptune is about 30 times farther from the Sun than Earth. Gravity there is much weaker, so Neptune moves slowly, and it also has a much longer path to travel. One Neptune year is about 165 Earth years.
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Closer means faster. In an oval orbit, an object speeds up as it falls toward the Sun and slows down as it moves away.
Comets show how all this fits together. Comets with very long orbits are thought to come from the Oort cloud, and most of those with shorter orbits are thought to come from the Kuiper belt. Gravity from passing objects nudges an icy body inward, and the Sun's gravity then pulls it on a long loop. The tail forms only near the Sun, where the ice warms and turns to gas. Astronomers infer this from the shapes and directions of comet orbits.
Earth orbits the Sun at an average distance of about $150{,}000{,}000$ km, and Neptune orbits at about $4{,}500{,}000{,}000$ km. (a) How many times farther from the Sun…
Earth orbits the Sun at an average distance of about $150{,}000{,}000$ km, and Neptune orbits at about $4{,}500{,}000{,}000$ km. (a) How many times farther from the Sun is Neptune than Earth? (b) Using what you know about gravity, explain why Neptune takes much longer than Earth to complete one orbit.
Aaliyah says, "If the Sun suddenly disappeared, Earth would keep moving in a circle around where the Sun used to be." Evaluate her claim. In your answer, explain (a)…
Aaliyah says, "If the Sun suddenly disappeared, Earth would keep moving in a circle around where the Sun used to be." Evaluate her claim. In your answer, explain (a) what force keeps Earth in orbit now, (b) what Earth's motion would be if that force vanished, and (c) why Earth's orbit is not a straight line even though Earth is always moving forward.
A comet travels in a long, oval orbit around the Sun. It moves very fast when it passes close to the Sun and very slowly when it is far away, near the orbit of Neptune.…
A comet travels in a long, oval orbit around the Sun. It moves very fast when it passes close to the Sun and very slowly when it is far away, near the orbit of Neptune. (a) Explain this change in speed using gravity. (b) Describe where the comet's tail points when it is near the Sun and why a tail forms only there. (c) Explain why astronomers think some comets come from the Kuiper belt or the Oort cloud.
Back to the start: the quick streak you saw was most likely a meteor, a tiny bit of rock burning up in Earth's atmosphere, not a star.
Summary
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The Sun is a star at the center. Eight planets orbit it: four small rocky inner planets and four large outer planets.
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A moon orbits a planet. Asteroids are rocky (mostly in the belt between Mars and Jupiter). Comets are icy and grow tails near the Sun. A meteor is a burning meteoroid in Earth's atmosphere.
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The Kuiper belt is a flat region of icy objects beyond Neptune. The Oort cloud is a distant sphere of icy objects around the whole system.
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Gravity pulls objects toward the Sun and keeps them in curved orbits. Without it, they would move in straight lines.
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Closer to the Sun means stronger gravity and faster motion. Farther away means slower motion and a longer orbit.