Click image to open full size
The Solar System is the Sun and the many objects held by its gravity, including eight planets, dwarf planets, moons, asteroids, comets, dust, and gas. It matters because it is our cosmic neighborhood and the best natural laboratory for understanding gravity, motion, atmospheres, and planetary formation. By comparing the planets, students can see how distance from the Sun, size, composition, and orbital motion shape very different worlds. The Sun is the central energy source, providing the light and heat that make life on Earth possible.
Key Facts
- Planet order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Gravity provides the centripetal force that keeps planets in orbit around the Sun.
- Average Earth to Sun distance: 1 AU = 149.6 million km.
- Kepler's third law for planets orbiting the Sun: T^2 = a^3 when T is in years and a is in AU.
- Orbital speed depends on distance from the Sun: v = 2πr / T for a nearly circular orbit.
- The inner planets are rocky, while the outer planets are giant planets made mostly of gas, ice, and deep atmospheres.
Vocabulary
- Astronomical Unit
- An astronomical unit, or AU, is the average distance from Earth to the Sun, equal to about 149.6 million kilometers.
- Orbit
- An orbit is the curved path an object follows around another object because of gravity.
- Terrestrial Planet
- A terrestrial planet is a rocky planet with a solid surface, such as Mercury, Venus, Earth, or Mars.
- Gas Giant
- A gas giant is a very large planet made mostly of hydrogen and helium, such as Jupiter or Saturn.
- Dwarf Planet
- A dwarf planet is a round object that orbits the Sun but has not cleared its orbital region of other debris.
Common Mistakes to Avoid
- Thinking the planets are evenly spaced is wrong because distances grow very large in the outer Solar System, and the gaps between planets are not equal.
- Drawing the planets to scale and the distances to scale on the same small diagram is misleading because the planets are tiny compared with the space between their orbits.
- Calling Pluto the ninth planet is outdated for most modern astronomy contexts because Pluto is classified as a dwarf planet by the International Astronomical Union.
- Assuming seasons are caused by Earth being closer to or farther from the Sun is wrong because seasons are mainly caused by Earth's axial tilt.
Practice Questions
- 1 Earth orbits the Sun at 1 AU and takes 1 year to complete one orbit. Using v = 2πr / T, find Earth's average orbital speed in AU per year.
- 2 Mars has an average orbital distance of about 1.52 AU. Using T^2 = a^3, estimate Mars's orbital period in Earth years.
- 3 Jupiter is much farther from the Sun than Earth but is also much more massive. Explain why Jupiter still orbits the Sun rather than the Sun orbiting Jupiter in the same way.