What is in the Solar System?
The Solar System is a family of objects held together by the gravity of the Sun, a star at its centre. Orbiting the Sun are the eight planets — in order of increasing distance: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small, rocky and dense; the four outer planets are large gas giants of lower density. Also present are dwarf planets (such as Pluto), moons that orbit planets, asteroids (mostly in a belt between Mars and Jupiter) and comets that follow long, highly elliptical orbits.
Key idea
The gravitational attraction of the Sun provides the centripetal force that keeps each planet in orbit. Orbital speed v = 2πr ÷ T, where r is the orbit radius and T the orbital period.
Orbits and gravity
Planets move in slightly elliptical (almost circular) orbits. The Sun's gravitational field strength weakens with distance, so planets further out feel a weaker pull, travel more slowly and take much longer to complete one orbit. A comet speeds up as it nears the Sun (stronger gravity) and slows as it moves away. The Solar System formed about 4.6 billion years ago when a cloud of gas and dust was pulled together by gravity.
Worked example
A planet orbits at radius r = 5.0×10¹¹ m with period T = 3.14×10⁸ s. Orbital speed v = 2πr ÷ T = (2 × 3.142 × 5.0×10¹¹) ÷ (3.14×10⁸) = 3.14×10¹² ÷ 3.14×10⁸ = 1.0×10⁴ m/s.
Remember
- Order out from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Gravity provides the centripetal force; field strength decreases with distance.
- Further planet → slower orbital speed → longer period.