Eight planets orbit the Sun, mostly in the same plane. Inner four are rocky; outer four are gas/ice giants.
Why do the planets orbit the Sun in roughly the same flat plane, all going the same way round?
The shaded wedge is the area swept by the radius in a fixed slice of time — it stays the same size all the way round (Kepler's 2nd law), so the planet must speed up when it's close to the Sun and crawl when it's far. e = 0 ⇒ a circle; bigger e ⇒ a more stretched ellipse with the Sun further off-centre.
The line-up
- Inner four — Mercury, Venus, Earth, Mars: small, rocky, few or no moons.
- Asteroid belt — rubble that never coalesced into a planet (Jupiter's gravity kept stirring it).
- Outer four — Jupiter, Saturn (gas giants), Uranus, Neptune (ice giants): huge, ringed, many moons.
- Beyond — the Kuiper Belt (Pluto's home) and the distant Oort cloud of comets.
Kepler's three laws describe the orbits (ellipses, equal areas in equal times, period² ∝ radius³). Newton later showed all three fall out of gravity plus F = ma.
Light takes about 8 minutes to reach Earth from the Sun. Roughly how far is that?
Kepler's three laws describe planetary orbits — Newton derived them from gravity and F = ma.