Summary
The Solar System is the Sun together with everything that orbits it: eight planets, their moons, dwarf planets like Pluto, asteroids and comets. The Sun is a star and holds almost all the mass; its gravity is what keeps every body turning in its orbit.
The planets are ordered by their distance from the Sun. Mercury, Venus, Earth and Mars are the rocky planets: small and close. Jupiter, Saturn, Uranus and Neptune are the gas giants: huge and far away. Earth takes a year to go once around the Sun (its orbit) and a day to spin on itself (its rotation); that rotation causes day and night, and the tilt of its axis causes the seasons.
Key points
- The Sun is a star and holds 99% of the Solar System's mass.
- Eight planets: 4 rocky (Mercury, Venus, Earth, Mars) and 4 gas (Jupiter, Saturn, Uranus, Neptune).
- Earth's rotation (24 hours) produces day and night.
- The orbit (365 days) and the tilt of the axis produce the year and the seasons.
- The Moon is Earth's satellite and orbits around it.
- Between Mars and Jupiter lies the asteroid belt.
- Pluto has been considered a dwarf planet since 2006.
The planets in order
| Planet | Type | Mean distance from the Sun | What makes it stand out |
|---|---|---|---|
| 1. Mercury | rocky | 58 million km | The smallest and the closest to the Sun |
| 2. Venus | rocky | 108 million km | The hottest: its atmosphere traps the heat |
| 3. Earth | rocky | 150 million km | Liquid water on the surface, and life |
| 4. Mars | rocky | 228 million km | The red planet, because of iron oxide |
| 5. Jupiter | gas giant | 778 million km | The largest; the Great Red Spot is a storm |
| 6. Saturn | gas giant | 1,430 million km | The biggest, most visible rings |
| 7. Uranus | ice giant | 2,870 million km | It spins on its side, with its axis almost horizontal |
| 8. Neptune | ice giant | 4,500 million km | The farthest, with the strongest winds |
- Mercury, Venus, Earth and Mars are the inner or rocky planets; Jupiter, Saturn, Uranus and Neptune, the outer or giant planets. Between the two groups lies the asteroid belt.
Dwarf planets
They are round bodies that orbit the Sun but have not «cleared» their orbit of other objects. The International Astronomical Union recognises five: Ceres (in the asteroid belt), Pluto, Haumea, Makemake and Eris (beyond Neptune).
Frequently asked questions
How many planets are in the Solar System?
Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, in that order from the Sun.
Why is Pluto no longer a planet?
In 2006 the International Astronomical Union defined what a planet is, and one of the conditions is having cleared its orbit. Pluto shares its orbit with many other bodies, so it became a dwarf planet.
Which is the hottest planet?
Venus, even though Mercury is closer to the Sun: its thick carbon dioxide atmosphere traps the heat (greenhouse effect) and goes above 400 °C.