The solar system is the Sun plus everything that orbits it: eight planets and their moons, several dwarf planets, millions of asteroids, and huge numbers of comets and smaller bits of rock and ice. On a clear night from Ontario you can see five of the planets without a telescope (Mercury, Venus, Mars, Jupiter and Saturn); they look like bright "stars" that slowly drift against the background stars from week to week. The word planet comes from a Greek word meaning "wanderer." Spacecraft have now visited every planet, so we know them as real worlds with volcanoes, storms, rings and ice.
What counts as a planet?
In 2006 the International Astronomical Union (IAU), the world organization of astronomers, set out three rules. A planet must (1) orbit the Sun, (2) have enough mass that its own gravity has pulled it into a nearly round shape, and (3) have cleared its orbit, meaning it is by far the largest object in its path and has swept up or flung away most other material there. Pluto meets the first two rules but shares its region with many other icy bodies in the Kuiper belt, so it fails the third rule. It was reclassified as a dwarf planet. The IAU currently recognizes five dwarf planets: Ceres (in the asteroid belt), Pluto, Haumea, Makemake and Eris.
Terrestrial planets and giant planets
- Made mostly of rock and metal
- Small and dense (about 4 to 5.5 g/cm³)
- Solid surfaces with mountains, craters and volcanoes
- Few or no moons (0 to 2) and no rings
- Close to the Sun, with short years
- Jupiter and Saturn (gas giants) are mostly hydrogen and helium; Uranus and Neptune (ice giants) contain lots of water, ammonia and methane
- Huge, with low density (about 0.7 to 1.6 g/cm³)
- No solid surface to stand on
- Many moons and ring systems (Saturn's are the brightest)
- Far from the Sun, with long years
| Planet | Average distance from Sun (AU) | Diameter (Earth = 1) | Type | Something notable |
|---|---|---|---|---|
| Mercury | 0.39 | 0.38 | Terrestrial | Smallest planet; almost no atmosphere, so about 430 °C by day and −180 °C at night |
| Venus | 0.72 | 0.95 | Terrestrial | Hottest planet (about 465 °C): its thick carbon dioxide atmosphere traps heat |
| Earth | 1.00 | 1.00 | Terrestrial | Only world known to have liquid water on its surface and life |
| Mars | 1.52 | 0.53 | Terrestrial | Red from iron oxide (rust) dust; Olympus Mons, the largest known volcano in the solar system |
| Jupiter | 5.20 | 11.0 | Gas giant | Largest planet; the Great Red Spot is a storm wider than Earth |
| Saturn | 9.54 | 9.1 | Gas giant | Brightest ring system, made of ice and rock pieces; less dense than water |
| Uranus | 19.2 | 4.0 | Ice giant | Axis tilted about 98°, so it rolls around the Sun on its side |
| Neptune | 30.1 | 3.9 | Ice giant | Fastest winds measured in the solar system, about 2000 km/h |
Smaller bodies of the solar system
- Moons (natural satellites): bodies that orbit a planet or dwarf planet. Mercury and Venus have none, Earth has one, Mars has two small ones, and each giant planet has many, from more than a dozen for Neptune to well over a hundred for Saturn, with new small moons still being discovered.
- Asteroids: rocky or metallic objects, from a few metres to hundreds of kilometres across. Most orbit in the asteroid belt between Mars and Jupiter (about 2.2 to 3.2 AU from the Sun).
- Comets: "dirty snowballs" of ice, dust and rock a few kilometres across. Most come from the Kuiper belt (an icy region about 30 to 50 AU from the Sun) or the much more distant Oort cloud. When a comet nears the Sun, its ice turns to gas, forming a glowing cloud (the coma) and long tails that point away from the Sun.
- Meteoroids: small pieces of rock or metal travelling through space, often broken off asteroids or left behind by comets.