The Sun holds about 99.8% of all the mass in the solar system, and its gravity keeps everything else in orbit. Those other objects fall into a few families.
- Rocky inner planets: Mercury, Venus, Earth and Mars. Small, made of rock and metal, with few or no moons.
- Giant outer planets: Jupiter and Saturn (gas giants, mostly hydrogen and helium) and Uranus and Neptune (ice giants). Huge, with rings and many moons.
- Dwarf planets: round, but too small to clear their orbits of other objects. Pluto, Eris and Ceres are examples.
- Asteroids: rocky and metal chunks, most in the asteroid belt between Mars and Jupiter.
- Comets: balls of ice and dust from the far edges of the solar system. Near the Sun they grow tails that point away from the Sun.
- Moons: natural objects that orbit a planet or a smaller body, like our Moon or Jupiter's Ganymede.
Key idea
Four small rocky planets lie close to the Sun; four giant planets lie farther out. Dwarf planets, asteroids, comets and moons fill in the rest.
Why it works
When the solar system formed, it was too warm near the young Sun for ice to stay frozen, so only rock and metal could clump into the inner planets. Farther out it was cold enough for ice too, so there was far more solid material. The outer planets grew big enough to pull in huge amounts of hydrogen and helium gas, which is why they became giants.
Worked example: Which family?
Ceres is round, orbits the Sun in the asteroid belt and shares its orbit with many asteroids. What is it?
- It is round and orbits the Sun, like a planet.
- But it has not cleared its orbit of other objects.
- So it is a dwarf planet (the largest object in the asteroid belt).
Answer: A dwarf planet
Worked example: The hottest planet
Mercury is about 58 million km from the Sun and Venus about 108 million km. Mercury's average temperature is about 167 °C; Venus's is about 464 °C. Why is Venus hotter?
- Being closer to the Sun is not the whole story.
- Venus has a very thick atmosphere of carbon dioxide that traps heat (a strong greenhouse effect).
- Mercury has almost no air, so its heat escapes quickly, especially at night.
Answer: Venus's thick carbon dioxide atmosphere traps heat
Common mistake
Guessing that Mercury is the hottest planet because it is closest to the Sun. Venus is hotter everywhere on its surface, day and night, because its thick carbon dioxide air traps heat. Check a planet table from NASA before trusting a guess.
Find these objects in the sky with the Night Sky Lab.