The hierarchy from Earth to Universe
Everything in space sits inside a tidy nested structure. Working from biggest to smallest:
- The Universe is everything that exists. All the matter, energy, space and time, taken together. The Universe contains roughly 100 billion galaxies
- A galaxy is a vast collection of billions of stars held together by their mutual gravity. Our galaxy is the Milky Way, and it contains around 200 billion stars
- A star is a huge ball of hot gas (mostly hydrogen) powered by nuclear fusion in its core. The Sun is one such star, fairly average in size and brightness
- A planetary system is a star together with everything that orbits it. Our planetary system is the Solar System
- A planet is a roughly spherical body that orbits a star, big enough for its own gravity to pull it into a sphere, but not big enough for fusion. The Earth is the third of the eight planets in the Solar System
- A moon is a smaller body that orbits a planet. Earth has one moon; Jupiter has 95 known moons
Other objects in the Solar System
- Asteroids are small rocky bodies, mostly orbiting the Sun in a band between Mars and Jupiter called the asteroid belt
- Comets are small icy bodies that orbit the Sun on highly stretched paths, swinging in from the outer Solar System
- Artificial satellites are human-built objects placed in orbit around the Earth or another planet, for communications, weather, GPS, science, and spying
Where everything orbits what
| Orbiting body | What it orbits |
|---|
| Planet | The Sun |
| Moon | A planet |
| Comet | The Sun |
| Asteroid | The Sun |
| Artificial satellite | The Earth (or another planet) |
- Smaller bodies always orbit larger bodies. The bigger gravitational pull belongs to the larger object, so the smaller one is the one that swings around