A family around a star
What is the Solar System?
The Solar System is the Sun and everything held in orbit by its gravity. The Sun is a star, and it is enormous: 99.8% of all the mass in the Solar System.
Going around it are:
- 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Dwarf planets: round worlds too small to clear their path around the Sun. Ceres, Pluto, Haumea, Makemake and Eris are the best known.
- Moons: 461 known moons orbit the planets and Pluto (JPL's list). New small moons are still being found every year.
- Asteroids: rocky and metal leftovers, most of them in the asteroid belt between Mars and Jupiter.
- Comets and icy bodies: frozen leftovers in the Kuiper belt beyond Neptune and, much further out, the Oort cloud.
Two kinds of planet
The four inner planets (Mercury, Venus, Earth, Mars) are small and rocky. The four outer planets are giants:
- Gas giants (Jupiter, Saturn): mostly hydrogen and helium, with no solid surface to stand on.
- Ice giants (Uranus, Neptune): smaller, with more water, ammonia and methane "ices" inside.
Close to the young Sun it was too hot for ice, so only rock and metal could clump together. Further out, ice was plentiful too, so planets grew big enough to pull in huge amounts of gas.
How far apart?
Distances in space are so large that astronomers use the astronomical unit (AU): the average distance from Earth to the Sun, 149.6 million km. Light from the Sun takes about 8 minutes 20 seconds to reach us.
| Planet | Average distance from the Sun |
|---|---|
| Mercury | 57.9 million km (0.39 AU) |
| Venus | 108.2 million km (0.72 AU) |
| Earth | 149.6 million km (1.00 AU) |
| Mars | 228.0 million km (1.52 AU) |
| Jupiter | 778.5 million km (5.20 AU) |
| Saturn | 1,432.0 million km (9.57 AU) |
| Uranus | 2,867.0 million km (19.16 AU) |
| Neptune | 4,515.0 million km (30.18 AU) |
Notice how the gaps grow: Neptune is about 30 times further from the Sun than Earth.
How it formed
About 4.6 billion years ago (the Sun's age: about 4.6 billion years), a cloud of gas and dust collapsed under its own gravity. Most of it fell to the centre and became the Sun. The rest spun into a flat disc, where grains stuck together into rocks, rocks into planet-building blocks, and those into planets. That is why the planets all go around the Sun the same way, in nearly the same flat plane. The asteroids and comets are leftovers that never became planets.
Facts: NASA NSSDCA fact sheets, JPL Solar System Dynamics and the JPL Small-Body Database, as of 6 October 2026. Planet maps: Solar System Scope (CC BY 4.0), from NASA data. Moon maps: USGS Astrogeology (public domain) from Galileo, Cassini, Voyager and New Horizons.
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