What is the solar system?
The solar system is the Sun and everything held in orbit by its gravity: eight planets and their moons, dwarf planets such as Pluto, asteroids, comets and dust. The Sun contains about 99.8 % of all the mass, which is why everything else orbits it. The planets all travel in the same direction and in nearly the same flat plane, a clue to how the solar system formed.
The eight planets
The four inner, rocky planets are Mercury, Venus, Earth and Mars. They are small and dense, with solid surfaces. The four outer giant planets are Jupiter and Saturn, the gas giants made mostly of hydrogen and helium, and Uranus and Neptune, the ice giants with large amounts of water, ammonia and methane ices. All four giants have rings and many moons. The asteroid belt between Mars and Jupiter separates the two groups.
| Planet | Distance (AU) | Diameter (Earth = 1) | Year |
|---|---|---|---|
| Mercury | 0.39 | 0.38 | 88 days |
| Venus | 0.72 | 0.95 | 225 days |
| Earth | 1.00 | 1.00 | 365 days |
| Mars | 1.52 | 0.53 | 1.9 years |
| Jupiter | 5.20 | 11.2 | 11.9 years |
| Saturn | 9.54 | 9.4 | 29.4 years |
| Uranus | 19.2 | 4.0 | 84 years |
| Neptune | 30.1 | 3.9 | 165 years |
A sense of scale
Most pictures of the solar system, including the “Fit everything” view here, are not to scale, because a true-scale picture would show almost nothing. If the Sun were the size of a football (22 cm), Earth would be a peppercorn about 24 m away, Jupiter a large marble about 120 m away, and Neptune a small bead nearly 700 m away. Light, the fastest thing in the universe, takes about 8 minutes to reach us from the Sun and over 4 hours to reach Neptune.
How the planets move
Gravity from the Sun pulls each planet towards it, while the planet’s sideways speed carries it forward, so it falls around the Sun in an orbit. Planets closer to the Sun feel stronger gravity and move faster. Johannes Kepler discovered that the square of a planet’s orbital period is proportional to the cube of its distance (Kepler’s third law): Jupiter is about 5.2 times further from the Sun than Earth, and 5.2³ ≈ 141 ≈ 11.9², so its year is about 11.9 Earth years. Real orbits are slightly stretched ellipses; this model uses circles, which is a good approximation for most planets.
How it formed
About 4.6 billion years ago a cloud of gas and dust collapsed under its own gravity. Most of the material fell to the centre and became the Sun, while the rest flattened into a spinning disc. Near the hot young Sun only rock and metal could stay solid, forming small rocky planets. Further out, beyond the “frost line”, ices could also form, so planets grew big enough to pull in huge amounts of hydrogen and helium gas.
Beyond the planets
Past Neptune lies the Kuiper Belt, home to icy dwarf planets such as Pluto and Eris. Much further out, the Oort Cloud is thought to surround the solar system in a giant shell of icy bodies and is the source of long-period comets. Pluto was reclassified as a dwarf planet in 2006 because it has not cleared its orbit of other objects.
Key terms
- Astronomical unit (AU)
- The average Earth–Sun distance, about 149.6 million km.
- Orbital period
- The time for one complete orbit: a planet’s year.
- Dwarf planet
- A round body orbiting the Sun that has not cleared its orbit of other objects.
Common misconceptions
- “The planets line up neatly in a row.” They are spread around their orbits; true alignments are rare.
- “Space between the planets is small.” The planets would fit between Earth and the Moon several times over.
- “Summer happens when Earth is closest to the Sun.” Seasons are caused by Earth’s tilt; Earth is actually closest in January.
Key takeaways
- The Sun holds almost all the mass, and its gravity keeps the planets in orbit.
- Four small rocky inner planets, four giant outer planets.
- Further planets orbit more slowly and have much longer years (Kepler’s third law).
- The solar system is enormous and mostly empty space.
Model note: orbits are drawn as circles in a single plane. Positions start from today’s date using each planet’s average position, so they can differ from the real sky by a few degrees. Moon counts keep rising as new small moons are discovered.