Coaching Centre
The solar system to scale
The Sun, the eight planets, dwarf planets, moons, asteroids and comets; rocky inner planets and giant outer planets; the astronomical unit and converting kilometres to AU; light travel times; and a true-scale model of the solar system.
Most pictures of the solar system squash the planets in a neat row, nearly touching. A true-scale picture is almost impossible to print. If the Sun were a basketball sitting on the try line at one end of a rugby field, Earth would be a grain of sand 26 metres away, Jupiter a cherry tomato more than 130 metres away, and Neptune a small marble about 800 metres away. Almost all of the solar system is empty space.
The Sun's family
The solar system is the Sun and everything held in orbit around it by its gravity. The Sun contains about 99.8% of all the mass in the solar system.
| Planet | Average distance from Sun (million km) | Distance (AU) | Type | Known moons |
|---|---|---|---|---|
| Mercury | 58 | 0.4 | Rocky | 0 |
| Venus | 108 | 0.7 | Rocky | 0 |
| Earth | 150 | 1.0 | Rocky | 1 |
| Mars | 228 | 1.5 | Rocky | 2 |
| Jupiter | 779 | 5.2 | Gas giant | more than 90 |
| Saturn | 1434 | 9.6 | Gas giant | more than 270 |
| Uranus | 2873 | 19.2 | Ice giant | 29 |
| Neptune | 4495 | 30.0 | Ice giant | 16 |
- The four inner planets are small and made of rock and metal.
- Between Mars and Jupiter lies the asteroid belt, millions of rocky lumps left over from when the planets formed.
- The four outer planets are giants. Jupiter and Saturn are mostly hydrogen and helium gas; Uranus and Neptune contain more water, ammonia and methane ices. All four have rings.
- Beyond Neptune is the Kuiper belt of icy bodies, including the dwarf planets Pluto, Eris, Haumea and Makemake. (Ceres, in the asteroid belt, is a dwarf planet too.)
- Comets are lumps of ice and dust on long, stretched orbits. When one comes close to the Sun, its ice turns to gas and forms a glowing tail that points away from the Sun.
Moon counts keep changing as telescopes find more small moons, so the numbers in the table are only a snapshot.
Measuring space
Kilometres are too small a unit for the solar system. Astronomers use the astronomical unit (AU), the average distance from the Earth to the Sun: about 150 million km.
Distance in AU = distance in km ÷ 150 million km.
Saturn: 1434 million km ÷ 150 million km = 9.6 AU. Saturn is almost 10 times further from the Sun than we are.
Pluto averages about 5900 million km from the Sun: 5900 ÷ 150 ≈ 39 AU.
For distances to stars, even the AU is too small, so we use the light-year: the distance light travels in a year, about 9.5 million million km. Light from the Sun takes about 8 minutes 20 seconds to reach us, so you always see the Sun as it was 8 minutes ago. The nearest star after the Sun, Proxima Centauri, is 4.2 light-years away, which is about 270 000 AU.
- Start with the inner planets, then zoom out to the whole solar system. What happens to Mercury, Venus, Earth and Mars?
- Pick Mars, then Neptune. How long would light take to get there? How long would a car take?
- In the basketball model, how far away is Neptune? Could you fit it in your school grounds?
Test yourself
One astronomical unit (AU) is
About 150 million km.
Jupiter is 779 million km from the Sun. In AU this is about
779 ÷ 150 ≈ 5.2.
Which is a dwarf planet?
Pluto was reclassified in 2006.
Light from the Sun takes about 8 minutes to reach Earth. This means
The light we see left the Sun 8 minutes earlier.
The asteroid belt is found between
Between the inner and outer planets.
Sort the objects.
Order the planets from closest to the Sun to furthest.
Uranus is about 2873 million km from the Sun. (a) Convert this to AU. (b) Explain why astronomers use AU rather than kilometres for distances in the solar system.
(a) 2873 million km ÷ 150 million km = about 19.2 AU. (b) Distances in the solar system are so large that in kilometres they become huge numbers that are hard to write and compare. In AU the numbers are small and easy to compare with the Earth's distance: for example, Uranus is about 19 times further from the Sun than we are.