The solar system and the Milky Way
What orbits what
Our solar system is one star, the Sun, and everything that orbits it.
- Planets: eight of them, from Mercury out to Neptune, all orbiting the Sun.
- Dwarf planets: smaller bodies such as Pluto and Ceres that also orbit the Sun.
- Moons: natural satellites that orbit planets. The Earth has one; Jupiter and Saturn have dozens.
- Artificial satellites: made and launched by people, such as the International Space Station, weather and GPS satellites.
A satellite is anything that orbits a larger body. Gravity keeps every one of these orbits going.
The solar system formed from a nebula, a cloud of dust and gas pulled together by gravity. Most of it became the Sun; the rest formed the planets.
From the Moon to the universe
Each thing in this list sits inside the next:
- the Moon orbits the Earth
- the Earth is one of eight planets orbiting the Sun
- the Sun is one of billions of stars in our galaxy, the Milky Way
- the Milky Way is one of billions of galaxies in the universe
A galaxy is a huge group of billions of stars held together by gravity. Distances between stars are so big that astronomers measure them in light-years: the distance light travels in one year.
1 light-year = 9.46 × 10¹⁵ m
Questions give you this value when you need it. The nearest star to the Sun is just over 4 light-years away.
Big numbers in standard form
Space distances have lots of zeros, so write them in standard form: one digit, the decimal point, then × 10 to a power.
- The Earth is 150 000 000 km from the Sun
- 1 km = 1000 m, so that is 150 000 000 000 m
- move the decimal point 11 places: 1.50 × 10¹¹ m
- light takes 1.50 × 10¹¹ ÷ 3.00 × 10⁸ = 500 s to reach us, about 8.3 minutes
Change km into m before you use the speed of light in m/s. A million km is 10⁹ m, not 10⁶ m.
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