The solar system and the Milky Way

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.

Each thing in this list sits inside the next:

1. the Moon orbits the Earth
2. the Earth is one of eight planets orbiting the Sun
3. the Sun is one of billions of stars in our galaxy, the Milky Way
4. 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.

Space distances have lots of zeros, so write them in standard form: one digit, the decimal point, then × 10 to a power.

1. The Earth is 150 000 000 km from the Sun
2. 1 km = 1000 m, so that is 150 000 000 000 m
3. move the decimal point 11 places: 1.50 × 10¹¹ m
4. 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.