24.1 - The Solar System & the Universe

The Solar System and the objects in it

• The Solar System is the Sun together with everything that orbits it.
• At its centre is the Sun, our nearest star: a hot sphere of gas.

Objects that orbit the Sun

• Planets - Large bodies that orbit a star. To count as a planet, a body must be massive enough that its gravity has cleared its orbit of other objects (apart from its own moons).
• Dwarf planets - Planet-like bodies that orbit the Sun but aren't big enough to count as planets, such as Pluto.
• Moons - Natural satellites: bodies that orbit a planet, the way planets orbit the Sun.
• Artificial satellites - Objects humans have built and put into orbit, most of which orbit the Earth.
• Asteroids - Lumps of rock and metal that orbit the Sun, mostly in the asteroid belt between Mars and Jupiter.
• Comets - Balls of rock, dust and ice that orbit the Sun, often on long, stretched-out paths.

A satellite is any object that orbits a larger one, so both moons (natural satellites) and artificial satellites are types of satellite.

The eight planets in order from the Sun

• There are eight planets in the Solar System.
• In order of distance from the Sun, moving outwards: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
• The four inner planets (Mercury, Venus, Earth, Mars) orbit closest to the Sun.
• The four outer planets (Jupiter, Saturn, Uranus, Neptune) lie beyond the asteroid belt.
• All eight orbit the Sun in roughly the same plane.

Galaxies and the scale of the Universe

• A galaxy is a huge collection of billions of stars, held together by gravity.
• Our galaxy is the Milky Way. The Sun is just one of its many billions of stars, lying about halfway along one of its spiral arms.
• Other stars in the Milky Way are vastly further from Earth than the Sun is, which is why they appear as tiny points of light while the Sun looks large and bright.
• Distances between neighbouring stars are typically hundreds of thousands of times greater than the distance from the Earth to the Sun.

The Universe

• The Milky Way is just one of billions of galaxies. Together, all of these galaxies make up the Universe.
• Galaxies are separated by even larger distances than the stars within them, so the Universe is mostly empty space.
• The ladder of scale:
  • The Universe contains billions of galaxies.
  • Each galaxy contains billions of stars.
  • Around a star like the Sun lies a planetary system.

Units of astronomical distance

• Distances between stars and galaxies are so vast that measuring them in kilometres is impractical.
• A light-year is the distance that light travels through space in one year. It's used for the large gaps between stars and galaxies.
• The astronomical unit (AU) is the mean distance from the Earth to the Sun. It's used for distances within a planetary system.