The solar systemIB MYP Physics: Revision notes

The Sun is a star at the centre of the solar system. Eight planets orbit it. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

• Rocky planets (Mercury, Venus, Earth, Mars) are small, dense and have solid surfaces. They are closest to the Sun.
• Gas giants (Jupiter, Saturn, Uranus, Neptune) are much larger and made mostly of gases such as hydrogen and helium. They have no solid surface, and have many moons and rings.

• Dwarf planets such as Pluto and Ceres orbit the Sun but are too small to have cleared their orbit.
• Moons are natural satellites that orbit planets. The Earth has one; Jupiter has dozens.
• Asteroids are rocky lumps. Most are in the asteroid belt between Mars and Jupiter.
• Comets are balls of ice and dust. When a comet gets near the Sun its ice evaporates, forming a bright tail.

Gravity from the Sun keeps the planets in orbit. Planet orbits are nearly circular. Comets have very elliptical (long, thin) orbits that take them close to the Sun and far beyond Neptune.

The time to complete one orbit is the orbital period. The further a planet is from the Sun, the longer its period, because it has a longer path to travel and the Sun's gravity is weaker there, so it moves more slowly. Earth takes 1 year; Neptune takes 165 years.

A satellite is an object that orbits a larger one. The Moon is a natural satellite. Artificial satellites are built by people and launched into orbit. They are held in orbit by gravity. The closer the orbit, the faster the satellite moves and the shorter its period.

• Communication: television, phone and internet signals are relayed.
• GPS: several satellites send signals that a receiver uses to find its position.
• Weather and Earth observation: images of clouds, oceans and land.

A geostationary orbit is about 36 000 km above the equator with a period of 24 hours, so the satellite stays above one point. It suits communications. A low orbit (a few hundred kilometres) is much closer and orbits in about 90 minutes. It suits weather monitoring and imaging, because the satellite sees more detail and covers the whole Earth as it goes over the poles.

We explore space using telescopes, orbiting satellites, robotic probes and rovers, and crewed missions such as the International Space Station.

• Robots: cheaper and can go to dangerous places, but cannot adapt like humans.
• Crewed missions: very expensive and risky, but astronauts can make decisions and do complex work.

Exploration gives new knowledge and technology, but costs a great deal of money, so choices must be weighed.