The Solar System

The Sun is a medium-sized star at the centre, holding everything in orbit through gravity. About 99.8% of the Solar System's mass is in the Sun.

The 8 planets in order from the Sun (mnemonic: My Very Educated Mother Just Served Us Noodles):

1. Mercury — small, rocky, no atmosphere
2. Venus — rocky, thick CO₂ atmosphere, hottest planet
3. Earth — rocky, has liquid water and life
4. Mars — rocky, thin atmosphere, polar ice
5. Jupiter — gas giant, largest, has many moons
6. Saturn — gas giant, famous rings
7. Uranus — ice giant, tilted on its side
8. Neptune — ice giant, strongest winds

Between Mars and Jupiter lies the asteroid belt — rocky debris that never formed a planet. Beyond Neptune is the Kuiper belt (where Pluto lives) and further out the Oort cloud, source of long-period comets.

Without gravity, every planet would fly off in a straight line. The Sun's gravitational pull constantly bends each planet's path into an almost-circular orbit. Same for moons around planets, and artificial satellites around Earth.

A useful fact: closer orbits move faster. Mercury orbits in 88 days; Earth in 365 days; Neptune in 165 years. This is because the closer you are to the Sun, the stronger its gravitational pull — the faster you need to move to balance it and stay in orbit.

For circular orbits, three relationships are worth noting:

• Closer → stronger gravity → larger required orbital speed.
• Closer → shorter orbital period.
• Heavier central body → stronger gravity → faster orbital speeds at any given radius.

For Earth, low-orbit satellites (like the ISS at ~400 km) zip around in ~90 minutes; geostationary satellites at 35 786 km take 24 hours to match Earth's rotation.

Natural satellites are moons. The 8 planets between them have around 200 known moons. Earth's Moon is large for an inner planet; Jupiter has 95+, including the four big Galilean moons (Io, Europa, Ganymede, Callisto).

Artificial satellites are objects humans have put in orbit. Their orbits depend on the job:

• Low Earth orbit (LEO), ~400–2 000 km: Imaging satellites, the ISS, Starlink communications. Short orbital period (~90 minutes).
• Polar orbit: passes near both poles, scanning the whole Earth strip by strip every day. Used for weather and Earth observation.
• Geostationary orbit, 35 786 km above the equator: 24-hour period, so the satellite appears fixed in the sky. Used for satellite TV and weather forecasting.
• GPS orbit, ~20 000 km: 12-hour period; a constellation of 24+ satellites allows position-finding anywhere on Earth.

Modern life depends on artificial satellites: GPS navigation in your phone, mobile signals carried via satellite in remote areas, weather forecasting, climate monitoring, internet access, and astronomical telescopes (Hubble, James Webb).