Welcome to Space: Exploring the Solar System!
On a clear night in a Country Park, you may spot the Moon, bright planets such as Venus and Jupiter, and many stars. In this chapter you will learn why the Sun is a star, meet the eight planets, compare the Earth, the Moon and other planets, and find out where life might be possible. Then you will discover our country's space achievements and how Hong Kong scientists have helped.
1. The Sun: Our Very Own Star
The Sun is a star
A star is a huge, very hot ball of glowing gas that makes its own light. The Sun is the star nearest to the Earth. Other stars look like tiny dots only because they are extremely far away.
The Sun is mainly hydrogen and helium. Nuclear reactions deep inside it release huge amounts of energy, and its surface is about 5,500 °C.
Light and heat for the Solar System
The Sun gives out light energy and thermal energy to all members of the Solar System. The energy travels through almost-empty space as radiation. Sunlight takes about 8 minutes to reach the Earth, about 150 million km away. It warms the Earth and drives the water cycle (see "The water cycle").
Planets and moons do not make their own light. We see them because they reflect sunlight, like a road sign reflecting car headlights.
Key takeaway: The Sun is a star. It gives light energy and thermal energy to every member of the Solar System.
2. Meet the Eight Planets
What is in the Solar System?
The Solar System is the Sun plus everything that travels around it: eight planets, their moons, and smaller objects such as dwarf planets, asteroids and comets. The Sun's gravity keeps them moving in paths called orbits. A planet orbits the Sun; a moon (natural satellite) orbits a planet.
The planets in order
From the nearest to the Sun:
1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune
Memory aid: "My Very Educated Mother Just Served Us Noodles."
Rocky planets and giant planets
The four inner planets (Mercury, Venus, Earth, Mars) are small rocky planets with solid surfaces. The four outer planets are giant planets: Jupiter and Saturn are gas giants (mostly hydrogen and helium), while Uranus and Neptune are ice giants. Giant planets have no solid surface.
Key takeaway: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The inner four are rocky; the outer four are giants.
3. Comparing the Earth, the Moon and Other Planets
What do scientists compare?
1. Composition of atmosphere – which gases surround it.
2. Average surface temperature
3. Presence of water – and in which state (ice, liquid or vapour).
4. Mass – how much matter it contains.
5. Gravity – how strongly it pulls objects towards it.
6. Distance from the Sun
7. Period of revolution – time to travel once around the Sun (its "year").
8. Period of rotation – time to spin once on its own axis (its "day").
Memory aid: Rotation is a dancer spinning on the spot; revolution is a runner going round a track.
Planet fact file
Approximate values; mass and gravity are compared with the Earth's, which are taken as 1.
Mercury – 58 million km from the Sun; year 88 days; rotation 59 days; almost no atmosphere; 430 °C by day, −180 °C at night; no liquid water; mass 0.06; gravity 0.4.
Venus – 108 million km; year 225 days; rotation 243 days; very thick atmosphere, about 96% carbon dioxide; about 460 °C, the hottest planet; no liquid water; mass 0.8; gravity 0.9.
Earth – 150 million km; year 365 days; rotation 24 hours; mainly nitrogen and oxygen; about 15 °C; lots of liquid water; mass 1; gravity 1.
Mars – 228 million km; year 687 days; rotation 24.6 hours; very thin atmosphere, about 95% carbon dioxide; about −65 °C; water ice at the poles and underground; mass 0.1; gravity 0.4.
Jupiter – 778 million km; year 12 Earth years; rotation 10 hours (fastest); mainly hydrogen and helium; about −110 °C; no solid surface; mass 318 (largest); gravity 2.5.
Saturn – 1,430 million km; year 29 years; rotation 11 hours; mainly hydrogen and helium; about −140 °C; mass 95; gravity 1.1.
Uranus – 2,870 million km; year 84 years; rotation 17 hours; hydrogen, helium, some methane; about −195 °C; mass 15; gravity 0.9.
Neptune – 4,500 million km; year 165 years; rotation 16 hours; hydrogen, helium, some methane; about −200 °C; mass 17; gravity 1.1.
The Moon: Earth's closest neighbour
The Moon is about 384,000 km from the Earth. It revolves around the Earth about once every 27 days and rotates once in the same time, so the same side always faces us.
The Moon has almost no atmosphere, so its surface reaches about 120 °C in sunlight and −170 °C at night. There is no liquid water, only some water ice in dark craters near its poles. Its mass is about 1% of the Earth's, and its gravity is about one-sixth of the Earth's.
Spotting patterns
1. Further from the Sun → longer year. Distant planets have a longer path and move more slowly.
2. Further from the Sun → usually colder. The exception is Venus: it is hotter than Mercury because its thick carbon dioxide atmosphere traps heat (a strong greenhouse effect).
3. More mass → usually stronger gravity. Jupiter has the most mass and the strongest gravity.
4. An atmosphere keeps temperatures steady. Airless Mercury and the Moon have scorching days and freezing nights.
Key takeaway: Distance from the Sun, atmosphere and mass explain many differences between the planets.
4. Could Life Survive There?
What living things need
1. Liquid water – the life processes in cells happen in water.
2. A suitable temperature – not too hot, not too cold, so water stays liquid.
3. An atmosphere – supplies gases such as oxygen and carbon dioxide, keeps temperatures steady and blocks some harmful radiation.
Life also needs an energy source, such as sunlight.
Why the Earth is "just right"
The Earth is not too close to the Sun (like Venus) and not too far (like the outer planets). This region, where liquid water can exist on a planet's surface, is the habitable zone, nicknamed the "Goldilocks zone". The Earth is the only place where we know life exists.
Mars: a possible candidate
Mars is the main place scientists search for signs of life:
1. It has water ice at its poles and underground.
2. Dried-up river valleys show that liquid water flowed there long ago.
3. Its day (24.6 hours) is like ours, and it has seasons.
But its atmosphere is thin and mostly carbon dioxide, it is very cold (about −65 °C), and harmful radiation reaches the ground. So far, no life has been found on Mars.
Elsewhere is worse: Mercury and the Moon have almost no air, Venus is far too hot, and the giant planets have no solid surface.
Did you know? Jupiter's moon Europa is thought to have an ocean of liquid water under its icy crust, so it is another place to search for life.
Key takeaway: Liquid water, a suitable temperature and an atmosphere are key conditions for life. Mars is a candidate, but no life has been found there yet.
5. Our Country's Space Achievements
China Manned Space Program and Shenzhou
The China Manned Space Program began in 1992. On 15 October 2003, astronaut Yang Liwei flew into space on Shenzhou-5, making China the third country to send a person into space on its own, after the former Soviet Union and the United States. Shenzhou spacecraft are launched on Long March rockets.
Tiangong Space Station
The Tiangong Space Station orbits about 400 km above the Earth. Its core module Tianhe was launched in 2021, and the Wentian and Mengtian laboratory modules completed it in 2022. Crews of three usually stay about six months, doing experiments such as growing rice in space.
In the Tiangong Classroom, astronauts give live science lessons from the station, and students in Hong Kong have joined in.
Did you know? Gravity in Tiangong is still nearly 90% as strong as on the ground! Astronauts float because they and the station are falling around the Earth together, so they feel weightless.
Chang'e: exploring the Moon
Named after Chang'e, the Moon goddess in Chinese legend, China's Lunar Exploration Program follows three steps: orbit, land and return.
Chang'e-4 (2019): first ever soft landing on the far side of the Moon.
Chang'e-5 (2020): brought about 1.7 kg of lunar samples back to Earth.
Chang'e-6 (2024): brought back the first ever samples from the far side.
Tianwen-1: journey to Mars
Tianwen-1 was launched in 2020. In 2021 it orbited Mars, landed, and released the Zhurong rover, all in China's first Mars mission.
Key takeaway: Major achievements include Shenzhou crewed flights, the Tiangong Space Station, the Chang'e lunar missions and Tianwen-1 to Mars.
6. Hong Kong Scientists in National Space Research
Lunar surface sampling
A team at The Hong Kong Polytechnic University helped design and build the Surface Sampling and Packing System for Chang'e-5, which collected lunar soil from the surface and packed it for the trip home.
Deep space exploration
Hong Kong university teams have also helped develop instruments, such as cameras, for Moon and Mars missions, and have helped analyse images to map the ground and study possible landing sites.
Space agriculture research
Astronauts on long journeys will need to grow food. Scientists, including researchers in Hong Kong, study how weightlessness and space radiation affect plants and how crops could be grown in space.
Key takeaway: Hong Kong scientists contribute to lunar surface sampling, deep space exploration and space agriculture research.
Common Mistakes
Common mistake 1: "The Sun is a planet." Correct: The Sun is a star; it makes its own light.
Common mistake 2: "The Moon gives out its own light." Correct: The Moon reflects sunlight.
Common mistake 3: "Mercury is the hottest planet." Correct: Venus is hottest, because its thick carbon dioxide atmosphere traps heat.
Common mistake 4: "On the Moon my mass gets smaller." Correct: Your mass stays the same; you only weigh about one-sixth as much.
Quick Review
1. The Sun is a star giving light energy and thermal energy to the Solar System.
2. Planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
3. Compare planets by atmosphere, temperature, water, mass, gravity, distance, revolution and rotation.
4. Life needs liquid water, a suitable temperature and an atmosphere; Mars is a candidate.
5. Our country: Shenzhou-5, Tiangong, Chang'e lunar samples and Tianwen-1, with help from Hong Kong scientists.
Great job, space explorer! Keep looking up!