Science notes, chapter 12
Solar system
Textbook Science Form 2 (DLP), pages 250–267
What this chapter covers
- Compare the distances between the Sun and the planets using astronomical units (A.U.) and light years (ly) (12.1.1)
- Compare and contrast the planets of the solar system and the Earth using a table of their characteristics (12.1.2)
- Explain relationships between planet characteristics (temperature, gravity, orbit time), including anomalies (12.1.3)
- Reason about hypothetical situations, such as the Earth rotating slower or stopping (12.1.4)
- Justify why the Earth is the most suitable planet for life (12.1.5)
12.1The solar system and distances in space
The solar system is the Sun and everything that moves around it: eight planets and their moons, plus dwarf planets, asteroids and comets. In 2006 Pluto was reclassified as a dwarf planet.
Solar system: textbook Figure 12.1, p. 252.
Memory aid for the order: My Very Energetic Mother Just Served Us Noodles. The four inner planets are small and rocky; the four outer planets are giant gas planets.
Two units for huge distances
Distances between the planets and the Sun are so great that kilometres become awkward. Two bigger units are used.
Astronomical unit (A.U.) is the average distance between the Earth and the Sun.
1 A.U. = 1.5 × 108 km
Light year (ly) is the distance travelled by light in one year. Light moves at 300 000 km every second.
1 ly = 9.5 × 1012 km
1 A.U. = 1.5 × 108 km
Light year (ly) is the distance travelled by light in one year. Light moves at 300 000 km every second.
1 ly = 9.5 × 1012 km
| To find | Formula |
|---|---|
| Distance in A.U. | distance in km ÷ (1.5 × 108 km) |
| Distance in ly | distance in km ÷ (9.5 × 1012 km) |
| Distance in km from ly | distance in ly × 9.5 × 1012 km |
Worked example: Mars
Mars is 2.28 × 108 km from the Sun.In A.U.: 2.28 × 108 ÷ 1.5 × 108 = 1.52 A.U.
In ly: 2.28 × 108 ÷ 9.5 × 1012 = 2.4 × 10−5 ly
Worked example: a star in ly
A star is 4.37 ly away. In km: 4.37 × 9.5 × 1012 = 4.15 × 1013 km.In A.U.: 4.15 × 1013 ÷ 1.5 × 108 = 2.77 × 105 A.U.
Dividing when you should multiply. Going from km to A.U. or ly, you divide. Going from ly to km, you multiply. Also keep the powers of ten: 108 ÷ 1012 = 10−4, so the ly answer is a very small number.
| Planet | Distance (km) | A.U. | ly |
|---|---|---|---|
| Mercury | 5.79 × 107 | 0.39 | 6.1 × 10−6 |
| Venus | 1.08 × 108 | 0.72 | 1.1 × 10−5 |
| Earth | 1.50 × 108 | 1.0 | 1.58 × 10−5 |
| Mars | 2.28 × 108 | 1.52 | 2.4 × 10−5 |
| Jupiter | 7.78 × 108 | 5.19 | 8.2 × 10−5 |
| Saturn | 1.43 × 109 | 9.5 | 1.51 × 10−4 |
| Uranus | 2.87 × 109 | 19.1 | 3.0 × 10−4 |
| Neptune | 4.5 × 109 | 30 | 4.7 × 10−4 |
Table values: textbook Activity 12.1, p. 253 (completed and rounded).
Uranus is 2.87 × 109 km from the Sun. How many A.U. is this?
Show answerHide answer
2.87 × 109 ÷ 1.5 × 108 = 19.1 A.U.
12.2Characteristics of the planets
| Planet | Key facts |
|---|---|
| Mercury | Closest to the Sun and the smallest planet. No atmosphere, so the sky looks dark and the surface is covered in craters like the Moon. |
| Venus | The "greenhouse" planet: its atmosphere is rich in carbon dioxide. Rotates east to west, so the Sun rises in the west. About 460 °C. |
| Earth | The only planet with living things. Atmosphere protects it from solar wind, ultraviolet rays and radiation. More than 71% of the surface is water. |
| Mars | The "Red Planet", with two moons (Phobos and Deimos). Polar caps of frozen water and carbon dioxide. |
| Jupiter | The largest planet, mass almost 320 times the Earth's. Its strong gravity can deflect large objects away from the Earth. |
| Saturn | Second largest, a giant gas planet with a ring system of mostly ice. Its moon Titan is bigger than Mercury. |
| Uranus | Giant gas planet with thin, dark rings. Its axis is tilted so much that it rotates on its side. Takes 84 years to orbit the Sun. |
| Neptune | The eighth planet, a giant gas planet. Takes almost 165 years to orbit the Sun. |
Comparing the planets
| Planet | Mass (× Earth) | Density (g cm−3) | Gravity (m s−2) | Surface temperature (°C) | Orbit time | Moons |
|---|---|---|---|---|---|---|
| Mercury | 0.055 | 5.4 | 3.7 | 167 | 88 days | 0 |
| Venus | 0.815 | 5.2 | 8.87 | 457 | 224.7 days | 0 |
| Earth | 1 | 5.5 | 9.8 | 14 | 365 days | 1 |
| Mars | 0.107 | 3.9 | 3.71 | −55 | 687 days | 2 |
| Jupiter | 317.8 | 1.3 | 24.79 | −153 | 11.9 years | 67 |
| Saturn | 95.159 | 0.7 | 10.44 | −185 | 29.5 years | 62 |
| Uranus | 14.536 | 1.27 | 8.69 | −214 | 84 years | 27 |
| Neptune | 17.147 | 1.6 | 11.15 | −225 | 164.8 years | 14 |
Data: textbook Table 12.2, pp. 256–257 (selected rows).
Two groups. Inner planets (Mercury to Mars): small, rocky, high density, hard surface, few or no moons. Outer planets (Jupiter to Neptune): huge, low density, covered in gas with no hard surface, many moons.
Name the planet that rotates from east to west and state the effect seen on its surface.
Show answerHide answer
Venus. The Sun would rise in the west and set in the east.
12.3Relationships between planet characteristics
Temperature and distance from the Sun
You might expect the nearest planet to be the hottest. In reality the atmosphere matters too.
| Planet | Why its temperature is what it is |
|---|---|
| Mercury (no atmosphere) | No air to hold heat. The side facing the Sun is above 427 °C and the dark side falls to −173 °C. |
| Venus (thick atmosphere) | Mostly carbon dioxide traps heat (greenhouse effect), reaching 462 °C. It is hotter than Mercury although it is further from the Sun. This is an anomaly. |
| Earth | Clouds reflect some sunlight; the atmosphere traps some heat. The temperature suits life. |
| Mars | Atmosphere is very thin (less than 1/100 of Earth's pressure), so it holds little heat: −143 °C to 35 °C. |
| Jupiter to Neptune | Far from the Sun and receive little sunlight, so they are extremely cold. |
Mass, density and gravity
- The gravity on the surface of a planet depends on its mass and density.
- Mercury and Mars have less mass than the Earth, so their gravity is lower (about 3.7 m s−2).
- Venus has almost the same mass as the Earth, so its gravity is almost the same (8.87 against 9.8 m s−2).
- Jupiter has an enormous mass, so its gravity (24.79 m s−2) is much higher than the Earth's.
- Saturn, Uranus and Neptune have a lot more mass than the Earth, but they are low-density gas planets, so their gravity is only close to the Earth's (and Uranus is lower).
An object released near the Earth's surface falls with an acceleration of 9.8 m s−2.
Distance from the Sun and orbit time
The further a planet is from the Sun, the longer it takes to orbit the Sun. Mercury (nearest) takes 88 days; Neptune (furthest) takes 164.8 years.
Rotation
All planets rotate on their axes at different tilts. Most rotate from west to east; Venus rotates east to west and Uranus rotates on its side (axis tilted 98°).
Planet tilts: textbook Figure 12.3, p. 260.
Thinking that "closest to the Sun means hottest" or "biggest mass means biggest gravity" always works. Check for anomalies: Venus is hotter than Mercury (greenhouse effect), and Saturn has a huge mass, but its gravity is much lower than Jupiter's because its mass is far smaller and its density is very low.
Why is the surface gravity of Saturn lower than that of Jupiter although Saturn has a huge mass?
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Saturn's mass is much smaller than Jupiter's (95 against 318 times the Earth's) and it has a very low density (0.7 g cm−3), so its gravity is lower.
12.4The Earth and its Moon; "what if" situations
A natural satellite is an object that moves around a planet in its own orbit. The Moon is the only natural satellite of the Earth.
- The Moon rotates on its axis and orbits the Earth in about the same time (about 27 days), so the same side of the Moon always faces the Earth.
- The Earth is about four times bigger than the Moon, so the Earth would look much larger in the Moon's sky.
If the Earth rotated slower or stopped
- The length of day and night would be longer in different parts of the Earth.
- The side facing the Sun would become hotter and drier, with more deserts.
- The side facing away from the Sun would get colder because it receives no sunlight.
- The high and low tides would change.
Give one reason why we see the Sun rise in the east on the Earth.
Show answerHide answer
The Earth rotates on its axis from west to east.
12.5The Earth: the planet for life
The Earth is the only planet known to have life. It has these features.
- Plenty of water for the life processes.
- Enough oxygen in the air for respiration.
- A suitable temperature range, not too hot or too cold.
- Sunlight for plants to carry out photosynthesis.
- An atmosphere that blocks harmful ultraviolet rays.
- Suitable gravity that keeps objects and the atmosphere from floating away.
Why not Venus, the "twin" of the Earth?
Venus has a similar size and density, but its atmosphere is mostly carbon dioxide and its temperature is about 460 °C, with no liquid water. Living things could not survive there.Ecological footprint
The ecological footprint measures how much land and water humans use for food, water, shelter and energy, and how much the Earth can absorb our waste and renew its resources.
If our footprint is larger than the Earth can renew, resources will run out. We can reduce it by recycling, saving energy and water, and wasting less food.
State two features of the Earth that make it suitable for living things.
Show answerHide answer
Any two: liquid water; oxygen for respiration; suitable temperature; sunlight; atmosphere that blocks ultraviolet rays.
Summary
| Remember | Key fact |
|---|---|
| 1 A.U. | 1.5 × 108 km, the Earth–Sun distance |
| 1 ly | 9.5 × 1012 km, the distance light travels in a year |
| Order of planets | Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune |
| Hottest planet | Venus (greenhouse effect), not Mercury |
| Orbit time | Longer for planets further from the Sun |
| Rotation | West to east, except Venus (east to west) and Uranus (on its side) |
| Gravity | Depends on mass and density; highest on Jupiter |
| Earth for life | Water, oxygen, temperature, sunlight, atmosphere, gravity |