Planets in the Solar System - Lecture Notes Review
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Planets in the Solar System (Overview per planet)
Mercury
Closest planet to the Sun: distance from Sun ≈ 57.9 imes 10^{6} ext{ km}.
Smallest planet in the solar system. Its diameter is about 0.60 of Earth's diameter (Mercury: ~4879 km; Earth: ~12756 km).
Its surface resembles the Moon’s: cratered terrain with cliffs tens of kilometers high.
Absence of atmosphere => no scattering of light => sky appears dark in outer space.
Note: Mercury’s day–night temperature extremes are extreme due to lack of atmosphere.
Venus
Second planet from the Sun.
Known as the 'greenhouse' planet due to the high CO₂ content in its atmosphere.
Rotational direction: Venus rotates from east to west (retrograde); Sun would rise in the west.
Size/age similar to Earth, but climate is extreme with surface temperatures around 460^{\circ}C.
Earth
Third planet from the Sun; the only known place to host life.
Atmosphere protects surface from solar wind, UV rays, and space radiation.
Surface composition: more than 71\% water and 29\% land.
Mars
Fourth planet; the 'Red Planet'.
Two moons: Phobos and Deimos.
Surface area ~25\% of Earth; mass ~10\% of Earth.
Atmospheric features: two zones when viewed from Earth — brighter, dusty reddish area and polar regions with frozen CO₂ and water.
Surface temperature ranges: varies widely due to thin atmosphere; typical range from about -143^{\circ}C to +35^{\circ}C.
Jupiter
Fifth planet from the Sun and the largest planet of all eight.
Mass ≈ 320 times that of Earth; in total, Jupiter’s mass is larger than the sum of all other planets.
Described as Earth’s protector because its strong gravity can deflect or capture potential impactors, shielding Earth from large objects.
Saturn
Sixth planet from the Sun; second largest in the solar system.
Classified as a 'giant gas' planet.
Characteristic ring system composed largely of ice with some rocky material and dust.
To date, at least 62 moons have been found orbiting Saturn; Titan (Saturn’s largest moon after Ganymede) is larger than Mercury.
Uranus
Seventh planet from the Sun; interior dominated by ice and rock, with a thick atmosphere.
Also a 'giant gas' planet.
Possesses a ring system that is thinner and darker than Saturn’s.
Has many moons.
Uniquely, its axis of rotation is tilted almost on its side, causing extreme seasonal variations.
Orbital period ≈ 84\text{ years}.
Neptune
Eighth planet from the Sun; another 'giant gas' planet.
Takes ≈ 165\text{ years} to orbit the Sun.
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Planetary data (summary table from Page 2)
Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Distance from Sun (million km):
Mercury: 57.9\times 10^{6}
Venus: 108.2\times 10^{6}
Earth: 149.6\times 10^{6}
Mars: 227.9\times 10^{6}
Jupiter: 778.3\times 10^{6}
Saturn: 1429\times 10^{6}
Uranus: 2871\times 10^{6}
Neptune: 4504\times 10^{6}
Relative mass (x Earth):
Mercury: 0.055
Venus: 0.815
Earth: 1
Mars: 0.107
Jupiter: 317.8
Saturn: 95.159
Uranus: 14.536
Neptune: 17.147
Diameter (km):
Mercury: 4879
Venus: 12\,104
Earth: 12\,756
Mars: 6794
Jupiter: 142\,984
Saturn: 120\,536
Uranus: 51\,118
Neptune: 49\,528
Density (g cm⁻³):
Mercury: 5.4
Venus: 5.2
Earth: 5.5
Mars: 3.9
Jupiter: 1.3
Saturn: 0.7
Uranus: 1.27
Neptune: 1.60
Gravity (m s⁻²):
Mercury: 3.7
Venus: 8.87
Earth: 9.8
Mars: 3.71
Jupiter: 24.79
Saturn: 10.44
Uranus: 8.69
Neptune: 11.15
Average surface temperature (°C):
Mercury: -167
Venus: 457
Earth: 14
Mars: -55
Jupiter: -153
Saturn: -185
Uranus: -214
Neptune: -225
Time to orbit Sun (Earth years):
Mercury: 0.24\text{ yr} (88 days)
Venus: 0.615\text{ yr} (224.7 days)
Earth: 1.0\text{ yr} (365 days)
Mars: 1.88\text{ yr} (687 days)
Jupiter: 11.9\text{ yr}
Saturn: 29.5\text{ yr}
Uranus: 84\text{ yr}
Neptune: 164.8\text{ yr}
Time to rotate once on axis (Earth days):
Mercury: 59\text{ d}
Venus: 243\text{ d}
Earth: 24\text{ h}
Mars: 25\text{ h}
Jupiter: 10\text{ h}
Saturn: 11\text{ h}
Uranus: 17\text{ h}
Neptune: 16\text{ h}
Velocity of rotation on axis (km/h):
Mercury: \sim 10.89
Venus: \sim 6.52
Earth: 1674.4
Mars: 868.2
Jupiter: \text{45300}
Saturn: \text{35500}
Uranus: \text{9320}
Neptune: \text{9660}
Main atmospheric content / surface characteristics (summary):
Mercury: None (no atmosphere); surface with craters, dust, plains, mountains, valleys.
Venus:
96.5% carbon dioxide;
3.4% nitrogen; 0.1% argon, helium, sulphur dioxide, water vapour
Earth: Nitrogen ~78%, Oxygen ~21%; a mix of inert gases; substantial surface oceans (~71%), 0.03% Carbon dioxide
Mars: 96% Carbon dioxide , 1.9% nitrogen, 1.9% argon, 0.2% oxygen, Carbon monoxide, red sandy and rock, has big plains, volcanoes, and wide craters
Jupiter, Saturn, Uranus, Neptune: No solid surface; composed of gas/ice; strong gravity; ring systems present (especially Saturn).
Notable notes:
Titan, Saturn’s largest moon (after Ganymede), is bigger than Mercury.
The Earth’s gravity is 9.8 m s⁻².
The outer giant planets have high masses but relatively low surface gravity due to low density.
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Rotational direction of the planets
All planets rotate on their axes at different axial tilts.
All planets rotate from west to east (prograde rotation) except
Venus: rotates from east to west (retrograde rotation).
Uranus: axis is tilted to the point that it essentially rotates on its side.
Why do Venus and Uranus rotate differently?
The transcript asks this question but does not provide a definitive mechanism. Common hypotheses (not stated in the text) include large impacts or substantial gravitational/tidal interactions that altered their spins in the past.
Natural satellites
Natural satellites are objects that orbit planets.
The Moon is the Earth’s natural satellite.
The Moon orbits the Earth and also rotates on its axis; the Moon’s rotation and orbital periods are similar (about 27 days), causing the same surface to face Earth.
Hypothetical situation related to the Solar System
What happens if Earth’s rotation slows down or stops?
Day and night cycles would be affected; tides would change; climate and weather patterns would alter.
The Earth as a Planet for Living Things
Earth is the only planet known to harbor life due to several features, including:
Presence of water
Essential minerals
Suitable temperature range
Protective atmospheric content
Discussion prompts in the text invite reflection on the potential for life elsewhere and on humanity’s role in conserving Earth.
Earth’s surface and tides (visuals in the text)
The Earth experiences tidal effects due to gravitational interactions with the Moon and the Sun.
Changes in tides and temperature depend on sunlight exposure.
Ecological discussion: Living on Earth
The ecological footprint measures the capacity of Earth to renew resources and absorb wastes across six areas: carbon dioxide waste treatment, land use (forests, agricultural spaces, and farming), construction areas, and fishing areas, often represented by a human-foot-like footprint.
If the ecological footprint exceeds Earth’s renewability, resources become depleted, and countries differ in footprint size.
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Units and conversions: Astronomical Unit (AU), Light Year (ly) and kilometres
Converting km to AU:
Distance in km to AU: ext{Distance (km)} \;\div\; (1\text{ AU in km}) = \text{Distance in AU}
1 AU ≈ 1.5 \times 10^{8}\text{ km}.
Converting km to light-years:
Distance in km to ly: \text{Distance (km)} \;\div\; (9.46\times 10^{12}\text{ km/ly}) = \text{Distance in ly}
1 ly ≈ 9.5 \times 10^{12}\text{ km}.
The speed of light: c \approx 3.0\times 10^{5}\ \text{km/s}.
Example 1: Distance of the Earth from the Sun.
Distance in km: 1.50\times 10^{8}\text{ km}.
Distance in AU: \dfrac{1.50\times 10^{8}}{1.50\times 10^{8}} = 1.0 \text{ AU}.
Distance in ly: \dfrac{1.50\times 10^{8}}{9.46\times 10^{12}} \approx 1.58\times 10^{-5} \text{ ly}.
Example 2: Saturn’s distance from the Sun is 1.43\times 10^{9}\text{ km}.
Distance in AU: \dfrac{1.43\times 10^{9}}{1.50\times 10^{8}} \approx 9.5\text{ AU}.$n- Distance in ly: \dfrac{1.43\times 10^{9}}{9.46\times 10^{12}} \approx 1.51\times 10^{-4} \text{ ly}.
Ecological footprint (revisited)
Definition: A measure of how much of Earth’s resources households require to meet their needs (food, water, shelter, energy, waste absorption, etc.).
It varies by country due to consumption patterns and resource use.
Light-year concept (recap)
A light-year measures distance, not time: the distance that light travels in one year.
Light travels at c = 3.0\times 10^{5}\ \,\text{km/s}, which amounts to approximately 9.46\times 10^{12}\text{ km} per year.
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Relationship between Temperature of a Planet and the Sun
Intuition: Closer planets receive more heat from the Sun than distant planets.
Reality is more complex because of atmospheres, greenhouse effects, albedo, atmospheric composition, and internal heat.
Examples (from the data above):
Earth’s average surface temperature ≈ 14\,^{\circ}\text{C}; atmosphere and oceans moderate temperatures.
Venus experiences extreme heating (≈ 462^{\circ}\text{C}) due to a thick CO₂-rich atmosphere causing a strong greenhouse effect.
Mercury has no substantial atmosphere; daylight surfaces can exceed 427^{\circ}\text{C} while night sides plunge to around -173^{\circ}\text{C}.
Giant planets (Jupiter, Saturn, Uranus, Neptune) have little or no solid surface and receive very little sunlight, resulting in very low effective surface temperatures.
Relationship between distance, time and speed (summary)
Distance from the Sun and orbital period are related: the further a planet is, the longer its orbital period around the Sun.
Mercury: distance ≈ 57.9\times 10^{6}\text{ km}; orbital period ≈ 88\text{ days}.
Neptune: distance ≈ 4.504\times 10^{3}\times 10^{6}\text{ km}; orbital period ≈ 164.8\text{ years}.
The Earth’s orbital period is 1 year; the Moon orbits Earth approximately every 27 days (sidereal month).
Density and gravity relationships
The surface gravity on a planet depends on its mass and density.
Example (from the table):
Earth: mass ≈ 1\,M_\oplus; density ≈ 5.5\ \text{g cm}^{-3}; gravity ≈ 9.8\ \text{m s}^{-2}.
Jupiter: mass ≈ 317.8\,M_\oplus; density ≈ 1.3\ \text{g cm}^{-3}; gravity ≈ 24.79\ \text{m s}^{-2}.
Venus: mass ≈ 0.815\,M_\oplus; density ≈ 5.2\ \text{g cm}^{-3}; gravity ≈ 8.87\ \text{m s}^{-2}$$.
Giant gas planets can have very high mass but lower surface gravity due to low density; Jupiter/Saturn/Uranus/Neptune show this trend.
Summary: Distance, time, and speed relationships
The Sun–planets distance grows outward, and orbital periods increase with distance.
The listed orbital times (in Earth days or years) illustrate this trend:
Mercury: 88 days, 0.24 years
Venus: 224.7 days, 0.615 years
Earth: 365 days, 1.0 year
Mars: 687 days, 1.88 years
Jupiter: 11.9 years
Saturn: 29.5 years
Uranus: 84 years
Neptune: 164.8 years