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GCSE level Physics exam revision notes on
astronomy
ASTRONOMY: part 2.
Our
contemporary model of our Solar System - a sun, planets (table of data), their moons and beyond
into the cosmos (and the world of cosmology -
see Big Bang Theory)
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INDEX physics notes on
ASTRONOMY
See also notes on the geology and
atmosphere of our moon and the planets of our solar system
 2. Our
contemporary model of our Solar System and beyond
The 8 major planets, minor planets and asteroids orbit the Sun in slightly
elliptical orbits (our 'Solar System'), but our Sun is just one of
millions-billions of stars in our galaxy (we see part of it as the 'Milky
Way') and in turn the observable universe itself contains billions of other
galaxies.
(a) Our
Solar System - the Sun (a
main sequence star) and 8 orbiting major planets, and minor planets and asteroids.
The movement of the planets and asteroids
has been observed from visible light (reflected sunlight) for thousands of
years, initially with the naked eye and from the early 16th century onwards,
with telescopes.
|
8
major PLANETS |
Distance from Sun in Mkm |
Mass relative to Earth |
Surface gravitational field constant in m/s2
or N/kg |
Size relative to Earth |
Time to orbit Sun (days or years) |
Axis rotation time |
Average surface temperature
oC |
|
Mercury |
58 |
0.05 |
3.7 |
0.4 |
88 d |
58.6
d |
+350 |
|
Venus |
108 |
0.8 |
8.8 |
0.9 |
225 d |
242
d |
+480 |
|
Earth |
150 |
1 |
9.8 |
1 |
365 d |
24 h |
+22 |
|
Mars |
228 |
0.1 |
3.8 |
0.5 |
687 d |
24.7
h |
-23 |
|
Jupiter |
778 |
318 |
25 |
11 |
12 y |
9.8
h |
-153 |
|
Saturn |
1430 |
95 |
10.5 |
9.4 |
29 y |
10.8
h |
-185 |
|
Uranus |
2870 |
15 |
10.4 |
4 |
84 y |
17.3
h |
-214 |
|
Neptune |
4500 |
17 |
12.8 |
3.8 |
165 y |
16 h |
-225 |
|
Pluto (dwarf planet) |
5915 |
0.003 |
0.49 |
0.2 |
248 y |
153
h |
-236 |
The force of gravity
Generally speaking the
gravitational field constant at the surface of a planet increases
with its mass.
The gravitational constant of our
Moon is 1.7 m/s2 or 1.7 N/kg.
The gravitational constant of our
Sun is 293 m/s2 or 293 N/kg.
The strength of the gravitational
pull decreases the further you are from the centre of the planet
(the attractive force decreases according to an inverse square law:
force
1 / distance2)
With modern techniques, the Sun, at
the centre of our Solar System, can be
observed by detecting emissions in various regions of the electromagnetic
spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray
emissions.
Orbital paths
The orbits are not quite perfect
circles, but slightly 'squashed' into an elliptical shape.
The Earth is the 3rd planet from
the Sun - see above data table on the planets.
The Sun is ~150 million km away
from us and sunlight takes ~8 minutes to reach us.
Mercury, Venus, Earth and Mars
are the four inner planets, relatively small and consisting mainly
of rock.
The gas giant planets Jupiter,
Saturn, Neptune and Uranus have gases such as
hydrogen, ammonia, methane and carbon dioxide
their atmosphere, may have rocky cores?
(b) Our
Milky Way - is our view of looking through
our own galaxy
The name 'Milky Way' comes from the
profusion of bright starlight from our galaxy when you look through its
centre against the background of the relatively dark night sky.
Our solar system is just one small
part of a galaxy - which is a massive collection of billions of
stars held together by gravity.
The Milky Way rotates around the
central core of the galaxy and astronomers think there is a massive
black hole there.
Until relatively recently, the Milky Way
galaxy, has been observed with the naked eye and then telescopes on Earth,
but now it can be viewed through powerful telescopes on satellites eg the
Hubble Space Telescope.
Our galaxy, and for that matter distant galaxies,
can be continually observed using everything from giant radio telescopes,
huge optical\visible light telescopes to gamma ray burst detectors.
(c)
'Outer space'
and nebulae
Beyond the Earth and beyond our own
solar system and galaxy, it is far from being a vacuum of 'emptiness'.
Interstellar space (between stars)
contains huge clouds of dust and a mixture of gases, mainly hydrogen and
helium gases, but traces of lots of other molecules including organic
molecules.
These clouds are where stars are
formed and are called
nebulae.
A
nebula is an enormous cloud of dust and
gas occupying the space between stars and acting as a nursery for
new stars.
(d) The
Universe is everything -
see separate page
Cosmology - the Big Bang Theory of the Universe
The
cosmos is a term used to
describe the universe seen as a well-ordered whole.
A mathematics note on distances
- a sort of perspective on 'everything'!
The distance from planet Earth to the
Sun is 150 million kilometres.
150 Mkm, 150 000 000 km, and in
standard form 1.5 x 108 km.
The distance from the Sun to the
dwarf planet Pluto is 5915 million km.
5915 Mkm, 5 915 000 000 km, in
standard form ~5.9 x 109 km.
The diameter of our galaxy, the
'Milky Way', ~1 000 000 000 000 000 000 km, in standard form ~1.0 x 1018
km.
Some derived calculations,
taking the speed of light to be 3.0 x 108 m/s.
Ex. 1. How long does it take
light to travel from the Sun's surface to the Earth?
speed = distance / time,
time = distance / speed
time = 1.5 x 108 x
1000 / 3 x 108 = 500 s,
8 minutes and 20 seconds.
Ex. 2. How long does it take light to
cross from one side of our galaxy to the other?
time = (1018 x 1000) /
3 x 108 = ~3.33 x 1012 s
1 Earth year is 365.25 x 24 x 60
x 60 = 31557600 s
time = 3.33 x 1012 /
31557600 = ~106 000 years!
More on the planets - just out of interest
(NOT required for GCSE exams!)
Let’s take a grand tour of the Solar
System and break down the distinctive characteristics
of each planet, from the scorched surface of Mercury to the icy winds of
Neptune.
Mercury
- Closest
planet to the Sun
- No atmosphere
to retain heat → extreme temperature swings (−180°C to 430°C)
- Surface:
Rocky, cratered like the Moon
- Orbit:
Fastest (88 Earth days)
Venus
- Similar size
to Earth but with a thick
CO2 atmosphere
- Hottest planet
due to runaway greenhouse effect (~465°C)
- Rotates backwards
(retrograde rotation)
- Surface:
Volcanic plains and sulfuric acid clouds
Earth
- Only known planet
to support life
- Atmosphere:
Nitrogen and oxygen-rich
- Surface:
70% water, tectonic activity
- Moon:
Stabilizes Earth’s tilt and tides
Mars
- Known as the Red Planet
due to iron oxide (rust) on its surface
- Thin atmosphere
(mostly CO₂)
- Evidence of past water:
dry riverbeds, polar ice caps
- Moons:
Phobos and Deimos
Jupiter
- Largest planet
in the Solar System
- Gas giant:
Mostly hydrogen and helium
- Great Red Spot:
A giant storm larger than Earth
- Moons:
Over 90, including Ganymede (largest in the Solar System)
Saturn
- Famous for its
spectacular ring system
- Gas giant
with low density (could float in water!)
- Moons:
Titan (thick atmosphere), Enceladus (ice geysers)
- Atmosphere:
Hydrogen, helium, ammonia clouds
Uranus
- Ice giant
with a pale blue-green color (methane in atmosphere)
- Rotates on its side
(axial tilt ~98°)
- Coldest atmosphere
of any planet (−224°C)
- Rings:
Faint and dark
Neptune
- Farthest
planet from the Sun
- Deep blue
due to methane
- Strongest winds
in the Solar System (up to 2,100 km/h)
- Moon:
Triton (retrograde orbit, possible captured object)
Quick Comparison Table
| Planet |
Type |
Atmosphere |
Moons |
Notable Feature |
| Mercury |
Rocky |
None |
0 |
Extreme temperature variation |
| Venus |
Rocky |
Thick CO2 |
0 |
Hottest surface temperature |
| Earth |
Rocky |
Nitrogen, Oxygen |
1 |
Supports life |
| Mars |
Rocky |
Thin CO2 |
2 |
Red surface, signs of past water |
| Jupiter |
Gas Giant |
Hydrogen, Helium |
90+ |
Great Red Spot, largest planet |
| Saturn |
Gas Giant |
Hydrogen, Helium |
80+ |
Rings, low density |
| Uranus |
Ice Giant |
Hydrogen, Methane |
27 |
Tilted axis, coldest atmosphere |
| Neptune |
Ice Giant |
Hydrogen, Methane |
14 |
Fastest winds, deep blue color |
Planetary science has come a
long way, and each discovery has reshaped how we understand our Solar
System. Here’s a timeline-style breakdown of key planetary
discoveries that often feature in GCSE/IGCSE Physics and Astronomy
content:
Historical Milestones about the
planets
- 1609 – Galileo’s Telescope:
First to observe moons orbiting Jupiter (e.g. Io,
Europa), proving not everything orbits Earth.
- 1619 – Kepler’s Laws:
Revealed that planets move in elliptical orbits, not
perfect circles.
- 1687 – Newton’s Law of
Gravitation: Explained
planetary motion using universal gravity.
Modern Planetary Discoveries
| Year |
Discovery |
Significance |
| 1781 |
Uranus
(by William Herschel) |
First planet discovered with a
telescope |
| 1846 |
Neptune
(by Johann Galle) |
Predicted by irregularities in
Uranus’s orbit |
| 1930 |
Pluto
(by Clyde Tombaugh) |
Initially classified as the 9th
planet |
| 2006 |
Pluto reclassified
as a dwarf planet |
Led to clearer definitions of what
makes a planet |
| 2015 |
New Horizons flyby of
Pluto |
Revealed complex geology and
possible subsurface ocean |
| 2020s |
Exoplanet discoveries |
Thousands of planets found
orbiting other stars using Kepler and TESS missions |
Scientific Advances in Planetary
Understanding
- Atmospheric studies:
Venus’s thick CO₂ atmosphere and Mars’s thin one helped us understand
greenhouse effects and climate.
- Water on Mars:
Evidence of ancient riverbeds and polar ice caps suggests Mars once had
liquid water.
- Moons with oceans:
Europa (Jupiter) and Enceladus (Saturn) may have subsurface
oceans, raising hopes for extraterrestrial life.
- Ring systems:
Saturn’s rings are the most famous, but Jupiter, Uranus, and
Neptune also have faint rings.
See also notes on the geology and
atmosphere of our moon and the planets of our solar system
INDEX of my physics notes on
ASTRONOMY
Key points for Physics -
Our
contemporary model of our Solar System - a sun, planets, their moons and beyond
into the cosmos!
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks
and syllabus-specifications for students taking the UK AQA,
Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and
CIE GCSE
physics 9-1 level science examinations.
GCSE/IGCSE Physics Revision
Notes: Our Contemporary Model of the Solar System
Topic:
The Sun, Planets, Moons,
and Beyond
Structure of the Solar System
| Component |
Description |
| The Sun |
A medium-sized star at the center;
contains over 99% of the Solar System’s mass |
| Planets |
8 major planets orbit the Sun in
elliptical paths |
| Moons |
Natural satellites orbiting
planets (e.g. Earth’s Moon, Jupiter’s 63+ moons) |
| Dwarf Planets |
Smaller planetary bodies (e.g.
Pluto, Eris) that haven’t cleared their orbit |
| Asteroids |
Rocky bodies, mostly in the
Asteroid Belt between Mars and Jupiter |
| Comets |
Icy bodies with elliptical orbits;
develop tails when near the Sun |
| Artificial Satellites |
Man-made objects orbiting Earth or
other celestial bodies (e.g. ISS) |
The Planets (in order from the Sun)
- Terrestrial (rocky) planets:
Mercury, Venus, Earth, Mars
- Small, dense, solid surfaces
- Gas giants:
Jupiter, Saturn
- Massive, mostly hydrogen and
helium
- Ice giants:
Uranus, Neptune
- Icy compositions, strong winds,
faint rings
Mnemonic:
My Very Educated Mother Just
Served Us Nachos
Other Solar System Features
- Asteroid Belt:
Between Mars and Jupiter; home to many rocky bodies
- Kuiper Belt:
Beyond Neptune; contains dwarf planets and icy bodies
- Oort Cloud
(theoretical): A distant shell of icy objects, source of long-period
comets
Key Concepts Across Exam Boards
|
Focus Areas |
| Classification of planets, moons,
satellites; asteroid/comet characteristics |
| Structure and formation of the
Solar System; natural versus artificial satellites |
| Orbital motion, gravitational
forces, and Solar System components |
| Planetary orbits, satellite
motion, and Solar System structure |
| Planet types, orbital speeds, and
distances; dwarf planets and comets |
| Orbits, centripetal force, and
Solar System evolution |
Student Tips
- Compare planet data:
Mass, distance, temperature, number of moons
- Understand orbits:
Elliptical paths, centripetal force, orbital speed
- Differentiate satellites:
Natural (moons) versus artificial (ISS, GPS)
- Use diagrams:
Sketch the Solar System to visualize scale and structure
- Practice data interpretation:
Especially for orbital periods and distances
- Link to gravity:
Gravitational pull explains orbits and satellite motion
Keywords, phrases and learning objectives for astronomy
Be able to describe the contemporary model of our Solar System
of our sun,
planets and moons.
Beyond our solar system be able to describe what the
following are: stars, galaxies and the universe.
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ASTRONOMY
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