Earth in the Universe · The Universe and the Solar System
The solar system
The solar system formed about 4.6 billion years ago, when gravity collapsed a cloud of gas and dust —already enriched with heavy elements from earlier generations of stars— into a rotating disc. The Sun formed at its centre; in the surrounding disc, the planets and the rest of the bodies that orbit it today.
The solar system is made up of the Sun, eight planets, the moons that orbit them, dwarf planets, asteroids and comets. Imagine looking up at the stars from the Wicklow Mountains on a clear, dark night: the bright "star" you see low in the sky may be a planet, and the Sun is the star that keeps them all in orbit.
The Sun as a star
Every day we feel the heat and light of the Sun, a gigantic source of energy that has been emitting radiation for about 4.6 billion years without running out, even though it is surrounded by the vacuum of space: where does this continuous source of energy come from if there cannot be any chemical fire out there?
Comparing Earth with other bodies
The table compares Earth with some other worlds. Use it to see how mass, gravity, size and composition vary.
Body
Diameter (km)
Mass (Earth = 1)
Gravity (m s^-2)
Composition
Earth
12,742
1
9.8
rock and metal
Moon
3,474
0.012
1.6
rock
Mercury
4,879
0.055
3.7
rock and metal
Venus
12,104
0.82
8.9
rock and metal
Mars
6,779
0.11
3.7
rock
Jupiter
139,820
318
24.8
hydrogen and helium gas
Saturn
116,460
95
10.4
hydrogen and helium gas
Uranus
50,724
14.5
8.7
water, ammonia and methane ices
Neptune
49,244
17
11.2
water, ammonia and methane ices
The Moon has much less mass than Earth, so its surface gravity is about one sixth of Earth's. Surface gravity depends on both the mass and the size of the body: more mass pulls harder, but a larger radius spreads that pull out. The table also shows the two groups of planets: the terrestrial planets are small, low in mass, rocky and have modest gravity, while the giant planets are huge, hundreds of times more massive in the case of Jupiter, and made of gases and ices.
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List the components of the Solar System and explain in one sentence what each type of body is.
List the components of the Solar System and explain in one sentence what each type of body is.
When we measure the planets, a striking contrast appears: the Earth has a density of $5.5\ \text{g/cm}^3$, whereas Saturn has barely $0.7\ \text{g/cm}^3$ (it would float on water): why are the four worlds closest to the Sun dense rocks and the four outer ones enormous gaseous spheres?
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The diagram shows the Solar System on a qualitative scale (the orbits and sizes are not realistic). Name the planets labelled with numbers from 1 to 8, from closest to…
The diagram shows the Solar System on a qualitative scale (the orbits and sizes are not realistic). Name the planets labelled with numbers from 1 to 8, from closest to furthest from the Sun.
For decades it was thought that there were nine planets, but from the 1990s onwards telescopes discovered hundreds of icy worlds comparable to Pluto orbiting beyond Neptune: how did astronomers decide on the exact boundary between what is a planet and what is a minor body?
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In 2006, Pluto came to be considered a dwarf planet. Explain which requirement of the definition of a planet it does not meet and name another dwarf planet in the Solar…
In 2006, Pluto came to be considered a dwarf planet. Explain which requirement of the definition of a planet it does not meet and name another dwarf planet in the Solar System.
Earth is currently the only planet with known life. Explain three characteristics of Earth that make it suitable for life, such as its distance from the Sun, the…
Earth is currently the only planet with known life. Explain three characteristics of Earth that make it suitable for life, such as its distance from the Sun, the presence of liquid water or its atmosphere.
A space telescope has detected three rocky exoplanets orbiting a red dwarf star that is cooler and smaller than the Sun, whose habitable zone lies between…
A space telescope has detected three rocky exoplanets orbiting a red dwarf star that is cooler and smaller than the Sun, whose habitable zone lies between $0.15\ \text{AU}$ and $0.30\ \text{AU}$:
• Planet A: Distance to the star: $0.06\ \text{AU}$. Mass: $1.1$ Earth masses. Dense atmosphere rich in water vapor.
• Planet B: Distance to the star: $0.22\ \text{AU}$. Mass: $0.9$ Earth masses. Detectable magnetic field and atmosphere with $N_2$, $CO_2$, and water vapor.
• Planet C: Distance to the star: $0.25\ \text{AU}$. Mass: $0.02$ Earth masses (similar to the Moon). No noticeable atmosphere.
a) Analyze with reasoning which of them is the ideal candidate to harbor liquid water and life, justifying why you rule out the other two.
b) Explain why being located within the "habitable zone" is a necessary but not sufficient condition for a planet to be habitable.
You are going to design a scale model of the Solar System in the hallway of your school, which is $50\ \text{m}$ long. You decide to represent the Sun with a basketball…
You are going to design a scale model of the Solar System in the hallway of your school, which is $50\ \text{m}$ long. You decide to represent the Sun with a basketball of $24\ \text{cm}$ in diameter (the actual diameter of the Sun is approximately $1\,390\,000\ \text{km}$).
a) At that same scale, calculate what diameter the Earth should have (actual diameter: $12\,742\ \text{km}$) and compare it with a small everyday object (such as a pinhead or a grain of sand).
b) Knowing that the average Earth-Sun distance is $1.5 \times 10^8\ \text{km}$ and Neptune's distance to the Sun is about $4.5 \times 10^9\ \text{km}$, calculate at what distance from the ball you would have to place both planets.
c) Based on the results, justify whether the complete model fits in the hallway while simultaneously maintaining the scale of sizes and the scale of distances, and explain why Solar System diagrams in textbooks are often misleading.
A robotic probe is exploring Neptune, at $30\ \text{AU}$ from the Sun ($1\ \text{AU} = 150\ \text{million km}$). At a time when Earth and Neptune are aligned on the same…
A robotic probe is exploring Neptune, at $30\ \text{AU}$ from the Sun ($1\ \text{AU} = 150\ \text{million km}$). At a time when Earth and Neptune are aligned on the same side relative to the Sun (minimum distance of $29\ \text{AU}$), the probe detects a rock on a collision course that will strike it in $6\ \text{hours}$.
a) Calculate the minimum time (in hours) it would take for an alarm signal to travel from the probe to Earth and for the corrective command to return to Neptune, knowing that radio signals travel at $300\,000\ \text{km/s}$.
b) Based on your calculation, predict whether the probe would survive if it depended on a command sent from ground control and justify why spacecraft in the outer Solar System must operate with real-time autonomous navigation systems.