Physics · 6 Space physics

6.1.2 The Solar System

Before you start

Speed = distance ÷ time, and rearranging it to time = distance ÷ speed (1.2).
Gravitational field strength $g = \frac{W}{m}$ , in N/kg, which is 9.8 N/kg at the Earth's surface; weight $W = m g$ (1.3).
With no resultant force, a moving object carries on in a straight line at constant speed (1.5.1).
Kinetic and gravitational potential energy stores, and the conservation of energy (1.7.1).
Electromagnetic waves, including light and radio waves, travel at $3.0 \times 10^{8}$ m/s in a vacuum (3.3).
Average orbital speed $v = \frac{2 π r}{T}$ (6.1.1).
Working with numbers in standard form, such as $1.5 \times 10^{11}$ m.

Syllabus objectives covered (10)

Describe the Solar System as containing: (a) one star, the Sun (b) the eight named planets and know their order from the Sun (c) minor planets that orbit the Sun, including dwarf planets such as Pluto and asteroids in the asteroid belt (d) moons, that orbit the planets (e) smaller Solar System bodies, including comets and natural satellites
Know that, in comparison to each other, the four planets nearest the Sun are rocky and small and the four planets furthest from the Sun are gaseous and large, and explain this difference by referring to an accretion model for Solar System formation, to include: (a) the model’s dependence on gravity (b) the presence of many elements in interstellar clouds of gas and dust (c) the rotation of material in the cloud and the formation of an accretion disc
Know that the strength of the gravitational field: (a) at the surface of a planet depends on the mass of the planet (b) around a planet decreases as the distance from the planet increases
Calculate the time it takes light to travel a significant distance such as between objects in the Solar System
Know that the Sun contains most of the mass of the Solar System and this explains why the planets orbit the Sun
Know that the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun
Know that planets, minor planets and comets have elliptical orbits, and recall that the Sun is not at the centre of the elliptical orbit, except when the orbit is approximately circular
Analyse and interpret planetary data about orbital distance, orbital duration, density, surface temperature and uniform gravitational field strength at the planet’s surface
Know that the strength of the Sun’s gravitational field decreases and that the orbital speeds of the planets decrease as the distance from the Sun increases
Know that an object in an elliptical orbit travels faster when closer to the Sun and explain this using the conservation of energy

Part 1 of 7

What the Solar System contains

The Solar System is the Sun and everything that orbits it. An orbit is the curved path one object follows round another, held on that path by gravity.
There is one star: the Sun. It is the only star in the Solar System. It gives out its own light. You see the planets and moons because they reflect sunlight.
There are eight planets. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Earth is the third. A sentence whose words start with the same letters helps you keep the order: My Very Easy Method Just Speeds Up Naming.
Minor planets also orbit the Sun, but they are much smaller than the eight planets. The objective names two kinds, and you need both: dwarf planets and asteroids.
Pluto is a dwarf planet. It was once counted as a ninth planet, but it is now classed as a dwarf planet, so do not put it in the list of eight.
Asteroids are lumps of rock, some containing metal, much smaller than the planets. Most of them orbit the Sun in the asteroid belt, which lies between the orbits of Mars and Jupiter.
Moons orbit planets, not the Sun directly. They travel round the Sun with their planet. Earth has one moon, the Moon.
A moon is a natural satellite. A satellite is any object in orbit round a larger one. 'Natural' separates it from the artificial satellites that people launch.
Comets are small bodies of ice and dust that orbit the Sun. Many have very elongated orbits, so they spend most of their time far from the Sun. When a comet comes close to the Sun, some of its ice turns to gas and it can form a glowing tail that points away from the Sun.
The trap in a Describe answer is to name the bodies but not say what each one orbits. Planets, minor planets and comets orbit the Sun. Moons orbit planets.

Part 2 of 7

Small and rocky inside, large and gaseous outside

First the pattern. Compared with each other, the four planets nearest the Sun (Mercury, Venus, Earth, Mars) are small and rocky. The four furthest from the Sun (Jupiter, Saturn, Uranus, Neptune) are large and gaseous, with thick atmospheres of hydrogen and helium.
The size difference is big. Earth is the largest of the inner four, yet Neptune, the smallest of the outer four, has nearly four times Earth's diameter.
The accretion model explains the pattern. 'Accretion' means growing by gathering material. The Solar System formed about 4.6 billion years ago from an interstellar cloud of gas and dust.
The cloud was mostly hydrogen and helium, but it contained many other elements in smaller amounts, such as carbon, oxygen, silicon and iron. These elements were made in earlier stars. Rock and metal are made mainly from them, so without them there could be no rocky planets.
The model depends on gravity. Every particle of the cloud attracts every other particle. Gravity pulled the cloud inwards and it collapsed. Most of its mass collected at the centre and became the Sun.
The cloud was rotating slowly. As it collapsed, it rotated faster, in the same way that a spinning skater spins faster when she pulls her arms in. The spinning flattened the material round the young Sun into a disc of gas and dust: the accretion disc.
forces diagram — not drawn yet
A flat disc of gas and dust seen at a slight angle with a bright young Sun at its centre; a curved arrow round the outer edge labelled 'rotation'; several short arrows in the disc pointing towards the Sun labelled 'gravity'; small clumps drawn in the disc labelled 'clumps grow by accretion'; the inner part of the disc labelled 'hot: only rock and metal solid' and the outer part labelled 'cold: ice and rock solid, plenty of gas'.
The rotating cloud has flattened into an accretion disc round the young Sun. The temperature falls with distance from the Sun, and that decides what each planet is made of.
The disc explains a fact you can check: all eight planets orbit the Sun in the same direction and in nearly the same flat plane. They all formed from the same rotating disc.
Inside the disc, dust particles collided and stuck together. The clumps grew. Once a clump was big enough, its own gravity pulled in more material. Over a long time the largest clumps became the planets.
Now the key idea: temperature. Close to the young Sun the disc was hot. Only materials with high melting points, rock and metal, could be solid there. Hydrogen, helium and substances such as water stayed as gas. In the model, the young Sun also gave out a strong stream of particles (the solar wind) that pushed much of this gas out of the inner Solar System.
So the inner planets formed only from rock and metal. These come from the heavier elements, which were a small fraction of the cloud, so there was not much of them. The result was small, rocky planets, whose gravity was too weak to hold on to much hydrogen and helium.
Far from the Sun the disc was cold. Ices, such as frozen water, could be solid there as well as rock, so there was far more solid material. Large cores grew, and their strong gravity pulled in and held huge amounts of hydrogen and helium gas. The result was large, gaseous planets.
The trap here is to write 'the outer planets are gaseous because they are far from the Sun' and stop. Distance is not the reason on its own. Say what distance changes: lower temperature, so ice and gas could collect, giving large cores whose gravity pulled in more gas.

Part 3 of 7

Gravitational field strength at and around a planet

Recall from 1.3: the gravitational field strength $g$ is the force per unit mass, in N/kg. At the surface of the Earth it is 9.8 N/kg, so each kilogram is pulled down with a force of 9.8 N.
$g = \frac{W}{m}$
(a) At the surface, the field strength depends on the mass of the planet. A planet with more mass pulls harder on each kilogram. Mars has about one-tenth of the mass of Earth, and the field strength at its surface is only 3.7 N/kg. On the Moon it is 1.6 N/kg.
(b) Around a planet, the field strength decreases as the distance from the planet increases. A spacecraft moving away from Earth feels a weaker and weaker pull from Earth for every kilogram of its mass.
Weaker does not mean zero. Earth's field still holds the Moon in orbit about 380 000 km away. 'There is no gravity in space' is wrong physics.
Put (a) and (b) together for big planets. The surface of a large planet is a long way from its centre. Uranus has about 15 times the mass of Earth, but its radius is about 4 times Earth's, so the field at its surface is 8.7 N/kg, a little weaker than Earth's. So use mass alone to compare surface field strengths only when the planets are the same size.
Close to a planet's surface, over heights that are small compared with the planet, the field strength hardly changes. The field there is uniform. That is why a data table gives one value of $g$ for each planet's surface.
To find a weight on another planet, use that planet's $g$ in $W = m g$ . The mass does not change from planet to planet. Only the weight does.

Part 4 of 7

Why everything orbits the Sun

The Sun contains most of the mass of the Solar System: more than 99.8% of it. All the planets, moons, asteroids and comets together make up less than 0.2%.
At the same distance, a bigger mass has a stronger gravitational field. With so much more mass than everything else, the Sun pulls on each planet far more strongly than anything else does. That is why the planets orbit the Sun.
Now ask what the force actually does. Recall from 1.5.1: with no resultant force, a moving object carries on in a straight line at constant speed. A planet would fly off in a straight line if nothing pulled on it.
The Sun pulls the planet towards itself all the time. That pull keeps turning the planet's direction of motion, so the planet follows a closed curve round the Sun instead of a straight line.
Say it precisely: the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun. Name the Sun. The word 'gravity' alone does not say whose gravity it is. The same rule applies to a planet, a dwarf planet, an asteroid or a comet.
The Sun's field gets weaker further out. This is the idea from the last section, applied to the Sun. A planet far from the Sun is pulled with a smaller force on each kilogram than a planet close to it.
A weaker pull can only keep a planet in orbit if the planet moves more slowly. So the orbital speeds of the planets decrease as the distance from the Sun increases. The average orbital speeds, from Mercury outwards, are about 47, 35, 30, 24, 13, 9.7, 6.8 and 5.4 km/s. Mercury is the fastest and Neptune the slowest.
graph diagram — not drawn yet
Points for the eight planets with average distance from the Sun on the x-axis from 0 to 4500 million km and average orbital speed on the y-axis from 0 to 50 km/s: Mercury (58, 47), Venus (108, 35), Earth (150, 30), Mars (228, 24), Jupiter (778, 13), Saturn (1430, 9.7), Uranus (2870, 6.8), Neptune (4500, 5.4); each point labelled with the planet's name and joined by a smooth curve that falls steeply at first and then levels off.
Orbital speed falls as distance from the Sun increases, because the Sun's gravitational field is weaker further out.
Two things make an outer planet's year long: it has a much longer path to travel round, and it moves more slowly along it. Neptune takes about 165 Earth years to go round once.

Part 5 of 7

Elliptical orbits, and why speed changes

Orbits are not perfect circles. Planets, minor planets and comets all move in ellipses. An ellipse is a stretched circle, like a circle that has been squashed from two sides.
The Sun is not at the centre of the ellipse. It sits at a point off-centre, nearer one end. So the distance between the object and the Sun changes as it goes round.
The planets' orbits are only slightly stretched, so they are approximately circular. For an approximately circular orbit the Sun is approximately at the centre. A comet's orbit can be very stretched, and then the Sun is close to one end of it.
forces diagram — not drawn yet
A long, narrow ellipse with the Sun drawn inside it close to the right-hand end; the centre of the ellipse marked with a small cross labelled 'centre of ellipse'; a comet drawn at the point closest to the Sun with a long arrow along the orbit labelled 'fastest', and at the far left end with a short arrow labelled 'slowest'; at both positions a thin arrow from the comet towards the Sun labelled 'gravitational attraction of the Sun'.
The Sun is off-centre, near one end of the ellipse. The comet moves fastest at the point closest to the Sun and slowest at the point furthest away.
An object in an elliptical orbit travels faster when it is closer to the Sun and slower when it is further away. The objective asks you to explain this with the conservation of energy, so you need two energy stores.
The kinetic energy store depends on speed: more speed, more kinetic energy. The gravitational potential energy store depends on how far the object is from the Sun: the further away, the more gravitational potential energy.
Moving away from the Sun is like a ball thrown upwards. It gains gravitational potential energy, so it must lose kinetic energy. It slows down.
Moving towards the Sun is the reverse. Its gravitational potential energy decreases, and that energy is transferred to its kinetic energy store. It speeds up.
The total stays the same. In space there is no air resistance and nothing else to transfer energy away, so kinetic energy plus gravitational potential energy is constant all the way round the orbit.
$kinetic energy + gravitational potential energy = constant$
So where the gravitational potential energy is least, at the point closest to the Sun, the kinetic energy is greatest and the object is fastest. At the furthest point it is slowest.
The trap: 'it goes faster because the Sun's pull is stronger when it is closer' does not use energy at all. Name both stores, say which one decreases and which increases, and say that the total stays constant.

Part 6 of 7

How long light takes to cross the Solar System

Light travels through space at $3.0 \times 10^{8}$ m/s. All electromagnetic waves travel at this speed in a vacuum, radio waves included. You met this in 3.3. Recall it: a question may not give it to you.
Time comes from the speed equation you already know (1.2), rearranged. Divide both sides of distance = speed × time by the speed, because that leaves time on its own.
$time = \frac{distance}{speed}$
Units first. The speed is in metres per second, so the distance must be in metres and the answer comes out in seconds. A distance in km must be multiplied by 1000 ( $10^{3}$ ) before you divide.
Light from the Sun travels $1.5 \times 10^{11}$ m to reach Earth. That takes $1.5 \times 10^{11} \div 3.0 \times 10^{8} = 500$ s, about 8.3 minutes. The sunlight you see left the Sun over eight minutes earlier.
Light from the Moon travels about $3.8 \times 10^{8}$ m to Earth, which takes about 1.3 s.
The distance between two planets changes as they move round their orbits, so a question gives you the distance at that moment. If a signal goes out and an answer comes back, the light travels the distance twice, so double the time.
If the question asks for minutes, divide the seconds by 60. Check the size of your answer against known values: light takes about 1.3 s from the Moon to Earth, about 8 minutes from the Sun to Earth and about 4 hours from the Sun to Neptune.

Part 7 of 7

Reading planetary data

A planetary data table gives you numbers for each planet. Your job is to find a pattern, describe it in words, and name any planet that does not fit. Here are approximate values for the eight planets. Read each column down from Mercury to Neptune.
$\begin{matrix} Planet & Distance from Sun / million km & Orbital period / Earth years & {Density / kg/m}^{3} & Temperature /^{\circ} C \\ Mercury & 58 & 0.24 & 5400 & 170 \\ Venus & 108 & 0.62 & 5200 & 460 \\ Earth & 150 & 1.00 & 5500 & 15 \\ Mars & 228 & 1.88 & 3900 & - 65 \\ Jupiter & 778 & 11.9 & 1300 & - 110 \\ Saturn & 1430 & 29.4 & 690 & - 140 \\ Uranus & 2870 & 84 & 1300 & - 195 \\ Neptune & 4500 & 165 & 1600 & - 200 \end{matrix}$
Orbital distance and orbital duration. The orbital period increases every time the distance increases. Describe it as a relationship: 'the further a planet is from the Sun, the longer its orbital period'.
Do not call it proportional. Neptune is 30 times as far from the Sun as Earth ( $4500 \div 150 = 30$ ), but its period is about 165 years, not 30 years.
Density. The inner four planets have densities from 3900 to 5500 kg/m³, typical of rock and metal. The outer four have densities from 690 to 1600 kg/m³, because they are mostly gas. Saturn is less dense than water (1000 kg/m³). This is the evidence for the rocky/gaseous split in the accretion section.
Surface temperature. In general it falls as distance from the Sun increases, because less of the Sun's energy reaches each square metre further out. The exception is Venus: it is further from the Sun than Mercury, yet hotter. Venus has a very thick atmosphere, mostly carbon dioxide, which traps heat.
When you describe a trend, name any exception and give a reason for it. 'Temperature decreases with distance' is incomplete if Venus is in the table.
Gravitational field strength at the surface. If a table gives $g$ for each planet, do not look for a pattern with distance from the Sun. The value of $g$ depends on the planet's own mass and size, not on where it orbits. Mercury and Mars both have a surface field strength of about 3.7 N/kg, yet Mars is about four times as far from the Sun.
Orbital speed from the data. Recall from 6.1.1 that the average orbital speed is $v = \frac{2 π r}{T}$ , where $r$ is the average orbital radius and $T$ is the orbital period. Put $r$ in metres and $T$ in seconds, and $v$ comes out in m/s.
$v = \frac{2 π r}{T}$
A period in days becomes seconds by multiplying by $24 \times 3600 = 86 400$ . A distance in million km becomes metres by multiplying by $10^{9}$ . Do the conversions before you substitute.

Worked examples

Example 1. The average distance from the Sun to the Earth is $1.5 \times 10^{8}$ km. Calculate the time taken for light to travel from the Sun to the Earth. Give your answer in minutes.

Show the working

Units first. The speed of light is in m/s, so turn the distance into metres. There are $10^{3}$ m in 1 km.
$1.5 \times 10^{8} km \times 10^{3} = 1.5 \times 10^{11} m$
Recall the speed of light in space: $3.0 \times 10^{8}$ m/s. It is not given in the question, so you must know it.
Time is distance divided by speed. Dividing powers of ten means subtracting the indices: $11 - 8 = 3$ .
$t = \frac{1.5 \times 10^{11}}{3.0 \times 10^{8}} = 0.5 \times 10^{3} = 500 s$
The question asks for minutes, so divide by 60.
$\frac{500}{60} = 8.3 minutes$
Check the size: a few minutes is sensible for light crossing part of the Solar System.

Answer: 500 s, which is 8.3 minutes (8 minutes 20 seconds).

Example 2. An astronaut has a mass of 75 kg. The gravitational field strength at the surface of Mars is 3.7 N/kg. (a) Calculate the weight of the astronaut on the Earth and on Mars. (b) Mars has about one-tenth of the mass of the Earth. Explain why the astronaut's weight on Mars is less than on the Earth. (c) The astronaut leaves Mars in a spacecraft. State how the gravitational field strength due to Mars changes as the spacecraft moves away.

Show the working

(a) Weight is mass times gravitational field strength. On Earth $g$ is 9.8 N/kg.
$W = m g = 75 \times 9.8 = 735 N$
On Mars use the field strength of Mars. The mass is still 75 kg: mass does not change from planet to planet.
$W = 75 \times 3.7 = 277.5 N \approx 280 N$
(b) The field strength at a planet's surface depends on the planet's mass. Mars has much less mass than Earth, so its surface field is weaker, and each kilogram of the astronaut is pulled with a smaller force.
Notice that Mars is also smaller than Earth, with about half Earth's radius. That alone would make its surface field stronger, but the much smaller mass is the bigger effect.
(c) The field strength around a planet decreases as the distance from the planet increases. It gets weaker, but it does not drop suddenly to zero.

Answer: (a) 735 N on the Earth; about 280 N (277.5 N) on Mars. (b) The field strength at a planet's surface depends on its mass; Mars has much less mass, so its field is weaker and the pull on each kilogram is smaller. (c) It decreases.

Example 3. Mars orbits the Sun at an average distance of $2.28 \times 10^{11}$ m. Its orbital period is 687 Earth days. (a) Calculate the average orbital speed of Mars. (b) The average orbital speed of the Earth is $3.0 \times 10^{4}$ m/s. Explain why Mars has a lower orbital speed than the Earth.

Show the working

(a) The equation needs $T$ in seconds. One day is $24 \times 3600$ s.
$T = 687 \times 24 \times 3600 = 5.94 \times 10^{7} s$
Recall the average orbital speed equation from 6.1.1. The distance round one orbit is $2 π r$ , and the planet covers it in one period $T$ .
Substitute, with $r$ already in metres.
$v = \frac{2 π r}{T} = \frac{2 π \times 2.28 \times 10^{11}}{5.94 \times 10^{7}} = 2.4 \times 10^{4} m/s$
That is about 24 km/s, which matches the speed for Mars in the list of orbital speeds in the lesson: between Earth's 30 km/s and Jupiter's 13 km/s.
(b) Mars is further from the Sun than the Earth. The Sun's gravitational field is weaker further away, and a weaker pull keeps a planet in orbit only at a lower speed.

Answer: (a) $2.4 \times 10^{4}$ m/s (about 24 km/s). (b) Mars is further from the Sun, where the Sun's gravitational field is weaker, so it orbits at a lower speed than the Earth.

Example 4. A comet moves around the Sun in a very elongated elliptical orbit. (a) State where the Sun is in this orbit. (b) Explain, using the conservation of energy, why the comet travels fastest when it is closest to the Sun.

Show the working

(a) The orbit is far from circular, so the Sun is not at its centre. It is off-centre, close to one end of the ellipse.
(b) Name the two energy stores first: the kinetic energy store, which depends on speed, and the gravitational potential energy store, which is greater the further the comet is from the Sun.
As the comet moves towards the Sun, its gravitational potential energy decreases. That energy is transferred to its kinetic energy store, so its speed increases.
The total stays constant: there is no air resistance in space and nothing else transfers energy away. So the kinetic energy is greatest where the gravitational potential energy is least.
Gravitational potential energy is least at the point closest to the Sun, so that is where the comet is fastest. Check that each sentence of your answer mentions energy, not just 'the pull is stronger'.

Answer: (a) Off-centre, near one end of the ellipse, not at its centre. (b) As the comet moves towards the Sun its gravitational potential energy decreases and is transferred to kinetic energy, so it speeds up. The total energy stays constant, so its kinetic energy and speed are greatest where its gravitational potential energy is least: at the point closest to the Sun.

Where people go wrong

Mistake: Putting the Sun at the centre of every orbit.

Instead: In an elliptical orbit the Sun is off-centre, nearer one end. It is approximately at the centre only when the orbit is approximately circular, as the planets' orbits are.

If the Sun were at the centre, the distance to the Sun would never change. The changing distance is what makes the object speed up and slow down around the orbit.

Mistake: Assuming a planet with more mass, or a bigger planet, always has a stronger gravitational field at its surface.

Instead: The surface field depends on the planet's mass, but the surface of a large planet is far from its centre. Use mass alone to compare only planets of the same size.

Uranus has about 15 times the mass of Earth, but its radius is about 4 times Earth's, so its surface field (8.7 N/kg) is a little weaker than Earth's (9.8 N/kg).

Mistake: Saying there is no gravity in space.

Instead: A gravitational field gets weaker with distance, but it does not vanish.

Earth's field holds the Moon in orbit about 380 000 km away, and the Sun's field holds Neptune in orbit 4500 million km away.

Mistake: Dividing a distance in kilometres by $3.0 \times 10^{8}$ m/s.

Instead: Multiply a distance in km by $10^{3}$ to get metres before you divide by the speed of light.

The speed is in metres per second. Mixing km with m/s gives a time 1000 times too small.

Mistake: Writing that the outer planets are gaseous 'because they are far from the Sun' and stopping there.

Instead: Say what the distance changes: it was colder far from the Sun, so ice and gas could collect, large cores formed, and their gravity pulled in hydrogen and helium.

The accretion model explains the difference through temperature and gravity. Distance on its own explains nothing.

Check yourself

State the eight planets in order, starting with the one nearest the Sun.
Answer
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Pluto and the asteroids are both minor planets. State the type of minor planet that Pluto is, and state where the asteroid belt is.
Answer
Pluto is a dwarf planet. The asteroid belt is between the orbits of Mars and Jupiter.
Explain why the cloud of gas and dust that formed the Solar System became a flat disc.
Answer
The cloud was rotating. As gravity pulled it inwards, it rotated faster, and the spinning flattened it into a disc of gas and dust round the young Sun: the accretion disc.
Planets X and Y have the same radius. X has three times the mass of Y. Deduce which planet has the stronger gravitational field at its surface.
Answer
Planet X. For planets of the same size, the one with more mass has the stronger field at its surface.
The Moon is $3.8 \times 10^{8}$ m from the Earth. Calculate the time taken for light reflected from the Moon to reach the Earth.
Answer
$t = 3.8 \times 10^{8} \div 3.0 \times 10^{8} = 1.3$ s.
State the force that keeps the Earth in its orbit, and explain why the Earth orbits the Sun rather than any other object.
Answer
The gravitational attraction of the Sun. The Sun contains most of the mass of the Solar System, so its gravitational pull on the Earth is far stronger than that of anything else.
State where the Sun is for (a) a planet in an approximately circular orbit and (b) a comet in a very elongated elliptical orbit.
Answer
(a) Approximately at the centre of the orbit. (b) Not at the centre: off-centre, close to one end of the ellipse.
Use the planetary data table. (a) Venus is further from the Sun than Mercury but has a higher surface temperature. Suggest why. (b) Neptune is further from the Sun than Uranus. State how Neptune's orbital speed compares with that of Uranus.
Answer
(a) Venus has a very thick atmosphere, mostly carbon dioxide, which traps heat. (b) Neptune's orbital speed is lower, because the Sun's gravitational field is weaker further from the Sun.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. The Solar System contains one star, eight planets and many smaller bodies.

(a) State the names of the four planets nearest the Sun, in order, starting with the nearest. 1 mark No calculator

Mark scheme

B1 Mercury, Venus, Earth, Mars, all four in this order

(b) Compare the four planets nearest the Sun with the four planets furthest from the Sun. 2 marks No calculator

Mark scheme

B1 the four nearest are smaller / the four furthest are larger, ORA
B1 the four nearest are rocky / the four furthest are gaseous, ORA

(c) Explain, using the accretion model of how the Solar System formed, why the four planets nearest the Sun are rocky. 3 marks No calculator

Mark scheme

B1 gravity pulled a rotating interstellar cloud of gas and dust inwards, forming an accretion disc around the (young) Sun
B1 near the Sun the temperature was high, so only rock and metal (materials with high melting points) could be solid / gases could not stay there or were pushed outwards
B1 solid particles collided and stuck together / were pulled together by gravity, building up rocky planets

Question 2. At one moment, the distance between the Earth and Jupiter is $6.3 \times 10^{11}$ m.

(a) Calculate the time taken for light from Jupiter to reach the Earth. 3 marks

Mark scheme

C1 speed of light $3.0 \times 10^{8}$ m/s used
C1 time = distance ÷ speed in any form, or $6.3 \times 10^{11} \div 3.0 \times 10^{8}$
A1 2100 s / $2.1 \times 10^{3}$ s / 35 minutes; unit required

(b) A space probe travels from the Earth towards Jupiter. State how the strength of the Earth's gravitational field at the probe changes as the probe moves further from the Earth. 1 mark No calculator

Mark scheme

B1 it decreases / gets weaker; 'it becomes zero' is insufficient

(c) Explain why the planets orbit the Sun. 2 marks No calculator

Mark scheme

B1 the Sun contains most of the mass of the Solar System
B1 so the gravitational attraction of the Sun is the force that keeps each planet in orbit, owtte

Question 3. Astronomers have found five planets, P, Q, R, S and T, orbiting a distant star. The table shows some data about them. $\begin{matrix} Planet & Average orbital distance / million km & Orbital period / Earth days & Average surface temperature /^{\circ} C \\ P & 45 & 67 & 260 \\ Q & 80 & 160 & 450 \\ R & 130 & 330 & 20 \\ S & 600 & 3300 & - 150 \\ T & 1300 & 10 400 & - 190 \end{matrix}$

(a) Describe how the orbital period of a planet changes as its average orbital distance increases. 1 mark No calculator

Mark scheme

B1 the period increases as the distance increases / the further the planet, the longer its orbital period; 'directly proportional' contradicts the data and scores nothing

(b)(i) Identify the planet whose surface temperature does not fit the pattern shown by the other planets. 1 mark No calculator

Mark scheme

B1 Q

(b)(ii) Suggest a reason for the surface temperature of this planet. 1 mark No calculator

Mark scheme

B1 it has a thick atmosphere that traps heat / greenhouse effect, owtte

(c) Calculate the average orbital speed of planet R. 3 marks

Mark scheme

C1 $T = 330 \times 24 \times 3600 = 2.85 \times 10^{7}$ s seen, or days converted to seconds correctly at any stage
C1 $v = \frac{2 π r}{T}$ used with $r = 1.3 \times 10^{11}$ m
A1 $2.9 \times 10^{4}$ m/s (accept 28 600 to 28 700 m/s, or the same value in km/s, such as 29 km/s); unit required

(d) Predict whether the average orbital speed of planet T is greater than or less than that of planet R. 1 mark No calculator

Mark scheme

B1 less than R (T is further from the star, where its gravitational field is weaker)

Question 4. A comet orbits the Sun. Its orbit is a very elongated ellipse.

(a) State where the Sun is in the comet's orbit. 1 mark No calculator

Mark scheme

B1 not at the centre: off-centre, close to one end of the ellipse / at a focus of the ellipse, owtte

(b) Explain, using the conservation of energy, why the comet travels fastest when it is closest to the Sun. 3 marks No calculator

Mark scheme

B1 as the comet moves towards the Sun, its gravitational potential energy decreases
B1 this energy is transferred to kinetic energy, so the comet's speed increases
B1 the total energy stays constant (no energy is transferred away in space), so kinetic energy is greatest where gravitational potential energy is least, at the point closest to the Sun

How this is examined

Reading a planetary data table: describing a trend in words, naming the planet that does not fit, and calculating an orbital speed: a table of planets followed by short parts: a one-mark Describe of a trend, an Identify and a Suggest about the planet that does not fit, and a Calculate using $v = \frac{2 π r}{T}$ A trend written as a relationship: 'the greater the orbital distance, the longer the orbital period' (not 'proportional'). The odd one out named with a physical reason, such as 'hotter than a planet nearer the star because a thick atmosphere traps heat'. For the speed: the period converted to seconds and the distance to metres before substituting, and a unit on the answer.
Why the inner planets are small and rocky and the outer planets large and gaseous: a two-mark Compare of the two groups of planets, then an Explain that asks you to use the accretion model Comparison: 'the inner four are smaller and rocky; the outer four are larger and gaseous'. Explanation points: 'gravity pulled a rotating cloud of gas and dust into an accretion disc'; 'near the Sun it was hot, so only rock and metal could be solid'; 'solid particles collided and stuck together'; 'far out it was cold, so ice and gas collected and large cores pulled in hydrogen and helium'.
Why an object in an elliptical orbit is fastest when it is closest to the Sun: a three-mark Explain that tells you to use the conservation of energy Three linked points: 'gravitational potential energy decreases as it moves towards the Sun'; 'this is transferred to kinetic energy, so it speeds up'; 'total energy is constant, so kinetic energy is greatest where gravitational potential energy is least, closest to the Sun'.
How long light takes to travel between objects in the Solar System: a two- or three-mark Calculate, which may or may not give you the speed of light The speed $3.0 \times 10^{8}$ m/s recalled; time = distance ÷ speed, with the distance in metres; the answer with its unit, converted to minutes if the question asks.
What keeps the planets in orbit, and why it is the Sun they orbit: a one-mark State of the force, or a two-mark Explain of why the planets orbit the Sun 'The gravitational attraction of the Sun', with the Sun named; 'the Sun contains most of the mass of the Solar System', linked to its gravitational pull being the force that keeps each planet in orbit.
How gravitational field strength depends on a planet's mass and on distance from it: a one-mark State or Deduce about field strength at a planet's surface or further from it 'More mass, stronger field at the surface' (for planets of the same size); 'the field gets weaker as the distance from the planet increases'. 'Becomes zero' does not score.

Words to know

Solar System: The Sun and everything that orbits it: the eight planets, minor planets, moons and comets.
Orbit: The curved path of one object around another, kept on that path by gravitational attraction.
Planet: One of the eight large bodies orbiting the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Minor planet: A body that orbits the Sun but is much smaller than the eight planets, such as a dwarf planet or an asteroid.
Dwarf planet: A minor planet, such as Pluto, that orbits the Sun but is not counted as one of the eight planets.
Asteroid: A small rocky body orbiting the Sun; most asteroids are in the asteroid belt between the orbits of Mars and Jupiter.
Comet: A small body of ice and dust orbiting the Sun, often in a very elongated elliptical orbit.
Natural satellite: A natural object in orbit around a larger one; a moon is a natural satellite of a planet.
Accretion: The growth of a body as particles collide and stick together and gravity pulls in more material.
Accretion disc: The rotating, flattened disc of gas and dust around the young Sun from which the planets formed.
Gravitational field strength: The gravitational force per unit mass, $g = \frac{W}{m}$ , measured in N/kg.
Elliptical orbit: An orbit shaped like a stretched circle, with the Sun off-centre, nearer one end.
Orbital period: The time taken for one complete orbit.

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