Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 6: SPACE PHYSICS
Our Solar System consists of 8 planets that orbit a central star - the Sun. You should have met this topic before and you may well have a good grasp of the structure of the Solar System already. For this course, you should be able to name the planets in order.
Figure 1: The planets in our Solar System
Distances not to scale!
WP | CC BY-SA 3.0 via Wikimedia Commons
In addition to the 8 planets, there are also many minor 'dwarf' planets that orbit the Sun. These are smaller objects that could - at some stage - collide with other objects such as the major planets. For this reason, Pluto is now classified as a dwarf planet as it could, at some stage in the future, collide with the much larger planet Neptune.
Moons orbit most of the planets. We only have 1 large moon, but this number varies widely. For example, Jupiter has 79 known moons at the time of writing.
In addition, there are smaller Solar System bodies, including comets which characteristically have highly elliptical orbits (see below), and other natural satellites.
In between Mars and Jupiter lies the asteroid belt. This is a ring of various sizes of rocky objects, some that are so large to be classed as minor (dwarf) planets (e.g. Ceres and Vesta.) Instead of gravity pulling these rocks together to form another rocky planet, it is believed that Jupiter's strong gravitational influence broke up any small planets and prevented their formation, leaving a large ring of rocky asteroids.
Figure 2: The asteroid belt (shown by black dots between the orbits of Mars and Jupiter)
NASA/McREL
Scientists think that our solar system formed from a giant cloud of dust and gas called a nebula. Gravity started to pull the atoms in the nebula together. As the cloud collapsed, any minor rotation that it had to start with will have made it spin faster and faster. This spinning motion acting along with gravity flattens the cloud into a disk called an accretion disk.(figure 3).
Figure 3: An accretion disc around a protostar (artist's impression)
NASA
The core of this disc gets hotter and hotter as gravity forces hydrogen and helium gas inwards, until eventually a fusion reaction starts. A star - the Sun - is born! (See the next section on stars for more details). Close to the Sun, only elements that stay solid at high temperatures, such as metals (like iron and aluminium) and rocky silicate compounds, will start to coalesce into clumps, forming the inner planets. Scientists believe this is why the four inner planets are rocky in composition, with a high concentration of metals.
The video below (an artists impression) shows inner planets being formed from materials in the early accretion disc around a star.
YouTube - Formation of Planets in a Protoplanetary Disk
NASA Video
The more volatile compounds (ones that form vapours easily) like methane and water turn into ices further from the Sun, and begin to form planets in the outer part of the accretion disc where temperatures are lower. As the total mass of these compounds is much larger than the heavier metals and rocky substances in the early solar system, the outer four planets are larger and gaseous in nature, called gas giants. In addition, once they started forming, their strong gravity allowed hydrogen and helium to be pulled inwards, adding to the mass. The Sun's radiation causes a pressure that pushes dust and gases outwards, and eventually the Solar System is cleared of nearly all dust and gas, leaving only objects we see today in orbit around the Sun.
In summary, you should be able to describe that the four planets nearest the Sun are rocky and small,
and the four planets furthest from the Sun are
gaseous and large. You should be able to 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.
The motion of galaxies, stars and planets are all ruled by the force of gravity. This force between any 2 objects depends on the masses of the objects and also the distance between them. The pull of gravity makes:
(An orbit is the path taken by an astronomical object as it moves around another).
In section 1.3 we met the concept of gravitational field strength (g). On Earth, this is always about 9.8 N/kg, but varies on other planets. For example, on the surface of Mars, g= 3.7 N/kg. This is much lower than on Earth because Mars has a much lower mass than Earth.
The further you move away from a massive object like a star or a planet, the weaker the gravitational field strength becomes. On the Earth's surface, g=9.8 N/kg, but if we could stand on a stationary platform 6000 km above the Earth's surface, the value of g would only be 2.6 N/kg. We would feel only about ¼ of our normal weight, and could jump 4 times higher!
The Sun contains most of the mass of the entire Solar System. This is why the Sun stays in the centre of our Solar System whilst the force of gravity makes the planets orbit around it.
Questions:
1. State the name of a planet in our solar system that is:
2. Choose words from the list below to complete this sentence:
Mars orbits the Sun in a near circular path because of the force of ____________ from ________.
Mars orbits the Sun in a near circular path because of the force of gravity from the Sun.
3. With reference to the original formation of our Solar System, explain why the inner planets are rocky and contain many metals.
The early solar system was formed from an accretion disc, containing dust and gas consisting of many elements and compounds. Only substances with high melting points could remain solid close to the Sun, so the inner planets formed from metals and rocky (silicate) compounds that clumped together ('coalesced') due to the force of gravity.
In the previous section on The Earth, you will have met the idea that all astronomical objects orbit in a path called an ellipse. Moons generally travel around planets in an ellipse that is close to a circular orbit. This means that they keep approximately the same distance from the planet at all times. This is also true of planets as they orbit the Sun. The Earth stays at approximately the same distance from the Sun all year as it travels around in its orbit.
Moons are natural satellites in orbit around planets. They tend to orbit the planets above the equator. However artificial satellites can take pretty much any orbit that is required. They can pass over the poles of a planet and can have a path that is not circular. Artificial satellites have been launched into space by humans and so their path and velocity can be fixed by rocket engines to the required orbit.
However, comets are very different. The distance from the Sun and the speed of a comet varies dramatically, as shown in figure 4:
Figure 4: The orbit of a comet
As a comet orbits the Sun, it's speed changes dramatically. This is because as gravity from the Sun pulls the comet inwards, it loses gravitational potential energy, and gains kinetic energy. (This is due to the law of conservation of energy - see section 1.7a.)
You can see this in action in this simulation, shown again here from the section on The Earth:
Figure 5: Elliptical Orbits explained
Phoenix7777 CC BY-SA 4.0
Notice how the planet with the highly elliptical orbit shown in purple speeds up as it nears the Sun.
One other point to note is the position of the Sun in figure 5. For the elliptical orbits, the Sun is positioned to one side, NOT in the centre of the ellipse. However for the red circular orbit shown, the Sun is indeed in the centre of the shape, as it is for Jupiter's orbit as shown in figure 4 above.
Did you know that Mercury is the closest planet to the Sun, and also has the fastest orbital speed? The further away from the Sun a planet is found, the lower the orbital speed. Different orbital paths have a speed that is fixed by the force of gravity at that point. As Mercury is close to the Sun, it experiences a stronger gravitational field than the Earth, and this means it orbits at a higher velocity.
Notice that in the simulation shown here in fig. 6, the inner planet overtakes the outer planet, and it is actually travelling significantly faster due to the higher gravitational field strength close to the blue star.
Figure 6: Simulation of 2 planets in orbit
GRC NASA
Note that as gravity pulls on a planet or moon during a circular orbit, it makes the direction change, but not the speed. This change in direction leads to a change in the velocity, as velocity is a vector quantity. (See section 1.1 for a recap about vectors). Therefore the velocity changes, but not the speed of the orbit.
For this course, you should be able to analyse and interpret data on the Solar System. Here is some information on the planets, followed by some typical exam questions:
Source:NASA
Questions:
4. Using data from the table above, state the name of the planet with:
5. Which planet has approximately four times the diameter of the Earth?
The Earth has a diameter of 12,800 km to 3 sig. figs.
12,800 x 4 = 51,200 km.
The closest to this figure is the planet Uranus.
6. Explain why the planets Saturn and Neptune are travelling at different speeds around the Sun.
Saturn and Neptune are in different orbits. (Neptune is much further from the Sun). The gravitational field strength from the Sun is weaker for Neptune than Saturn, and causes it to orbit at a much slower speed than Saturn.
As Neptune orbits at a much slower speed than Saturn, and also has much further to travel, so the time period for 1 orbit is MUCH longer than the time period for Saturn as shown in the data table.
Here's a quick 10 minute quiz about the Solar System:
