Which are the 4 Inner Planets and which are the 4 Outer Planets? Give 3 differences between these 2 types of planets. The inner planets mercury, Venus, Earth and Mars

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Year 10 Astronomy Revision Name: _________________________________

  1. Arrange the 8 planets in order from the Sun.
    1. Mercury 2. Venus 3. Earth 4. Mars 5. Jupiter 6. Saturn 7. Uranus 8. Neptune

  2. Which are the 4 Inner Planets and which are the 4 Outer Planets?
    Give 3 differences between these 2 types of planets.

Mercury, Venus, Earth and Mars

Jupiter, Saturn, Uranus and Neptune

Small size

Earth is the largest of the Inner Planets, with a diameter of 12,756 km (7,926 miles). Mercury is the smallest with a diameter of 4,878 km (3,031 miles)


Jupiter, the largest planet, has a diameter of 142,984 km (88,846 miles). Neptune is the smallest of the Outer Planets with a diameter of 49,532 km (30,779 miles)

Have solid surfaces and thin/no atmospheres

In theory, it would be possible to stand on each of the Inner Planets, although you would only survive on Earth.

Balls of gas with no surface

Most of the Outer Planets are made of gas. It is likely that they have a much smaller solid or liquid centre. It would be impossible to stand on any of the Outer Planets.

Greater Density

The size and composition of the planets is caused by the density of the elements that make up the planets. The elements in the Inner Planets are more closely packed together, causing them to be smaller on solid.

Smaller Density

Despite being larger, the elements that make up the Outer Planets are less densely packed together causing them to be quite light for their size.

Varied atmospheres

The contents of the atmospheres of the Inner Planets varies from planet to planet. Mercury has no atmosphere although Sodium and Helium can be detected above the surface. Venus' atmosphere is mostly Carbon Dioxide with a very small amount of Nitrogen. Earth's atmosphere is mostly Nitrogen with a smaller amount of Oxygen and even smaller amounts of other gases. Mars has a similar composition of carbon dioxide and nitrogen as Venus although has a much thinner atmosphere.

Similar atmospheres

The atmospheres of the Outer Planets consist mostly of Hydrogen and Helium, with Methane also being present in the atmospheres of Uranus and Neptune. Other gases are present although in much smaller quantities.

Known by the ancients

The existence of the Inner Planets has been known about for thousands of years. The earliest astronomers didn't know that the four objects (including Earth) were planets, but they knew they existed.

Not known by the ancients

Of the Outer Planets, only Jupiter and Saturn were observed by ancient astronomers. The existence of Uranus and Neptune was not known until relatively recently. Uranus was discovered in 1781 and Neptune in 1846.

Spin slowly

Compared to the much larger Outer Planets, the Inner Planets spin quite slowly. Earth spins the quickest, taking 23 hours and 56 minutes to spin on its axis. Venus takes 243 days to spin on its axis, spinning in an opposite direction to the other planets.

Spin quickly

All of the Outer Planets spin quicker than the Inner Planets. Uranus spins slowest, taking 17 hours and 14 minutes to spin on its axis. Jupiter takes only 9 hours and 55 minutes to spin on its axis. This rapid rotation causes Jupiter and Saturn to appear squashed, wider across the equator than from top to bottom.

Orbit the Sun quickly

Because they are quite close to the Sun, the Inner Planets complete an orbit quickly. Mercury takes only 88 days to orbit the Sun. Mars takes 687 days.

Orbit the Sun slowly

The Outer Planets orbit the Sun from millions of miles and have a much greater distance to cover to complete an orbit, so take much longer to do so. Jupiter takes almost 12 years to complete an orbit and Neptune takes over 164 years.

Few Moons

Only Earth and Mars have moons orbiting them. One moon orbits Earth and two small moons orbit Mars.

Lots of Moons

All of the Outer Planets have many moons orbiting them. There are 63 moons known to orbit Jupiter, 60 orbiting Saturn, 27 orbiting Uranus and 13 orbiting Neptune.

No rings
None of the Inner Planets have rings orbiting them


All of the Outer Planets have rings orbiting them. The rings are thin discs of dust and rocks possibly caused by moons being broken up or not being completely formed while orbiting the planet. Saturn has the most visible ring system of any of the planets.

Multiple space craft visitors

Due to being close to Earth, there have been several missions to the other Inner Planets, especially to Mars and Venus. Mercury has been visited by two spacecrafts.

All Outer Planets visited by one space craft

There have been multiple visits to Jupiter and Saturn, but Uranus and Neptune have only been visited once. This was by Voyager 2 (which also visited Jupiter and Saturn).

  1. Why Pluto is no longer a Planet (since 2006)?
    It occurred during the 2006 International Astronomical Society convention. Pluto was re-classified as a 'Dwarf Planet' because of even larger objects discovered further out from Pluto. It's thought that up to *50,000* Pluto-sized objects and larger could exist in the Kuiper belt, so, rather than have to come up with names for that many planets, Pluto's classification was changed, and it's now the first "Kuiper Belt Object" discovered.
    The orbit is not in the same plane as the orbits or other planets.

  2. What is the meaning of orbit in astronomy? Give 3 examples of celestial objects that orbit others.
    Orbit means to revolve around another object.

Planets orbit the Sun, comets orbit the Sun, asteroids orbit the Sun, moons orbit planets, artificial satellites orbit planets, stars can orbit each other (any 3).

  1. Arrange these celestial objects according to their size in descending order.
    Milky Way, Super-cluster, Universe, Earth, Meteoroid, Jupiter, Sun, Local Group, Mars, Asteroid, Solar System, our Moon.
    Meteoroid  Asteroid  Our Moon  Mars  Earth  Jupiter  Sun  Solar System  Milky Way Galaxy  Local Group  Super-cluster  Universe

  2. What are the 2 opposite forces the control or maintain the integrity of a star? Explain briefly.
    Gravitational force as a result of the mass, pulling the star inward, against the outward radiation force from fusion of hydrogen.

  1. Describe briefly the structure of our Sun.
    Much like the earth, the Sun has many different layers that define its structure. Unlike the earth, the Sun is completely gaseous, there is no solid surface on the Sun. Although the Sun is completely made of gas, the density and temperature of the gas changes drastically as you travel from the center to the outermost regions.

  2. What is Nuclear Fission in the core of the Star?
    Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright.
    Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium.

  1. What determines the formation of other elements in stars?

A star's mass determines what other type of element undergoing fusion in its core (or during explosive changes in its life cycle)

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