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INNER
PLANETS |
OUTER
PLANETS |
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Small
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) |
Huge!
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) |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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 |
Rings
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). |
