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The Inner Planets v The Outer Planets
Our Solar System has eight known planets divided into two categories – the inner and the outer planets.
The first group consists of the four ones that are closest to the Sun: Mercury, Venus, Earth, and Mars.
They are also called terrestrial or rocky planets and are separated from the second group by the
asteroid belt, a region occupied by numerous asteroids. Further from this area and the Sun, lie the
outer or gaseous planets: Jupiter, Saturn, Uranus, and Neptune. They are also called the gas giants or
the Jovian planets.
During the first stages of formation, the gases near the Sun were blow away by the solar wind. The
result was that the planets closer to the Sun were left only with their rocky and heavy elemental
materials, while the most distant ones maintained their gaseous form and continued to grow by
accumulating even more gas.
Surface Composition The inner planets have rocky surfaces and dense metal cores. The outer planets
consist of gas layers and have liquid cores. Theoretically, you could stand on the four inner planets
(though you may not survive anywhere but Earth) but you could not stand on the outer planets because
their surfaces are not solid, they are gas.
Density The inner planets are denser, smaller, and more solid than the outer planets. That is because
the elements of the inner planets are closer together and densely packed. The elements of the outer
planets are loosely packed and are therefore less dense.
Atmosphere The atmospheres of the inner planets vary a lot. Earth has an atmosphere mostly consisting
of nitrogen, a small percentage of oxygen and smaller amount of other gases. Carbon dioxide and small
amounts of nitrogen can be found on both Venus and Mars, and Mercury almost lacks any significant
amount of gas. All outer planets have similar atmospheres mainly consisting of hydrogen and helium.
The atmospheres of Uranus and Neptune have methane in addition to hydrogen and helium.
Size One of the basic differences is related to the size of the planets. The inner planets are small;
Mercury is the smallest with a diameter of 3,031 miles and Earth the biggest one with a diameter of
7,926 miles. The outer planets are much larger with Jupiter having the largest diameter among them
(88,846 miles) and Neptune the smallest (30,779 miles).
Position to the Sun Mercury is the planet closer to the Sun. The next planet is Venus, then Earth, then
Mars. Those are the inner planets. The next planet (a good distance away) is Jupiter. Then Saturn, next
is Uranus, and finally is Neptune. Those are the outer planets.
Moons The inner planets have few moons. Only one moon orbits the Earth and two moons orbit Mars.
The situation is very different for the outer planets. Jupiter alone has 63 known moons and Saturn has
60. Uranus and Neptune have 27 and 13 moons respectively.
Rings A common feature among the outer planets is the fact that thin rings or discs of dust particles
and rocks orbit around them.
Relative Temperature The outer planets emit more heat than the inner planets; however, the inner
planets are warmer because of their position to the Sun.
Revolution/Orbit (Length of a Year) A complete orbit for the planets close to the Sun ranges from 88 to
687 days. The time scale for the outer planets is measured in years due to the longer orbits the planets
have to cover. Jupiter makes a complete revolution in 12 years and Neptune in 164 years.
Rotation The small planets rotate at a much slower pace than the bigger ones. To make some
comparisons, Venus takes 243 days to spin around its axis, whereas Jupiter takes only 9 hours and 55
minutes.
Annotation Guide:
1. How does a planet’s position to the Sun affect their composition?
Use evidence from the text to support your answer. [Hint: Why are the
inner planes known as the “rocky planets” and why are the outer
planets known as the “gas giants?”
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2. Why can we only live on Earth? Provide three reasons and use
evidence from the text.
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3. Compare and contrast the average length of a year and the average
length of a day for inner planets and outer planets. Use evidence from
the text to support your answer.
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