The Inner Planets
Mercury, Venus, Earth and Mars
The Outer Planets
Jupiter, Saturn, Uranus and Neptune
All of the inner planets are quite small, with Earth being the largest of the inner planets. It has 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 in the solar system, has a diameter of 142,984 km (88,846 miles) and is more than 10 times wider than Earth. Neptune is the smallest of the outer planets with a diameter of 49,532 km (30,779 miles), and is over 4 times wider than Earth.
Have solid surfaces and thin or no atmospheres
Balls of gas with no surface
In theory, it would be possible to stand on each of the inner planets, although you would only survive on Earth.
The outer planets are mostly 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.
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 and solid.
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.
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.
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
Not 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.
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.
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.
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
Orbit the Sun slowly
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.
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.
Lots of Moons
Only Earth and Mars have moons orbiting them. One moon orbits Earth and two small moons orbit Mars.
All of the outer planets have many moons orbiting them. There are 79 moons known to orbit Jupiter, 82 orbiting Saturn, 27 orbiting Uranus and 13 orbiting Neptune.
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 spacecraft visitors
All outer planets visited by one spacecraft
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 spacecraft with a third on its way.
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).