The Solar System: Order, Sizes and Facts of the Planets
Our solar system consists of the Sun and everything bound to it by gravity - eight planets, their moons, dwarf planets, asteroids, comets and meteoroids. In examinations this topic is asked through the order of the planets, the difference between the inner and the outer planets, and the special record of each planet, such as the hottest, the largest or the one that spins on its side. The facts are direct, so a clear list of the eight planets and their unique features is the key to scoring.
Order of the planets
There are eight planets in our solar system. In order of increasing distance from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury is the closest planet to the Sun and Neptune is the farthest planet from the Sun. Between the orbits of Mars and Jupiter lies the asteroid belt, a region of rocky objects. The Sun contains about 99.86 per cent of the total mass of the solar system, so the planets together are only a tiny fraction.
The inner or terrestrial planets
The inner planets are Mercury, Venus, Earth and Mars. They are also called the terrestrial planets because they are small, rocky planets with solid surfaces. Mercury is the smallest planet and the closest to the Sun, and it is a world of extremes, very hot by day and very cold by night. Venus is the hottest planet because of its thick carbon dioxide atmosphere that causes a runaway greenhouse effect; it is also called Earth's twin because of its similar size and mass, and it appears as the Morning Star and Evening Star. Earth is the only planet known to support life, is the densest planet, and is called the Blue Planet because of its abundant water. Mars is called the Red Planet because of the iron oxide (rust) on its surface, and it has the largest volcano in the solar system, Olympus Mons, about 22 kilometres high.
The outer or giant planets
The outer planets are Jupiter, Saturn, Uranus and Neptune, which are large and mostly made of gas and ice. Jupiter is the largest planet, and it is famous for the Great Red Spot, a giant storm larger than the Earth. Saturn is famous for its spectacular ring system made of ice and rock particles, and it has the most confirmed moons (over 140 in recent counts), though Jupiter also has many. Uranus is the planet that rotates on its side, with an axial tilt of about 98 degrees, and Neptune has the strongest winds in the solar system, reaching up to about 2,100 kilometres per hour; Neptune is also blue because of methane in its atmosphere.
Special records and movements
Several planetary records are asked again and again. Mercury and Venus have no moons. Venus and Uranus spin in the opposite direction (retrograde motion) compared with most planets, and Venus has a day longer than its year - one Venusian day takes about 243 Earth days while its year takes about 225 Earth days. Jupiter, Saturn, Uranus and Neptune are much larger than the four inner planets, and the four giants together account for most of the planetary mass. Pluto, once counted as the ninth planet, was reclassified as a dwarf planet in 2006, which is why the solar system now has eight planets.
Quick revision
For quick revision, remember the order "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune" and separate the small rocky inner planets from the large gas and ice giants. Fix the records: Jupiter the largest, Mercury the smallest and closest to the Sun, Venus the hottest and Earth's twin, Earth the densest and the only one with life, Mars the Red Planet with Olympus Mons, Saturn with the most moons and the brightest rings, Uranus tilted on its side and Neptune the windiest and farthest. Finally note that Mercury and Venus have no moons, Venus and Uranus rotate in the opposite direction, and Pluto is now a dwarf planet.
How to prepare this topic
- Memorise the order of the eight planets from the Sun with a mnemonic, and always add the asteroid belt between Mars and Jupiter.
- Make two columns - inner rocky planets and outer giant planets - and write one key feature against each planet.
- Learn the record-holders: largest, smallest, hottest, densest, most moons, strongest winds and the planet that rotates on its side.
- Keep the tricky facts ready: Venus has a day longer than its year, Venus and Uranus are retrograde, and Mercury and Venus have no moons.
- Remember that Pluto is a dwarf planet, so there are eight planets and not nine.
Common exam traps
- Venus, not Mercury, is the hottest planet, even though Mercury is closer to the Sun.
- Saturn has the most confirmed moons, not Jupiter, although Jupiter is the largest planet.
- Uranus rotates on its side, while Venus rotates backwards; both are exceptions to the usual direction.
- Mercury and Venus have no moons; Mars has two small moons.
- Pluto is a dwarf planet since 2006, so the solar system has eight planets.
Frequently Asked Questions
What is the correct order of the planets and which are the inner and outer planets?
In order of increasing distance from the Sun, the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The inner or terrestrial planets are Mercury, Venus, Earth and Mars, which are small and rocky. The outer or giant planets are Jupiter, Saturn, Uranus and Neptune, which are large and mostly made of gas and ice. The asteroid belt lies between the orbits of Mars and Jupiter.
Which is the largest, smallest, hottest and densest planet?
Jupiter is the largest planet in the solar system, and Mercury is the smallest and the closest to the Sun. Venus is the hottest planet because of its thick carbon dioxide atmosphere that causes a runaway greenhouse effect, and it is also called Earth's twin. Earth is the densest planet and the only planet known to support life.
Which planets have noteworthy special features or records?
Mars is called the Red Planet because of the iron oxide on its surface and has the largest volcano, Olympus Mons. Saturn is famous for its bright ring system and has the most confirmed moons, while Jupiter is the largest planet and has the Great Red Spot. Uranus rotates on its side with a tilt of about 98 degrees, and Neptune has the strongest winds, reaching up to about 2,100 kilometres per hour, and is the farthest planet.
Which planets have no moons and which planets rotate in the opposite direction?
Mercury and Venus have no moons. Venus and Uranus spin in the opposite or retrograde direction compared with most planets. Venus also has a day that is longer than its year - one Venusian day takes about 243 Earth days, while its year takes about 225 Earth days. Mars, by contrast, has two small moons.
Why does the solar system have eight planets and what is the asteroid belt?
The solar system has eight planets because Pluto, once counted as the ninth planet, was reclassified as a dwarf planet in 2006. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The asteroid belt is a region of rocky objects that lies between the orbits of Mars and Jupiter.