Definition · Plain-language
The planets in order
The eight planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
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The order from the Sun outwards
Starting nearest the Sun and moving outward, the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A popular memory aid is the sentence “My Very Easy Method Just Speeds Up Naming”, where each initial letter matches a planet in order. Mercury is the closest and smallest; Neptune is the most distant. All eight orbit the Sun in the same direction and roughly the same flat plane, a legacy of forming from the same spinning disc of gas and dust.
Inner rocky planets
The four inner planets — Mercury, Venus, Earth and Mars — are the terrestrial planets: relatively small, dense worlds with solid rocky surfaces. They formed close to the Sun where it was too hot for ices to survive, so they are made chiefly of rock and metal. Earth is the only one known to support life and to have abundant surface water. Mars and Venus have atmospheres; Mercury has almost none. These rocky worlds are separated from the giants beyond by the asteroid belt.
Outer giant planets
Beyond the asteroid belt lie the four giant planets. Jupiter and Saturn are gas giants — vast worlds made mostly of hydrogen and helium, with no solid surface to stand on. Uranus and Neptune are ice giants, containing more water, ammonia and methane ices alongside their gases. All four are far larger than the rocky planets, have many moons and possess ring systems, though only Saturn’s rings are bright enough to be famous. They formed in the cold outer Solar System where ices could survive.
Where Pluto fits in
Pluto was regarded as the ninth planet from its discovery in 1930 until 2006, when the International Astronomical Union adopted a formal definition of a planet. To qualify, a body must orbit the Sun, be massive enough to be roughly round, and have cleared its orbital neighbourhood of other objects. Pluto meets the first two tests but shares its region with many similar icy bodies, so it was reclassified as a dwarf planet. Other dwarf planets include Ceres, Eris, Haumea and Makemake.
Key facts
At a glance
- Order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Inner four: small rocky (terrestrial) planets
- Outer four: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants)
- Closest: Mercury — Farthest: Neptune
- Memory aid: “My Very Easy Method Just Speeds Up Naming”
- Pluto: reclassified as a dwarf planet by the IAU in 2006
Common misconceptions
What people often get wrong
Often heard: Pluto is still the ninth planet of the Solar System.
Actually: The International Astronomical Union reclassified Pluto as a dwarf planet in 2006, because it has not cleared its orbital region of other icy bodies. The Solar System now has eight recognised planets.
Often heard: The largest planets are also the closest to the Sun.
Actually: The opposite is true. The four nearest planets are the small rocky worlds; the giants — Jupiter, Saturn, Uranus and Neptune — orbit far out, beyond the asteroid belt, where ices could survive as they formed.
Often heard: You could stand on the surface of a gas giant like Jupiter.
Actually: Gas giants have no solid surface to stand on — they are mostly hydrogen and helium that thicken with depth into liquid and exotic states. Only the rocky inner planets have firm ground.
