What makes a world a planet?
What Is a Planet?
In 2006, the International Astronomical Union (IAU) gave planets a formal definition for the first time. To qualify, a body must:
- Orbit the Sun
- Have enough mass for gravity to pull it into a roughly spherical shape
- Have "cleared the neighbourhood" around its orbit — meaning its gravity dominates that region of space
Eight objects in our solar system meet all three criteria. Pluto meets the first two but not the third — which is why it was reclassified as a "dwarf planet" that same year.
Simple way to remember: Think of a planet as the boss of its neighbourhood. It is not just orbiting the Sun — it has gravitationally swept up or ejected everything else nearby.
The Rocky Planets (Terrestrial Planets)
Did you know?
The inner planets are called rocky planets because they have solid surfaces made mainly of rock and metal.
The four inner planets — Mercury, Venus, Earth, and Mars — are smaller and denser than the outer planets. They are the worlds most similar to a solid ground beneath your feet, even though their conditions are very different from Earth.
The Gas Giants
Did you know?
Jupiter and Saturn are so large that their gravity shapes the movement of moons, comets and asteroids around them.
Jupiter and Saturn are the gas giants. They are made mostly of hydrogen and helium and do not have a solid surface like Earth. Their clouds, storms, rings and moons make them some of the most fascinating worlds in the Solar System.
The Ice Giants
Did you know?
Uranus and Neptune are called ice giants because they contain large amounts of water, ammonia and methane deep inside them.
Uranus and Neptune are far from the Sun, extremely cold and still not fully understood. They show us that the outer Solar System is not empty; it is filled with strange, beautiful and powerful worlds.
What About Pluto?
Pluto was discovered in 1930 and called the ninth planet for 76 years. When astronomers began finding other Pluto-sized objects in the outer solar system (like Eris, which is slightly larger), they realised the problem: if Pluto was a planet, so were dozens of other objects.
In 2006, the IAU voted to reclassify Pluto as a dwarf planet. It sits in the Kuiper Belt — a ring of icy bodies beyond Neptune. Pluto has five moons, the largest being Charon, which is so big relative to Pluto that the two bodies effectively orbit each other.
The debate continues: Many planetary scientists still argue Pluto should be a planet. The 2006 vote was controversial — only 4% of the IAU's members voted on it. The question of what counts as a planet is genuinely unresolved in science.
Planet Comparison at a Glance
| Planet |
Type |
Diameter (km) |
Distance from Sun |
Moons |
| Mercury |
Rocky |
4,879 |
57.9M km |
0 |
| Venus |
Rocky |
12,104 |
108.2M km |
0 |
| Earth |
Rocky |
12,742 |
149.6M km |
1 |
| Mars |
Rocky |
6,779 |
227.9M km |
2 |
| Jupiter |
Gas giant |
139,820 |
778.5M km |
95 |
| Saturn |
Gas giant |
116,460 |
1.43B km |
146 |
| Uranus |
Ice giant |
50,724 |
2.87B km |
27 |
| Neptune |
Ice giant |
49,244 |
4.5B km |
16 |
Go beyond the list
Do not compare planets by size alone—ask what made each world different
All eight planets formed from the same broad disc of material around the young Sun, yet they did not receive the same ingredients or develop under the same conditions. Close to the Sun, heat favoured rock and metal. Farther out, colder conditions allowed more ice and gas to collect. Size then changed what each planet could hold: stronger gravity helped the giant planets retain thick atmospheres, while smaller worlds lost gases more easily.
A useful planet comparison therefore follows a chain: where did it form, what is it made from, how strong is its gravity, what atmosphere did it keep, and how has heat changed it? That chain explains more than memorising diameters.
Which planet is hottest?
Venus, not Mercury. Mercury receives more direct sunlight, but Venus has a massive carbon-dioxide atmosphere that traps heat through an extreme greenhouse effect.
Which worlds have ground?
Mercury, Venus, Earth and Mars have rocky surfaces. Jupiter and Saturn become denser with depth but do not offer a normal solid surface. Uranus and Neptune are also deep fluid worlds.
Where might water exist?
Earth has surface oceans, Mars stores large amounts of ice, and several moons around giant planets may hide oceans beneath ice. “Water exists” does not automatically mean “humans can live there.”
Why study hostile planets?
Extreme worlds test ideas about atmospheres, climate, geology and magnetic fields. Venus helps scientists think about runaway warming; Mars preserves evidence of a wetter past; giant planets show how whole systems of moons form.
Evidence, not guesswork
How can scientists know what a planet is made of without landing on it?
Telescopes separate reflected or emitted light into a spectrum. Different atoms and molecules leave recognisable patterns in that light, allowing scientists to identify gases such as hydrogen, methane or carbon dioxide. A planet’s movement reveals its orbit, while the gravitational effect on moons or spacecraft helps calculate its mass.
Spacecraft add close measurements. Cameras map clouds and surfaces, radar can look through thick cloud or measure terrain, magnetometers detect magnetic fields, and radio signals reveal small changes in motion. On Mars, landers and rovers can examine rock directly. The strongest explanations combine several independent measurements rather than depending on one photograph.
That is why these guides distinguish between what has been measured, what a model explains well, and what remains uncertain. Continue with gravity and orbits to understand how motion reveals mass, or open an individual planet below for its evidence and mission history.
Frequently Asked Questions
Questions that make the eight planets easier to understand
Why are the four inner planets rocky?
The inner Solar System was too warm for many volatile materials to freeze when the planets formed. Rock and metal could remain solid, so those materials became the main building blocks.
Why are Jupiter and Saturn called gas giants?
Most of their visible and bulk composition is hydrogen and helium. They do not have an accessible solid surface like Earth, although pressure creates very dense and unusual material deep inside.
Why are Uranus and Neptune called ice giants?
Their interiors contain a larger proportion of materials astronomers call “ices,” including water, ammonia and methane, compared with Jupiter and Saturn. Under planetary pressure these materials are hot, dense fluids rather than ordinary ice cubes.
Could another planet replace Earth as a human home?
None is a ready replacement. Every other planet has serious combinations of temperature, pressure, radiation, atmosphere, gravity or distance. Exploration may allow small protected habitats, but protecting Earth remains far easier than making another planet Earth-like.