Last update: August 29, 2025

Rocky planets and gas giants: two families, two histories

Defining what a planet is

A planet is a celestial body that orbits the Sun, has sufficient mass for its own gravity to give it a spherical shape — a state called hydrostatic equilibrium — and has cleared its orbital neighborhood of other bodies. This definition, adopted on August 24, 2006 during the 26th IAU General Assembly, resulted from lengthy discussions among several hundred scientists.

An organized and remarkably flat solar system

If we observed the Solar System from a point far above Earth's North Pole, we would see the planets orbiting the Sun counterclockwise. Their orbits form a surprisingly flat disk, reflecting their formation within a single primordial cloud. Only Mercury stands out with a notable orbital inclination of about 7° relative to the general plane.

Two great families of worlds

The rocky planets — Mercury, Venus, Earth, and Mars — are located close to the Sun and have a solid surface. Farther out, the gas and ice giants — Jupiter, Saturn, Uranus, and Neptune — dominate by their size and composition rich in gases and ices. These two families testify to different formation processes, revealing the diversity and complexity of our Solar System.

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