Characteristics of Planets: What Makes Them Unique

Mass and Density

The mass of the planets varies considerably, from small Mercury to massive Jupiter. In parallel, density reveals their internal nature: terrestrial planets (Mercury, Venus, Earth, Mars) are denser than gas giants. This reflects their composition dominated by metallic and silicate elements, unlike the giants rich in hydrogen and helium.

Diameter

The planetary diameter reflects the physical size of a planet. It is strongly linked to mass but also depends on composition. Jupiter, the largest, exceeds 140,000 km, while Mercury is less than 5,000 km. The giants have an extended atmosphere, inflating their apparent radius.

Distance to the Sun, Aphelion and Perihelion

The average distance of a planet from the Sun defines its overall temperature, but extreme distances (aphelion and perihelion) modulate seasonal variations. Mercury and Pluto (not listed here) have strong eccentricities, while Venus and Neptune have nearly circular orbits.

Orbital Speed and Revolution Period

The average orbital speed depends on the distance from the Sun, via Kepler's third law. The closer a planet is to the Sun, the higher its speed and the shorter its revolution period. Mercury completes its orbit in 88 days, compared to 165 years for Neptune.

Inclination and Eccentricity

Inclination determines the angle of the orbit relative to the ecliptic plane, and eccentricity measures its elongation. These two factors influence the seasonal and orbital climate variations of the planets. Uranus, with its highly inclined axis, experiences extreme seasons.

Escape Velocity

Escape velocity is the minimum speed needed to escape a planet's gravitational attraction. It is proportional to its mass and inversely to its radius. This conditions the retention of an atmosphere: only massive and cold planets can retain light gases like hydrogen.

Comparative Table of Physical and Orbital Parameters of the Planets