Characteristics of Planets: What Makes Them Unique

Earth moves like a spinning top around its orbit. All of Earth's movements are irregular and vary over time. Comparison of the planets in the Solar System according to several physical and orbital parameters. Image source: astronoo.com (new window)

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

Planet	Mass (10²⁴ kg)	Density (kg/m³)	Diameter (km)	Average Distance (10⁶ km)	Aphelion / Perihelion (10⁶ km)	Orbital Speed (km/s)	Revolution Period (days)	Inclination (°)	Eccentricity	Escape Velocity (km/s)
Mercury	0.330	5427	4879	58	69.8 / 46.0	47.9	88	7.0	0.206	4.25
Venus	4.87	5243	12104	108	108.9 / 107.5	35.0	225	3.4	0.007	10.36
Earth	5.97	5514	12756	150	152.1 / 147.1	29.8	365.2	0.0	0.017	11.19
Mars	0.642	3933	6792	228	249.2 / 206.6	24.1	687	1.8	0.094	5.03
Jupiter	1898	1326	142984	778	816.6 / 740.5	13.1	4331	1.3	0.049	59.5
Saturn	568	687	120536	1430	1514.5 / 1352.6	9.7	10747	2.5	0.057	35.5
Uranus	86.8	1270	51118	2870	3003.6 / 2741.3	6.8	30589	0.8	0.046	21.3
Neptune	102	1638	49528	4495	4558.9 / 4444.5	5.4	59800	1.8	0.010	23.5