What is the nature of the planets in our solar system?

Terrestrial planets

: Mainly composed of rocks and metals, they have a solid surface and a generally thin atmosphere. They are located near the Sun, particularly in the inner system. Among them, Earth, Venus, Mars, and Mercury stand out for their dense composition and complex geology. They often have natural satellites, such as the Moon for Earth, or Phobos and Deimos for Mars.

• Gas giants: Massive and composed mainly of hydrogen and helium, these planets have a thick atmosphere and little discernible solid surface. They occupy the outer zone of the system, including Jupiter, Saturn, Uranus, and Neptune. Their environment is punctuated by rings and numerous multiple satellites, enriching their complexity.
• The fundamental difference therefore lies in their chemical composition, density, and mass. The presence or absence of water, ice, or gas directly influences their behavior and evolution. For example, Jupiter and Saturn, whose masses exceed all the other planets combined, are gas giants that dominate the system through their gravitational force. While Mercury and Venus, smaller, have a rocky surface, sometimes molten or covered with active volcanoes, as on Venus, notably analyzed by CNES during its numerous missions. Category

Examples

Main composition	Relative mass	Average distance from the Sun	Terrestrial planets	Mercury, Venus, Earth, Mars
Rocks, metals	Less than 1 Earth mass	0.4 to 1.5 astronomical units	Gas giants	Jupiter, Saturn, Uranus, Neptune
Hydrogen, helium	More than 15 Earth masses	5 to 30 astronomical units	Terrestrial planets: rocky worlds with striking characteristics	The planets referred to as terrestrial extend from Mercury to Mars, including Venus and Earth. Their internal structure is full of peculiarities, with each stage of their compilation offering surprises. The planet Mercury, visited by the NASA probe, has a liquid iron core and a cratered surface, bearing witness to a tumultuous past markedly different from Earth’s.

Venus, nicknamed Earth’s twin sister, has an atmosphere composed of 96% carbon dioxide, which causes an extreme greenhouse effect, making its surface inaccessible to any life as we know it. The temperature there reaches 735 K, enough to melt certain materials, a truth revealed during CNES expeditions. On its surface, high mountains, volcanoes, and endless plains bear witness to active volcanism, documented by various space missions.

Planet Earth, our cradle, remains the only place where life thrives. Its oxygen-rich atmosphere, abundant oceans, and active tectonics create a unique environment in the solar system. The presence of a natural satellite, the Moon, plays a crucial role in climate stabilization and tidal generation. Mars, meanwhile, remains a key target for space exploration, sparking hypotheses about past and present life. Planet Diameter (km)Main Composition

Average Temperature (K)

Satellites

Mercury	4,878	Iron, silicates	90–700 depending on location	0
Venus	12,104	Silicates, metals	735 (surface)	0
Earth	12,756	Silicates, iron, oxygen	288 (average)	1 (the Moon)
Mars	6,779	Silicates, iron oxides	-63 to 20 (average temperature)	2 (Phobos, Deimos)
Gas giant planets: silent but dynamic worlds	Gas giants are distinguished by their predominantly gaseous composition and impressive size. Jupiter, their queen, has a complex system of thin rings, violent winds, and a myriad of satellites, including Ganymede, the largest moon in the solar system. The Great Red Spot, a centuries-long storm of fascination, is presented in detail in the analyses of planetary news.	Saturn, famous for its incredibly luminous rings, is home to a multitude of moons, including Titan, which has a dense atmosphere and methane lakes. The probe is exploring Saturn’s internal structure in detail to understand its evolution. Its rich environment is still partly a mystery, but its powerful magnetic field produces spectacular auroras.	Discover the mysteries of our solar system and beyond with our fascinating exploration of the planets. Learn more about the unique characteristics, atmospheres, and curiosities of each planet, and delve into the captivating world of celestial bodies. Ice Giants: Mysterious Worlds, Between Water and Ice	Uranus and Neptune, often referred to as « ice giants, » reveal compositions rich in water, ammonia, and methane. The atypical rotation of the former, which turns on its side, reveals the complexity of the formation of these globes: their exceptional inclination increases the interest of astrophysicists. Their cold and inactive atmosphere contrasts with the turbulent Neptune, which exhibits a dark storm similar to the Great Red Spot.

A remarkable feature is that their magnetic system is distorted, as highlighted in the dedicated article

Among their many moons, notably Miranda and Triton, some may harbor subterranean oceans, reviving questions about microbial life in extreme environments. Their study is being entrusted to new European and French missions, notably those of theSociété Française de Propulsion.

Planet Diameter (km) Main composition

Known satellites

Uranus

50,724Ices, silicates49 (extreme cold) 27 (several including Miranda, Oberon)Neptune

49,244	Ices, hydrogen	55 (extreme cold)	14 (including Triton)	A future perspective: the exploration of exoplanets and the link with our system
Discoveries in astrophysics, notably through the	Habitable Exoplanets 2025	project, offer a new dimension to understanding the nature of planets. European planetary missions, in close collaboration with NASA and CNES, seek to characterize these distant worlds that could resemble or even surpass the complexity of our own neighborhood. The search for habitability, magnetic fields, and active volcanoes allows us to better understand the processes that govern our planets, while avoiding anthropocentrism. These studies remain essential for anticipating the future of our own star—the Sun—and the destiny of Earth. A detailed understanding of the atmospheres, chemical composition, and geological mechanisms of outer planets like Neptune or Uranus, which can be visualized, for example, on planetary maps, opens the way to the search for similar worlds in distant systems. This is no longer a mere curiosity, but a major challenge for astrobiology, in partnership with the luminous objects sector and the synthesis of space data.	Discover everything you need to know about the planets in our solar system and beyond. Explore their characteristics, their atmospheres, and the latest scientific discoveries that fuel our understanding of these fascinating worlds.	Frequently Asked Questions: Everything you need to know about the nature of the planets in the solar system
What criteria define a planet in the solar system?	A planet must orbit the Sun, have sufficient mass to form a spherical shape, and have cleared its vicinity of smaller objects, explains the International Astronomical Union. Most of its characteristics also fit within the scientific model, enriched by ESA Satellite missions, which allow us to study their atmosphere, geology, and internal composition.	Could gas planets one day become rocky?	This is unlikely in the near future, as their mass and composition give them their own dynamics. However, billions of years from now, the partial loss of gas could facilitate the formation of denser cores, similar to the hypotheses for Neptune and Uranus. More generally, the boundary between rocky and gaseous planets is still a subject of debate in studies conducted at the Astrophysics Research Center.	What is the most dangerous composition of planetary atmospheres?

Atmospheres rich in toxic or corrosive gases, such as carbon dioxide or sulfuric acid, constitute extreme environments. Venus is a perfect example, where the pressure is equivalent to that of a deep ocean floor, making human exploration difficult. The composition and stability of these atmospheres fascinate researchers seeking to understand planetary evolution, with a critical eye on possible habitability elsewhere in the galaxy.