Planets of the Solar System UPSC: Inner and outer planets

The solar system comprises eight planets that orbit the Sun, each with unique characteristics that offer significant scientific insight and exploration potential. These planets are divided into two categories: the inner or terrestrial planets, composed mostly of rock and metal, and the outer or gas giant planets, composed mostly of hydrogen and helium with thick atmospheres. Understanding these planets’ composition, atmospheric conditions, surface features, and exploration missions is essential for civil services aspirants. In this article, we explore each planet’s key characteristics, their role in the solar system, and their relevance to UPSC preparation.

Inner Planets (Terrestrial Planets)

The four inner planets—Mercury, Venus, Earth, and Mars—are primarily composed of rocky material and metals, with relatively high densities. These planets have solid surfaces, and three of them (Venus, Earth, and Mars) have atmospheres substantial enough to support weather patterns.

Mercury

✎ Position: Closest planet to the Sun.

✎ Surface Temperature: Extremely hot (+427°C) during the day and freezing (-173°C) at night.

✎ Rotation: Takes 58 days to rotate once, and a full revolution around the Sun takes 87 days.

✎ Distance from Sun: 0.4 AU.

✎ Atmosphere: Extremely thin exosphere, composed mainly of hydrogen, helium, oxygen, and sodium.

✎ Surface: Heavily cratered with no active atmosphere, resembling the Moon. It has large temperature fluctuations due to the lack of atmospheric insulation.

✎ Exploration: NASA’s MESSENGER spacecraft revealed evidence of water ice at its poles and volcanic activity.

✎ Significance: Mercury’s surface conditions and lack of atmosphere provide a unique understanding of planetary evolution in the absence of weather systems.

Venus

✎ Position: Second planet from the Sun.

✎ Surface Temperature: The hottest planet in the solar system, with surface temperatures around 480°C.

✎ Rotation: A day on Venus lasts 243 Earth days, longer than its year (224 Earth days). It has retrograde rotation, meaning it rotates clockwise.

✎ Distance from Sun: 0.7 AU.

✎ Atmosphere: Thick atmosphere composed mainly of carbon dioxide, with clouds of sulfuric acid, creating a runaway greenhouse effect.

✎ Surface: Venus has a surface obscured by thick clouds, with mountains, volcanoes, and plains, similar to Earth’s surface features.

✎ Significance: Venus’ extreme temperature and atmospheric pressure offer valuable lessons about the greenhouse effect and planetary climates.

Earth

✎ Position: Third planet from the Sun.

✎ Surface Temperature: Averages around +22°C, supporting liquid water.

✎ Rotation: Takes 23 hours and 56 minutes to complete one rotation, with a year of 365 days.

✎ Distance from Sun: 1 AU.

✎ Atmosphere: A nitrogen-oxygen atmosphere that sustains life, protects from solar radiation, and regulates temperatures.

✎ Surface: Earth has diverse landforms, oceans, mountains, and active tectonics. The only planet known to support life.

✎ Significance: Earth serves as the benchmark for studying habitability, climate science, and the potential for life on other planets.

Characteristic	Mercury	Venus	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Sun
Diameter (km)	4,879	12,104	12,756	6,779	139,820	116,460	51,120	48,600	1,391,000
Distance from Sun (AU)	0.39	0.72	1	1.52	5.2	9.6	19.2	30	~1
Rotation Period	58 days	243 days	24 hrs	1.025 days	9.9 hrs	10.7 hrs	17 hrs	16 hrs	24.47 days
Revolution Period	87 days	224 days	365 days	687 days	11.9 years	29 years	84 years	164 years	~225 million years
Surface Gravity (m/s²)	3.7	8.87	9.8	3.7	24.9	10.4	8.8	11.1	274
Atmosphere	Thin (O, Na)	Thick (CO2)	O₂, N₂	CO₂, Ar	H₂, He	H₂, He	H₂, He, Methane	H₂, He, Methane	H₂, He, Plasma
Major Composition	Rock, Metal	Rock, Metal	Rock, Water	Rock, Metal	Gas (H₂, He)	Gas (H₂, He)	Ice, Gas	Ice, Gas	Plasma
Moons	0	0	1	2	79	82	27	14	0
Rings	No	No	No	No	Yes	Yes	Yes	Yes	Yes

Mars

✎ Position: Fourth planet from the Sun.

✎ Surface Temperature: Cold, with an average temperature of -23°C.

✎ Rotation: A day on Mars is 24.6 hours, nearly the same as Earth.

✎ Distance from Sun: 1.5 AU.

✎ Atmosphere: Thin atmosphere, composed mainly of carbon dioxide (95%), with trace amounts of nitrogen and argon.

✎ Surface: Known as the “Red Planet,” Mars has volcanoes, canyons, and polar ice caps. It features the largest volcano (Olympus Mons) and canyon (Valles Marineris) in the solar system.

✎ Exploration: Mars has been explored by numerous missions, including rovers like Curiosity and Perseverance, which seek signs of past life.

✎ Significance: Mars is a prime candidate for future human exploration and colonization, with studies focused on its climate, water potential, and geology.

Outer Planets (Gas Giants and Ice Giants)

The outer planets, composed mainly of hydrogen, helium, and various ices (like water, ammonia, and methane), are divided into gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune). These planets are significantly larger than the inner planets and have thick atmospheres, numerous moons, and powerful magnetospheres.

Jupitar

✎ Position: Fifth planet from the Sun.

✎ Surface Temperature: -150°C.

✎ Rotation: A day on Jupiter lasts about 9.9 hours, the shortest day among all planets.

✎ Distance from Sun: 5.2 AU.

✎ Atmosphere: Composed mainly of hydrogen and helium, with visible cloud bands and the Great Red Spot, a massive storm.

✎ Moons: Jupiter has 79 known moons, including the Galilean moons (Io, Europa, Ganymede, and Callisto).

✎ Exploration: NASA’s Juno spacecraft is currently studying Jupiter’s atmosphere and magnetic field.

✎ Significance: Jupiter’s immense size, complex atmospheric patterns, and magnetic field provide crucial insights into the dynamics of gas giants.

Saturn

✎ Position: Sixth planet from the Sun.

✎ Surface Temperature: -180°C.

✎ Rotation: A day on Saturn lasts about 10.7 hours.

✎ Distance from Sun: 9.6 AU.

✎ Atmosphere: Primarily composed of hydrogen and helium, with visible cloud bands and a rich system of rings.

✎ Moons: Saturn has 82 known moons, including Titan, the second-largest moon in the solar system, which has a dense nitrogen-rich atmosphere.

✎ Exploration: NASA’s Cassini mission provided extensive data about Saturn, its rings, and moons before its end in 2017.

✎ Significance: Saturn’s rings and large moon systems are key to understanding planetary ring dynamics and moon formation.

Uranus

✎ Position: Seventh planet from the Sun.

✎ Surface Temperature: -214°C.

✎ Rotation: Uranus has an unusual rotation, as its axis is tilted at nearly 98 degrees, causing it to roll on its side.

✎ Distance from Sun: 19.2 AU.

✎ Atmosphere: Composed mainly of hydrogen, helium, and methane, giving it its blue-green color.

✎ Moons: Uranus has 27 known moons, with Miranda, Ariel, and Titania being some of the largest.

✎ Exploration: Voyager 2 provided the only close-up images of Uranus during its flyby in 1986.

✎ Significance: Uranus’ unique axial tilt and composition make it an important planet for studying planetary atmospheres and magnetic fields.

Neptune

✎ Position: Eighth planet from the Sun.

✎ Surface Temperature: -220°C.

✎ Rotation: A day on Neptune lasts about 16 hours.

✎ Distance from Sun: 30.0 AU.

✎ Atmosphere: Mainly hydrogen, helium, and methane, giving it its distinctive blue color.

✎ Moons: Neptune has 14 known moons, with Triton being the largest, possessing a retrograde orbit.

✎ Exploration: Voyager 2’s flyby in 1989 provided the only close-up images of Neptune.

✎ Significance: Neptune’s extreme winds (the fastest in the solar system) and its moon Triton’s retrograde orbit make it an interesting subject for understanding the behavior of icy bodies.

Dwarf planet Pluto

Pluto, once considered the ninth planet, is now classified as a dwarf planet by the International Astronomical Union (IAU). It is located in the Kuiper Belt, a region beyond Neptune.

✎ Surface Temperature: -223°C.

✎ Rotation: Takes 6.39 days to complete one rotation.

✎ Distance from Sun: 39.5 AU.

✎ Exploration: NASA’s New Horizons mission provided the first detailed images of Pluto in 2015, revealing a diverse surface with mountains, plains, and signs of possible cryovolcanism.

✎ Significance: Pluto’s reclassification and exploration have raised important questions about what constitutes a planet and the characteristics of distant celestial bodies.

Highest, smallest and other superlatives among planets

Superlative	Planet
Largest Planet	Jupiter
Smallest Planet	Mercury
Most Massive Planet	Jupiter
Most Dense Planet	Earth
Planet with the Strongest Gravity	Jupiter
Planet with the Weakest Gravity	Saturn
Planet with the Longest Day	Venus (243 days)
Planet with the Longest Year	Neptune (164 years)
Hottest Planet	Venus (480°C)
Coldest Planet	Neptune (-220°C)
Planet with the Most Moons	Jupiter (79 moons)
Planet with the Most Rings	Saturn
Planet with the Highest Surface Temperature	Mercury (427°C)
Planet with the Highest Winds	Neptune (2,100 km/h)
Planet with the Most Similar Tilt to Earth	Mars (25.19°)
Planet with the Most Similar Composition to Earth	Venus
Planet with the Largest Volcano	Mars (Olympus Mons)
Planet with the Largest Storm	Jupiter (Great Red Spot)
Planet with the Largest Moon	Jupiter (Ganymede)
Planet with the Thickest Atmosphere	Venus
Planet with the Thinnest Atmosphere	Mercury
Planet with the Fastest Rotation	Jupiter (10 hours)
Planet with the Slowest Rotation	Venus (243 days)