Planets of our Solar System - Clarity Desk Hub

At its simplest:

“A planet is a celestial body that moves in an elliptical orbit around a star.”

In our solar system, the Sun is that central star, and the eight planets orbit it in well-defined paths. But all planets are not the same — they can be grouped into two broad families.

🪐 Classification of Planets

1. Inner Planets / Terrestrial Planets

Mercury, Venus, Earth, Mars
Located between the Sun and the Asteroid Belt
Made of rock and metals → dense, compact
Called terrestrial = “Earth-like” (terrestrial = terra = earth)

Watch 🎥: Animation of the Inner planets orbiting around the sun

2. Outer Planets / Jovian Planets / Gas Giants

Jupiter, Saturn, Uranus, Neptune
Located beyond the asteroid belt
Made mostly of hydrogen and helium
Large size, low density, thick atmospheres
Jovian = “Jupiter-like”

Watch 🎥: Animation of the Outer planets orbiting around the sun

🔁 Orbital Motion and Spin

All eight planets revolve counter-clockwise around the Sun (as seen from above the Sun’s north pole).
6 out of 8 planets also rotate counter-clockwise on their axis.
But Venus and Uranus are exceptions:
- Venus: rotates clockwise — retrograde rotation
- Uranus: rotates almost on its side (extreme axial tilt), also retrograde

🧠 Such exceptions are useful for tricky UPSC MCQs!

🧮 Astronomical Unit (AU)

The average distance between Earth and the Sun
1 AU = ~150 million km
Used as a cosmic scale to compare distances in the solar system

Inner Planets — The Rocky Siblings

These four planets have:

Crust and mantle made of silicates
Core made of iron and nickel
Most have impact craters, tectonic features, and volcanism
Venus, Earth, and Mars have substantial atmospheres
Do not confuse inner planets with inferior planets
→ Inferior planets = closer to the Sun than Earth (i.e., Mercury & Venus)

☿ Mercury — The Scorched Mini World

Surface is full of craters, like our Moon → no atmosphere = no erosion
Geologically inactive for billions of years
Extreme temperature variation:
- −173 °C at night to 427 °C in day
Seen only near horizon in the early morning or evening
Smaller than Ganymede (Jupiter’s moon) and Titan (Saturn’s moon)
→ But more massive than both due to higher density
MESSENGER mission (2004):
- Discovered evidence of pyroclastic flows (explosive volcanism)
- Found water ice at poles

♀ Venus — The Fiery Twin of Earth

Brightest planet visible from Earth
→ Called Morning Star / Evening Star in ancient texts
Why so bright?
→ High albedo due to sulfuric acid clouds (high reflectivity)
Sometimes even visible in daylight to the naked eye!

Earth’s Twin — But How Much?

Similarities:

Similar size, mass, density, and composition
Physical features: high plateaus, mountain belts, volcanoes

Differences:

Atmosphere: 96% CO₂ with sulfuric acid clouds
Thickest atmosphere among terrestrial planets
Surface pressure: 92 times Earth’s → like being 900m underwater on Earth
Hottest planet in the solar system
→ Not Mercury! Because greenhouse effect traps heat
One day = 243 Earth days, longer than its year (224 Earth days)
Rotates in reverse (retrograde — clockwise)

🔴 Mars — The Red Hope

“Mars is often seen as humanity’s next home, but it’s also a planet that lost its heart.”

🔹 Why “Red”?

Mars’s surface is rich in iron oxide (rust) → gives it the reddish hue

🔹 Atmosphere & Magnetosphere:

96% CO₂, with traces of methane, water vapor
Lost its magnetosphere 4 billion years ago
→ Solar wind stripped away the atmosphere → caused cooling

Methane mystery:
Despite being short-lived, methane exists on Mars.

This suggests there must be ongoing sources → either geological (e.g., volcanic) or biological (e.g., microbes)

🔹 Earth-like Seasons:

Axial tilt = 25.19° → similar to Earth → has seasons
Day = 1.03 Earth days, but year = ~687 Earth days
Gravity = 38% of Earth → walking would feel like bouncing!

🌊 What Happened to Mars’s Water?

Once had liquid water, now:

Atmosphere too thin → water can’t remain liquid
Water is locked in polar ice caps

🏔️ Surface Features:

Olympus Mons: Tallest volcano (24 km) in Solar System
Valles Marineris: One of the largest canyons in the Solar System
Geologically dead → no tectonic recycling of materials

🔹 Moons:

Phobos and Deimos: Probably captured asteroids

🌐 Mars vs Earth — Quick Comparison

Parameter	Mars
Diameter	53% of Earth’s
Mass	10% of Earth’s
Surface Gravity	38%
Day Length	1.03 Earth days
Axial Tilt	25.19°
Atmosphere	95% CO₂ (very thin)
Life?	No evidence yet, but conditions were once favorable

🪐 The Outer Planets — The Titans Beyond the Belt

These are the gas and ice giants, formed far from the Sun where it was cool enough for gases to condense.

🔹 Why Are They Gaseous?

Close to the Sun → too hot → gases couldn’t condense
Solar winds blew off gases from inner planets
Outer planets, being farther, retained their gases

🌟 Jupiter — The Giant

Largest planet, composed of hydrogen and helium
No solid surface → just swirling gases
Fastest rotation (10 hours) → bulges at equator (oblate spheroid)
Home to the Great Red Spot (storm) and Galilean moons:
- Ganymede (largest moon in Solar System, even larger than Mercury)
- Io, Europa, Callisto
Visited by NASA’s Juno mission

💍 Saturn — The Ringed Beauty

Least dense planet (less than water)
Iconic rings made of ice and rock
Titan: 2nd largest moon, has a thick atmosphere (mainly nitrogen)

🔄 Uranus — The Tilted Planet

Rotates on its side (axial tilt ~98°)
Unusual orientation → extreme seasons

🔵 Neptune — The Windy Twin

Often paired with Uranus as ice giants
Strongest winds in Solar System (~2100 km/h)
Thick atmosphere with ices (water, methane, ammonia)

🎯 Final Insights

Category	Inner Planets	Outer Planets
Distance from Sun	Closer	Farther
Composition	Rocky	Gaseous/Icy
Atmosphere	Thin/Moderate	Thick (H₂, He)
Moons	Few or none	Many
Rings	None	All have rings (Saturn’s most visible)
Density	High	Low
Surface	Solid	No solid surface (except moons)

🌕 The Moon — Earth’s Loyal Companion

The Moon is more than just a glowing object in the night sky — it’s Earth’s gravitational partner, orbital stabiliser, and a future candidate for human colonisation.

🔹 Physical & Orbital Facts:

Distance from Earth: ~3,84,400 km
Diameter: ¼ of Earth’s diameter
Tidally locked → Rotational period = Orbital period (~27 days)
→ That’s why we always see only one side of the Moon from Earth.

🔹 Cosmic Significance:

The Moon stabilises Earth’s axial tilt (currently 23.5°).
- Without it, the tilt could swing wildly up to 85°, destabilising Earth’s climate over time.

🌑 Formation — The Giant Impact Hypothesis

Also called the “Big Splat”, it suggests:

A Mars-sized body collided with early Earth → debris was ejected into orbit → coalesced to form the Moon (~4.44 billion years ago)

Watch ▶️ this beautiful animation by NASA showing the formation of Moon.

Interesting point:

At that time, a day on Earth was just 6 hours!
Moon was much closer than today → it’s now drifting away by 4 cm per year

🌊 Effects on Earth:

Moon’s gravity causes tides in oceans
Tidal friction → slows Earth’s rotation slightly
In return, the Moon slowly moves away

🧪 Lunar Exploration:

1959: Soviet Luna 2 → first artificial impact on Moon
1968: Apollo 8 → first human orbit
1969: Neil Armstrong & Buzz Aldrin → first humans on Moon (Apollo 11)

India’s Milestone:

Chandrayaan-1 (2009) discovered water at lunar poles
→ Lunar soil has 0.1% water by weight

🏠 Colonising the Moon — Dream vs Reality

✅ Pros:

Could serve as a launch base for deeper space missions
Lunar poles receive continuous sunlight → useful for solar energy

❌ Challenges:

Long lunar nights (350+ hrs) → solar energy is unreliable
Lacks carbon, nitrogen, hydrogen — essentials for life
No atmosphere → extreme temperatures, solar radiation, meteor risks
Growing food is difficult → poor soil, no pollinators, temperature swings