Planets

The term planet originates from the Greek word meaning “wanderer.” Unlike stars, planets appear to shift positions in the sky because they revolve around the Sun in elliptical orbits. There are eight planets in our solar system, each with distinct physical and orbital characteristics: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Recognition of Dwarf Planets

The International Astronomical Union (IAU) has identified five major dwarf planets:

• Ceres
• Pluto
• Haumea
• Makemake
• Eris (sometimes referred to as Sedna)

These bodies orbit the Sun and possess planetary features but lack the gravitational dominance to clear their orbital paths.

Classification of Planets

Planets in the solar system are categorized based on composition, size, and location into two principal types:

1. Terrestrial Planets

These are rocky, metallic, and dense planets located closer to the Sun. They are also termed inner planets and exhibit a solid surface with thinner atmospheres.

• It Includes: Mercury, Venus, Earth, and Mars
• Characteristics: Lower radii and masses, fewer or no moons, absence of rings
• Composition: Primarily silicate rocks and metals

2. Jovian Planets

Also known as gas giants (or ice giants in the case of Uranus and Neptune), these planets are significantly larger and composed primarily of gaseous elements.

• It includes Jupiter, Saturn, Uranus, and Neptune
• Characteristics: Thick gaseous envelopes, multiple natural satellites, ring systems, lower densities
• Composition: Predominantly hydrogen, helium, methane, and ammonia

Formation of Planets: Nebular Hypothesis

Planetary formation is best explained through the nebular hypothesis, which outlines the sequential stages of planetary evolution:

1. Gravitational Collapse of a localized gas lump within a nebula leads to the formation of a dense core and a rotating disc.
2. The disc undergoes condensation, forming tiny spherical particles which evolve into planetesimals through cohesion.
3. Repeated collisions and accretion among planetesimals lead to the formation of larger celestial bodies.
4. These large aggregates ultimately mature into full-fledged planets, differentiated into core, mantle, and crust layers.

Detailed Study of Individual Planets

Mercury

• Proximity: Closest planet to the Sun (58 million km or 0.39 AU)
• Size: Smallest planet in the solar system
• Rotation Period: One day equals 59 Earth days
• Nature: Terrestrial planet with a thin exosphere composed of oxygen, sodium, hydrogen, helium, and potassium
• Temperature Extremes: Daytime – 430°C; Nighttime –180°C
• Exploration: Visited by Mariner 10 (1974–75) and MESSENGER (orbited in 2011)

Venus

• Second planet from the Sun (108 million km or 0.72 AU)
• Rotation: Retrograde rotation; one day equals 243 Earth days
• Surface: Volcanically modified, with extensive lava plains
• Atmosphere: Thick, toxic; predominantly carbon dioxide and nitrogen, with sulfuric acid clouds
• Temperature: Around 480°C due to extreme greenhouse effect
• Missions: Over 40 missions including NASA’s Magellan (mapped 98% of surface)

Earth

• Third Planet: 150 million km from the Sun or 1 Astronomical Unit (AU)
• Atmosphere: 78% nitrogen, 21% oxygen, and 1% other gases
• Distinguishing Feature: Only known planet supporting life, with a stable hydrosphere and biosphere
• Satellite: One natural satellite – the Moon
• Protective Shield: Atmosphere burns most meteoroids before impact

Mars

• Distance: 228 million km or 1.52 AU
• Rotation: Slightly over 24 Earth hours
• Surface: Altered by volcanic activity, meteor impacts, tectonic shifts, and dust storms
• Atmosphere: Thin; consists mainly of carbon dioxide, nitrogen, and argon
• Moons: Two – Phobos and Deimos
• Significance: Known as the Red Planet due to oxidation of iron minerals
• Exploration by: Mariner 4 (1965) was the first successful mission

Jupiter

• Position: Fifth planet; 778 million km or 5.2 AU from the Sun
• Status: Largest planet in the solar system
• Rotation speed: One day equals 10 Earth hours
• Composition: Predominantly hydrogen and helium; lacks solid surface but may have a solid core
• Moons: 67 identified (50 confirmed + 17 awaiting confirmation)
• Ring System: Faint, discovered by Voyager 1 (1979)
• Missions: Juno spacecraft arrived in 2016
• Some moons (like Europa) may have subsurface oceans

Saturn

• Distance: 1.4 billion km or 9.5 AU
• Rotation speed: 10.7 hours
• Composition: Gas giant with atmosphere dominated by hydrogen and helium
• Rings: Most prominent ring system with seven distinct bands
• Moons: 62 total (53 confirmed + 9 provisional)
• Exploration by: Pioneer 11, Voyager 1 & 2, and Cassini-Huygens (2004–2017)

Uranus

• Distance: 2.9 billion km or 19.19 AU
• Rotation: 17 hours; tilted at 98 degrees, hence rotates on its side
• Nature: Classified as an ice giant, composed of water, methane, and ammonia
• Atmosphere: Hydrogen, helium, and methane
• Moons: 27, named after characters from Shakespeare and Pope
• Rings: 13 known rings
• Exploration by: Voyager 2 remains the only mission (1986)

Neptune

• Distance: 4.5 billion km or 30.07 AU
• Rotation: 16 hours
• Classification: Ice giant; composition similar to Uranus
• Atmosphere: Rich in hydrogen, helium, and methane
• Moons: 13 (e.g., Triton, which orbits in a retrograde path)
• Exploration by: Only by Voyager 2 (1989)

Pluto

• Formerly the ninth planet; now a dwarf planet
• Located in the Kuiper Belt, a region of icy objects
• Average distance: 3.6 billion miles from the Sun
• Has five moons: Charon (largest), Kerberos, Styx, Nix, and Hydra

Kuiper Belt

• Extends beyond Neptune, from 30 AU to 50 AU
• Similar to the Asteroid Belt but far more massive
• Composed of frozen bodies, dwarf planets, and remnants of early solar system formation
• Source of short-period comets

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Subject: Geography

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