Physical Geography · Topic 1 · GS Paper 1

The Universe & the Solar System — From the Big Bang to Neptune

The complete UPSC Physical Geography opener — origin of the Universe (Big Bang, Pulsating, Steady State), galaxies and stars, the Sun, the eight planets, asteroids, comets, meteors and meteorites — with labelled diagrams, exam-skeleton reproductions, mnemonics, and Mains-answer templates aligned to NCERT Class XI Fundamentals of Physical Geography.

Physical Geography · Topic 1 · ~25 min read · Updated June 2026

Why this topic matters for UPSC

The Universe-Solar System chapter is the foundational opener of UPSC Physical Geography. Prelims (CSAT & GS-1) regularly tests definitions and characteristics — terrestrial vs Jovian planets, asteroid belt location, comet structure, meteor showers. Mains GS-1 maps it to "salient features of the World's Physical Geography" and to current-affairs items (Chandrayaan-3 lunar south pole landing 2023, Aditya-L1 launch 2023, JWST discoveries 2022-26). Essay paper has set themes on cosmology, "the place of humanity in the cosmos", and scientific temperament. Strong fundamentals here also unlock subsequent chapters — Earth's motions, geomorphology, climatology.

1. Origin of the Universe D1D7D30

NCERT XI · Fundamentals of Physical Geography · Ch. 1 "The Earth in the Solar System" · pp. 1-3

The Universe is the totality of matter, energy, space and time. Its currently estimated age is 13.8 billion years, and the observable Universe spans ~93 billion light-years across (because space itself has expanded since light was emitted). Three classical theories compete to explain its origin.

1.1 Timeline of the evolution of the Universe

Fig 1.1 — Compressed cosmic timeline from the Big Bang singularity through inflation, nucleosynthesis, recombination (release of the Cosmic Microwave Background), first stars, formation of the Solar System (4.6 Gya), to the present.

1.2 The Big Bang Theory D1D7

The currently accepted standard model of cosmology. Proposed by Belgian priest and astrophysicist Georges Lemaître (1927) as the "primeval atom" hypothesis; mathematical foundations from Friedmann's solutions to Einstein's General Relativity (1922). The name "Big Bang" was coined sarcastically by Fred Hoyle on a 1949 BBC broadcast — and the label stuck.

Key propositions

All matter, energy, space and time originated ~13.8 billion years ago from an extremely hot, dense singularity.
The Universe is still expanding — galaxies are moving apart from each other (not into pre-existing space; space itself is stretching).
Three classical phases — Inflation (10⁻³⁶ to 10⁻³² seconds, exponential expansion by factor ≥10²⁶) → Nucleosynthesis (first 3 minutes — formation of H, He, trace Li) → Recombination (~380,000 years — electrons combined with nuclei to form neutral atoms; photons escaped and form today's Cosmic Microwave Background).

Empirical evidence

Evidence	Discovery	What it shows
Galactic redshift	Vesto Slipher 1912; Edwin Hubble 1929	Distant galaxies recede; recession velocity ∝ distance (Hubble-Lemaître Law). Implies expansion.
Cosmic Microwave Background (CMB)	Penzias & Wilson 1964 (Nobel 1978); COBE 1992; WMAP 2003; Planck 2013	Uniform 2.725 K microwave radiation from all directions — the "echo" of recombination 380,000 yrs after Big Bang.
Abundance of light elements	Big Bang Nucleosynthesis predictions (Alpher-Bethe-Gamow 1948)	Observed cosmic ratios of H (~75%), He (~25%) match predictions exactly.
Large-scale structure	Sloan Digital Sky Survey, etc.	Galaxy distribution matches predictions from CMB fluctuations seeded in inflation.

Mnemonic"I-NRC + HCN" — phases Inflation, Nucleosynthesis, Recombination, Cooling; evidence Hubble redshift, CMB, Nuclear abundance.

1.3 The Pulsating (Oscillating) Universe Theory

Proposed by Allan Sandage (1965) as a variant of the Big Bang. The Universe alternately expands and contracts in cycles ("Big Bang" → expansion → "Big Crunch" → "Big Bounce" → next Big Bang). Each cycle may last hundreds of billions of years.

Requires the Universe to have sufficient mass-density for gravity to halt and reverse expansion (Ω > 1, a "closed" universe).
Modern observations (1998 onwards — Perlmutter, Schmidt, Riess; Nobel 2011) show that expansion is in fact accelerating, driven by dark energy. This rules out the simple pulsating model in its original form.
The idea survives in modern cosmology as "cyclic" models (Steinhardt-Turok, ekpyrotic universe) — but remains speculative.

1.4 The Steady State Theory (for completeness)

Proposed by Hermann Bondi, Thomas Gold, Fred Hoyle (1948). The Universe has no beginning and no end; matter is continuously created (at a tiny rate — about one H atom per cubic metre every billion years) so that mean density remains constant despite expansion. Disproved after the 1964 discovery of the CMB — a steady-state universe would not produce a hot relic background.

UPSC examiner's framing. Big Bang vs Pulsating vs Steady State has appeared as a Prelims MCQ (definitional) and as Mains analytical sub-parts. Always cite (a) the proposer, (b) the date, (c) one supporting evidence or one falsifying observation. The CMB discovery (1964) is the single most-cited piece of evidence in UPSC answers.

In the news James Webb Space Telescope (launched Dec 2021, first images July 2022) has imaged galaxies as far back as ~300 million years after the Big Bang (JADES-GS-z14-0, 2024) — pushing the frontier of observational cosmology and challenging some early-galaxy formation models. India's Aditya-L1 (launched Sep 2023, halo orbit at L1 from Jan 2024) studies the Sun's outer atmosphere — solar-stellar continuum.

Mains answer template — 150-word skeleton (10 marks)

Q: Discuss the Big Bang Theory and the evidences in its favour.

Intro (2 lines) — Define Big Bang as standard cosmological model; date proposal (Lemaître 1927).
Body — three phases — Inflation → Nucleosynthesis → Recombination (use Fig 1.1 keywords).
Body — four evidences — Hubble redshift, CMB (Penzias-Wilson 1964), light-element abundances, large-scale structure.
[Diagram cue: cosmic timeline arrow with 4 ticks — Big Bang · Recombination · First stars · Today]
Conclusion (2 lines) — Mention recent reinforcement by JWST (2022-) and ongoing puzzles (dark matter ~27%, dark energy ~68% of Universe).

2. Components of the Universe D1D7D30

NCERT XI · Ch. 1 · pp. 3-5 · Goh Cheng Leong Ch. 1 · Savindra Singh Ch. 1

The Universe is composed of galaxies (the gravitationally bound assemblies of stars, gas, dust, and dark matter) which together form clusters, superclusters, and the cosmic web. Approximately 2 trillion galaxies are estimated in the observable Universe.

2.1 Galaxies — types & structure

Edwin Hubble's classification (1926), the "Hubble tuning-fork diagram", divides galaxies by visual morphology into four broad classes.

Fig 1.2 — Four major galaxy morphologies after Hubble (1926). The Milky Way is a barred-spiral galaxy of class SBbc.

Type	Form	Population & gas	Examples
Spiral (S)	Central bulge + flat rotating disc + spiral arms	Population I (young, blue stars in arms) + Pop II (old, in bulge); abundant gas/dust → active star formation	Andromeda M31, M81, M101
Barred spiral (SB)	As above, plus a straight bar across the bulge from which arms emerge	Similar to spiral	Milky Way (SBbc), NGC 1300
Elliptical (E)	Smooth ellipsoid, no arms, no bar	Pop II only (old, red stars); little gas; minimal new star formation	M87 in Virgo cluster, M32, ESO 146-IG 005
Irregular (Irr)	No defined symmetry, often distorted by gravity	Rich in gas, active star formation; small dwarfs	Large & Small Magellanic Clouds (LMC, SMC)
Lenticular (S0)	Intermediate — disc + bulge but no clear arms	Old stars, little gas (transitional)	NGC 2787, NGC 5866

Mnemonic"Some Bored Engineers Love Iridescent galaxies" — Spiral · Barred spiral · Elliptical · Lenticular · Irregular.

The Milky Way Galaxy — our home

Type — Barred spiral (SBbc); diameter ~100,000 light-years; thickness ~1,000 ly at disc, ~10,000 ly at central bulge.
Contains 100-400 billion stars; total mass ~10¹² solar masses (including dark matter halo).
The Sun lies in the Orion-Cygnus arm, about 26,000 light-years from the galactic centre (Sagittarius A*, a supermassive black hole of ~4 million solar masses).
The Solar System orbits the galactic centre at ~230 km/s, completing one orbit ("cosmic year") in ~225-250 million years.
Andromeda (M31) is our nearest large galactic neighbour at ~2.5 million ly; the two galaxies will collide in ~4.5 billion years.

2.2 Stars D1D7

A star is a luminous sphere of plasma held together by gravity, generating energy through nuclear fusion in its core (mainly H → He in the proton-proton chain or CNO cycle). The Sun is an average star — one of ~10²² (10 sextillion) stars in the observable Universe.

2.2.1 Birth, life and death of a star

Fig 1.3 — Stellar life cycle. A star's mass at birth determines its fate — low-mass stars (≤8 M☉) end as white dwarfs; high-mass stars explode as supernovae leaving neutron stars or black holes.

2.2.2 Hertzsprung-Russell (H-R) diagram — classifying stars

The H-R diagram (Hertzsprung 1911 & Russell 1913, independently) plots stars by luminosity (y-axis) against surface temperature / spectral class (x-axis). ~90% of stars including the Sun lie on the diagonal "main sequence". The spectral classification — O B A F G K M (hot to cool, blue to red) — is the standard.

Mnemonic"Oh Be A Fine Girl/Guy, Kiss Me" — spectral classes from hottest (O, ~30,000 K, blue) to coolest (M, ~3,000 K, red). The Sun is class G2V.

2.2.3 End states of stellar evolution

Remnant	Progenitor mass	Defining feature
White dwarf	0.08-8 M☉ (like the Sun)	Earth-sized, density ~10⁹ kg/m³; supported by electron degeneracy pressure
Neutron star	8-20 M☉	~20 km diameter; density ~10¹⁷ kg/m³; pulsars are rapidly-rotating neutron stars (Hewish & Bell, 1967)
Black hole	>20 M☉	Escape velocity exceeds c; event horizon = Schwarzschild radius. First image captured by Event Horizon Telescope, M87* (April 2019); Sgr A* image (May 2022)

Mains template — 250-word answer (15 marks)

Q: Describe the life cycle of a star and explain the role of mass in determining its end state.

Intro — define a star; energy source = nuclear fusion (H → He in p-p chain).
Stage 1-3 — Nebula → Protostar → Main sequence (90% of stellar lifetime; the Sun has spent 4.6 Gyr here with ~5 Gyr remaining).
Stage 4 branches by mass — Low mass (≤8 M☉) → red giant → planetary nebula → white dwarf. High mass (>8 M☉) → red supergiant → supernova → neutron star or black hole.
[Diagram cue: 5-box flowchart with branch at "main sequence" for low-mass vs high-mass path]
Conclusion — stellar nucleosynthesis seeds heavy elements (C, O, Fe) in the interstellar medium → next-generation star & planet formation; we are "made of star-stuff" (Sagan).

In the news The Event Horizon Telescope released the first image of the supermassive black hole at the centre of M87 in April 2019, and of Sagittarius A* (the Milky Way's central black hole) in May 2022. LIGO-Virgo detected the first gravitational waves in September 2015 (GW150914 — binary black hole merger; Nobel 2017), and the first binary neutron-star merger (GW170817, Aug 2017) — opening the era of multi-messenger astronomy.

3. The Solar System — origin & the Sun D1D7D30

NCERT XI · Ch. 1 · pp. 5-7 · Goh Cheng Leong Ch. 1

The Solar System comprises the Sun, eight planets, five officially-recognised dwarf planets (Pluto, Eris, Ceres, Haumea, Makemake), ~200 known moons, the asteroid belt, the Kuiper Belt, the Oort Cloud, and countless comets, meteoroids, and dust. Its age is ~4.6 billion years, dated from primitive meteorites.

3.1 Origin of the Solar System — the Nebular Hypothesis

Multiple theories have been proposed; the modern consensus is a refined version of the Nebular Hypothesis.

Theory	Proposer (year)	Core idea	Status
Nebular Hypothesis	Immanuel Kant (1755) & Pierre-Simon Laplace (1796)	Solar System formed from a slowly rotating gas-dust nebula that contracted, flattened into a disc, and fragmented — central mass = Sun; peripheral rings = planets	Modified version (Solar Nebular Disc Model, SNDM) accepted today
Tidal / Planetesimal Hypothesis	Chamberlin & Moulton (1900); James Jeans & Harold Jeffreys (1917)	A passing star pulled out matter from the Sun in a "cigar-shaped" filament, which condensed into planetesimals → planets	Discarded — requires extremely unlikely stellar encounters
Binary Star Hypothesis	H.N. Russell (1937), R.A. Lyttleton (1938)	Sun had a companion star; passing third star pulled matter from companion; companion was destroyed	Discarded
Modern SNDM	Otto Schmidt (1944), Carl von Weizsäcker (1944), Safronov (1969)	~4.6 Gya a giant molecular cloud collapsed (triggered by nearby supernova); ~99.8% of mass → Sun; remainder formed protoplanetary disc; planets grew by accretion of planetesimals	Accepted standard model

Mnemonic"K-L-J-J-S" — Solar System theories in order: Kant · Laplace · Jeans-Jeffreys · Schmidt. Modern model = "Solar Nebular Disc Model" (SNDM).

3.2 The Sun — our star

Fig 1.4 — Layered structure of the Sun. Energy generated by H→He fusion in the core takes ~170,000 years to travel through the radiative zone, then minutes through the convective zone, before being emitted from the photosphere as sunlight.

Basic data — the Sun

Parameter	Value
Diameter	1.39 × 10⁶ km (~109 × Earth)
Mass	1.989 × 10³⁰ kg (~333,000 × Earth; 99.86% of Solar System mass)
Composition (by mass)	H ~73%, He ~25%, heavier elements (O, C, Fe, Ne, N) ~2%
Surface temperature (photosphere)	~5,500 °C (5,778 K)
Core temperature	~15 million °C
Corona temperature	1-2 million °C (paradoxically hotter than the surface — "coronal heating problem")
Rotation period	25 days at equator, ~35 days at poles (differential rotation)
Age	4.6 billion years (currently ~halfway through main-sequence life)
Spectral class	G2V (yellow dwarf)
Distance from Earth	149.6 × 10⁶ km = 1 Astronomical Unit (AU); light takes 8 min 20 s

3.2.1 Internal layers (centre outward)

Core — radius ~0.25 R☉, temperature ~15 million °C, density ~150 g/cm³. Site of nuclear fusion (proton-proton chain: 4 H → He + 2 e⁺ + 2 ν + γ, releasing 26.7 MeV per cycle).
Radiative zone — 0.25 to 0.7 R☉. Energy transported outward as photons that take ~170,000 years (random walk through dense plasma).
Convective zone — 0.7 R☉ to surface. Plasma circulates in convection cells (granulation visible on photosphere).
Photosphere — visible "surface", ~400 km thick; temperature 5,500 °C.
Chromosphere — pinkish layer above photosphere, ~2,000 km thick, visible during total solar eclipse.
Corona — outermost layer, extends millions of km; only seen during eclipse; temperature 1-2 million °C (coronal heating still incompletely understood).

3.2.2 Solar activity

Sunspots — cooler (~3,800 °C) magnetic regions on photosphere; follow an 11-year cycle (Schwabe cycle, 1843). Solar Cycle 25 began Dec 2019; peaks 2024-25.
Solar wind — continuous stream of charged particles (mainly protons, electrons) at 400-800 km/s; carves the heliosphere.
Solar flares — sudden electromagnetic radiation bursts; classified X, M, C, B, A (most to least intense).
Coronal Mass Ejections (CMEs) — huge plasma+magnetic-field releases; can cause geomagnetic storms on Earth, aurorae, and disrupt satellites/power grids (e.g., Carrington Event, 1-2 Sep 1859 — largest recorded).
Aurorae — solar-wind particles excite atmospheric gases at poles → Aurora Borealis (N) & Aurora Australis (S).

In the news Aditya-L1 — ISRO's first dedicated solar mission, launched 2 Sep 2023; reached Sun-Earth Lagrange Point 1 (L1) on 6 Jan 2024; halo orbit ~1.5 million km Sun-ward of Earth. Carries 7 payloads (VELC, SUIT, ASPEX, etc.) studying corona, photosphere, chromosphere, solar wind, CMEs. NASA's Parker Solar Probe (2018-) has flown to ~6.1 million km from the Sun (Dec 2024) — closest human-made object to a star.

Mains template — 250-word answer (15 marks)

Q: Describe the internal structure of the Sun and discuss the impact of solar activity on Earth.

Intro — Sun = G2V yellow-dwarf star; ~99.86% of Solar System mass; energy source = p-p fusion.
Six layers (centre outward) — Core (15 M °C, fusion) → Radiative zone (170,000-yr photon transit) → Convective zone → Photosphere (5,500 °C, visible surface) → Chromosphere → Corona (1-2 M °C).
[Diagram cue: half-circle cross-section with 6 labelled concentric layers + sunspot + prominence]
Solar activity — Sunspots (11-yr cycle), solar wind, flares (X-A), CMEs.
Earth impacts — Aurorae, geomagnetic storms (satellite damage, power-grid trips — Quebec 1989 blackout, Carrington Event 1859), GPS/HF radio disruption, climate forcing (Maunder Minimum ~1645-1715 correlated with "Little Ice Age").
Conclusion — Mention Aditya-L1 (2023-24) and Parker Solar Probe contributions; importance for space weather forecasting and India's growing space economy.

4. The Eight Planets — terrestrial & Jovian D1D7D30D90

NCERT XI · Ch. 1 · pp. 6-9 · Goh Cheng Leong Ch. 1

Per the 2006 International Astronomical Union (IAU) definition (Prague Assembly, 24 August 2006), a planet is a celestial body that (a) orbits the Sun, (b) has sufficient mass for its self-gravity to assume hydrostatic equilibrium (near-round shape), and (c) has cleared the neighbourhood around its orbit. Bodies meeting (a)-(b) but not (c) are dwarf planets (Pluto demoted in 2006).

Fig 1.5 — Order of planets from the Sun. The asteroid belt separates the rocky terrestrial planets (Mercury–Mars) from the gas/ice giant Jovian planets (Jupiter–Neptune).

Mnemonic"My Very Educated Mother Just Served Us Noodles" — Mercury · Venus · Earth · Mars · Jupiter · Saturn · Uranus · Neptune. (Earlier "Pizzas" included Pluto, now demoted.)

4.1 Terrestrial vs Jovian — the great division

Terrestrial (Inner) — Mercury, Venus, Earth, Mars

Small, dense, rocky (density 3.9-5.5 g/cm³)
Iron-nickel core + silicate mantle/crust
Thin / no atmospheres
Few or no moons (0-2)
No rings
Slow rotation (24 hr - 243 days)
Formed inside "frost line" — only refractory materials survived

Jovian (Outer) — Jupiter, Saturn, Uranus, Neptune

Large, low-density, gaseous/icy (density 0.7-1.6 g/cm³)
Hydrogen-helium (gas giants) or water-ammonia-methane (ice giants)
Thick, deep atmospheres
Many moons (14-95+)
All have rings (Saturn most prominent)
Fast rotation (10-17 hr)
Formed beyond frost line — accreted icy volatiles & H₂/He

4.2 The eight planets — characteristics

4.2.1 Mercury (☿)

Smallest planet; closest to Sun (0.39 AU); diameter 4,879 km (smaller than Ganymede & Titan moons).
Orbital period 88 Earth days; 3:2 spin-orbit resonance — rotates 3 times for every 2 orbits; one solar day = 176 Earth days.
Surface temperature range −173 °C (night) to +427 °C (day) — largest of any planet.
No atmosphere (exospheric trace); heavily cratered (Caloris Basin); shrinking — surface contains lobate scarps.
Has a relatively large iron core (~85% of radius) — densest planet after Earth.
Missions — Mariner 10 (1974-75), MESSENGER (2011-15), BepiColombo (ESA/JAXA, en route — orbit insertion 2026).

4.2.2 Venus (♀)

"Earth's twin" by size (diameter 12,104 km, ~95% Earth's); rotates retrograde (east to west).
Slowest rotation — 243 Earth days; orbital period 225 days, so Venusian day > Venusian year.
Atmosphere — 96.5% CO₂, surface pressure 92 × Earth; runaway greenhouse effect → surface 462 °C (hottest planet, hotter than Mercury despite being farther from Sun).
Sulfuric acid clouds; permanent yellowish haze; no magnetic field.
Brightest object in night sky after Moon; "morning/evening star".
Missions — Mariner 2 (1962, first interplanetary flyby), Venera series (USSR, 1961-83), Magellan, Akatsuki (JAXA), Parker Solar Probe flybys, ISRO's planned Shukrayaan-1 (~2028).

4.2.3 Earth (♁)

Third planet; only known abode of life; 1 AU from Sun; diameter 12,742 km.
Atmosphere — N₂ 78%, O₂ 21%, Ar 0.9%, CO₂ 0.04% + variable water vapour.
71% surface covered by water (unique); active plate tectonics; protective magnetic field generated by liquid iron-nickel outer core.
One natural satellite — the Moon (3,474 km diameter; 384,400 km away; tidally locked).
Detailed treatment in A2 — The Earth.

4.2.4 Mars (♂)

"Red planet" — iron oxide dust surface; diameter 6,779 km (~half of Earth).
Orbital period 687 Earth days; rotation 24 hr 37 min (nearest to Earth's day).
Tenuous atmosphere (CO₂ 95%, surface pressure <1% Earth); surface temperature −143 to +35 °C.
Olympus Mons — Solar System's tallest volcano (~22 km, ~2.5× Everest).
Valles Marineris — Solar System's largest canyon system (4,000 km long, 7 km deep).
Two small irregular moons — Phobos & Deimos (probably captured asteroids).
Evidence of ancient surface water; ice at poles; subsurface liquid-water lakes (Mars Express radar, 2018).
Missions — Viking 1 & 2 (1976), Mars Pathfinder (1997), Mangalyaan / Mars Orbiter Mission (ISRO, 2014-22) — made India the first nation to reach Mars on its maiden attempt; NASA's Curiosity (2012-), Perseverance + Ingenuity helicopter (2021-), China's Tianwen-1 + Zhurong rover (2021-).

4.2.5 Jupiter (♃) — King of planets

Largest planet — diameter 139,820 km (~11 × Earth); mass 318 × Earth (2.5 × all other planets combined).
Gas giant — ~90% H, ~10% He (by atoms); no solid surface; may have a small rocky core.
Orbital period 11.86 yrs; rotates fastest of all planets — 9 hr 56 min — causing equatorial bulge.
Great Red Spot — anticyclonic storm observed since 1665; ~16,000 km wide (currently shrinking).
Bands & zones — alternating cloud belts driven by jet streams.
95 known moons (as of 2023); four Galilean moons (Io, Europa, Ganymede, Callisto) discovered by Galileo Jan 1610. Ganymede is the largest moon in the Solar System (bigger than Mercury). Europa & Ganymede have subsurface oceans — astrobiology priority targets.
Faint ring system (1979 Voyager 1 discovery).
Missions — Pioneer 10/11, Voyager 1/2, Galileo (1995-2003), Juno (2016-, ongoing), Europa Clipper (NASA, launched Oct 2024), JUICE (ESA, launched Apr 2023, arrives 2031).

4.2.6 Saturn (♄)

Second-largest planet — diameter 116,460 km; lowest density of any planet (0.687 g/cm³ — would float in water).
Gas giant — H/He composition similar to Jupiter.
Orbital period 29.46 yrs; rotation 10 hr 33 min.
Spectacular ring system — A, B, C, D, E, F, G rings discovered telescopically by Galileo (1610) & resolved by Cassini-Huygens; composed mostly of water ice + dust; ~282,000 km wide but only ~10 m thick.
146 known moons (Saturn overtook Jupiter in 2023 count). Titan — second-largest moon in Solar System; only moon with thick atmosphere (N₂-rich); hydrocarbon lakes (Huygens probe landing, 14 Jan 2005). Enceladus — geysers from subsurface ocean, prime astrobiology target.
Missions — Pioneer 11 (1979), Voyager 1/2 (1980-81), Cassini-Huygens (2004-17, deliberately deorbited 15 Sep 2017), Dragonfly (NASA, launches 2028, Titan rotorcraft).

4.2.7 Uranus (⛢)

First planet discovered by telescope — William Herschel, 13 March 1781; named after the Greek sky god.
"Ice giant" — H/He envelope over water-ammonia-methane mantle; pale blue-green colour from methane absorption.
Diameter 50,724 km; orbital period 84 Earth years; rotation 17 hr 14 min (retrograde).
Extreme axial tilt of 97.77° — effectively rolls on its side; each pole faces the Sun for ~42 years at a time.
27 known moons named after Shakespearean & Pope characters (Titania, Oberon, Miranda, Ariel, Umbriel).
13 known rings.
Only one visit — Voyager 2 flyby, 24 Jan 1986. NASA Uranus Orbiter and Probe mission proposed for ~2030s.

4.2.8 Neptune (♆)

Mathematically predicted before being observed — Le Verrier & Adams (1845-46) from perturbations of Uranus's orbit; Johann Galle observed it 23 September 1846.
Farthest planet from Sun (30 AU); orbital period 164.8 years — has completed only one orbit since discovery (2011).
Diameter 49,244 km; rotation 16 hr 6 min.
Ice giant; deep blue colour from methane; strongest winds in Solar System (up to 2,100 km/h).
"Great Dark Spot" storm observed by Voyager 2 (1989); later vanished.
14 moons — Triton largest, retrograde orbit (probably captured Kuiper Belt object), one of the coldest objects in Solar System (−235 °C), has nitrogen geysers.
Only one visit — Voyager 2 flyby, 25 Aug 1989.

4.3 Quick comparison — orbital & physical data

Planet	Distance (AU)	Diameter (km)	Mass (Earth=1)	Rotation	Revolution	Moons
Mercury	0.39	4,879	0.055	58.6 d	88 d	0
Venus	0.72	12,104	0.815	243 d (R)	225 d	0
Earth	1.00	12,742	1.000	23 h 56 m	365.25 d	1
Mars	1.52	6,779	0.107	24 h 37 m	687 d	2
Jupiter	5.20	139,820	317.8	9 h 56 m	11.86 yr	95
Saturn	9.58	116,460	95.2	10 h 33 m	29.46 yr	146
Uranus	19.2	50,724	14.5	17 h 14 m (R)	84 yr	27
Neptune	30.0	49,244	17.1	16 h 6 m	164.8 yr	14

(R = retrograde rotation. Moon counts as of 2024-25 IAU updates.)

4.4 Dwarf planets

IAU recognises five dwarf planets — Ceres (in asteroid belt), Pluto, Haumea, Makemake, Eris (last four in Kuiper Belt / scattered disc). Possibly hundreds more; ~10² candidates beyond Neptune.

Pluto — diameter 2,377 km; orbital period 248 yrs; discovered by Clyde Tombaugh, 18 Feb 1930; demoted from planet to dwarf planet by IAU on 24 August 2006. NASA's New Horizons flyby on 14 July 2015 revealed a complex world with nitrogen ice plains (Sputnik Planitia), water-ice mountains, and a thin atmosphere.
Eris — slightly more massive than Pluto; its discovery (2005) triggered the IAU redefinition.
Ceres — only dwarf planet in inner Solar System; orbits within asteroid belt; visited by Dawn mission (2015-18).

5. Small bodies — asteroids, comets, meteors & meteorites D1D7D30

NCERT XI · Ch. 1 · pp. 9-10 · Goh Cheng Leong Ch. 1

5.1 Asteroids

Small rocky/metallic bodies orbiting the Sun, mostly in the main asteroid belt between Mars and Jupiter (2.2-3.2 AU). Probably remnant planetesimals whose accretion into a planet was prevented by Jupiter's gravitational perturbation.

~1.1-1.9 million asteroids >1 km; total mass < 4% of the Moon.
Largest: Ceres (940 km; ~⅓ of belt mass; reclassified as a dwarf planet 2006), followed by Vesta, Pallas, Hygiea.
Composition classes — C-type (carbonaceous, 75%), S-type (silicaceous), M-type (metallic).
Trojan asteroids — share orbit of a planet at Lagrange points L4 / L5 (60° ahead / behind). Jupiter has the largest Trojan swarms (~12,000 known); Mars, Neptune, Earth (just 2) also have them.
Near-Earth Asteroids (NEAs) — perihelion < 1.3 AU. ~34,000 catalogued (NASA CNEOS, 2024); subset Potentially Hazardous Asteroids (PHAs) — >140 m diameter, MOID < 0.05 AU.
Missions — NEAR-Shoemaker (Eros, 2001), Hayabusa-1 (Itokawa, 2010) & Hayabusa-2 (Ryugu, 2020) — sample return; OSIRIS-REx (Bennu, sample return 24 Sep 2023); Dawn (Vesta & Ceres); Lucy (Trojans, ongoing); Psyche (16-Psyche, launched Oct 2023). NASA's DART mission (26 Sep 2022) deliberately impacted asteroid Dimorphos — first planetary-defence kinetic-deflection test, successfully altered orbit by ~32 min.

5.2 Comets — "dirty snowballs"

Fig 1.6 — Comet anatomy. The nucleus (1-50 km of ice + dust) sublimates near the Sun, producing a vast coma and two tails — a curved dust tail and a straight ion tail — both directed away from the Sun by radiation pressure and the solar wind.

Definition — small icy bodies (water, CO₂, CH₄, NH₃ ices + silicate dust) that develop a coma and tail when nearing the Sun.
Origin reservoirs:
- Kuiper Belt (30-50 AU) — source of short-period comets (period <200 yrs; e.g., Halley).
- Oort Cloud (~2,000-100,000 AU; ~1 ly out) — spherical shell hypothesised by Jan Oort (1950); source of long-period comets (period >200 yrs; e.g., Hale-Bopp, NEOWISE).
Structure — nucleus (cold, dark core); coma (luminous gas/dust envelope around nucleus); hydrogen envelope (huge invisible cloud); dust tail (curved, yellowish, follows orbit); ion (plasma) tail — straight, bluish, directly away from Sun.
Famous comets:
- Halley's Comet (1P/Halley) — period 76 yrs; last seen 1986; next 2061. Edmund Halley (1705) predicted its return.
- Hale-Bopp (C/1995 O1) — visible to naked eye 1996-97 for 18 months.
- Shoemaker-Levy 9 — broke up & impacted Jupiter Jul 1994 (first directly observed extraterrestrial collision).
- 67P/Churyumov-Gerasimenko — Rosetta mission orbited 2014-16; Philae lander 12 Nov 2014.
- NEOWISE (C/2020 F3) — last bright naked-eye comet visible from India (Jul 2020).
- C/2023 A3 (Tsuchinshan-ATLAS) — visible Oct 2024.

MnemonicDust tail = "Curvy & Cream-coloured"; Ion tail = "Straight & Sky-blue". Both always point away from the Sun — so when a comet is moving away from the Sun, the tail goes in front of it.

5.3 Meteors & meteorites

Fig 1.7 — Three-stage life of an interplanetary rock. A meteoroid in space becomes a meteor ("shooting star") when it burns up in Earth's atmosphere; if a fragment survives and lands, it is a meteorite.

Term	Definition
Meteoroid	Small rocky or metallic body (mm to ~1 m) in space — debris from asteroids or comets
Meteor	Streak of light produced when a meteoroid enters Earth's atmosphere (typically 80-120 km altitude) and burns up due to friction
Meteorite	The surviving fragment that reaches Earth's surface
Bolide / Fireball	Exceptionally bright meteor (brighter than Venus)

5.3.1 Types of meteorites

Stony meteorites (~94%) — chondrites (with chondrules — primitive, unchanged since Solar System formation, 4.56 Gya) and achondrites (differentiated).
Iron meteorites (~5%) — iron-nickel alloy; show Widmanstätten pattern; fragments of differentiated asteroid cores.
Stony-iron meteorites (~1%) — pallasites & mesosiderites; rarest, most prized.

5.3.2 Major meteor showers

Annual events that occur when Earth passes through the dust trail left by a comet (or rarely, asteroid). Named after the radiant constellation.

Shower	Peak date	Parent body	Notes
Quadrantids	3-4 January	Asteroid 2003 EH₁ (likely dead comet)	Short, intense peak
Lyrids	22-23 April	Comet C/1861 G1 Thatcher	Active for 2,700 yrs (oldest recorded)
Eta Aquariids	5-6 May	1P/Halley	Best viewed from Southern Hemisphere
Perseids	12-13 August	Comet 109P/Swift-Tuttle	Among brightest; reliable; ~100 meteors/hr at peak
Orionids	21-22 October	1P/Halley	Two showers per year from Halley's debris (with Eta Aquariids)
Leonids	17-18 November	Comet 55P/Tempel-Tuttle	Periodic meteor storms every 33 yrs (last 1999-2002)
Geminids	13-14 December	Asteroid 3200 Phaethon	Strongest annual shower (~150/hr); only major shower from an asteroid

Mnemonic"Quick Lyric Eta Persuades Orion's Leo Genius" — Quadrantids · Lyrids · Eta Aquariids · Perseids · Orionids · Leonids · Geminids. Two most reliable: Perseids (Aug) & Geminids (Dec).

5.3.3 Famous impacts

Tunguska event — 30 June 1908, Siberia — asteroid or comet airburst (~10-15 MT TNT-equivalent) flattened ~2,150 km² of forest; no crater found.
Chelyabinsk meteor — 15 Feb 2013, Russia — ~20 m asteroid airburst, ~500 kT energy; 1,500+ people injured by shattered glass.
Chicxulub impact — ~66 Mya, Yucatán, Mexico — ~10 km asteroid; created 180-km-diameter crater; caused the K-Pg extinction that ended the non-avian dinosaurs.
Vredefort crater, South Africa (~2.02 Gya) — largest confirmed impact structure on Earth (~300 km diameter).
Lonar Lake, Buldhana, Maharashtra — only meteoritic crater in Indian basalt (~52,000 yrs old; ~1.83 km diameter); declared Ramsar Site in November 2020.

In the news NASA's DART mission (Sep 2022) successfully altered the orbit of asteroid Dimorphos — first demonstration of planetary defence via kinetic impactor. OSIRIS-REx returned ~250 g of Bennu samples to Earth on 24 Sep 2023; analysis (2023-25) confirmed water-bearing minerals and organic compounds. The Bennu material findings bear on the origin of water and life on Earth. India's Chandrayaan-3 (23 Aug 2023) made India the fourth nation to soft-land on the Moon and the first to land near the lunar south pole.

Mains template — 250-word answer (15 marks)

Q: Distinguish between asteroids, comets, meteors and meteorites. What do they tell us about the formation and evolution of the Solar System?

Intro — these are the "small bodies" of the Solar System — leftover building blocks of planet formation 4.6 Gya.
Asteroids — rocky/metallic; main belt 2.2-3.2 AU; ~1.9 M known; types C/S/M; Trojans; NEAs.
Comets — icy "dirty snowballs"; Kuiper Belt (short-period) & Oort Cloud (long-period) reservoirs; structure (nucleus, coma, dust tail, ion tail); famous — Halley, Hale-Bopp, Shoemaker-Levy 9.
Meteor / Meteorite — meteoroid → meteor (atmospheric burn) → meteorite (surface impact); types stony/iron/stony-iron; major showers Perseids & Geminids.
[Diagram cue: 3-panel flow showing meteoroid → meteor → meteorite OR comet anatomy with labelled coma/nucleus/tails]
Significance — chondrites preserve 4.56-Gyr-old Solar System material; comets & carbonaceous asteroids may have delivered water and organic compounds to early Earth (panspermia debate); Chicxulub impact rewrote evolutionary history.
Conclusion — Mention DART (planetary defence 2022), OSIRIS-REx Bennu return (2023), Indian Lonar crater + Chandrayaan-3 lunar south pole landing (Aug 2023). Highlight India's growing planetary-science capability.

UPSC Previous Year & model questions

2024 Consider the following pairs of celestial body and feature (Prelims question on Jovian moons & subsurface oceans).
2023 Aditya-L1, launched by ISRO, was placed in which Lagrangian point and why is L1 chosen? (GS-3 link, also Prelims MCQ on Lagrange points)
2022 With reference to "Carbon nanotubes", why are they suitable for use in [tech context] — Prelims (recurring "what is in the news" pattern on space tech).
2021 Prelims — match planets with their distinguishing feature (e.g., Venus retrograde rotation, Saturn's rings, Jupiter's Great Red Spot).
2020 "Mars Orbiter Mission (MOM)" — what were its scientific objectives and significance? (GS-3 / GS-1 overlap)
2019 Discuss the work being done in India in space exploration. How can it help India's economic growth and security? (GS-3)
2018 Prelims — Kuiper Belt and Oort Cloud questions; Halley's Comet next return year.
2017 Prelims — meteors and meteorites; difference between asteroid and comet.
2016 Prelims — Cassini-Huygens mission and Saturn.
2013 Big Bang Theory — Prelims definitional MCQ.

Honest attribution note. UPSC's exact questions on Universe/Solar System have appeared mostly in Prelims (definitional MCQs on Big Bang, planets, asteroids, comets, meteor showers, ISRO missions). Direct Mains questions on the chapter are rare; the topic recurs as background to GS-3 Space & Technology questions on Indian space missions (Chandrayaan, Mangalyaan, Aditya-L1, Gaganyaan), planetary defence, and dark sky / astronomy debates. Always cross-verify with official UPSC papers and standard compilations.

Additional model questions (Mentor's own)

Discuss the Big Bang Theory and the evidences supporting it. What questions remain unanswered? (150 words, 10 marks)
Describe the life cycle of a star and explain how a star's mass determines its end state. (250 words, 15 marks)
Compare and contrast the terrestrial and Jovian planets with respect to their physical and chemical characteristics. (250 words, 15 marks)
Why is Venus called Earth's twin, and yet why is it the hottest planet despite Mercury being closer to the Sun? (150 words, 10 marks)
Describe the internal structure of the Sun. Discuss the impact of solar activity (sunspots, flares, CMEs) on the Earth. (250 words, 15 marks)
What are asteroids, comets, meteors and meteorites? What is their significance for understanding the formation of the Solar System? (250 words, 15 marks)
Discuss the importance of the Kuiper Belt and the Oort Cloud as reservoirs of comets. (150 words, 10 marks)
Critically examine the IAU's 2006 redefinition of "planet" that demoted Pluto. (150 words, 10 marks)
Trace the evolution of Indian planetary exploration from Mangalyaan (2014) to Chandrayaan-3 (2023) and Aditya-L1 (2024). What scientific and strategic objectives do these missions serve? (250 words, 15 marks)
"Planetary defence is no longer science fiction." Discuss in light of NASA's DART mission (2022) and the implications for India's space programme. (250 words, 15 marks)

Mentor's note. These 10 supplementary questions are model questions designed by the MentorsDaily team aligned to standard UPSC GS-1 (Physical Geography), GS-3 (Space & Technology) and Essay paper themes. Use them for answer-writing practice alongside genuine PYQs.

15 must-know facts for revision

Age of Universe — 13.8 billion years (Planck 2013 measurement); Age of Solar System & Sun — 4.6 billion years.
Big Bang Theory — proposed by Georges Lemaître (1927); named (sarcastically) by Fred Hoyle (1949).
Cosmic Microwave Background — discovered by Penzias & Wilson (1964; Nobel 1978); temperature 2.725 K; direct evidence of Big Bang.
Hubble's Law (1929) — recession velocity of distant galaxies ∝ distance; H₀ ≈ 70 km/s/Mpc. Implies expanding Universe.
Hubble's galaxy classification (1926) — Spiral · Barred Spiral · Elliptical · Lenticular · Irregular. The Milky Way is a barred spiral (SBbc); ~100,000 ly across; Sun is ~26,000 ly from galactic centre (Sagittarius A*).
Stellar spectral classification — O B A F G K M (hottest blue to coolest red). The Sun is class G2V.
Sun's basic data — diameter 1.39 × 10⁶ km (109 × Earth); 99.86% of Solar System mass; surface 5,500 °C; core 15 million °C; sunspots follow 11-yr Schwabe cycle.
Eight planets — Mercury · Venus · Earth · Mars · Jupiter · Saturn · Uranus · Neptune ("My Very Educated Mother Just Served Us Noodles"). Pluto demoted to dwarf planet by IAU on 24 August 2006.
Terrestrial (Mercury–Mars) = small, dense, rocky, few moons. Jovian (Jupiter–Neptune) = large, low-density, gaseous/icy, many moons, all have rings.
Venus — hottest planet (462 °C); rotates retrograde; day > year. Mars — Olympus Mons (tallest volcano) & Valles Marineris (largest canyon). Jupiter — largest, Great Red Spot, 95 moons, Ganymede biggest moon. Saturn — least-dense (0.69 g/cm³, would float), 146 moons, Titan with thick atmosphere. Uranus — discovered by Herschel 1781, axial tilt 97.77° (rolls on side). Neptune — mathematically predicted; strongest winds (2,100 km/h).
Asteroid belt — between Mars & Jupiter, 2.2-3.2 AU; ~1.9 million bodies; largest = Ceres (also dwarf planet). NASA's DART mission (Sep 2022) — first kinetic-impact planetary defence test (Dimorphos).
Comet reservoirs — Kuiper Belt (30-50 AU, short-period comets), Oort Cloud (~2,000-100,000 AU, long-period comets). Halley's Comet — period 76 yrs; last 1986; next return 2061.
Meteor showers — most reliable annual showers are Perseids (12-13 Aug, parent: 109P/Swift-Tuttle) and Geminids (13-14 Dec, parent: asteroid 3200 Phaethon — only asteroidal major shower).
Famous impacts — Chicxulub (~66 Mya) caused K-Pg dinosaur extinction; Tunguska (30 June 1908); Chelyabinsk (15 Feb 2013); Lonar Lake, Maharashtra (~52,000 yrs ago) — only Indian basaltic meteoritic crater; Ramsar site since Nov 2020.
Indian space milestones — Mangalyaan / MOM (Sep 2014, first nation to reach Mars on maiden attempt); Chandrayaan-3 (23 Aug 2023, first soft landing near lunar south pole); Aditya-L1 (launched 2 Sep 2023, reached L1 on 6 Jan 2024).

Key numbers

13.8 GyrAge of Universe
93 GlyDiameter of observable Universe
2.725 KCMB temperature today
2 trillionGalaxies in observable Universe
~70 km/s/MpcHubble constant H₀
100,000 lyMilky Way diameter
26,000 lySun's distance from galactic centre
230 km/sSun's orbital speed in galaxy
225 MyrCosmic year (Sun's orbit)
4.5 GyrMilky Way-Andromeda collision
4.24 lyDistance to Proxima Centauri (nearest star)
4.6 GyrAge of the Sun & Solar System
1.39 × 10⁶ kmSun's diameter (109 × Earth)
5,500 °CSun's surface temperature
1.5 × 10⁷ KSun's core temperature
150 × 10⁶ kmEarth-Sun distance (1 AU)
8 min 20 sSunlight travel-time to Earth
~10²² starsIn observable Universe
G2VSun's spectral class
~5 GyrSun's remaining main-sequence life
88 daysMercury's orbital period
243 daysVenus's rotation period (longest)
365.25 daysEarth's orbital period
687 daysMars's orbital period
11.86 yrsJupiter's orbital period
29.46 yrsSaturn's orbital period
84 yrsUranus's orbital period
164.8 yrsNeptune's orbital period
2-4 AUMain asteroid belt range
30-50 AUKuiper Belt range
2,000-100,000 AUOort Cloud range
12 AugPerseid meteor shower peak
13-14 DecGeminid meteor shower peak
76 yrsHalley's Comet period (next 2061)
29 Jun 1908Tunguska event (asteroid airburst)
15 Feb 2013Chelyabinsk meteor airburst
23 Aug 2023Chandrayaan-3 lunar south pole landing
2 Sep 2023Aditya-L1 launched (ISRO)
Jul 2022JWST first science images

Keyword cloud

UniverseCosmologyBig BangLemaîtreHubbleHubble's LawHubble constantRedshiftDoppler effectInflationNucleosynthesisRecombinationCMBPenzias-WilsonCOBEWMAPPlanckJWSTPulsating TheoryBig CrunchBig BounceSteady StateHoyleBondiGoldDark matterDark energyAccelerating universeNobel 2011GalaxySpiralBarred spiralEllipticalLenticularIrregularHubble tuning forkMilky WaySBbcAndromedaM31M87Magellanic CloudsLMCSMCSagittarius A*Local GroupVirgo SuperclusterLaniakeaCosmic webStarPlasmaNuclear fusionp-p chainCNO cycleNebulaProtostarMain sequenceRed giantRed supergiantWhite dwarfNeutron starPulsarBlack holeSupernovaPlanetary nebulaSchwarzschild radiusEvent horizonEHTLIGOGravitational wavesH-R diagramHertzsprung-RussellOBAFGKMG2VPopulation IPopulation IISolar SystemSunPhotosphereChromosphereCoronaSunspotSolar windSolar flareCMEAuroraMercuryVenusEarthMarsJupiterSaturnUranusNeptuneTerrestrial planetJovian planetGas giantIce giantPlutoDwarf planetErisCeresHaumeaMakemakeAsteroidAsteroid beltVestaNEANEOTrojanCometHalleyHale-BoppShoemaker-LevyNEOWISEComaNucleusDust tailIon tailKuiper BeltOort CloudMeteoroidMeteorMeteoriteBolideIron meteoriteStony meteoriteChondriteAchondriteTunguskaChelyabinskPerseidsGeminidsLeonidsQuadrantidsLyridsOrionidsAditya-L1Chandrayaan-3MangalyaanISRONASAESAJAXAGaganyaanShukrayaan-1SPADEXL1 point

Quick quiz

Who first proposed the Big Bang Theory and in what year?
Year and discoverers of the Cosmic Microwave Background?
What is the value of the CMB temperature today?
Name the four major galaxy types in Hubble's classification.
What type of galaxy is the Milky Way? Sun's distance from the galactic centre?
Mnemonic for the spectral classification of stars (O B A F G K M)?
What is the Sun's spectral class and how long has it been on the main sequence?
What determines whether a star becomes a white dwarf, neutron star, or black hole?
What is the temperature at the Sun's core vs photosphere?
What is the difference between a meteoroid, meteor and meteorite?

The Universe & the Solar System — From the Big Bang to Neptune

Why this topic matters for UPSC

On this page

1. Origin of the Universe D1D7D30

1.1 Timeline of the evolution of the Universe

1.2 The Big Bang Theory D1D7

Key propositions

Empirical evidence

1.3 The Pulsating (Oscillating) Universe Theory

1.4 The Steady State Theory (for completeness)

Mains answer template — 150-word skeleton (10 marks)

2. Components of the Universe D1D7D30

2.1 Galaxies — types & structure

The Milky Way Galaxy — our home

2.2 Stars D1D7

2.2.1 Birth, life and death of a star

2.2.2 Hertzsprung-Russell (H-R) diagram — classifying stars

2.2.3 End states of stellar evolution

Mains template — 250-word answer (15 marks)

3. The Solar System — origin & the Sun D1D7D30

3.1 Origin of the Solar System — the Nebular Hypothesis

3.2 The Sun — our star

Basic data — the Sun

3.2.1 Internal layers (centre outward)

3.2.2 Solar activity

Mains template — 250-word answer (15 marks)

4. The Eight Planets — terrestrial & Jovian D1D7D30D90

4.1 Terrestrial vs Jovian — the great division

Terrestrial (Inner) — Mercury, Venus, Earth, Mars

Jovian (Outer) — Jupiter, Saturn, Uranus, Neptune

4.2 The eight planets — characteristics

4.2.1 Mercury (☿)

4.2.2 Venus (♀)

4.2.3 Earth (♁)

4.2.4 Mars (♂)

4.2.5 Jupiter (♃) — King of planets

4.2.6 Saturn (♄)

4.2.7 Uranus (⛢)

4.2.8 Neptune (♆)

4.3 Quick comparison — orbital & physical data

4.4 Dwarf planets

5. Small bodies — asteroids, comets, meteors & meteorites D1D7D30

5.1 Asteroids

5.2 Comets — "dirty snowballs"

5.3 Meteors & meteorites

5.3.1 Types of meteorites

5.3.2 Major meteor showers

5.3.3 Famous impacts

Mains template — 250-word answer (15 marks)

UPSC Previous Year & model questions

Additional model questions (Mentor's own)

15 must-know facts for revision