The solar system is the Sun and everything bound to it by gravity: eight planets, five recognised dwarf planets, hundreds of moons, and countless asteroids, comets and meteoroids.
Stars are luminous, making energy by thermonuclear fusion of hydrogen into helium; planets are non-luminous and shine by reflected starlight.

The Universe and the Stars

Universe

The universe is everything that exists, from galaxy superclusters to subatomic particles.
The Big Bang theory, backed by the expansion found by Edwin Hubble (1929), is the accepted account of its origin.

Aspect	Current value or view
Age	About 13.8 billion years (Planck satellite, cosmic microwave background)
Galaxies	Hundreds of billions in the observable universe, grouped into clusters and superclusters (“archipelagos of stellar islands”)
Open question	The “Hubble tension”: expansion measured from nearby stars (about 73 km/s per megaparsec) exceeds that inferred from the early universe (about 67)

Galaxies and the Milky Way

A galaxy is a vast system of stars, gas, dust and dark matter held together by gravity; by shape galaxies are spiral, elliptical or irregular.
The Milky Way (Akash Ganga) is our galaxy, a barred spiral; seen edge-on from inside, its disc looks like a river of milky light across the night sky.

Feature	Value
Diameter	about 100,000 light years
Stars	roughly 100–400 billion
Sun’s distance from the centre	about 26,000 light years, in the Orion Arm
Galactic year (one orbit of the Sun)	about 230 million years; only about two orbits since the Palaeozoic began
Nearest large galaxy	Andromeda (spiral), about 2.5 million light years
Nearest galaxies	dwarf satellites such as the Large Magellanic Cloud (about 160,000 light years)

Stars

A star’s colour tracks its surface temperature: red is coolest (about 3,000 °C), yellow like the Sun intermediate (about 5,500 °C), blue-white hottest (above 10,000 °C).
After the Sun, the nearest star is Proxima Centauri (4.24 light years).
A nebula is a cloud of interstellar gas and dust: emission (glowing ionised gas, as in Orion), reflection (scattered starlight) or dark (blocking light behind it).

Life Cycle of a Star

A star begins as a nebula that contracts into a protostar; fusion starts once the core is hot and dense enough, and mass decides the ending.
- A white dwarf cannot exceed about 1.4 solar masses, the Chandrasekhar limit set by Subrahmanyan Chandrasekhar in the early 1930s.

Star mass	Stages after hydrogen runs out	End state
Low to Sun-like (below about 8 Suns)	Red giant; helium fuses to carbon; outer layers shed as a planetary nebula	White dwarf (shines by stored heat) → black dwarf on cooling
Massive (above about 8 Suns)	Red supergiant → supernova explosion	Neutron star, or a black hole for the heaviest cores

Constellations

A constellation is a group of stars forming an imaginary figure; the IAU recognises 88.
Their fixed seasonal positions long guided night navigation.

Constellation	Indian name	Where and when seen
Ursa Major (Great Bear; contains the Big Dipper)	Saptarishi	Northern sky, early night in summer; two “pointer” stars lead to Polaris
Ursa Minor (Little Bear)	Laghu Saptarishi	Holds Polaris (Dhruva Tara), the pole star
Orion	Mriga	Late evening in winter
Cassiopeia (W-shaped)	Sharmishtha	Northern sky in winter

The Sun

The Sun is a yellow dwarf star whose gravity holds every planet in orbit and whose radiation, from gamma rays to radio waves, powers Earth’s surface systems.

Property	Value
Age	About 4.6 billion years
Composition (by mass)	About 73% hydrogen, 25% helium, 2% heavier elements
Share of solar-system mass	99.86% (about 2 × 10³⁰ kg)
Diameter	109 times Earth’s
Distance from Earth	149.6 million km (1 astronomical unit, AU); light takes about 8 min 20 s

Structure of the Sun

Fusion energy leaves the core by radiation, then rises by convection to the surface.
The corona is far hotter than the surface; its heat strips iron, nickel and calcium of electrons, giving its spectral lines.

Layer	Temperature (approx.)	Key point
Core	15 million °C	Hydrogen fuses into helium
Radiative zone	falls to about 2 million °C	Energy moves outward as radiation
Convection zone	about 2 million °C to 5,500 °C	Hot plasma rises, cools and sinks
Photosphere	about 5,500 °C	Visible surface; sunspots
Chromosphere	6,000 to 20,000 °C	Reddish layer seen at eclipses
Corona	1 to 3 million °C	Outer atmosphere; source of the solar wind

Solar Activity

Sunspots are dark, cooler patches of intense magnetism on the photosphere; flares and ejections erupt from around them.
Their number rises and falls in an 11-year sunspot cycle, from solar minimum to solar maximum.
- Solar Cycle 25 peaked in October 2024, stronger than forecast.

Phenomenon	What it is	Effect on Earth
Solar wind	Continuous outflow of electrons, protons and alpha particles from the corona	Shapes the magnetosphere; drives auroras
Solar flare	Sudden burst of radiation near sunspots	Radio blackouts; hazard to satellites and astronauts
Coronal mass ejection (CME)	Huge cloud of magnetised plasma thrown into space	Main cause of geomagnetic storms
Solar storm	Umbrella term for these disturbances spreading through the heliosphere	Space weather (short term) and space climate (long term)

Geomagnetic Storms and Auroras

A geomagnetic storm is a temporary disturbance of the magnetosphere when a CME or solar-wind shock strikes Earth’s magnetic field.
Charged particles funnelled towards the poles light the upper atmosphere as auroras: Aurora Borealis in the north, Aurora Australis in the south.
Indian example: the extreme storm of May 2024 produced red auroras photographed from Hanle, Ladakh.
Aditya-L1, India’s first solar observatory (launched 2 September 2023), has orbited the L1 point, 1.5 million km sunward, since 6 January 2024; it recorded two CMEs colliding during the May 2024 storm.

The Solar System

Members and Their Motions

All planets revolve anticlockwise (seen from above the north pole), in nearly the same plane, along slightly elliptical orbits; most moons revolve the same way.
Most planets also rotate in that direction; Venus and Uranus are the exceptions.
The Titius–Bode rule predicts that each planet lies roughly twice as far from the Sun as the one inside it.
- It fits most planets and the asteroid belt, not Neptune.
The same process formed all the planets, so theories of the origin of the Earth are theories of the whole system.

Terrestrial and Jovian Planets

Planet size increases towards Jupiter from either side, Mars being the exception.

Feature	Terrestrial (inner)	Jovian (outer)
Planets	Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus, Neptune
Composition	Rock and metal	Gas giants (H, He); ice giants (water, methane, ammonia over a rocky core)
Size and density	Small, dense (3.9–5.5 g/cm³)	Huge, low density (0.7–1.6 g/cm³)
Moons and rings	Few moons (0–2), no rings	Many moons, all have rings
Rotation	Slow	Fast (10–17 hours)

Planetary Data

Planet	Mean distance (million km / AU)	Diameter (km)	Mass (Earth = 1)	Density (g/cm³)	Rotation	Revolution
Mercury	57.9 / 0.39	4,879	0.055	5.43	58.6 days	88 days
Venus	108.2 / 0.72	12,104	0.815	5.24	243 days (retrograde)	224.7 days
Earth	149.6 / 1.00	12,756	1	5.51	23 h 56 min	365.26 days
Mars	227.9 / 1.52	6,792	0.107	3.93	24 h 37 min	687 days
Jupiter	778.5 / 5.2	142,984	317.8	1.33	9.9 h	11.9 years
Saturn	1,432 / 9.5	120,536	95.2	0.69	10.7 h	29.5 years
Uranus	2,867 / 19.2	51,118	14.5	1.27	17.2 h (retrograde)	84 years
Neptune	4,515 / 30.1	49,528	17.1	1.64	16.1 h	164.8 years

Planet	Axial tilt	Moons (mid-2026)	Atmosphere	Notable features and missions
Mercury	0.03°	0	Thin exosphere (O, Na, H, He, K)	Smallest planet; 430 °C by day, −180 °C at night; Mariner 10, MESSENGER; BepiColombo arriving 2026
Venus	177°	0	96% CO₂, sulphuric-acid clouds	Hottest planet (about 465 °C) from a runaway greenhouse; Sun rises in the west; Magellan mapped 98%; ISRO’s Venus Orbiter Mission approved 2024
Earth	23.4°	1	78% N₂, 21% O₂	Only known life; liquid water; plate tectonics; average density highest of all planets
Mars	25.2°	2 (Phobos, Deimos)	Thin CO₂	Red from iron oxide; Olympus Mons; Mangalyaan (2014); Perseverance in Jezero Crater, an ancient river delta (2021)
Jupiter	3.1°	115	H, He	About 1,300 Earths fit inside; Great Red Spot; faint rings (Voyager 1, 1979); Ganymede is the largest moon; Juno (2016), JUICE and Europa Clipper en route
Saturn	26.7°	293 (most of any planet)	H, He	Least dense planet; brightest rings, first seen as “handles” by Galileo Galilei (1610); Titan has a thick atmosphere; Cassini–Huygens (2004–17)
Uranus	97.8°	29	H, He, methane	Ice giant that rolls on its side; 13 rings; moons named after characters of Shakespeare and Pope; only Voyager 2 (1986)
Neptune	28.3°	16	H, He, methane	Ice giant with the fastest winds; found by prediction (1846); five main rings; Triton orbits backwards; only Voyager 2 (1989)

The Moon

Earth’s only natural satellite orbits at about 384,400 km, rotating and revolving in the same 27.3 days, so one face always points to Earth.
It is rocky and cratered, with only a thin exosphere; twelve Apollo astronauts walked on it (1969–72).
Indian example: Chandrayaan-1 (2008) helped detect surface water, and Chandrayaan-3 made the first landing near the lunar south pole on 23 August 2023.
Artemis II flew four astronauts around the Moon in April 2026, the first crewed lunar voyage since 1972.

Dwarf Planets and the Kuiper Belt

The IAU defined a planet on 24 August 2006; a body must:
1. Orbit the Sun.
2. Be massive enough for gravity to pull it into a near-spherical shape.
3. Have cleared the neighbourhood of its orbit, dominating it by absorbing or ejecting other bodies.
Pluto (discovered 1930) fails the third test: it has only about 0.08 times the mass of the other objects in its orbit, while Earth has 1.7 million times.
- It was reclassified as a dwarf planet, catalogued as 134340 Pluto.
The Kuiper Belt (Edgeworth–Kuiper belt) is a disc of icy bodies from Neptune’s orbit (30 AU) to about 50 AU, far larger than the asteroid belt; New Horizons flew past Pluto (2015) and Arrokoth (2019).

Dwarf planet	Location	Mean distance (AU)	Diameter (km)	Moons	Note
Ceres	Asteroid belt	2.8	940	0	First asteroid found (Giuseppe Piazzi, 1801); orbited by Dawn (2015)
Pluto	Kuiper Belt	39.5	2,377	5 (Charon, Nix, Hydra, Kerberos, Styx)	Largest dwarf planet by size; a day lasts 6.4 Earth days
Haumea	Kuiper Belt	43	about 1,600 (elongated)	2	Fast spin; has a ring
Makemake	Kuiper Belt	45.5	about 1,430	1	Methane-ice surface
Eris	Scattered disc	68	2,326	1 (Dysnomia)	Most massive dwarf planet; its discovery (2005) forced the 2006 definition

Small Bodies

These rocky and icy fragments are leftovers of planet formation.
Indian example: Lonar crater (Maharashtra) is a meteorite-impact crater in Deccan basalt.
3I/ATLAS (found 1 July 2025) is the third known interstellar object to cross the solar system.

Body	Nature	Where found	Key facts
Asteroids	Solid, rocky or metallic, irregular; no atmosphere or rings	Main belt between Mars and Jupiter; near-Earth asteroids	Ida and its moon Dactyl (1993) was the first asteroid–moon pair; Dawn orbited Vesta (2011); DART deflected Dimorphos (2022); OSIRIS-REx returned Bennu samples (2023)
Meteoroids, meteors, meteorites	Meteoroid: small rock in space; meteor: its streak (“shooting star”) in the air; meteorite: the piece that lands	Everywhere; showers when Earth crosses comet debris	Stony, iron or stony-iron; largest known is Hoba (Namibia); may have delivered amino acids
Comets	“Dirty snowballs” of ice, rock and dust; near the Sun they grow a coma and tails pointing away from the Sun	Short-period from the Kuiper Belt; long-period from the Oort Cloud	Halley returns every 75–76 years (last 1986, next 2061); Perseids come from comet Swift–Tuttle; may have brought water and organics to Earth

Kármán Line

The Kármán line, 100 km above sea level, is the conventional boundary between the atmosphere and outer space.
- It separates airspace law from space law; the US uses a lower mark of about 80 km.