Origin and Evolution of the Earth and the Solar System

1. Early theories of the Earth's origin

Read the detailed note →

• Nebular hypothesis: the planets formed from a cloud of material around a young sun that rotated slowly. The German philosopher Immanuel Kant proposed it, and the mathematician Laplace revised it in 1796.
• Revised nebular hypothesis (1950): put forward by Otto Schmidt (Russia) and Carl Weizsacker (Germany).
• The sun was surrounded by a solar nebula of mostly hydrogen and helium, plus dust.
• Particles rubbed and collided, which formed a disk-shaped cloud.

• The planets grew from this disk by accretion (small bodies sticking together).

• Beyond NCERT, the "passing star" (binary) theories:

• Chamberlin and Moulton's planetesimal hypothesis (early 1900s): a wandering star passed near the sun and pulled out cigar-shaped strands of material. These cooled into small bodies (planetesimals), which joined to make planets.
• Jeans and Jeffreys' tidal hypothesis (1919) also relied on a passing star.

• Why they failed: a passing star cannot explain the planets' angular momentum (their spin and orbital motion). The modern solar-nebula model brought back an improved nebular idea.

• Later, scientists moved from asking how the Earth began to asking how the universe began.

2. The origin of the universe: Big Bang, expanding universe and steady state

Read the detailed note →

• Big Bang theory (also called the expanding universe hypothesis): the universe began as a tiny point that was unimaginably hot and dense and then expanded violently.
• All matter sat in one "tiny ball" (singular atom) with tiny volume and infinite temperature and density.
• It exploded. The fastest expansion came within fractions of a second, and after that it slowed. Some energy turned into matter.
• The first atoms began to form within 3 minutes.

• Within 300,000 years the temperature fell to 4,500 K, atomic matter formed and the universe became transparent. (Now: about 380,000 years at about 3,000 K.)

• Age: NCERT gives 13.7 billion years. The current figure is about 13.8 billion years (Planck 2018).

• Expanding universe: galaxies are moving apart because the space between them is growing. Edwin Hubble gave the evidence (NCERT: 1920; his velocity-distance law came in 1929). Lemaitre proposed the "primeval atom" in 1927. The expansion is now speeding up, which is put down to dark energy.
• The balloon analogy is only partly correct: galaxies do not swell, but the dots on a balloon do.
• Steady state theory (Hoyle, Bondi, Gold, 1948): the universe looks roughly the same at all times, with new matter created as it expands.
• It lost because the evidence for expansion kept growing.
• The cosmic microwave background (CMB) was the decisive blow. This is leftover light from the early universe, found by Penzias and Wilson in 1965.
• Hoyle himself coined the name "Big Bang" (1949) as a jibe.

3. Galaxies, stars and cosmic distances

Read the detailed note →

• How stars formed: matter in the early universe was spread unevenly.
• Uneven density meant uneven gravity, which pulled matter together.
• Hydrogen gathered into a huge cloud, a nebula (a giant cloud of gas, mainly hydrogen, and dust).

• Clumps inside the nebula grew denser and became stars.

• Galaxy: a huge system of stars, gas and dust held together by gravity. Galaxies are 80,000–150,000 light years across.

• When stars formed: NCERT says "some 5–6 billion years ago". Now: the first stars formed within a few hundred million years of the Big Bang. Our Sun formed about 4.6 billion years ago.
• Light year: a unit of distance, not time. It is the distance light travels in one year at about 300,000 km/s, which is about 9.461 × 10¹² km (about 9.46 trillion km).
• Sun to Earth: about 149,598,000 km (1 astronomical unit, AU). Sunlight takes about 8.3 minutes to cover it (NCERT: 8.311 minutes).
• Beyond NCERT:
• The Milky Way is a barred spiral galaxy about 100,000 light years across. The Sun is about 26,000 light years from its centre.
• Proxima Centauri is the nearest star after the Sun, about 4.2 light years away.
• Andromeda is the nearest large galaxy.

4. Formation of the planets and the solar system today

Read the detailed note →

• How planets formed (three stages): 1. Gravity inside a gas lump forms a core, and a huge rotating disc of gas and dust forms around it. 2. Matter condenses into small rounded objects that stick together into planetesimals (small solid bodies). Collisions and gravity join them into larger bodies. 3. Many planetesimals come together by accretion (colliding and sticking under gravity) to form a few large planets.

	Terrestrial (inner)	Jovian (outer)
Planets	Mercury, Venus, Earth, Mars	Jupiter, Saturn (gas giants); Uranus, Neptune (ice giants)
Build	Small, dense, rocky	Large, low density, mostly gas, many moons and rings

• Why they differ:
• Near the Sun it was too hot for gases and ices to condense.
• The solar wind swept gas out of the inner zone.

• The inner planets' weaker gravity could not hold on to escaping gas.

• The asteroid belt lies between Mars and Jupiter.

• Dwarf planet: under IAU Resolution B5 (24 August 2006), a planet must orbit the Sun, be round under its own gravity and have cleared its orbit of other objects. A dwarf planet meets the first two conditions but not the third.
• Pluto was moved into this class.

• The others are Ceres (in the asteroid belt), Eris, Haumea and Makemake.

• Kuiper belt: a doughnut-shaped zone of icy bodies beyond Neptune, about 30–50 AU from the Sun. Pluto lives here, and it is the source of many short-period comets. The more distant Oort cloud supplies long-period comets.

• Lagrange points: five spots where the gravity of two large bodies (such as the Sun and Earth) balances the motion of a small craft, so it can stay in place.
• Aditya-L1, India's first solar observatory, launched on 2 September 2023. It entered a halo orbit at Sun-Earth L1, about 1.5 million km sunward of Earth, on 6 January 2024.
• The James Webb Space Telescope sits at L2.

5. The Moon and a layered Earth: age, giant impact and differentiation

Read the detailed note →

• Age of the Earth: about 4.6 billion years (4,600 million years; more precisely 4.54 billion years).

• This age comes from radiometric dating of meteorites and the oldest Earth minerals. The oldest are zircons from Jack Hills, Western Australia, about 4.4 billion years old.

• The early Earth: barren, rocky and hot, with a thin hydrogen-helium atmosphere.

• Layered structure: the atmosphere is the least dense layer, and density rises inward towards the core.
• Differentiation (how the layers formed):
• The young Earth was mostly volatile. As its density grew, its inside heated up.
• Material separated by weight. Heavy iron sank to the centre and lighter material rose.
• The Earth cooled, solidified and shrank, and a crust formed.

• Result: crust, mantle, outer core and inner core, with density rising inward. The detail of each layer is in earth-interior.

• Giant impact hypothesis: a Mars-sized body (Theia) hit the young Earth about 4.5 billion years ago. The debris came together to form the Moon. NCERT notes that the impact heated the Earth further.

• Evidence: Apollo Moon rocks match Earth's mantle in oxygen isotopes (forms of oxygen atoms), and the Moon has only a small iron core.

• Older ideas such as fission, capture and co-formation have been set aside.

• India hook: Chandrayaan-3 landed near the lunar south pole on 23 August 2023.

6. Evolution of the atmosphere and hydrosphere

Read the detailed note →

Evolution of the atmosphere happened in three stages:

1. Loss of the primordial hydrogen-helium atmosphere. It was stripped away by the solar wind (a stream of charged particles flowing out from the Sun). This happened to all the terrestrial planets.

2. Degassing: as the Earth cooled, gases and water vapour were released from its hot interior, helped by constant volcanic eruptions. - This second atmosphere had water vapour, nitrogen, carbon dioxide, methane and ammonia, with very little free oxygen.

3. Photosynthesis by living things changed the mix and added free oxygen.

• Origin of the oceans:
• Water vapour condensed into rain, and the rain dissolved carbon dioxide.
• The Earth cooled further, which brought more rain.
• Water filled low-lying basins (depressions).

• So oceans formed within 500 million years of the Earth's birth and are about 4,000 million years old.

• Oxygenation of the atmosphere:

• Photosynthesis evolved about 2,500–3,000 million years ago.
• Oxygen first saturated the oceans. Then, about 2,000 million years ago, it began to flood the atmosphere.

• Beyond NCERT, this is the Great Oxidation Event (about 2.4–2.0 billion years ago).

• Side effects:

• Iron dissolved in seawater reacted with oxygen and settled as banded iron formations. These are the source of much of India's iron ore (see minerals-rocks-rock-cycle).

• Oxygen also allowed the ozone layer to form.

• Today's atmosphere is mainly nitrogen and oxygen (see atmosphere-composition-weather).

7. The Earth's magnetic shield and space weather

Read the detailed note →

• Geodynamo: moving liquid iron in the outer core creates the Earth's magnetic field (detail in earth-interior).
• Magnetosphere: the region around the Earth controlled by this field.
• It shields the planet from the solar wind and cosmic rays.
• It is squashed on the side facing the Sun and stretched into a long tail on the night side.

• It traps charged particles in the Van Allen belts.

• Why it matters for Section 6: the magnetic field helped the Earth keep its second atmosphere and its water. Mars lost its global field and then lost most of its air.

• Geomagnetic storm: a major disturbance of the magnetosphere caused by a burst of solar wind or a coronal mass ejection (CME), a huge cloud of charged gas thrown out by the Sun.
• Effects: auroras seen at lower latitudes, and disruption to satellites, GPS, radio and power grids.

• NOAA scale: G1 (minor) to G5 (extreme).

• Key events:

• The Carrington Event (1859) is the largest storm on record.

• In May 2024 a G5 storm produced auroras over Ladakh, photographed from the Indian Astronomical Observatory at Hanle.

• Aditya-L1 studies CMEs and the solar wind to improve warnings.

8. The origin of life

Read the detailed note →

• Origin of life: the last phase of the Earth's evolution. The early Earth and its atmosphere were not suited to life.
• Life arose from chemical reactions. These first made complex organic molecules, and then the molecules were arranged so they could copy themselves. Lifeless matter thus became living substance.

• Life began about 3,800 million years ago and stayed in the oceans for a long time.

• Evidence: fossils preserved in rocks.

• Blue-green algae (cyanobacteria) are microscopic organisms that make food by photosynthesis.
• Structures closely like them are found in rocks older than about 3,000 million years.
• Their layered mats harden into stromatolites, some about 3.5 billion years old.

• They added oxygen to the oceans and the air.

• Beyond NCERT:

• The Miller-Urey experiment (1953) sent sparks through a copy of the early atmosphere and produced amino acids. This supports the Oparin-Haldane "primordial soup" idea.

• Hydrothermal vents on the sea floor are a leading alternative setting for the start of life.

• Life moved onto land only about 500–450 million years ago, after oxygen and the ozone layer had built up.

9. The geological time scale, impacts, extinctions and the Anthropocene

Read the detailed note →

• Geological time scale (kept by the International Commission on Stratigraphy):
• Eons: Hadean → Archean → Proterozoic → Phanerozoic.

• The Phanerozoic is split into eras: Palaeozoic → Mesozoic → Cenozoic. Eras are split into periods, and periods into epochs.

• Key markers:

• Cambrian explosion of life: about 539 million years ago.
• Pangaea breaks up: about 200 million years ago (see plate-tectonics).
• Non-bird dinosaurs die out: 66 million years ago.

• Holocene begins: 11,700 years ago.

• Mass extinction: a large share of species die out in a geologically short time. The Big Five:

• end-Ordovician
• Late Devonian
• end-Permian (about 252 million years ago, the worst; linked to Siberian Traps volcanism)
• end-Triassic

• end-Cretaceous (66 million years ago)

• Today's biodiversity loss is called a possible sixth mass extinction (see biodiversity-conservation).

• Chicxulub impact: an asteroid strike 66 million years ago that left a crater about 180 km wide under Mexico's Yucatan Peninsula. It was the main cause of the end-Cretaceous extinction, with Deccan Traps eruptions adding stress.
• Lonar crater: a near-circular crater lake made by a meteorite impact into Deccan basalt, in Buldhana district, Maharashtra. Its water is saline and alkaline. It became a Ramsar site in November 2020.

• Dhala (Madhya Pradesh) and Ramgarh (Rajasthan) are other impact structures (verify current).

• Meghalayan age: the latest of the Holocene's three ages (Greenlandian, Northgrippian, Meghalayan), ratified in 2018.

• It began about 4,200 years ago with a global megadrought that hit the Harappan civilisation, among others.

• Its reference marker is a stalagmite from Mawmluh Cave, Meghalaya.

• Anthropocene: a proposed epoch marked by human impact, such as radioactive fallout, plastics and carbon dioxide. The proposed reference site was Crawford Lake, Canada.

• It was rejected by the Subcommission on Quaternary Stratigraphy in March 2024, and the IUGS upheld the decision.
• It is still used as an informal term.

---

Exam angles

Prelims — high-yield facts and traps

• Who proposed what:
• Nebular hypothesis: Kant, revised by Laplace (1796).
• Revised nebular hypothesis: Otto Schmidt and Carl Weizsacker (1950).
• Planetesimal hypothesis: Chamberlin and Moulton.
• Tidal hypothesis: Jeans and Jeffreys (1919).

• Steady state: Hoyle (1948).

• Big Bang: about 13.8 billion years ago (NCERT: 13.7). First atoms within 3 minutes. The CMB (Penzias and Wilson, 1965) is the key evidence.

• "A light year is a unit of time" — FALSE. It is a distance, about 9.46 trillion km. Sunlight takes about 8.3 minutes to reach the Earth.
• "Differentiation is a stage in the evolution of the atmosphere" — FALSE. The three stages are solar wind stripping, degassing and photosynthesis.
• "Galaxies themselves expand as the universe expands" — FALSE. Only the space between galaxies grows.
• Timeline: Earth 4.6 billion years old → oceans about 4,000 million years old → life about 3,800 million years ago → photosynthesis 2,500–3,000 million years ago → oxygen in the air from about 2,000 million years ago.
• Dwarf planets: Pluto, Ceres (asteroid belt), Eris, Haumea, Makemake. IAU criteria from 2006. The Kuiper belt lies beyond Neptune at 30–50 AU.
• Mission positions: Aditya-L1 at Sun-Earth L1, about 1.5 million km from Earth. JWST at L2.
• India-linked pairs:
• Meghalayan age – Mawmluh Cave – 4,200 years ago.

• Lonar – Buldhana, Maharashtra – Deccan basalt – Ramsar site 2020.

• "The Anthropocene is now an official epoch" — FALSE. It was rejected in March 2024.

• Extinctions: Chicxulub (Yucatan, Mexico) – end-Cretaceous, 66 million years ago, alongside the Deccan Traps. The end-Permian (252 million years ago, Siberian Traps) was the worst.

Mains — GS-I themes

1. Explain the Big Bang theory and the evidence for an expanding universe. Why was the steady state theory abandoned?
2. Trace the Earth's evolution from a hot, barren body to a planet with oceans, an oxygen atmosphere and life. Cover differentiation, degassing and photosynthesis.
3. Why do terrestrial and Jovian planets differ in size, density and composition?
4. Should the Anthropocene be recognised as a geological epoch? Weigh the evidence and the March 2024 decision.
5. Space weather is a risk to infrastructure that depends on satellites. What role do missions like Aditya-L1 play?

Current-affairs hooks

• Aditya-L1 science results and space weather alerts (verify current).
• Strong geomagnetic storms and auroras over Ladakh (May 2024 and later events; verify current).
• The "Hubble tension": new results on how fast the universe is expanding and how old it is (verify current).
• Chandrayaan and other lunar missions testing the giant impact story (verify current).
• The Anthropocene debate after 2024; the Meghalayan age and records from Indian caves.
• Lonar Lake conservation and Ramsar issues (verify current).