Origin of the Earth (Geomorphology)

• The origin of the Earth cannot be separated from the origin of the solar system, because all the planets formed by the same process at the same time; every hypothesis on the one is a hypothesis on the other.
• The Earth is about 4.54 billion years old, and the oldest solids in meteorites date to about 4.567 billion years, both from radiometric dating.
• The Sun is one of hundreds of billions of stars of the Milky Way, about 26,000 light years from its centre, which it circles once in a galactic year of roughly 225–250 million years.
• Ideas on the Earth’s origin run from the nebular hypotheses of the 18th century, through the encounter (tidal) hypotheses of the early 20th century, to today’s solar nebula model, set within the Big Bang account of the universe.

Approaches to the Problem

Religious and Scientific Views

• Religious concepts explain creation by divine will and cannot be tested.
  • Archbishop James Ussher (1650) dated creation from biblical chronology to October 4004 BC, an estimate now of only historical interest.
• Scientific concepts rest on physical laws, above all gravitation and the conservation of angular momentum.

Classification of Scientific Theories

• By the initial state of matter:
  • Hot origin concepts: the matter was hot at the start or was heated during formation (Laplace, Jeans).
  • Cold origin concepts: the matter was cold, and the Earth was heated later by radioactive decay or by the pressure of the overlying load (Chamberlin, Schmidt).
• By the number of heavenly bodies involved:
  • Monistic (mono-parental): one body, a rotating nebula.
  • Dualistic (bi-parental): two bodies, the Sun and a passing or companion star.
  • Binary star or trihybrid: three bodies, the Sun, a companion and an intruding star.

Monistic (Nebular) Hypotheses

Gaseous Hypothesis of Kant (1755)

• The German philosopher Immanuel Kant set it out in Universal Natural History and Theory of the Heavens (1755), claiming to build on Newton’s laws of gravitation and motion.
  • It was later found dynamically unsound, but it was the first scientific attempt to explain the Earth’s origin; his confidence shows in the line “Give me matter and I will build a world out of it.”

Assumptions

• Primordial matter, supernaturally created, was scattered through space as very cold, solid, motionless particles, in modern terms close to absolute zero (0 K).
• The particles began to attract one another by gravitation.

Mechanism

• Attraction made the particles collide; collisions produced random motion and friction, and friction produced heat.
• Random motion supposedly turned into rotation, so the cold cloud became a hot, rotating, gaseous nebula.
• As it heated, the nebula expanded, and Kant held that its spin increased with its size.
• When centrifugal force exceeded gravity, rings were thrown off one after another, nine in all; the residual central mass became the Sun.
• Irregularities in each ring formed a knot round which its matter gathered into one planet; repetition around the planets formed satellites.

Evaluation

• Kant never explained the source of the primordial matter, nor the energy for its first motion: by Newton’s first law, bodies at rest stay at rest unless an external force acts.
• Collisions among particles cannot create rotation in a mass that had none, since internal forces cancel out.
• A nebula spinning faster as it expands violates the conservation of angular momentum.
  • Angular momentum is the product of mass, angular velocity and the square of the radius, and stays constant in an isolated system.
  • With mass fixed, angular velocity varies inversely with the square of the radius: an expanding nebula must slow down, and only a contracting one speeds up.
• Still, the idea of a rotating nebula paved the way for Laplace.

Nebular Hypothesis of Laplace (1796)

• The French mathematician Pierre-Simon Laplace proposed it in Exposition du système du monde (1796), without mathematical formulation, as a corrected version of Kant.
• It removed Kant’s three defects: cold collisions cannot make heat, collisions cannot make rotation, and a nebula cannot spin faster as it grows.

Assumptions

• A huge, hot, gaseous nebula existed in space.
• It was rotating from the start.
• It was cooling by radiation from its surface and contracting as it cooled.

Mechanism

• Cooling → contraction → faster rotation, as the conservation of angular momentum requires.
• At a critical speed, centrifugal force at the equator equalled gravity and the equatorial matter became weightless.
• The cooled outer layer could not keep pace with the contracting core and was left behind as a single ring (not nine, as in Kant).
• The ring split into nine rings, each condensing through a hot gaseous knot into a planet; the central nucleus became the Sun, and repetition produced satellites.
• Édouard Roche (mid-19th century) modified it, holding that the nine rings separated from the nebula itself.

Merits

• It explained why the planets revolve and rotate in the same direction and why their orbits lie nearly in one plane.
• It pictured the Earth passing from gas to liquid to solid, with a solid crust over a hot interior, an early explanation of internal heat and layering.
• Its simplicity kept it in favour for more than a century.

Evaluation

• It does not explain where the hot, rotating nebula came from.
• It cannot explain why exactly nine planets formed; dynamically, a ring would break into many fragments.
• Weak cohesion would make ring-shedding continuous, not intermittent as the theory needs.
• The Sun, as the residual nucleus, should keep an equatorial bulge, which it lacks.
• Some satellites of Jupiter and Saturn revolve in the opposite (retrograde) direction to their planets.
• James Clerk Maxwell (1859) and later Jeans showed that a ring’s mass was too small to condense into a planet by its own gravity.
• The angular momentum problem, the decisive objection:
  • The Sun holds about 99.8% of the mass of the solar system but only about 1% of its angular momentum.
  • A nebula conserving its angular momentum should have left the Sun spinning fastest, not the planets.
• This problem turned cosmogony towards two-body (dualistic) hypotheses in the early 20th century.

Dualistic and Multi-Star Hypotheses

Planetesimal Hypothesis of Chamberlin and Moulton (1905)

• The geologist Thomas Chrowder Chamberlin and the astronomer Forest Ray Moulton, both of the University of Chicago, proposed it in 1905.
  • It revived the collision hypothesis (1749) of Georges-Louis Leclerc, Comte de Buffon, who imagined a comet striking the Sun.
  • It tried to explain not only the Earth’s origin but also its interior, atmosphere, continents and ocean basins.

Mechanism

• An intruding star passed very close to the proto-Sun, and its tidal pull drew out jets of solar matter.
  • Some followed the star into space; the rest was captured into orbits around the Sun.
• The ejected matter cooled into countless small solid bodies, the planetesimals.
• The larger planetesimals became nuclei that swept up smaller ones and grew into planets by accretion; satellites formed the same way.
• Growing gradually, the Earth need never have been wholly molten, making this a cold origin concept.

Evolution of the Earth

• Chamberlin traced three overlapping stages:
  • Planetesimal accession: the Earth reached roughly its present size and shape.
  • Dominant vulcanism: internal heat caused selective melting and volcanism; continents and ocean basins began to form.
  • Actual geological period: folding, faulting, emergence and submergence built the ancient relief.
• Atmosphere came from gases captured from space as the Earth’s gravity grew, and from gases (carbon dioxide, water vapour, nitrogen) trapped in planetesimals and released by volcanic eruptions.
• Internal heat came from colliding planetesimals, compression under the growing load and heat-releasing chemical changes; high pressure kept the deep interior solid while rocks nearer the surface melted in patches.
• Oceans began as water in crevices and craters that merged into basins; heavy (basic) material washed into them left the continents lighter (acidic), and the basins deepened.

Evaluation

• Matter torn from the Sun would be vaporised and dispersed, not condensed: Lyman Spitzer (1939) showed such hot gas would expand and dissipate.
• It gives no reason for the ordered sizes of the planets, which increase outward to Jupiter (Mars excepted) and then decrease, nor for the low-density outer planets.
• A passing star could impart too little angular momentum to match what the planets carry.
• Stars are so far apart that a close encounter is extremely improbable.
• Harold Jeffreys objected that small nuclei could not hold an atmosphere, and meteorites are dry and gas-poor.
• Its account of continents and mountain building supplied too little compressional force.
• Even so, the planetesimal survives at the heart of the modern solar nebula model.

Tidal Hypothesis of Jeans and Jeffreys

• The British astronomer Sir James Jeans worked out the tidal hypothesis in 1917 and set it out fully in 1919; the British geophysicist Harold Jeffreys modified it in 1929.

Assumptions

• The primitive Sun was a hot, gaseous, rotating body.
• A much larger intruding star moved along a path bringing it close to the Sun, and its tidal force acted strongly on the Sun’s surface.

Formation of the Filament and Planets

• By Newton’s law of gravitation (1687), the pull between two bodies varies with the product of their masses and inversely with the square of their distance.
• As the star approached, the tidal bulge on the Sun grew; little matter left while it was distant, the most at closest approach, and little again as it receded.
  • The result was a cigar-shaped filament, thick in the middle and tapering at both ends.
• The filament first followed the star, failed to keep pace and began revolving around the Sun.
• On cooling it broke into pieces, each condensing into a planet: the largest (Jupiter, Saturn) from the thick middle, the smallest from the ends.
• Solar tides on the young planets raised satellites; large planets stayed gaseous longer and yielded many, while the smallest cooled too fast to yield any, the reason offered for Mercury and Venus having none.

Evidence in Favour

• Planet sizes increase from Mercury to Jupiter (Mars excepted) and fall again beyond it, matching the cigar shape.
• The satellites of Jupiter and Saturn repeat the pattern, and the giant planets have by far the most (Saturn 285 and Jupiter 101 confirmed by March 2026).
• All planets revolve in the same direction and nearly one plane, and all except Venus and Uranus rotate in that sense.
• An encounter outside the Sun’s equatorial plane explains the inclined axes of the planets.

Modification by Jeffreys (1929)

• Jeffreys proposed a collision among three stars: an intruding star struck the Sun’s companion star head-on and shattered it.
• Part of the debris scattered into space, the rest revolved around the Sun and formed the planets, and the impact let the intruding star escape.

Evaluation

• Boris Levin and others stressed that close encounters are so rare that planetary systems would be rare; yet more than 6,000 planets around other stars have been confirmed (NASA Exoplanet Archive, September 2025).
• The fate of the intruding star is unexplained.
• Filament matter should have stayed near the Sun, so the hypothesis cannot give the real planetary distances, hundreds to thousands of solar diameters.
• The planets, a tiny fraction of the mass, carry most of the angular momentum, which a passing star could not supply.
• The Sun is mostly hydrogen and helium, while the inner planets are rich in heavy elements.
• Mercury and Venus, nearest the Sun, should have been the first to gain tidal satellites.
• The condensation of hot filament matter was never explained, and Spitzer showed it would disperse.
• Jeffreys himself later admitted (1951) that the hypothesis needed substantial change.

Binary Star Hypothesis of Russell

• The American astronomer Henry Norris Russell proposed it in the mid-1930s to remedy the tidal hypothesis over angular momentum, heavy elements and distances; Raymond Lyttleton (1936) developed a parallel version.
• The primitive Sun had a companion star revolving at a great distance.
• A giant approaching star, moving opposite to the companion, passed close to the companion (not the Sun) and its tidal pull ejected matter from it.
• The matter revolved in the direction of the approaching star and condensed into planets; mutual pulls among the young planets yielded satellites.
• Formed far from the Sun, the matter explains the great distances and the large angular momentum of the planets.

Evaluation

• The fate of the rest of the companion is not explained.
• If the planets were captured by the Sun after the approaching star left, why was the nearer residual companion not captured too?
• The mechanism that brought the planets into the Sun’s gravitational field is not described.

Supernova Hypothesis of Hoyle (1944–46)

• Fred Hoyle of the University of Cambridge derived the planets from a companion star that exploded, first framed as a nova (1944) and then as a supernova, using nuclear physics.
• Stars shine by nuclear fusion, hydrogen fusing into helium; the giant companion burned its fuel far faster than the Sun, exhausted its hydrogen, collapsed and exploded.
• The recoil threw its core out of the Sun’s gravitational field, while its gas and dust remained as a rotating disc around the Sun.
• The explosion’s extreme temperatures fused heavy elements, and the planets condensed from this heavy-element-rich disc, not from the Sun.

Evaluation

• Merits: it explains the great distances, the angular momentum of the planets (supplied by the explosion) and their heavy elements.
• Limitations: it cannot explain the ordered sizes of the planets, their common direction and plane of motion, or the light outer planets.
• Current view: planets are no longer derived from a companion’s debris, but short-lived isotopes such as aluminium-26 and iron-60 in the oldest meteorites suggest that a nearby supernova seeded, and may have triggered the collapse of, the cloud that formed the Sun.

Interstellar Dust Hypothesis of Schmidt (1943–44)

• The Soviet scientist Otto Yulievich Schmidt held that the Sun, during its galactic revolution, captured a cloud of interstellar gas and dust carrying its own angular momentum; he did not explain the cloud’s origin.
• The captured matter settled into a flat rotating disc, shaped by the Sun’s rotation, the Sun’s gravity and the disc’s own angular momentum.
• Particles sorted themselves by mass and density: the Sun’s heat drove light gases outward, while heavy particles stayed in the inner bands.
• Collisions gathered dust round larger grains into embryos, which grew into asteroid-sized bodies and then planets; leftover matter formed satellites.
  • Hence dense inner planets and light outer planets, formed by the freezing out of gases.
• The planets were spaced roughly by the Titius–Bode rule, D = 0.4 + 0.3 × 2ⁿ astronomical units (n = −∞ for Mercury, 0 for Venus, 1 for Earth, 2 for Mars).
  • It fits out to Uranus but fails for Neptune, so it is now an empirical pattern, not a law.

Evaluation

• Merits: it accounts for near-circular, coplanar orbits in the Sun’s equatorial plane, ordered sizes and distances, the inner–outer contrast and the distribution of angular momentum.
• Criticisms: a single star cannot capture such a cloud (capture needs a binary system); some held that meteorites are fragments of broken planets, not their building blocks; and no remnant of the captured matter was found.
• Legacy: growth of planets from dust and planetesimals in a disc was developed by Viktor Safronov (1969) into the basis of the modern model; only the capture is abandoned.

Modern Views

Recent Hypotheses

• Mid-20th-century ideas returned to a rotating disc around the young Sun:
  • Carl Friedrich von Weizsäcker (1944): turbulent eddies in the nebula gathered matter into planets at regular spacings.
  • Gerard Kuiper (1949): protoplanets condensed from a massive nebula by gravitational instability.
  • Hannes Alfvén: electromagnetic forces in an ionised cloud moved angular momentum from the Sun to the planetary matter.
  • Others include Ross Gunn’s rotational-tidal hypothesis, the Cepheid hypothesis of A. C. Banerjee of Allahabad University, Fesenkov’s globule concept (1951), Voitkevich’s protoplanetary chondrule concept (1971) and Drobyshevski’s Jupiter–Sun binary hypothesis (1974).
• The angular momentum problem was eased by magnetic braking: the young Sun’s magnetic field and wind passed its spin to the disc and to space.
• By the early 1980s, dusty discs around young stars had restored the nebular idea as the solar nebula (nuclear disc) model, a neo-Laplacian revival.

Modern Solar Nebula Model

• The Sun and planets are products of one event: the collapse of a fragment of a cold interstellar molecular cloud about 4.568 billion years ago.
• The stages:
  1. Collapse and spin-up: the cloud spun faster and flattened into a protoplanetary disc; most mass fell to the centre as the proto-Sun.
  2. Condensation sequence: as the disc cooled, metals and silicates condensed near the Sun and ices beyond the frost (snow) line between the orbits of Mars and Jupiter.
  3. Planetesimals: dust grains stuck into kilometre-sized bodies that grew by runaway collisions into planetary embryos.
  4. Giant planets: beyond the frost line, cores of about ten Earth masses formed quickly and captured hydrogen and helium.
  5. Clearing: the young Sun’s T Tauri wind swept away the remaining gas within about 10 million years.
  6. Terrestrial planets: embryos collided over 10–100 million years to build Mercury, Venus, Earth and Mars.
• Later planetary migration (the Nice model and the Grand Tack hypothesis) explains the giant planets’ orbits, the small size of Mars and the scattering of small bodies into the asteroid belt and the Kuiper Belt (about 30–50 AU out).
• It explains what earlier hypotheses could not: the common plane and direction of motion (inherited from the disc), rocky inner and gaseous outer planets (from the condensation sequence) and the Sun’s slow spin (magnetic braking).
• Evidence includes meteorite inclusion ages, thousands of exoplanet systems and discs around young stars, most strikingly the ALMA image of HL Tauri (2014), a star about a million years old whose disc shows rings and gaps swept by forming planets.
• Planetary spacing, the retrograde spin of Venus and similar oddities are now read as results of later collisions and migration; Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006, leaving eight planets.

From Accretion to a Layered Earth

• Heat of accretion, compression and radioactive decay melted much of the young Earth into a magma ocean.
• Differentiation: molten iron and nickel sank to form the core within about the first 30 million years, and lighter silicates rose, giving the Earth its core, mantle and crust.
• Giant impact: a Mars-sized body, Theia, struck the proto-Earth about 4.5 billion years ago, and the debris formed the Moon.
  • Indian example: the APXS instrument on Chandrayaan-3’s Pragyan rover, built at the Physical Research Laboratory, Ahmedabad, found uniform ferroan anorthosite soil near the lunar south pole (Nature, August 2024), supporting the lunar magma ocean that followed the Moon’s birth.
• Atmosphere and oceans: a secondary atmosphere came from volcanic outgassing plus water-rich asteroids and comets; free oxygen came much later from photosynthesis, not from volcanoes.
• Oldest records: zircons from the Jack Hills of Western Australia, about 4.4 billion years old, show a solid crust and liquid water early in the Hadean eon.
  • Indian example: zircons from Champua, Kendujhar district, Odisha (Singhbhum craton), reported in 2018, are about 4.24 billion years old, the oldest mineral grains found in India.

Big Bang Theory

• The Big Bang theory explains the origin of the universe itself, within which the Sun and Earth formed much later.
• Georges Lemaître argued in 1927 that the universe is expanding and in 1931 traced it back to a “primeval atom”.
• Edwin Hubble (1929) showed that the farther a galaxy, the faster it recedes (Hubble’s law).
• George Gamow, with Ralph Alpher, worked out in 1948 how the light elements formed in the hot early universe.
• Fred Hoyle, a critic, coined the name “Big Bang” in a 1949 broadcast; with Hermann Bondi and Thomas Gold he had proposed the rival steady state theory (1948).

The Theory

• Everything emerged from an extremely hot, dense state (a singularity in the simplest models) about 13.8 billion years ago.
• Space itself has expanded ever since, so galaxies move apart as the space between them stretches.

• As it expanded and cooled, hydrogen, helium and a little lithium formed in the first minutes; about 380,000 years later atoms formed and radiation was set free as the cosmic microwave background.
• Gravity then gathered gas into galaxies and stars, which forged heavier elements and scattered them in supernovae; from such enriched clouds the Sun and Earth eventually formed.

Evidence in Support

• Red shift: light from distant galaxies is stretched to longer, redder wavelengths, showing that they recede faster with distance.
• Cosmic microwave background (CMB): discovered by Arno Penzias and Robert Wilson (1965), at about 2.7 K.
  • NASA’s COBE satellite (launched 1989) confirmed its perfect blackbody spectrum and found the tiny fluctuations that seeded galaxies; WMAP and Planck refined them.
• Light elements: about 75% hydrogen and 25% helium by mass, as predicted.

Current View

• The Planck mission (2018) puts the age of the universe at 13.787 ± 0.020 billion years.
• Open questions remain within the framework:
  • The Hubble tension: the expansion rate from the CMB (about 67 km/s/Mpc) disagrees with that from nearby stars and supernovae (about 73 km/s/Mpc).
  • DESI survey results of 2024–25 hint that dark energy may be weakening, not yet confirmed.
  • The James Webb Space Telescope has found massive galaxies surprisingly early, testing models of galaxy formation.