Theories of the Origin of the Universe and Planetary Formation

1. Nebular Hypothesis – Immanuel Kant (1755)

• Proposed by: Immanuel Kant, the eminent German philosopher
• Foundation: Newton’s Law of Universal Gravitation.
• Kant envisioned that the universe originated from an expansive distribution of cold, motionless solid particles, floating silently in space.
• Under the influence of gravitational attraction, these particles began to collide, generating heat and triggering rotational motion. Over time, this gave birth to a hot, rotating nebula.
• As the nebula spun faster, it developed strong centrifugal forces, causing rings of matter to detach. These rings gradually cooled and condensed into planets and satellites, including Earth.

Limitations:Kant’s theory did not explain the origin of the primordial matter. However, Kant claimed that the universe included primordial matterIt also failed to clarify what initiated the movement of the initially static particles.

2. Nebular Hypothesis – Pierre-Simon Laplace (1796)

• Proposed by: Pierre-Simon Laplace, renowned French mathematician and astronomer
• Building on Kant’s ideas, Laplace introduced a more scientifically grounded model. 
• He proposed that the early solar system originated from a vast, rotating gaseous nebula, primarily composed of hydrogen, helium, and dust particles.
• As this nebula contracted under its own gravity, it began to spin faster. 
• Most of the lighter gases gravitated towards the center, where intense pressure and temperature triggered nuclear fusion, giving rise to the Sun.

• Meanwhile, heavier elements drifted outward, cooling and coalescing to form planets. This explains why the Sun is predominantly made of lighter elements, whereas the planets are denser and more solid.

Supporting Evidence proposed by Laplace ;

• The disk-like structure of the solar system.
• The elemental composition difference between the Sun and the planets.

The Big Bang Theory

The Big Bang Theory posits that the universe began around 13.7 billion years ago from an infinitely dense and hot singularity — a point with zero volume and infinite energy.

• This singularity underwent a massive explosion, not in space, but of space itself, marking the birth of space, time, matter, and energy.
• According to this theory the timeline of Early Events:
  • 0 seconds – The Big Bang: a sudden, rapid expansion of space.
  • Fractions of a second – Rapid inflation; subatomic particles begin to form.
  • 3 minutes – Formation of light atomic nuclei (hydrogen, helium).
  • 300,000 years – Universe cools to ~4,500 K; atoms form, leading to cosmic microwave background radiation.
• As per the Big Bang theory the component of the Universe consists of Ordinary (visible) matter approx ~4%,  Dark matter  ~26%, Dark energy ~70%.
• Key Evidence in support of the theory;
  • Redshift of galaxies (expansion of the universe).
  • Cosmic Microwave Background Radiation (CMBR).
  • Proportions of light elements predicted by nucleosynthesis.

Step wise formation of the planet.

SOLAR SYSTEM ( UPSC PRE 2013 )

The Solar System is believed to have formed around 4.6 billion years ago from a massive rotating cloud of gas and dust known as the solar nebula.

• The collapse of this nebula, likely triggered by a nearby supernova, led to:
  • Core formation around 5–5.6 billion years ago, giving rise to the proto-Sun.
  • Surrounding matter flattened into a protoplanetary disk, leading to the formation of planets.
• The Solar System consist of ;
  • Sun at the center, containing 99.86% of the solar system’s total mass.
  • Planets-  8 planets, divided into inner (terrestrial) and outer (Jovian) types.
  • Moons– At least 63 confirmed natural satellites, orbiting various planets.
  • Other Bodies– Includes asteroids, comets, meteoroids, and interplanetary dust.

Term	Definition
Asteroids	Chunks of rock orbiting in a belt between Mars and Jupiter
Meteors	Streaks of light when a meteoroid enters Earth’s atmosphere
Comets	Balls of ice orbiting the outer solar system beyond Neptune

Classification of Planets

Characteristic	Inner (Terrestrial) Planets	Outer (Jovian) Planets
Examples	Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus, Neptune
Composition	Rock and metal	Gases (Hydrogen, Helium) and ices• Jupiter & Saturn: gas giants• Uranus & Neptune: ice giants
Density	Higher density	Lower density
Surface	Solid surfaces	No solid surface (gaseous)
Size	Smaller	Larger
Moons	Fewer or no moons	Many moons
Rings	No rings	Ring systems present
Formation Location	Close to the Sun (warmer region)	Far from the Sun (cooler region)
Temperature During Formation	Too warm for gases to condense into solid particles	Cool enough for gases and ices to condense
Solar Wind Effect	Intense solar winds blew off gases and dust	Weaker solar winds; retained gases
Gravity	Lower gravity; could not hold escaping gases	Higher gravity; retained thick atmospheres

Brief Review of the Solar System

Planet	Position	Key Characteristics
Mercury	1st (Closest to Sun)	No atmosphere; extreme temperature variation: +427°C (day) to –270°C (night)Smallest planetSurface is heavily cratered
Venus	2nd	Earth’s closest planetary neighbor (~40 million km)Thick atmosphere of 96% CO₂, SO₂, and COSurface temperature: ~480°C (due to runaway greenhouse effect)Brightest planet in the night skyRotates in retrograde direction
Earth	3rd	Only known planet to support lifePresence of water, oxygen-rich atmosphere, and moderate climate
Mars	4th	Known as the Red Planet (due to iron oxide)Atmosphere: 95% CO₂, thin and coldPresence of polar ice caps, largest volcano (Olympus Mons)Ongoing research on possible microbial life
Jupiter	5th	Largest planet in the Solar SystemPrimarily composed of hydrogen and heliumFastest rotation (10 hours)No solid surface; dense atmosphere with ammonia cloudsKnown for the Great Red Spot (giant storm)
Saturn	6th	Known for its spectacular ring systemMostly hydrogen and heliumContains hydrogen cyanide (toxic gas)Atmosphere also contains 90% nitrogenVery cold: average temperature ~ -184°C
Uranus	7th	Ice giant; mainly methane, hydrogen, heliumAppears bluish-green due to methaneUnique feature: rotational axis tilted at ~98°, rolls like a ballVery cold and distant
Neptune	8th	Smaller than Earth, dark and coldAtmosphere contains frozen methane, giving a deep blue colorStrongest winds in the solar system14 known moons; Triton is the largest