Early and modern theories

Each theory explains a different scale. In exams, first identify whether the question is about the universe, solar system, or Earth.

The planetesimal hypothesis assumes planets formed by joining of small bodies. In exams, treat it as an accumulation-based theory.

Kant explained the origin of the Sun and planets from a gaseous nebula. In exams, connect nebula with gas.

Laplace proposed that rings separated from a contracting nebula. In exams, connect the word ring with Laplace.

In the tidal theory, the attraction of an external star is the main cause. In exams, a passing star is the key clue.

The steady state theory assumes the universe remains the same on average. In exams, understand it as opposite to the Big Bang idea.

The Big Bang theory links the universe with a very hot and dense initial state. In exams, hot and dense is the key clue.

Hubble’s law states that, on a large scale, the recessional velocity of a distant galaxy is related to its distance from us: v = H₀d. Thus, galaxies that are farther away generally show a greater red shift and appear to be moving away faster. This observation supports the conclusion that space itself is expanding, rather than proving that Earth occupies a special central position. Option A is correct because the systematic red shift of distant galaxies is strong observational evidence for an expanding universe. Options B, C, and D concern Earth’s shape or physical features and have no connection with Hubble’s law or galactic red shift.

In the modern idea, gas and dust form small particles and then larger bodies. Remembering the sequence is very useful in exams.

Planets formed from a flat disk move nearly in the same plane. In exams, remember disk and orbital plane together.

Accretion turns small materials into larger planetary bodies. In exams, understand it as the growth process of planet formation.

The steady state theory proposed maintaining average density despite expansion. In exams, connect it with continuous creation.

This radiation is important support for the Big Bang theory. In exams, write it as cosmological evidence.

In differentiation, heavier materials moved toward the center and lighter materials upward. In exams, link it with formation of core and crust.

Matter drawn from the Sun is a key clue of the tidal theory. In exams, identify solar matter and external star together.

The idea of rotation increasing with contraction appears in the nebular hypothesis. In exams, study contraction and rotation together.

Gravity gathers gas, dust, and particles. In exams, understand gravity as the force behind accretion.

Kant is associated with the gaseous nebula idea. In exams, remember the scientist and main idea pair.

The Big Bang theory works at the cosmological scale. In exams, do not mix it with local events of Earth.

Red shift indicates galaxies moving away. In exams, connect it with evidence for expansion of the universe.

Modern theories have clearer evidence and physical processes. In exams, prefer an evidence-based view.

In the solar nebula, more matter collected at the center and formed the Sun. In exams, remember center and Sun together.

Condensation of water vapour led to rain and water bodies. In exams, connect hydrosphere with cooling and condensation.

Collision and joining of planetesimals form the basis of protoplanet formation. In exams, remember the order from small to large bodies.

Both include the idea of solar system formation from cloud-like material. In exams, study both similarities and differences.