Early and modern theories

In the planetesimal theory, large planets formed from small solid bodies. In exams, treat planetesimals as an intermediate stage of planet formation.

This theory assumes a passing star produced a tidal effect on the Sun. In exams, connect the word tide with gravitational pull.

The modern theory links planets with a gas-dust disk, not matter pulled from the Sun. In exams, keep it separate from the tidal theory.

Near the Sun, high temperature favored rocky materials, while farther colder regions retained more gases and ices. In exams, remember the relation between distance and composition.

Terrestrial planets formed in warmer inner regions where heavy rocky material could remain. In exams, identify inner planets as rocky planets.

In colder outer regions, light gases and ices could accumulate in larger amounts. In exams, remember outer planets as gaseous and large.

According to the Big Bang theory, as the universe expanded, its energy spread through a larger volume. Therefore, the temperature fell and the universe cooled. It did not remain constant because energy density decreases during expansion. Exam tip: remember that expansion of the universe is associated with cooling.

Light elements formed in the early universe and later helped in the development of stars and galaxies. In exams, remember basic matter before large bodies.

Hubble strengthened evidence for the recession of distant galaxies. In exams, connect Hubble with expansion of the universe.

This radiation is considered a leftover signal of the early hot universe. In exams, write it as major evidence of the Big Bang.

Evidence such as cosmic background radiation and expansion weakened the steady state theory. In exams, remember that evidence reviews theories.

Gravity attracts small particles and bodies to form larger bodies. In exams, remember gravity as the main force of accretion.

Protoplanets became larger and more stable planets by adding more matter. In exams, treat planet formation as gradual growth.

Heavy metals moved toward the center and helped form the core. In exams, remember heavy matter and core together.

Lighter materials remained above and helped form the crust. In exams, understand layered structure through differentiation.

Free oxygen was very low in the early atmosphere because biological oxygen production increased later. In exams, do not treat the early atmosphere as the same as today.

The release of volcanic and internal gases is called outgassing. In exams, remember gas release and atmosphere together.

On a cooler Earth, water vapour could condense into rain and water bodies. In exams, connect hydrosphere with cooling.

Early water was linked with gases from Earth and condensation of water vapour. In exams, connect ocean formation with outgassing and condensation.

Old theories are historically important, but modern theories have stronger evidence. In exams, mention the evidence basis while comparing.

The Big Bang is at the universe scale, while the solar nebula is at the solar system scale. In exams, identify the scale of the theory.

After the Big Bang, matter gathered to form galaxies and stars. In exams, remember the sequence from small matter to large structures.

Leftover material around a star like the Sun forms a disk for planets. In exams, understand star formation and planetary disk together.

Weizsäcker gave the idea of vortex-like motion in the solar nebula. In exams, connect him with modern planet-origin ideas.

Schmidt proposed planet formation from interstellar gas and dust. In exams, connect him with the modern dust-cloud idea.