Early and modern theories

Laplace's ring idea was simple but did not clearly explain the full physics of planet formation. In exams, separate historical idea and modern limitation.

Accumulation of planetesimals resembles modern accretion. In exams, remember the sequence from small bodies to larger planets.

A passing star was assumed to create tidal pull on the Sun. In exams, connect the word tidal with gravitational pull.

It is physically difficult for hot solar matter to easily form stable planets. In exams, remember the hot-matter problem.

Bodies formed from the same rotating disk show similar motion. In exams, connect angular momentum with orbital order.

Beyond the frost line, accumulation of ice and gases was more possible. In exams, study temperature, distance, and planet composition together.

In the hot inner region, rocky and metallic materials persisted more. In exams, treat inner planets as dense rocky planets.

Outer regions were colder, so gases and ices could accumulate more. In exams, connect outer planets with large gaseous planets.

The direct answer is C: rotation and gravitational collapse. A protoplanetary disk begins as a large cloud of gas and dust. Gravity pulls the material inward, so the cloud collapses and becomes smaller. At the same time, the cloud rotates. As it contracts, conservation of angular momentum makes the rotation faster, and collisions and motion tend to spread material into a flattened rotating disk around the young Sun. Therefore C is correct: both gravitational collapse and rotation are important parts of the process. Option A, soil erosion and deposition, occurs on planetary surfaces and does not form a protoplanetary disk. Option B, phases of the Moon, are caused by the changing portion of the Moon lit by the Sun as seen from Earth; they do not flatten a cloud of cosmic matter. Option D, ocean tides and ebbs, are caused mainly by gravitational effects of the Moon and Sun on oceans and are not the process that creates the early disk. The reasoning chain is: gas and dust cloud → gravity causes collapse → rotation increases during contraction → material forms a flattened disk. Memory cue: “collapse pulls inward; rotation spreads into a disk.”

The Sun at the center and planet material in the disk identify the modern solar nebula model. In exams, read clues in sequence.

This radiation is considered a remnant of the early hot universe. In exams, treat background radiation as Big Bang evidence.

The steady state theory proposed continuous creation of matter. In exams, identify it separately from the Big Bang.

Big Bang has a beginning and expansion, while steady state has average uniformity and continuous creation. In exams, remember the basic contrast.

The Big Bang explains the beginning of the universe, while Earth formed later in the solar system. In exams, identify scale and sequence.

Large cosmic structures formed by gravitational concentration of early matter. In exams, connect structure formation with gravity.

After the Big Bang, light elements formed and later became the basis for stars. In exams, connect chemical evolution with cosmic sequence.

Accretion builds the planetary body and differentiation organizes its interior. In exams, do not reverse the order.

Density-based differentiation formed the arrangement of core, mantle, and crust. In exams, remember heavy elements and core together.

In molten Earth, heavy materials could sink and lighter ones rise. In exams, treat melting as a condition for differentiation.

Both heat sources kept Earth hot and dynamic. In exams, connect heat sources with internal layers.

Outgassing released water vapour, carbon dioxide, and other gases. In exams, connect gas release with atmosphere.

The early atmosphere did not have oxygen like today. In exams, connect free oxygen with a later biological stage.

On a cooler Earth, water vapour condensed and helped develop water bodies. In exams, remember cooling and hydrosphere together.

The giant impact idea connects Moon origin with the impact-rich early Earth. In exams, identify it as a modern idea.

Radiometric method is a basis of scientific age determination. In exams, connect Earth's age with radioactive decay.