Star formation, formation of planets, and our solar system

The governing concept is the distinction between astronomical formation and biological evolution. Planets form through physical processes such as the gathering of dust and rock, accretion of planetesimals, gravitational growth, heating, and differentiation. The origin of life is a later question involving chemical complexity and biochemical processes under suitable environmental conditions. Option C is correct because it keeps these stages connected in sequence but does not treat them as identical. A incorrectly merges planetary and biological history, B reverses the order because Earth must exist before life can develop on it, and D has no scientific connection with planet formation.

The governing concept is explanatory integration: a strong scientific model accounts for several related observations through one physically plausible process. The modern solar nebula model proposes that the Sun and planets formed from a rotating disk of gas and dust. This framework helps explain the broadly orderly orbital arrangement, the contrasting compositions of inner and outer planets, and the existence of smaller leftover bodies such as asteroids and comets. Therefore, option A is correct. Options B and C are far too narrow and concern unrelated or incomplete subjects. Option D is not a scientific claim of the model, because planets are treated as physical bodies rather than living organisms.

Similar direction indicates origin from a common rotating disk. Exam tip: link direction and plane with nebular theory.

Conservation of angular momentum increases rotation during contraction and forms a disk. Exam tip: remember this as the cause of disk formation.

A protostar can glow because contraction heats it. Exam tip: remember fusion separately for a stable star's energy.

In a main sequence star hydrogen fusion gives stable energy. Exam tip: identify a stable star stage by fusion.

Metals and silicates could remain solid at higher temperatures. Exam tip: link this with the rocky composition of inner planets.

More icy solids helped outer cores become large. Exam tip: link the frost line with the large size of outer planets.

Temperature decreased with distance from the Sun so materials accumulated differently. Exam tip: remember distance temperature and composition together.

Gravity of large cores could attract gases like hydrogen and helium. Exam tip: connect gas accretion with mass.

As mass increases gravitational attraction becomes stronger. Exam tip: understand accretion through increasing mass.

Solar wind pushed light gases away from the inner region. Exam tip: remember this as a reason for the rocky nature of inner planets.

Planets formed from the disk revolve nearly in one plane. Exam tip: connect orbital plane with the protoplanetary disk.

When gravity becomes stronger than internal support the cloud can collapse. Exam tip: link star birth with gravitational instability.

Collisions can change planetary rotation and tilt. Exam tip: understand both the general rule and exceptions.

Jupiter's gravity prevented bodies from merging stably. Exam tip: remember the asteroid belt and Jupiter together.

A planet is dominant in its orbit while a dwarf planet is not. Exam tip: understand this through Pluto.

The Kuiper Belt is linked with distant icy bodies and the Oort Cloud with very distant comets. Exam tip: remember both distance and material.

Solar wind and radiation push comet gas dust away from the Sun. Exam tip: remember tail direction away from the Sun.

The Oort Cloud is considered a distant source of long period comets. Exam tip: link it with the very outer solar system.

Many meteorites give evidence of early solar system material. Exam tip: remember meteorites in age estimation.

Chondrites give information about old and less altered solar materials. Exam tip: remember them as evidence of early composition.

Heavy materials sank in the molten Earth. Exam tip: connect Earth's core with density differentiation.

The early Earth became hot through several interacting processes during its formation. Repeated collisions among planetesimals converted kinetic energy into heat, and the growing body was compressed by its own gravity, producing additional internal heating. Radioactive elements also released heat as they decayed. These sources helped create a hot, partly molten Earth, which later allowed heavier materials such as iron to sink and lighter materials to rise during planetary differentiation. Option D is correct because it includes the major mechanisms together. Cold rain and snowfall cannot explain the initial heating, present sunlight is not the only or principal internal source, and ocean waves are far too weak to heat a planet-sized body significantly.

Outgassing is the release of gases from Earth’s interior, mainly through volcanic activity. It supplied gases such as water vapour, carbon dioxide and nitrogen, helping to build the early atmosphere. The Sun’s core formed through the collapse of the solar nebula, not through Earth’s outgassing, so option B is incorrect. Exam tip: link outgassing with the early atmosphere and the later condensation of water vapour to form oceans.