Star formation, formation of planets, and our solar system

In planetary differentiation heavy and light materials separate to form layers. Exam tip: remember iron nickel in core and lighter materials above.

Matter in the disk plane made formation of planetesimals and planets possible. Exam tip: understand the solar disk as the birthplace of planets.

In a stable star internal pressure balances gravitational contraction. Exam tip: remember stable star with fusion and balance.

In cold dense clouds matter can gather more easily. Exam tip: connect star formation with gravitational contraction.

Fragmentation can make separate parts of a cloud become seeds of different stars. Exam tip: remember that one cloud can form many stars.

The disk around a protostar contains material for planet formation. Exam tip: treat the protoplanetary disk as the birthplace of planets.

Enough mass and contraction are needed for star formation. Exam tip: treat mass as a condition for star birth.

Sticking of tiny particles was an early step in planet formation. Exam tip: remember the sequence from tiny particles to planets.

The early solar disk contained materials that experienced different temperatures at different distances from the young Sun. As the disk cooled, substances condensed at the temperatures suitable for them. Refractory materials, including many rocks and metals, could condense in hotter inner regions, while volatile substances remained gaseous there and condensed farther away in cooler regions. This temperature gradient helps explain why inner planets are mainly rocky and outer planets contain much more gas and ice.

Option B is correct because differences in temperature and condensation temperatures produced different material behaviour. Rainfall on Earth, the Moon’s shadow, and human activity cannot explain the composition of planets forming in the solar disk. The answer should connect the changing temperature with the point at which different substances changed from gas into solid material. That process influenced the later composition of planets.

Inside the frost line volatile ices could not remain easily. Exam tip: connect inner planets with rocky materials.

Beyond the frost line colder regions had more solid ice and gas available. Exam tip: link this with the large size of outer planets.

As mass increased gravity became more effective. Exam tip: connect accretion with increasing gravity.

A more massive protoplanet attracts more material. Exam tip: remember gravitational attraction in planetary growth.

Matter organized in a rotating disk helped planets have regular orbits. Exam tip: connect elliptical orbits with the solar disk.

The correct answer is D. In the nebular hypothesis, the early solar system began as a large, rotating cloud of gas and dust. As gravity caused the nebula to contract, its radius decreased. If external torque was negligible, angular momentum remained conserved, expressed broadly as L = Iω. A smaller radius reduces the moment of inertia I, so the angular speed ω increases; this is why a contracting nebula rotates faster and tends to flatten into a rotating disc. Rainfall, ocean colour, and the Moon's surface are not consequences of this conservation principle in solar-system formation. The concept is similar to a skater spinning faster while drawing in the arms.

In a main sequence star hydrogen fusion runs steadily. Exam tip: remember stable star and fusion together.

A protostar can glow after being heated by contraction. Exam tip: separately remember fusion for a stable energy source.

Removal of light gases made rock and metal dominant. Exam tip: link solar wind with inner planet composition.

Large mass and strong gravity held light gases. Exam tip: remember hydrogen and helium in gas giant composition.

Icy materials are important in Uranus and Neptune. Exam tip: treat them as a subtype different from Jupiter and Saturn.

Direct answer: B, because rocky bodies there could not join into a complete planet. During planet formation, small solid bodies collided and combined through accretion. Between Mars and Jupiter, however, the strong gravitational influence of Jupiter disturbed the rocky material, so it did not easily gather into one large planet. The remaining objects form the asteroid belt and are viewed as leftover planet-building material. Option A is wrong because Earth’s rivers did not create the asteroid belt. Option B correctly explains why the material is called leftover. Option C is wrong because the belt is not the Sun’s core. Option D is wrong because it has no relation to the boundary of the Milky Way; the belt lies within our solar system. Memory cue: between Mars and Jupiter, unfinished rocky planet.

Jupiter's strong gravity prevented asteroids from joining into a large planet. Exam tip: remember Jupiter's influence on the asteroid belt.

The Kuiper Belt is linked with distant icy bodies. Exam tip: remember asteroid belt as rocky and Kuiper Belt as icy.

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

Solar wind pushes a comet's gas and dust outward. Exam tip: remember the tail direction away from the Sun.