Star formation, formation of planets, and our solar system

Temperature decreased with distance so different materials became solid in different regions. Exam tip: connect condensation sequence with composition.

The impact may have mixed materials of Earth and the impacting body. Exam tip: connect chemical clues with origin theory.

In a molten state materials could separate by density. Exam tip: connect differentiation with Earth's layers.

Motion of iron nickel in the liquid outer core is linked with magnetic field. Exam tip: remember core composition and magnetism together.

With low mass gases can escape more easily. Exam tip: understand planetary mass and atmosphere together.

Venus's atmosphere traps too much heat. Exam tip: understand temperature through both distance and atmosphere.

Suitable distance and atmosphere kept temperature favorable and water sources supplied water. Exam tip: write habitability as multi factor.

Direct answer: Option A, the ability to sustain stable hydrogen fusion. A star is a very large, hot ball of gas, mainly hydrogen and helium. Its strong gravity squeezes the central gas until the temperature and pressure become high enough for nuclear fusion. In stable hydrogen fusion, hydrogen nuclei combine to form helium and release enormous energy as light and heat. A brown dwarf forms like a star but does not have enough mass to maintain ordinary hydrogen fusion in its core, so it is sometimes called a failed star. Therefore the decisive scientific difference is the sustained hydrogen-fusion ability, not its orbit, appearance, or surface features. Option A is correct because it gives the physical test used to distinguish a normal star from a brown dwarf. Option B is wrong because stars and brown dwarfs are not defined by revolving around Earth; Earth itself revolves around the Sun. Option C is wrong because comet tails are produced when a comet approaches a star and its icy material is heated, not because an object is a star. Option D is wrong because making oceans is not a defining property of stars or brown dwarfs and requires a suitable planetary surface and conditions. Memory cue: star means sustained hydrogen fusion; brown dwarf means insufficient mass for it.

The Oort Cloud is linked with very distant comets and the Kuiper Belt with relatively nearer icy bodies. Exam tip: remember both sources separately.

Gravity of large planets can change orbits of small bodies. Exam tip: treat small bodies as clues of dynamic history.

Large impacts or tidal effects can change planetary rotation. Exam tip: identify general rules and exceptions separately.

Large planets can hold more small bodies by gravity. Exam tip: connect satellites with planetary mass.

Direct answer: Option A, they formed from common solar-nebula material. Nebular theory says that the Sun and planets developed from one large, rotating cloud of gas and dust called the solar nebula. As gravity pulled this material inward, the central part became the young Sun, while material in the surrounding disk clumped together and formed planetesimals and then planets. Since the Sun and planets came from the same original reservoir, they can show broad chemical similarities, even though their exact compositions differ because temperature, distance from the Sun, and later geological processes affected them. Option A is correct because a shared source naturally explains shared elements and some common chemical characteristics. Option B is wrong because Earth's rivers formed much later and cannot explain the origin of the Sun or all planets. Option C is wrong because the planets and Sun did not arrive from different galaxies in this theory; they formed within the same solar system. Option D is wrong because the Moon's surface is only a small later body and could not create the Sun and planets. Exam cue: common chemical similarity usually points to common origin in the solar nebula.

Over time oceans life and chemical processes changed the atmosphere. Exam tip: treat atmosphere as an evolving system.

Direct answer: Option A, heavy metals in the core and lighter layers above. Planetary differentiation means that, when the young Earth became sufficiently hot and partly molten, materials separated according to density. Dense substances moved downward under gravity, while lighter silicate materials remained nearer the surface. This produced broad internal layers: a metallic core, a mantle and a crust. Option A is correct because the concentration of heavy iron and nickel in the core, with lighter rocky materials above, is clear evidence of differentiation. Option B, rings of the Sun, is not an Earth structure and does not show Earth’s internal separation. Option C, a tail of the Moon, is not a normal lunar structure and is unrelated to planetary layering. Option D, oceans of the Milky Way, is scientifically meaningless; the Milky Way is a galaxy, not an object with oceans. Exam cue: differentiation means separation by density—heavy material sinks and light material rises.

A planet is not identified only by its size or by the fact that it moves around the Sun. Under the modern classification, a major planet must have cleared most other objects from its orbital neighbourhood. A dwarf planet also orbits the Sun and is nearly round, but it has not cleared that region. A crowded orbit can therefore indicate that the body belongs to the dwarf-planet category rather than the major-planet category.

Option A is correct because failure to clear the orbit is the relevant classification condition. The body does not become a star merely because other objects are nearby; becoming a star requires very different physical conditions. It cannot be called a galaxy, which is a huge system of stars, gas, and dust, and it is not Earth’s atmosphere. Thus orbital neighbourhood, not crowding alone in an everyday sense, is the key scientific idea.

Terrestrial planets are dense due to rock metal while gas giants have more light gases. Exam tip: understand composition and density together.

As core mass increased ability to capture gases increased. Exam tip: link gas giants with core accretion.

Planets formed orderly in the disk and comet orbits can be elongated by later gravitational effects. Exam tip: connect orbit shape with origin.

The Moon helps axial stability which affects long term climate. Exam tip: understand habitability as multi factor.

Dust has a role in cooling and forming solid particles. Exam tip: treat dust as material for star and planet formation.

A normal star needs enough temperature and pressure in the core. Exam tip: understand mass as a condition for star formation.

The solar disk explains orbits and temperature gradient explains composition. Exam tip: remember the model through two clues.

As hot Earth cooled the outer part could become solid. Exam tip: connect crust with the cooled upper part.

Less altered material gives information about the old solar disk. Exam tip: treat meteorites as records of solar history.