Star formation, formation of planets, and our solar system

The governing concept is the modern classification of objects orbiting the Sun. The Solar System currently has eight recognised planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In 2006, Pluto was reclassified by the International Astronomical Union as a dwarf planet because it does not meet all the criteria used for a full planet, particularly the requirement that it clear the neighbourhood around its orbit. Therefore option C, eight, is correct. Seven is one fewer than the accepted number, while nine reflects the older school list that included Pluto as a planet. Ten is not supported by the current classification. The answer should be based on present astronomical terminology, while remembering that dwarf planets and other smaller bodies still belong to the Solar System.

Option D is correct: Pluto is classified as a dwarf planet. The International Astronomical Union redefined the categories in 2006. Under the current definition, a full planet must orbit the Sun, be nearly round because of its own gravity, and have cleared the neighbourhood around its orbit. Pluto satisfies the first two conditions but has not cleared its orbital neighbourhood, so it is placed in the dwarf-planet category. It is not a star because it does not produce energy through sustained nuclear fusion. It is not a satellite because it orbits the Sun rather than serving as a natural satellite of a planet. The classification concerns Pluto’s astronomical characteristics, not its size alone.

A dwarf planet revolves around the Sun but has not cleared its orbit. Exam tip: take Pluto as its example.

The Kuiper Belt is linked with icy small bodies in the outer solar system. Exam tip: connect it with Pluto's region.

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

A body that burns and glows in the atmosphere is called a meteor. Exam tip: if it reaches the surface it is called a meteorite.

A body that reaches Earth's surface is called a meteorite. Exam tip: keep meteor and meteorite separate.

The governing concept is planetary differentiation. In its early molten or semi-molten stage, Earth was hot enough for materials to move and separate according to density. Dense substances, especially metallic materials such as iron and nickel, were pulled downward by gravity and concentrated toward the centre, while lighter silicate materials remained nearer the surface. Therefore, the correct answer is C, the difference in density. Light, rain and vegetation do not provide the physical mechanism for this large-scale internal separation. This process helped produce Earth’s layered structure, including the core, mantle and crust.

Direct answer: D, iron and nickel. Earth has layers because, when the young planet was very hot and partly molten, materials separated according to density. Dense materials moved downward toward the centre, while lighter rocky materials remained above. Earth’s core is therefore mainly metallic, with iron and nickel as its major components; the inner core is solid and the outer core is liquid because of different pressure and temperature conditions. Option A, sand and soil, are surface materials, not core metals. Option B, ice and water, are light surface or near-surface materials. Option C, wood and coal, are organic materials found near the crust and cannot form the metallic core. Option D is correct. Memory cue: heavy metal centre—iron and nickel.

Moon formation is linked with a collision between Earth and a large body. Exam tip: remember it as the giant impact hypothesis.

The Sun's large mass gives strong gravity. Exam tip: link planetary orbits with the Sun's gravity.

Planets formed from a rotating solar disk so their orbits are nearly in one plane. Exam tip: remember the disk shaped structure.

Solar wind pushed light gases away from inner regions. Exam tip: connect this with the rocky nature of inner planets.

Jupiter is the largest planet in the solar system and is a gas giant composed mainly of hydrogen and helium. Earth and Mars are terrestrial rocky planets, and Mercury is also a small rocky planet. Exam tip: remember Jupiter and Saturn as the main examples of gas giants.

Uranus and Neptune are outer planets because they lie far from the Sun, beyond the asteroid belt. They are not terrestrial planets, which are rocky inner planets. Although they have gaseous atmospheres, Uranus and Neptune are more specifically called ice giants; the term gas giants is used mainly for Jupiter and Saturn. Exam tip: planets beyond the asteroid belt are classified as outer planets.

Terrestrial planets are rocky and relatively dense. Exam tip: remember this feature for inner planets.

Gas giants have a thick gaseous cover. Exam tip: remember gas in the structure of outer planets.

Asteroids are considered small rocky leftovers. Exam tip: connect them with leftover material from planet formation.

The nebular theory considers the Sun and planets to have formed from a gas dust cloud. Exam tip: remember it as a key idea for solar system origin.

The solar system is considered about 4.6 billion years old. Exam tip: connect this number with Earth's origin.

Gravity pulled more matter toward the center and formed the Sun. Exam tip: remember Sun formation at the center.

In planet formation small particles first become planetesimals and then protoplanets. Exam tip: remembering the sequence helps score easily.

Leftover matter after Sun formation helped form planets satellites asteroids and comets. Exam tip: link leftover matter with planet formation.

In a stable star fusion continuously provides energy. Exam tip: write nuclear fusion as the cause of steady starlight.

Icy materials can remain more easily in distant cold regions. Exam tip: connect comets and the Kuiper Belt with the cold outer region.