Star formation, formation of planets, and our solar system

The nebular theory explains the common formation of the Sun and planets. Exam tip: treat it as a key idea for the origin of the solar system.

Stars form from huge clouds of gas and dust. Exam tip: connect the beginning of star formation with a nebula.

Gravity pulls the cloud inward. Exam tip: remember gravity as the basic force behind star and planet formation.

Direct answer: Option C is correct. When a large gas cloud contracts under gravity, its particles move closer together. Gravitational potential energy is converted into heat, so the central region becomes hotter. Because the same material occupies less space, its density also increases. The sequence is: gravity causes contraction; particles come closer; pressure and collisions increase; temperature and density rise; a hot, dense central region can develop into a protostar. Option A is wrong because contraction generally raises, not lowers, central temperature. Option B is wrong because seas are surface features of planets and do not form in a contracting stellar gas cloud. Option C correctly states both physical changes. Option D is wrong because forests are biological features that cannot grow in a hot contracting gas cloud. Memory cue: contraction under gravity makes the centre hot and dense.

A protostar is an early stage of star formation. Exam tip: treat it as the stage between a nebula and a star.

The governing scientific concept is nuclear energy generation in stars. In the hot, dense core of a star such as the Sun, hydrogen nuclei combine through a sequence of nuclear-fusion reactions to form helium. A small amount of mass is converted into a large amount of energy, according to the mass–energy relationship expressed by E = mc². This released energy travels outward and eventually reaches space as radiation, including the light and heat we observe. Therefore option A is correct. Evaporation is a change of state in matter, while erosion and weathering are surface processes that shape landforms; none can power a star. The distinction to remember is that fusion joins light atomic nuclei, whereas fission splits heavy nuclei.

Direct answer: D, the Sun. A planet is a large body that travels around a star; in our solar system, the Sun is the star and its gravity holds the planets in orbit. Thus Earth, Mars, Jupiter and the other planets revolve around the Sun, although they also rotate on their own axes. Option A, the Moon, is Earth’s natural satellite, not the central body of the solar system. Option B, Jupiter, is a planet and revolves around the Sun. Option C, Earth, is also a planet and revolves around the Sun; other bodies do not revolve around it as the main solar-system pattern. Option D is correct because the Sun’s strong gravity controls planetary orbits. Memory cue: “Solar” comes from the Sun.

Accretion changes small particles into larger bodies. Exam tip: remember this term for planet formation.

Planetesimals join to form larger bodies later. Exam tip: treat planetesimals as an early link in planet formation.

The rotating disk in the early solar system consisted mainly of gas and dust particles. As this material condensed and accreted, it formed planets and other bodies. Water alone was not the main material of this early disk. Exam tip: link a rotating disk of gas and dust with the solar nebula theory.

Inner planets are made of rocky and metallic materials. Exam tip: treat Mercury Venus Earth and Mars as terrestrial planets.

In distant cold regions light gases and ice could remain more easily. Exam tip: remember outer planets as giant and gaseous.

Mercury is the planet nearest to the Sun, so option B is correct. Venus is the second planet and Earth is the third planet from the Sun, so neither is the closest. Exam tip: remember the opening planetary order: Mercury, Venus, Earth, Mars.

The governing concept is the order of the planets according to their average distance from the Sun. Starting outward, the sequence is Mercury, Venus, Earth, and Mars, followed by Jupiter, Saturn, Uranus, and Neptune. Since Mercury is first and Venus is second, Earth occupies the third position; therefore option C is correct. Mars is the fourth planet, so it is a plausible distractor for students who remember the inner planets but miscount. Mercury and Venus are also incorrect because they lie closer to the Sun than Earth. Earth’s position is significant because it receives an appropriate range of solar energy and has liquid water, an atmosphere, and other conditions that support life. The answer depends on orbital order, not size or mass.

The planets of the solar system differ greatly in size. Jupiter is the largest of them, while Earth is much smaller and Saturn, though very large, is still less massive and smaller in diameter than Jupiter. Neptune is also an outer giant planet but does not exceed Jupiter in size. Remembering Jupiter as the largest planet is a useful basic astronomy fact, whereas the other choices are different planets with other characteristics.

Option D, Jupiter, is correct. The question asks for the largest planet, not the planet with the most visible rings or the greatest distance from the Sun. Saturn is famous for its rings, Earth is the planet on which we live, and Neptune is a distant ice giant, but none is the largest. Thus the supplied answer follows directly from the size comparison among the planets and is not ambiguous.

The asteroid belt lies between the orbits of Mars and Jupiter. It contains numerous small rocky bodies that revolve around the Sun. The Kuiper Belt is beyond Neptune's orbit, so it is not the correct answer. Exam tip: remember that the asteroid belt is between Mars and Jupiter.

Our solar system is located in the Milky Way galaxy. Exam tip: treat the solar system as a smaller level than a galaxy.

Rotation is the spinning of a body about its own axis. Earth’s rotation on its axis causes day and night. Revolution is the motion of a body in an orbit around another body, so it is the closest but incorrect option. Exam tip: around an axis means rotation; around an orbit means revolution.

Earth's movement around the Sun is revolution. Exam tip: link day and night with rotation and the year with revolution.

Direct answer: A, Mercury, Venus, Earth and Mars. Inner planets are the four planets nearest to the Sun. They formed from rocky material, so they are also called terrestrial or rocky planets. Starting outward from the Sun, the order is Mercury, Venus, Earth, Mars. Option A lists exactly these four. Option B lists Jupiter, Saturn, Uranus and Neptune, which are the outer planets and are much larger, mostly gaseous or icy. Option C mixes Saturn and Neptune with two inner planets, so it is not a complete inner-planet group. Option D mixes Uranus and Neptune with Mercury and Saturn and is also incorrect. Exam cue: the first four planets are the rocky inner planets.

Jupiter Saturn Uranus and Neptune are outer planets. Exam tip: remember them as large gaseous planets.

Planets do not make their own light but reflect sunlight. Exam tip: this is the basic difference between planets and stars.

Solar heat can release gas and dust from a comet's ice. Exam tip: remember a comet as a body of ice dust and gas.

The governing concept is the appearance of Earth when viewed from space. Oceans and other water bodies cover roughly 71 percent of Earth’s surface, and water absorbs many wavelengths of sunlight while reflecting and scattering blue light more strongly. The atmosphere also scatters shorter blue wavelengths, contributing to the planet’s blue appearance. Therefore option A is correct: abundant surface water is the principal reason Earth is called the Blue Planet. Earth is not the closest planet to the Sun; Mercury is. Earth has no prominent ring system like Saturn, and it is a rocky planet with a solid surface rather than a body made entirely of gas. The name is a visual and physical description, not a claim that every part of Earth is blue.

Venus appears very bright so this name is used. Exam tip: remember Venus is a planet not a star.