Star formation, formation of planets, and our solar system

A satellite is a celestial body that revolves around a planet or another larger body because of gravitational attraction. Earth’s Moon is the familiar natural example: it follows an orbit around Earth while Earth orbits the Sun. A star is a self-luminous body, and planets may orbit stars rather than being defined as bodies orbiting planets. A nebula is a vast cloud of gas and dust, not normally a single satellite body, while a galaxy is a huge system containing stars, gas, dust, and planetary systems. Therefore, option B is correct. The key concept is the orbital relationship between a satellite and its parent planet.

The solar system is considered to have formed from a nebula of gas and dust. Exam tip: connect it with the solar nebula idea.

Contraction increases temperature and density at the center. Exam tip: remember this step in star birth.

Outer planets are generally large and gaseous. Exam tip: remember inner planets as rocky and outer planets as gaseous.

Saturn is famous for its broad and clearly visible ring system. Its rings are made mainly of ice, dust, and rocky particles. Jupiter, Uranus, and Neptune also have rings, but they are much fainter and less distinct than Saturn’s rings. Exam tip: remember Saturn as the planet with the most prominent rings.

Earth's suitable distance keeps temperature favorable and water supports life. Exam tip: remember water and temperature for life.

Planetesimals can combine to form protoplanets. Exam tip: remember the gradual process from small to large bodies.

The planets remain bound to the solar system mainly because of the Sun’s gravitational attraction. The Sun contains almost all of the solar system’s mass, so its gravity provides the dominant force that keeps planets moving in stable orbits around it. An orbit results from the balance between a planet’s forward motion and the Sun’s inward gravitational pull; gravity continuously changes the direction of motion instead of simply letting the planet fly away. The Moon, comets, and asteroids also exert gravity, but their masses are far smaller and their influence on the planets is comparatively limited. Therefore, option C is correct.

As a planet’s distance from the Sun increases, the solar energy received per unit area generally decreases. The Sun radiates energy outward, and that energy spreads over increasingly larger spherical surfaces. Consequently, the intensity follows an inverse-square relationship: if distance becomes twice as large, the received intensity is approximately one-fourth, assuming other conditions are comparable. This is why planets farther from the Sun generally receive less insolation and tend to have lower average temperatures, although atmospheres and other factors also matter. Option A ignores distance, option B is opposite to the relationship, and option C confuses sunlight with night. Thus, option D is correct.

Venus is somewhat similar to Earth in size and structure. Exam tip: remember Venus as Earth's twin planet.

The solar system has many small bodies along with planets. Exam tip: include satellites comets and asteroids.

Matter around the young Sun can arrange in a disk. Exam tip: link this disk with planet formation.

Mars is a terrestrial (rocky) planet with a solid, rocky surface and is one of the inner planets. Jupiter, Saturn, and Neptune are giant planets made mainly of gases or icy materials, so they are not rocky planets. Exam tip: Remember the four terrestrial planets: Mercury, Venus, Earth, and Mars.

The correct answer is Mercury, option B. Mercury is the smallest of the eight officially recognised planets in the Solar System. It is a rocky terrestrial planet in the inner Solar System and has the smallest diameter and volume among those eight planets. Mars and Earth are also rocky planets, but both are larger than Mercury. Neptune is an outer ice-giant planet and is far larger than Mercury. Mercury is also the planet closest to the Sun, but these are two separate facts: being closest to the Sun does not by itself explain why it is the smallest. Pluto is not a competing answer because it is classified as a dwarf planet, not one of the eight planets. Thus B is the only correct option; A, C and D name planets that are all larger than Mercury.

Mars is called the Red Planet because its surface rocks and dust contain abundant iron oxide, which gives it a reddish appearance. Venus may appear very bright, but it is not known as the Red Planet. Exam tip: associate iron oxide with the red colour of Mars.

The Sun is the central star of the solar system. Its gravity keeps planets, dwarf planets and other bodies in orbit, and it produces its own light and heat. The Moon is Earth’s natural satellite, so option A is incorrect; the Sun is not a planet either. Exam tip: a celestial body that produces its own light and heat is called a star.

Leftover material around a new star can form planets. Exam tip: understand star and planet formation as a sequence.

In accretion particles collide and join to make larger bodies. Exam tip: write this process in planet formation.

Uranus and Neptune are outer planets because they lie far from the Sun, beyond the asteroid belt. Along with Jupiter and Saturn, they are placed in the group of outer planets. Uranus and Neptune are ice giants, so they are not inner terrestrial planets. Exam tip: remember that the outer-planet group begins after Mars and the asteroid belt.

Rotation can make matter disk shaped and relatively flat. Exam tip: link the rotating disk with planet formation.

A planet revolves around the Sun and is a dominant body in its orbit. Exam tip: remember this basic part of the definition.

The center of a nebula becomes denser and hotter. Exam tip: consider the center as the place where a star forms.

According to the current accepted view the solar system has eight planets. Exam tip: Pluto is considered a dwarf planet.

A dwarf planet is a celestial body that travels around the Sun and is large enough to be roughly round, but it has not cleared other objects from its orbital neighbourhood. Pluto belongs to this category. Jupiter, Saturn, and Mars are classified as planets because they meet the accepted criteria for planets in our Solar System.

Option D is correct because Pluto is recognised as a dwarf planet. The question asks for an example, not for a full explanation of the classification. The other three choices are major planets: Jupiter and Saturn are gas giants, while Mars is a terrestrial planet. Their classification is therefore different from Pluto’s.

Heavier materials helped form rocky planets near the Sun. Exam tip: remember inner planets as rocky.