Star formation, formation of planets, and our solar system

The early rotating disk of gas and dust surrounding the young Sun is called the solar nebula or protoplanetary disk. In the wording of this question, “solar disk” is the intended name, so option A is correct. Material in this disk moved around the forming Sun, collided, and gradually accumulated into larger bodies that eventually became planets, moons, asteroids, and other objects. The forest, water, and rock cycles are Earth-system concepts and do not describe the disk from which the solar system developed. A precise scientific treatment may distinguish the solar nebula from the later protoplanetary disk, but among these choices A is the only relevant answer.

Solar heat can change a comet's ice into gas and dust. Exam tip: remember comet as an icy body.

Jupiter's gravity made it difficult for asteroids to join into a large planet. Exam tip: connect the asteroid belt with Jupiter's position.

A meteor is seen when a small body burns while entering the atmosphere. Exam tip: keep meteor and meteorite separate.

A body that reaches Earth's surface is called a meteorite. Exam tip: reaching the surface is the key clue.

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

Continuous collisions and compression increased heat in early Earth. Exam tip: connect early Earth with a hot molten condition.

Gravity pulls matter together and forms larger bodies. Exam tip: write gravity as the basic force in planet formation.

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

From the Sun the first is Mercury the second is Venus and the third is Earth. Exam tip: practice the planet order repeatedly.

Earth is called the Blue Planet because of its abundant water. Exam tip: remember the link between Earth and water.

Venus appears very bright so it is called the morning and evening star. Exam tip: it is a planet not a star.

Direct answer: Option A, Venus, is correct. Venus has the hottest average surface temperature among the planets, even though Mercury is closer to the Sun. The key concept is the greenhouse effect. Venus has a very dense atmosphere containing much carbon dioxide. Sunlight enters, the surface becomes warm, and outgoing heat is trapped by the atmosphere instead of escaping easily. The reasoning is: dense atmosphere; strong trapping of heat; very high surface temperature. Option A is correct because Venus combines a thick atmosphere with an extreme greenhouse effect. Option B, Uranus, is a distant ice giant and has a very cold atmosphere, not the hottest surface. Option C, Mercury, is closest to the Sun but has almost no substantial atmosphere, so it cannot retain heat efficiently; its temperatures also vary greatly. Option D, Mars, has a thin atmosphere and is generally cold. Memory cue: closest does not always mean hottest—Venus is hottest because its atmosphere traps heat.

Gas giants are dominated by a thick cover of gases. Exam tip: remember this for Jupiter and Saturn.

Gravity pulled the cloud inward. Exam tip: consider gravity as the main cause in formation processes.

Asteroids are considered leftover rocky bodies from planet formation. Exam tip: also remember them between Mars and Jupiter.

The Kuiper Belt contains icy small bodies. Exam tip: connect it with Pluto and the outer solar system.

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

Collisions add matter and increase a planet's mass. Exam tip: link accretion with mass increase.

A protoplanet can later become a developed planet. Exam tip: understand the sequence as small bodies growing larger.

The Sun's gravity keeps planets in their orbits. Exam tip: define orbit as the path of revolution.

Terrestrial planets are denser because they are made of rock and metal. Exam tip: link density with composition.

Gravity pulls planets and their motion keeps them in orbit. Exam tip: understand orbit as a result of balance.

Planets with large mass can hold more satellites. Exam tip: remember strong gravity of outer planets.

Temperature was higher near the Sun and lower far away so materials collected differently. Exam tip: remember the link between temperature and planet composition.