Early and modern theories

The Nebular Hypothesis is an early explanation for the formation of the solar system, including Earth. Plate tectonics explains the movement of Earth's plates, not how the solar system formed.

The Nebular Hypothesis explains the formation of the solar system from a cloud of gas and dust. The other choices describe unrelated Earth processes or human geography topics.

Immanuel Kant proposed an early nebular account of the solar system in 1755. Laplace later developed a related version, so Kant is the answer when the question asks who first proposed an early form.

The expected school-level answer is Laplace. Kant proposed an early nebular idea in which the Solar System originated from a rotating cloud of matter, and Laplace later modified and developed this hypothesis, commonly called the Kant–Laplace nebular hypothesis. The question asks who modified the Nebular Hypothesis, so option D is the relevant historical association. Chamberlain and Moulton are linked mainly with the planetesimal hypothesis, a different explanation of planetary origin. Edwin Hubble contributed important observational evidence about galaxies and the expanding universe, but he was not the scientist identified in this textbook pairing. Therefore Laplace is the correct choice.

The classical Nebular Hypothesis describes the solar system forming from a rotating nebula—a cloud of gas and dust. As it contracted, material in it formed the Sun and planets.

The Planetesimal Hypothesis is an early theory of planet formation. It proposed that small solid bodies, called planetesimals, accumulated to form planets.

The Planetesimal Hypothesis is associated with geologists Thomas C. Chamberlin and Forest R. Moulton, who developed it in the early twentieth century.

In the Tidal Hypothesis, a star passing close to the Sun was thought to raise tides on it and draw out solar material. That material was then proposed to condense into planets.

The Tidal Hypothesis is associated with James Jeans and Harold Jeffreys. Their model proposed that a passing star drew material from the Sun, from which planets could form.

The Big Bang model is the most widely accepted scientific account of the universe's early evolution. It describes the universe expanding from an early hot, dense state; it does not mean an explosion into pre-existing empty space.

The Big Bang theory describes the universe’s early development and expansion from a hot, dense state. Therefore, it concerns the origin and evolution of the universe.

The Big Bang model describes the early universe as extremely hot and dense. As it expanded, it cooled over time.

Edwin Hubble found a relationship between a galaxy’s distance and its redshift. This supported the conclusion that distant galaxies are receding and the universe is expanding.

Hubble’s observations showed that distant galaxies are generally receding from us. This supports the idea that the universe is expanding.

The Big Bang model describes the universe as expanding from an early hot, dense state. This expansion began long before the formation of planets or humans.

The governing cosmological concept is expansion-related cooling. As the universe expands, matter and radiation become distributed through a larger volume, so the average energy density decreases. Radiation is also stretched to longer wavelengths, lowering its temperature. Therefore the general trend has been a decrease in temperature as the universe expanded. Option D expresses this cooling. The idea that temperature kept increasing is opposite to the standard model, and saying that it did not change ignores the effect of expansion on energy density and radiation. “Ended instantly” is not a physical description of cosmic evolution. The question asks for the broad trend, not a claim that every local region always cools at the same rate.

As the universe expanded and cooled, gravity drew matter together into denser regions. Over time, these regions contributed to the formation of stars and galaxies.

A galaxy is a very large system of stars, gas, dust and dark matter held together by gravity. Our galaxy is called the Milky Way. Option A refers to soil, while options C and D are landforms on Earth, so they cannot be galaxies. Exam tip: choose galaxy when a question describes a huge gravitationally bound system of stars.

Our solar system lies within the Milky Way, one of the many galaxies in the universe.

The nebular model proposes that the solar system formed from a rotating cloud of gas and dust. Material in this cloud gradually came together to form the Sun and planets.

Accretion is the gradual joining of smaller particles into larger bodies. In the early solar system, this process helped form planetesimals and, eventually, planets.

The correct answer is B. Planetesimals are small solid bodies that can collide and combine to build larger bodies, eventually contributing to the formation of planets.

The correct answer is D. The solar nebula was a cloud of gas and dust from which the Sun and the bodies of the Solar System formed.

The correct answer is A. As gas and dust in the nebula collapsed under gravity, the young Sun formed as a protostar before becoming a star powered by nuclear fusion.

The correct answer is A. Small particles collide and stick together, gradually building larger bodies through accretion. These bodies can then combine into planetesimals and planets.