In planetary classification, which one of the following planets is not a terrestrial (rocky) planet and instead belongs to the group of gas or ice giants?

Introduction / Context:
Planets in our solar system are broadly divided into terrestrial (rocky) planets and gas or ice giants based on their composition and structure. Terrestrial planets are small, dense and composed mostly of rock and metal, while gas and ice giants are large, low density planets dominated by gases and ices. This question asks you to identify which planet in the list is not terrestrial but instead belongs to the giant planet category.

Given Data / Assumptions:

• Mercury, Venus and Mars are known inner planets located relatively close to the Sun.
• Saturn is one of the outer planets and is known for its extensive ring system.
• Terrestrial planets include Mercury, Venus, Earth and Mars.
• Gas or ice giants include Jupiter, Saturn, Uranus and Neptune.

Concept / Approach:
Terrestrial planets are characterised by solid rocky surfaces, relatively small radii and higher densities. Mercury, Venus, Earth and Mars fall into this category. They are located in the inner part of the solar system. Saturn, on the other hand, is a gas giant composed mainly of hydrogen and helium with a small rocky or icy core, and has a much larger size and lower average density. It has an extensive system of rings and many moons. Because the question asks for the planet that is not terrestrial, we look for the one in the list that is clearly a gas giant, which is Saturn.

Step-by-Step Solution:
Step 1: Recall the classification of planets into terrestrial and gas or ice giants. Step 2: List the terrestrial planets: Mercury, Venus, Earth and Mars, all rocky and relatively small. Step 3: List the gas and ice giants: Jupiter, Saturn, Uranus and Neptune, all large and primarily gaseous or icy. Step 4: Compare these categories with the options: Mercury, Venus and Mars are clearly terrestrial planets. Step 5: Recognise that Saturn is a gas giant and therefore the only option that is not terrestrial, making it the correct answer.

Verification / Alternative check:
Planetary data tables show that Mercury, Venus and Mars have solid surfaces and diameters comparable to Earth, while Saturn has a diameter almost ten times that of Earth and a very low average density, indicating a gaseous composition. Images from spacecraft also show Saturn as a giant planet with rings, lacking a defined rocky surface that one could stand on. These observational facts and classifications from astronomy confirm that Saturn is not a terrestrial planet, while the other listed planets are.

Why Other Options Are Wrong:
Option a, Mercury, is a small rocky planet closest to the Sun and is a textbook example of a terrestrial planet.

Option b, Venus, has a rocky surface covered with a thick atmosphere and is categorised as a terrestrial planet.

Option c, Mars, is another rocky planet often called the red planet and is firmly placed in the terrestrial group.

Common Pitfalls:
Some learners may rely only on distance from the Sun and assume that any planet far away must be terrestrial or any planet with an atmosphere must be a gas giant. Others confuse the presence of rings with some other category. The key distinguishing features are size, composition and density. Remembering that only the four inner planets are terrestrial and that Saturn is grouped with the large outer gas giants helps quickly select the correct answer in multiple choice questions.

Final Answer:
The planet listed that is not a terrestrial planet is Saturn.