In planetary astronomy, which of the following is not classified as an inner terrestrial planet of our Solar System?

Introduction / Context:
Planets in our Solar System are often grouped into inner terrestrial planets and outer giant planets. Inner planets are closer to the Sun, are smaller and have rocky surfaces. This question checks whether you can correctly identify which listed planet does not belong to the inner terrestrial group and therefore should not be classified with Mercury, Venus and Earth in that category.

Given Data / Assumptions:

• Inner planets are those orbiting closest to the Sun and are rocky.
• The options are Mercury, Saturn, Venus and Earth.
• We must pick the one that is not an inner terrestrial planet.

Concept / Approach:
The four inner terrestrial planets are Mercury, Venus, Earth and Mars. They share characteristics such as smaller size, rocky crusts and relatively thin atmospheres. The outer planets, starting from Jupiter and Saturn, are gas giants or ice giants with thick atmospheres and many moons. Therefore, any planet from Jupiter outward cannot be considered an inner planet. By checking the standard ordering of planets from the Sun, we can quickly identify the odd one out in the options.

Step-by-Step Solution:
Step 1: Recall the sequence of planets from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Step 2: Identify the inner terrestrial planets as Mercury, Venus, Earth and Mars. Step 3: Compare each option with this list of inner planets. Step 4: Mercury is an inner terrestrial planet, so it is not the answer. Step 5: Venus is also an inner terrestrial planet, so it is not the answer. Step 6: Earth is an inner terrestrial planet, so it is not the answer. Step 7: Saturn is a gas giant far beyond Mars and is therefore not an inner planet, so it is the correct choice.

Verification / Alternative check:
Another way to verify is to recall physical properties. Saturn has a ring system, large size and a thick gaseous envelope mainly of hydrogen and helium. These are typical features of outer gas giants. Inner terrestrial planets lack such ring systems and are much smaller. So this physical difference confirms that Saturn cannot be grouped with Mercury, Venus and Earth as an inner rocky planet.

Why Other Options Are Wrong:
Mercury: This is the closest planet to the Sun and is one of the classic inner terrestrial planets, so it is not the correct answer.
Venus: Venus orbits second from the Sun and is a rocky inner planet, so it is incorrectly chosen if selected.
Earth: Earth is the third planet from the Sun and clearly an inner terrestrial planet, so it cannot be the odd one out in this list.

Common Pitfalls:
A common error is to confuse the term inner planet with small planet, or to misremember the sequence of planets, especially when including Mars and the asteroid belt. Some students also mix up the ideas of terrestrial and habitable, which are not the same. Terrestrial simply indicates a rocky composition, not the presence of life. Remembering the standard list of inner rocky planets as Mercury, Venus, Earth and Mars helps avoid these mistakes.

Final Answer:
The planet that is not classified as an inner terrestrial planet is Saturn.