In planetary science, which pair of planets listed below are both smaller in size than the Earth when compared by overall diameter and volume?

Introduction / Context:
The planets in our solar system differ significantly in size, mass and composition. Understanding which planets are larger or smaller than the Earth is a basic aspect of astronomy and helps in visualising the structure of the solar system. This question asks you to identify the pair of planets that are both smaller than the Earth in size when considering their diameters and overall bulk.

Given Data / Assumptions:

• The comparison is based on overall size, usually measured by diameter and volume.
• Earth is taken as the reference planet for comparison.
• Uranus and Neptune are outer gas or ice giants, while Venus and Mars are terrestrial planets.
• We use standard approximate planetary diameters accepted in basic astronomy.

Concept / Approach:
Earth has a mean diameter of roughly 12700 kilometres. Venus is very close in size to Earth but slightly smaller, with a diameter of about 12100 kilometres. Mars is significantly smaller, with a diameter of about 6800 kilometres, making both Venus and Mars smaller than Earth. Uranus and Neptune, on the other hand, are gas or ice giant planets with much larger diameters and volumes than Earth. Therefore, any option containing Uranus or Neptune as a partner in the pair cannot represent a pair of planets that are both smaller than Earth. The only pair where both members are smaller than Earth is Venus and Mars.

Step-by-Step Solution:
Step 1: Recall the approximate sizes of the relevant planets: Earth is a medium sized terrestrial planet, Venus is slightly smaller than Earth, Mars is smaller than both Earth and Venus, and Uranus and Neptune are large outer planets. Step 2: Recognise that Uranus and Neptune are giant planets with diameters several times greater than the Earth, so any pair containing them cannot be entirely smaller than Earth. Step 3: Check option a, Uranus and Mars; Uranus is larger than Earth, so this pair is invalid. Step 4: Check option b, Neptune and Venus; Neptune is larger than Earth, so this pair is also invalid. Step 5: Check option c, Venus and Mars; both these planets are known to be smaller than Earth, so this pair satisfies the condition, whereas option d includes Neptune, which is larger than Earth.

Verification / Alternative check:
Astronomy data tables list planetary diameters. Earth is about 12742 kilometres across, Venus about 12104 kilometres, Mars about 6779 kilometres, Uranus about 50724 kilometres and Neptune about 49244 kilometres. These values clearly show that Uranus and Neptune are much bigger than Earth, while Venus and Mars are smaller. Visual comparisons in textbooks often show scaled diagrams where Venus and Mars appear noticeably smaller than Earth, and Uranus and Neptune are much larger spheres. This evidence confirms that the only correct pair of planets smaller than Earth among the options is Venus and Mars.

Why Other Options Are Wrong:
Option a, Uranus and Mars, includes Uranus, which is a giant planet much larger than Earth, so the pair does not fulfil the condition.

Option b, Neptune and Venus, includes Neptune, another giant planet larger than Earth, so both members are not smaller than Earth.

Option d, Neptune and Mars, again includes Neptune, making the pair invalid since Neptune is larger than Earth even though Mars is smaller.

Common Pitfalls:
Learners sometimes focus only on whether a planet is terrestrial or gaseous and assume that all gas giants must be smaller or that all inner planets must be smaller than Earth. In reality, Venus is close in size to Earth and Mars is smaller, but Uranus and Neptune are significantly larger despite appearing less familiar in popular images. Remembering that gas and ice giants are generally much bigger than terrestrial planets helps avoid mistakes and leads to the correct identification of Venus and Mars as the two smaller planets in the list.

Final Answer:
The planets that are both smaller in size than the Earth are Venus and Mars.