In basic astronomy, which one of the following planets is smaller in size than the Earth?

Introduction / Context:
Knowing the relative sizes of planets in the solar system is a common requirement in school level astronomy and general knowledge. Some planets are giant gas planets, while others are rocky terrestrial planets similar in size to Earth. This question asks which listed planet is smaller than Earth, testing your understanding of planetary diameters and basic classification.

Given Data / Assumptions:

• The question compares planet sizes relative to Earth.
• Options include Neptune, Venus, Saturn, and Uranus.
• We must pick the planet whose overall size is slightly less than that of Earth.
• We assume knowledge of which planets are gas giants and which are terrestrial.

Concept / Approach:
Among the planets listed, Neptune, Saturn, and Uranus are all gas giant or ice giant planets that are much larger than Earth in radius and volume. Venus, however, is a terrestrial planet very similar to Earth in size, but slightly smaller. Its diameter and mass are a bit less than those of Earth, which is why it is sometimes called Earth twin. Therefore, of the options given, Venus is the only planet that is smaller than Earth, while the others are significantly larger.

Step-by-Step Solution:
Step 1: Identify which planets in the list are gas or ice giants. Saturn, Uranus, and Neptune fall into this category and are all much larger than Earth. Step 2: Recall that Saturn is famous for its rings and is very large in radius compared with Earth. Step 3: Remember that Uranus and Neptune are also large outer planets beyond Saturn, both significantly larger than Earth. Step 4: Consider Venus, which is a rocky terrestrial planet located closer to the sun than Earth but almost similar in size. Step 5: Note that Venus radius and diameter are slightly less than those of Earth, making it a little smaller overall. Step 6: Therefore, the correct choice for a planet smaller than Earth among the options is Venus.

Verification / Alternative check:
Comparing approximate planet radii confirms this reasoning. Earth radius is about 6370 kilometres. Venus radius is slightly under this figure, while Saturn, Uranus, and Neptune each have radii many times larger than Earth. This numerical comparison aligns with the general classification of Venus as nearly Earth sized but modestly smaller, confirming that Venus fits the condition described in the question, and the others do not.

Why Other Options Are Wrong:

• Neptune: An ice giant located far from the sun, clearly larger than Earth in size.
• Saturn: A gas giant with a huge radius and ring system, much bigger than Earth.
• Uranus: Another ice giant, larger than Earth and not smaller in dimension.

Common Pitfalls:
Some learners remember that Mercury and Mars are smaller than Earth but may not quickly recall Venus exact size. Others may think any inner planet is smaller than Earth, which is not accurate in this case. The safest method is to remember that of the terrestrial planets, Mercury and Mars are clearly smaller, Venus is just slightly smaller, and Earth is the largest of the four. The giant outer planets are all significantly bigger than Earth.

Final Answer:
The planet smaller than Earth among the options is Venus.