Within our solar system, which of the following planets has the least mass compared to the others listed in the options?

Introduction / Context:
This question tests basic knowledge of the planets in the solar system, specifically focusing on their relative masses. Understanding which planet has the least mass is important for general science and astronomy because mass affects gravity, atmosphere, and many other planetary characteristics. The learner needs to compare common planets and identify which one is physically the smallest in terms of mass among the listed options.

Given Data / Assumptions:

• The planets listed are Neptune, Jupiter, Mars, and Mercury.
• We consider their actual physical masses, not their volumes or distances from the Sun.
• Standard accepted planetary mass values from astronomy are assumed.

Concept / Approach:
The approach is to recall or look up the approximate masses of the listed planets. Jupiter and Neptune are both gas giants, whereas Mars and Mercury are smaller rocky planets. Generally, gas giants are far more massive than terrestrial planets. Among terrestrial planets, Mercury is known to be the smallest in both size and mass. The concept of comparing planetary properties such as size, mass, and composition is central to understanding this question.

Step-by-Step Solution:
Step 1: Recall that Jupiter is the largest and most massive planet in the solar system. Therefore, it cannot be the planet with the least mass. Step 2: Neptune is also a gas giant and is much more massive than any of the terrestrial planets, so it is not the correct answer. Step 3: Compare Mars and Mercury. Mars is larger than Mercury in diameter and has greater mass, though both are rocky planets. Step 4: Mercury is known as the smallest planet in the solar system in terms of both radius and mass after the reclassification of Pluto as a dwarf planet. Step 5: Therefore, among Neptune, Jupiter, Mars, and Mercury, the planet with the least mass is Mercury.

Verification / Alternative check:
To verify, consider approximate mass values. Jupiter has a mass more than 300 times that of the Earth. Neptune has a mass around 17 times the mass of the Earth. Mars has about one tenth of the Earth's mass. Mercury has even less mass than Mars, less than half of Mars mass, and is recognized as the smallest planet. This quick sanity check confirms that Mercury has the least mass of all planets listed in the options.

Why Other Options Are Wrong:

• Neptune: A gas giant and very massive compared to terrestrial planets, so it is far from being the least massive.
• Jupiter: The most massive planet in the solar system, making it the exact opposite of what the question asks for.
• Mars: Smaller and less massive than Earth, but still larger and more massive than Mercury.

Common Pitfalls:
Learners sometimes confuse the order of planets by distance from the Sun with their sizes or masses. Another common error is mixing up Mars and Mercury because both are relatively small terrestrial planets. Some may also incorrectly recall Pluto as a planet and think about it as the smallest, but Pluto is now officially classified as a dwarf planet. Remembering that Mercury is the smallest and least massive of the eight major planets is crucial for avoiding these mistakes.

Final Answer:
The planet in the solar system with the least mass among the given options is Mercury.