In our solar system, which planet has the lowest average density compared with the other major planets?

Introduction / Context:

This general knowledge physics question asks about the average density of planets in the solar system. Density connects mass and volume and helps us understand whether a planet is mainly rocky, icy, or made of light gases. Knowing which planet has the lowest density also creates a strong mental picture of how unusual that planet is compared with Earth and other worlds.

Given Data / Assumptions:

• We compare only the four planets listed: Earth, Jupiter, Saturn, and Neptune.
• Density refers to average bulk density, that is mass divided by volume for the whole planet.
• No numerical values are given, so the question relies on conceptual knowledge from basic astronomy.

Concept / Approach:

Terrestrial planets such as Earth are mostly rocky and metallic, so they have relatively high densities. Gas giants such as Jupiter and Saturn are made largely of hydrogen and helium, which are very light elements. Ice giants such as Neptune contain heavier ices and rock, so their densities are higher than those of pure gas giants. Among all planets, Saturn is well known for having an average density lower than that of water, which means that if there were a large enough ocean, Saturn would float. This makes Saturn the correct choice.

Step-by-Step Solution:

Verification / Alternative check:

An alternative conceptual check is to recall common textbook facts. Many textbooks highlight the interesting statement that Saturn would float in a sufficiently large water body because its density is less than 1 gram per cubic centimetre, while water has density around 1 gram per cubic centimetre. No similar statement is usually made for Jupiter or Neptune, so this reinforces the fact that Saturn has the minimum density.

Why Other Options Are Wrong:

• Earth: As a dense, rocky planet with an iron core, Earth has much higher density than the gas giants and ice giants.
• Jupiter: Although composed of light gases, Jupiter is highly compressed by its own gravity, giving it a density higher than that of Saturn.
• Neptune: Being an ice giant with significant amounts of heavier materials, Neptune has a density higher than both Jupiter and Saturn.

Common Pitfalls:

Students sometimes assume that the largest planet must have the lowest density or confuse mass with density. Jupiter is the most massive planet but not the least dense. Another mistake is to think that all gaseous planets have similar densities, ignoring the effect of gravitational compression and composition. Keeping the distinct categories of terrestrial, gas giant, and ice giant clear in memory helps avoid confusion.

Final Answer:

Saturn has the lowest average density among the planets listed.