Which planet in our solar system is popularly known as the twin of the Earth?

Introduction / Context:
This is a straightforward astronomy and world geography question that asks about a common nickname used for one of the planets. Some planets share similarities with the Earth in size, mass, and composition. Because of these similarities, one planet is often called the twin of the Earth in school level science and geography books. The goal is to recall which planet is described this way, even though its surface conditions are very different from those on Earth.

Given Data / Assumptions:

• The question refers to a popular nickname, not a strict scientific classification.
• The four options are Mars, Saturn, Uranus, and Venus.
• Similarities such as size, density, and composition are considered when using the word twin.
• We assume the context of standard school and exam level references.

Concept / Approach:
Venus is very similar to the Earth in size, mass, and overall composition. Its diameter is only slightly smaller than that of the Earth, and it is a rocky terrestrial planet, not a gas giant. For this reason, it is often called the twin of the Earth. Mars is also a rocky planet but much smaller. Saturn and Uranus are gas or ice giants with completely different structures and sizes, so they are not considered twins of the Earth in any common classification.

Step-by-Step Solution:
Step 1: Recall that Earth like or terrestrial planets include Mercury, Venus, Earth, and Mars. Step 2: Compare sizes. Venus has a diameter only slightly less than that of the Earth, making it very close in size and mass. Step 3: Compare the options. Mars is terrestrial but smaller and less massive than the Earth. Saturn and Uranus are much larger and are gas or ice giants with thick atmospheres and no solid surface like the Earth. Step 4: Remember the widely used phrase in textbooks that Venus is called the twin of the Earth because of the similarity in size and structure. Step 5: Select Venus as the correct answer from the four choices.

Verification / Alternative check:
Even if you only remember that Venus is often called the Earth like planet or the sister planet, that is enough to identify it as the twin. Mars is frequently called the red planet, Saturn is known for its rings, and Uranus is known for its tilted axis, not as Earth’s twin. Many exam oriented books explicitly state that Venus is about ninety five percent of Earth’s diameter and around eighty percent of its mass, which confirms its status as the closest twin in size and composition.

Why Other Options Are Wrong:
Option A, Mars, does share some features with Earth, such as being rocky and having polar ice caps, but it is significantly smaller and is not commonly called the twin of the Earth. Option B, Saturn, is a gas giant with prominent rings and a mass many times that of the Earth, so it cannot be considered a twin. Option C, Uranus, is an ice giant with a very different composition and an unusual tilt, which makes it physically and visually quite different from the Earth.

Common Pitfalls:
Students sometimes confuse Mars and Venus because both are close to the Earth and are often mentioned in space exploration news. Another mistake is to think that any planet with some similarity can be called a twin. The nickname here is fixed and conventional: Venus is called the twin of the Earth primarily because of its similar size and composition, even though its dense atmosphere and extremely high surface temperature make it a very hostile world.

Final Answer:
The planet popularly known as the twin of the Earth is Venus.