Why outer planets are called gas giants?

Explanation

**Step-by-Step Solution:**

1. **Identify the Outer Planets**: The outer planets in our solar system include Jupiter, Saturn, Uranus, and Neptune. These planets are located beyond the asteroid belt.

2. **Understand the Composition**: The outer planets are primarily composed of gases such as hydrogen and helium, along with other trace gases.

3. **Solid Surface Absence**: Unlike the inner planets (Mercury, Venus, Earth, and Mars), which have solid surfaces, the outer planets do not have a defined solid surface. Instead, they are made up of thick atmospheres.

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