The third planet in our solar system is Earth. It is the only planet known to support life and is the largest of the terrestrial planets. Here are some key points to help you understand more about the Earth:

1. Size and Composition

Earth has a diameter of about 12,742 kilometers, making it the fifth largest planet in the solar system.
It is composed of a rocky outer layer called the crust, a thick layer of mostly solid rock called the mantle, and a metallic core.

2. Atmosphere and Climate

Earth's atmosphere is made up of nitrogen (78%), oxygen (21%), and other gases such as carbon dioxide and water vapor.
The atmosphere plays a crucial role in regulating the planet's temperature and supporting life forms.

3. Geology and Landforms

Earth's surface is constantly changing due to processes such as erosion, plate tectonics, and volcanic activity.
It features a variety of landforms including mountains, valleys, plains, and bodies of water such as oceans, rivers, and lakes.

4. Biosphere and Biodiversity

Earth is home to a wide range of ecosystems that support diverse forms of life, including plants, animals, and microorganisms.
Biodiversity on Earth is influenced by factors such as climate, geography, and human activities.

Study Guide

Here are some study questions to help you review and understand the concept of the third planet, Earth:

What are the main components of Earth's atmosphere and their percentages?
How does the structure of the Earth contribute to its geologic activity?
Describe the role of the biosphere in supporting life on Earth.

By understanding the unique characteristics of Earth as the third planet, we can gain a greater appreciation for the complexity and diversity of our home planet.