Planets are large celestial bodies that orbit stars. They can be rocky, like Earth and Mars, or gas giants, like Jupiter and Saturn. Planets do not produce their own light; instead, they reflect the light of their parent star. Each planet has unique characteristics, such as size, atmosphere, and surface conditions. Stars are massive, luminous spheres of plasma held together by gravity. They generate energy through nuclear fusion, which produces light and heat. The closest star to Earth is the Sun, which provides the energy necessary for life. Stars come in various sizes and colors, indicating their temperature and age.