Astronomy Practice Exam

What two planets are often referred to as twins?

1. Earth and Mars

2. Venus and Earth

3. Jupiter and Saturn

4. Mercury and Venus

The correct answer is: Venus and Earth

Venus and Earth are often referred to as "twins" due to their similar size, composition, and proximity to the Sun. Both planets are categorized as terrestrial planets and have a similar diameter, with Venus being only about 95% the size of Earth. Additionally, they share comparable bulk densities and are primarily composed of rock and metal. However, despite their similarities, the planets have markedly different atmospheres and surface conditions. Earth has a breathable atmosphere rich in nitrogen and oxygen, supporting life, while Venus is shrouded in a thick atmosphere primarily composed of carbon dioxide, with extreme greenhouse gas effects leading to surface temperatures hot enough to melt lead. The contrasting environmental conditions highlight the uniqueness of each planet, even while their similarities earn them the "twin" designation. The other choices represent planets that do not share as many characteristics. Earth and Mars, while both rocky, have significant differences in size, atmosphere, and temperature. Jupiter and Saturn are gas giants, differing greatly in structure and composition from terrestrial planets, and Mercury and Venus, while close in proximity to the Sun, have little in common in terms of their atmospheric properties and surface conditions.