What planet is often referred to as Earth's twin due to its similar size and structure?

Venus is often referred to as Earth's twin due to its comparable size, mass, and density with Earth. Both planets have similar characteristics in terms of their rocky composition and internal structure, which includes a solid surface, a mantle, and a core. However, the environmental conditions on Venus are extremely different from those on Earth, primarily due to its thick atmosphere composed mainly of carbon dioxide, which leads to an intense greenhouse effect and surface temperatures hot enough to melt lead. This stark contrast highlights the uniqueness of Earth despite their similarities in size and structure, making Venus an intriguing subject of study in planetary science.