Which planet is referred to as Earth's "sister planet"?

Venus is often referred to as Earth's "sister planet" due to its similar size, composition, and proximity to Earth. Both planets are terrestrial—meaning they have a solid planetary surface—and are similar in diameter and mass, which results in comparable gravitational forces. Venus has about 95% of Earth's diameter and is only slightly less massive. This resemblance has led scientists to study Venus to gain insights into Earth's climate and geological processes.

Additionally, Venus exhibits a thick atmosphere primarily composed of carbon dioxide, leading to a strong greenhouse effect that keeps its surface temperature extremely high. Despite these harsh conditions, the similarities in basic physical characteristics merit the title of "sister planet." The other options represent planets with significant differences in composition and atmospheric characteristics, rendering them less relevant for this particular designation.