Which two planets are sometimes referred to as the "twin" planets?

Uranus and Neptune are often referred to as the "twin" planets due to their similar size, composition, and distance from the Sun. Both planets are classified as ice giants, which distinguishes them from the gas giants like Jupiter and Saturn. They have similar atmospheric compositions, consisting mainly of hydrogen, helium, and methane, giving them their characteristic blue color. Additionally, their diameters and masses are fairly comparable, which further solidifies their "twin" status in the context of planetary science.

In contrast, the other pairs of planets do not share the same close physical and compositional characteristics. Earth and Venus are sometimes called "sister planets" because of their similar size and mass, but they differ significantly in atmosphere and conditions. Saturn and Jupiter are gas giants and are more massive than Uranus and Neptune. Mercury and Mars vary considerably in terms of size, atmospheric composition, and distance from the Sun, making them less similar overall. Therefore, Uranus and Neptune are the planets that are most accurately described as "twins" due to their specific similarities.