What two planets are often referred to as the twin planets?

The correct pairing of twin planets refers to Uranus and Neptune. These two planets are often called twin planets because they share several similarities in terms of their size, composition, and atmospheric characteristics. Both Uranus and Neptune are classified as ice giants, a category distinct from terrestrial planets or gas giants, which highlights their unique formation and physical properties. They both have similar diameters and masses, and their atmospheres contain hydrogen, helium, and methane, which gives them their striking blue color.

Additionally, the similarities in their internal structures and the way they experience atmospheric phenomena further reinforce the idea that they are "twins" among the planets in our solar system. This term helps in conceptualizing their comparable attributes within the broader context of planetary science.