Neptune and Uranus are so similar that scientists sometimes refer to the distant, icy planets as planetary twins. But these ice giants have one big difference: their color.
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New space and ground-based telescope observations have revealed what's behind this difference in tone. The most distant planets from the sun in our solar system, Neptune and Uranus have similar sizes, masses and atmospheric conditions. Looking at the the two planets side by side, which was made possible after NASA's Voyager 2 spacecraft flew by them in the 1980s, Neptune has a bright blue appearance. Uranus is a paler shade of cyan blue.