It’s an intriguing question. Uranus and Neptune, the two outermost planets of our solar system, are both ice giants — cold worlds that are part gas, part ice, with similar chemical compositions.
They’re not far off in mass, either, Uranus being 15 times that of Earth, and Neptune 17 times. And they’re both about four times the size of Earth, Uranus being slightly larger.
Yet the two worlds look decidedly different. Uranus, as first revealed by NASA’s passing Voyager 2 spacecraft in 1986, is a featureless light-blue blob. When the same spacecraft encountered Neptune in 1989, it revealed a world with the most powerful winds in the solar system, which rip through a royal blue atmosphere, with giant storms and even a mysterious dark spot. Why the difference?
Patrick Irwin, a planetary physicist at Oxford University, and colleagues have now developed an answer. They pieced together a detailed understanding of each world’s atmosphere using the Gemini North telescope in Hawaii, the Hubble Space Telescope and other observations.