Think of Uranus and Neptune, the solar system’s outermost planets, and you may picture two distinct hues: pale turquoise and cobalt blue. But astronomers say that the true colors of these distant ice giants are more similar than their popular depictions.
Neptune is a touch bluer than Uranus, but the difference in shade is not nearly as great as it appears in common images, according to a study published in Monthly Notices of the Royal Astronomical Society.
The results help to “set the record straight,” said Leigh Fletcher, a professor of planetary science at the University of Leicester in England and an author of the study. “There is a subtle difference in the blue shade between Uranus and Neptune, but subtle is the operative word there.”
The deep blue attributed to Neptune dates to an artificial enhancement in the 1980s, when NASA’s Voyager 2 became the first (and still the only) spacecraft to visit the two planets. Scientists at that time cranked up the blue in images of Neptune made by Voyager’s cameras to highlight the planet’s many curiosities, such as its south polar wave and dark spots. But as many sky watchers have known for decades, both Neptune and Uranus appear pale greenish-blue to the human eye.
“Uranus, as seen by Voyager, was pretty bland, so they made it as near to true color as we can,” said Patrick Irwin, a University of Oxford professor and an author of the study. “But with Neptune, there’s all sorts of weird things” that “get a bit washed out” with proper color correction.
Enhanced images of Neptune often include captions that address the artificial color, but the vision of a deep-blue planet has endured.
Irwin and his colleagues used advanced instruments on the Hubble Space Telescope and on the Very Large Telescope in Chile to resolve the colors of the planets as accurately as possible. They also reviewed an observational record of both planets captured by Lowell Observatory in Arizona from 1950 to 2016.
The results confirm that Uranus is only slightly paler than Neptune, because of the thicker layer of aerosol haze that lightens its color.
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