Uranus and Neptune reveal their true colors

Think of Uranus and Neptune, the outer planets of the solar system, and you can imagine two different shades: pale turquoise and cobalt blue. But astronomers say the true colors of these distant ice giants are more similar than their popular images.

Neptune is slightly bluer than Uranus, but the color difference is not nearly as great as it appears in ordinary images, according to A study published Friday in Monthly Notices of the Royal Astronomical Society.

The results help “set the record straight,” said Leigh Fletcher, professor of planetary sciences at the University of Leicester in England and author of the study. “There is a subtle difference in the blue hue between Uranus and Neptune, but subtle is the key word there.”

The deep blue attributed to Neptune dates back 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 the time enhanced the blue in images of Neptune taken by Voyager’s cameras to emphasize the planet’s many landmarks, such as the south pole wave and dark spots. But as many observatories have known for decades, both Neptune and Uranus appear pale teal to the human eye.

“Uranus, as seen by Voyager, was quite dull, so they got as close to its true color as possible,” said Patrick Irwin, professor of planetary physics at the University of Oxford and author of the study. “But with Neptune there are all kinds of weird things,” he said, that “get a little washed out” with the right color correction.

Enhanced images of Neptune often include captions addressing the artificial color, but the vision of a deep blue planet has persisted.

Dr. Irwin and his colleagues used advanced instruments on the Hubble Space Telescope and on the Very Large Telescope in Chile to determine the colors of the planets as accurately as possible.

They also reviewed an immense observational record of both planets recorded by the Lowell Observatory in Arizona between 1950 and 2016.

The results confirm that Uranus is only slightly paler than Neptune, due to the thicker layer of aerosol haze that lightens its color.

The Lowell dataset also sheds new light on the mysterious color shifts that Uranus experiences during its extreme seasons.

For years, astronomers have wondered why Uranus is green during its solstices, but emits a bluer glow during its equinoxes. The pattern is related to Uranus’s strange position: tilted almost entirely on its side. Over the course of an 84-year orbit around the Sun, Uranus’ poles are immersed in decades of perpetual light or darkness in summer and winter, while the equatorial regions near the equinoxes face the Sun.

Uranus’ changing colors can be partly explained by methane in its atmosphere. Because methane absorbs red and green light, the equator ultimately reflects more blue light; in contrast, the poles, which contain half as much methane, are tinted light green. The new study confirms this dynamic, showing that a ‘cap’ of ice particles coalesces over the sunlit poles of the Uranian summer, amplifying the greening effect.

The study “opens the door for many future studies aimed at understanding Uranus’ atmosphere and seasons,” said Ravit Helled, professor of theoretical astrophysics at the University of Zurich, who was not involved in the research. This work, she added, could “improve our understanding of the planet’s internal structure and thermal evolution.”

For Heidi Hammel, an astronomer who worked on Voyager’s imaging team in 1989, the new study is the latest chapter in a long-standing quest to uncover the planet’s true color.

“For the public, I hope this article can help undo the decades of misinformation about the color of Neptune,” said Dr. Hammel, who is now vice president for science at the Association of Universities for Research in Astronomy. “Eliminate the word ‘azure’ from your vocabulary when talking about Neptune!”

The gap between public perception and Neptune’s reality illustrates just one of the many ways in which data is manipulated to emphasize certain features or increase the appeal of astronomical visualizations. For example, the stunning images released by the James Webb Space Telescope are false-color composites of the original infrared observations.

“There was never any attempt to deceive,” said Dr. Fletcher, “but there has been an attempt to tell a story with these images by making them aesthetically pleasing to the eye, so that people can enjoy these beautiful scenes in a way that is, perhaps more meaningful than a vague, gray, amorphous blob in the distance.”