Think of Uranus and Neptune, the outermost planets of the solar system, and you might imagine two distinct hues: soft turquoise and cobalt blue. But astronomers say the true colors of these distant ice giants are more similar than popular depictions.
Neptune is a touch bluer than Uranus, but the difference in shade is not as great as it appears in common images, according to a study published Friday in the Monthly Notices of the Royal Astronomical Society.
The results help “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 blue hue between Uranus and Neptune, but subtle is the operative 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 only) spacecraft to visit the two planets.
Scientists at the time tinted the blue in the images of Neptune taken by the Voyager cameras to highlight the planet’s many curiosities, such as south polar wave and dark spots. But as many sky watchers have known for decades, both Neptune and Uranus appear pale green-blue to the human eye.
“The sky, as seen by Voyager, was quite soft, so they made it as close to the true color as we can,” said Patrick Irwin, professor of planetary physics at the University of Oxford and an author of the study. “But with Neptune, there’s all the weird stuff,” he said, that “washes out a little bit” with the right color correction.
Enhanced images of Neptune often include captions referring to the artificial color, but the vision of a deep blue planet has endured.
Dr. Irwin and his colleagues used advanced instruments on the Hubble Space Telescope and the Very Large Telescope in Chile to determine the colors of the planets as precisely as possible.
They also looked at a vast observational record of both planets recorded by the Lowell Observatory in Arizona between 1950 and 2016.
The results confirm that Uranus is slightly paler than Neptune, due to a thicker layer of aerosol haze that brightens its color.
The Lowell data set also sheds new light on the mysterious color changes that Uranus experiences during its extreme seasons.
For years, astronomers have wondered why Uranus is green during its solstices, but radiates a bluer glow at its equinoxes. The pattern is linked to the peculiar position of Uranus — it is almost entirely tilted on its side. During an 84-year orbit around the sun, Uranus’ poles are plunged into decades of perpetual light or darkness in summers and winters, while the equatorial regions see the sun near the equinoxes.
The changing colors of the Sky can be partly explained by atmospheric methane. Because methane absorbs red and green light, the equator ends up reflecting more blue light. In contrast, the poles, which have half the amount of methane, are slightly green in color. The new study confirms this dynamic and shows that a “hood” of ice particles coalesces over Uranus’ summer-sunny poles, enhancing the greening effect.
The study “opens the door to many future studies aimed at understanding the atmosphere of Uranus and its seasons,” said Ravit Helled, a professor of theoretical astrophysics at the University of Zurich who was not involved in the research. This work, he added, can “improve our understanding of the planet’s internal structure and thermal evolution.”
For Heidi Hammel, an astronomer who worked on the Voyager imaging team in 1989, the new study is the latest chapter in a long-running quest to reveal the planet’s true color.
“For the public, I hope this paper can help undo decades of misinformation about Neptune’s color,” said Dr. Hamel, who now serves as vice president for science at the Association of Universities for Research in Astronomy. “Delete the word ‘blue’ from your vocabulary when discussing Neptune!”
The gap between public perception and reality of Neptune illustrates just one of the many ways in which data is manipulated to emphasize certain features or enhance the appeal of astronomical depictions. For example, the stunning images released by the James Webb Space Telescope are false-color composite versions of the original infrared observations.
“There has never been an attempt to deceive,” said Dr. Fletcher, “but an attempt has been made 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 to be, perhaps, more substantial than a murky, gray, shapeless blob in the distance.”