The color attributed to Neptune has long been an intense azure blue, while Uranus tends towards green. Thanks to the work of a team of scientists, we now have the most accurate depiction of what she looks like yet.
Our solar system consists of eight planets of different color combinations. We know these nuances mainly thanks to photos taken with terrestrial or space telescopes. However, the photos we find online are often filtered or digitally processed to reveal the nuances and characteristics of these planets. Recently, scientists conducted a comparative study aimed at accurately determining the color of twins Uranus and Neptune, known as the two blue planets of the solar system. To everyone's surprise, the results published on January 4, 2024 in the Royal Astronomical Society suggest that these two twins have much more in common than we thought.
Neptune is not as blue as we thought
Until today, it was known in the collective consciousness that Neptune was a deep azure blue. For what ? In the summer of 1989, NASA's Voyager 2 spacecraft transmitted the first images of this giant ice planet via radio transmission. But these first photos did not give a true reflection of reality. In a press release, one Patrick Irwin, the physicist and professor at Oxford University who is leading this study, explains that Neptune's colors have been “accentuated and enhanced” to particularly highlight its details and reliefs. its clouds, its bands and its winds. Result: The most distant known planet in the solar system has artificially turned blue. Although these adjustments were initially indicated in the captions, they were gradually forgotten.
Uranus is paler than its twin
To correct this misunderstanding and redefine the true color of Neptune as accurately as possible, scientists used current data. Those obtained with the STIS imaging spectrograph at NASA's Hubble Space Telescope and the second-generation wide-field 3D spectrograph (MUSE for short) at the European Southern Observatory (ESO) in Chile. Thanks to the processing capabilities of these sophisticated tools, the team was able to reprocess the images taken by Voyager 2 in the 1980s, rebalancing the colors captured. “This showed that Uranus and Neptune actually have a fairly similar green-blue hue,” the press release continued. The main difference is that Uranus is slightly paler than Neptune, partly due to a smaller amount of methane in the upper atmosphere, which absorbs more light from the Sun.
Explaining the color variations of Uranus
Additionally, this study allows us to better understand why Uranus' color varies regularly throughout its 84-year orbit around the Sun. Professor Irwin explains that it appears greener during its solstices, winter and summer, particularly due to a drop in methane levels in the polar regions. Conversely, its hue is slightly bluer during the equinox, when the sun overlooks the equator. Experts explain this seasonal color variation with the unusual rotation of Uranus. “It rotates almost sideways during its orbit, meaning that during the planet's solstice, its north pole or south pole points almost directly toward the sun and Earth,” the researchers explain.
This study is the first to explain why Uranus changes color during its orbit and also highlights the importance of these two planets for future space missions. “A mission to explore the uranium system – from its strange seasonal atmosphere to its diverse collection of rings and moons – is a top priority for space agencies in the coming decades,” says Professor Leigh Fletcher, a planetary specialist at the University of Leicester Co- Author of the new study.
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