New images reveal what Neptune and Uranus really look like

Recent research by Professor Patrick Irwin shows this Neptune And Uranus Both are a similar shade of teal, challenging previous perceptions of their colors. The study used modern telescopic data to correct historical errors in colors and explain subtle color changes in Uranus above its orbit.

Neptune is famous for its rich blue color and the green of Uranus, but a new study has revealed that the two ice giants are actually much closer in color than usually thought.

The correct shadows of the planets were confirmed with the help of research conducted by Professor Patrick Irwin of the University of California Oxford universitywhich is published today in the Monthly Notices of the Royal Astronomical Society.

He and his team found that both worlds actually have a similar shade of greenish blue, despite popular belief that Neptune is a deep sky blue and Uranus has a pale cyan appearance.

Voyager 2/ISS images of Uranus and Neptune released shortly after Voyager 2's flybys in 1986 and 1989, respectively, were compared to reprocessing the individual candidate images in this study to determine the best estimate of the true colors of these planets. Credit: Patrick Irwin

Misconception of planetary colors

Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors.

This misconception arose due to photographs taken of both planets during the 20th century, including by… NASAVoyager 2 mission, the only spacecraft to fly across these worlds – images recorded in separate colors.

Monochrome images were later recombined to create composite color images, which were not always precisely balanced for a 'true' color image, and were often made 'too blue' – especially in the case of Neptune.

Uranus as seen by HST/WFC3 from 2015-2022. During this sequence, the light green North Pole swings toward the Sun and Earth. In these images, the equator and latitude are marked at 35 north and 35 south. Credit: Patrick Irwin

In addition, the contrast of early images of Neptune from Voyager 2 has been greatly improved to better reveal the clouds, bands and winds that make up our modern view of Neptune.

Professor Irwin said: “Although the familiar Voyager 2 images of Uranus were published in a form closer to 'true' colour, the images of Neptune were actually stretched and enhanced, and thus artificially blue-shifted.”

“Although the artificially saturated color was known at the time among planetary scientists – and images were released with captions explaining it – this distinction has been lost over time.”

“By applying our model to the original data, we were able to reconstruct the most accurate representation to date of the color of both Neptune and Uranus.”

Explaining true colors through modern research

In the new study, the researchers used data from Hubble Space TelescopeThe Space Telescope Spectrograph Imager (STIS) and Multi-Unit Spectrograph Explorer (contemplation) on the European Southern Observatory Very large telescope. In both devices, each pixel is a continuous spectrum of colors.

This means that STIS and MUSE observations can be processed unambiguously to determine the true apparent color of Uranus and Neptune.

The researchers used this data to rebalance composite color images recorded by Voyager 2, as well as the Hubble Space Telescope's Wide Field Camera 3 (WFC3).

This revealed that Uranus and Neptune are actually somewhat similar shades of teal. The main difference is that Neptune has a slight hint of extra blue, which the model reveals is caused by a thinner haze layer on that planet.

Animation of seasonal color changes on Uranus during two Uranian years (one Uranian year is equivalent to 84.02 Earth years), extending from 1900 to 2068 and beginning just before the southern summer solstice, when the south pole of Uranus points almost directly toward the Sun.
The left disc shows the naked-eye appearance of Uranus, while the color of the right disc has been extended and enhanced to make atmospheric features more pronounced. In this animation, Uranus' rotation has been slowed by more than 3,000 times so that the planet's rotation can be seen, with separate storm clouds seen passing through the planet's disk.
As the planet moves toward the solstices, a pale polar “cap” of increasing cloud opacity and decreased methane abundance can be seen filling more of the planet's disk leading to seasonal changes in the planet's overall color.
The changing size of Uranus' disk is due to Uranus's changing distance from the Sun during its orbit.
Credit: Patrick Irwin, University of Oxford

Explain the color differences in Uranus

The study also provides an answer to the long-standing mystery of why Uranus changes color slightly during its 84-year orbit around the sun.

The researchers reached their conclusion after comparing images of the ice giant for the first time with measurements of its brightness, which were recorded by the Lowell Observatory in Arizona from 1950 to 2016 in blue and green wavelengths.

These measurements showed that Uranus appears slightly greener at the solstices (that is, summer and winter), when one of the planet's poles is pointing toward our star. But during the equinox – when the sun is above the equator – its color is somewhat bluer.

Part of the reason for this is that Uranus has a very unusual rotation.

It actually rotates almost on its side during its orbit, meaning that during the planet's solstices, its north or south pole points almost directly toward the Sun and Earth.

This is important, the researchers said, because any changes in the reflectivity of the polar regions would have a significant impact on Uranus' overall brightness when viewed from our planet.

What astronomers have been less clear about is how or why this reflection differs.

This prompted researchers to develop a model that compares the spectra of Uranus's polar regions with those of its equatorial regions.

She found that the polar regions are more reflective at green and red wavelengths than at blue wavelengths, in part because methane, which absorbs red, is about half as abundant near the poles as it is at the equator.

However, this wasn't enough to fully explain the color change, so the researchers added a new variable to the model in the form of a “cap” of gradually thickening icy fog that had previously been observed during summer, the sunlit pole like the planet. It moves from equinox to solstice.

Astronomers believe this is likely composed of methane ice particles.

When simulated in the model, ice particles had increased reflectance at green and red wavelengths at the poles, providing an explanation for why Uranus is greener at solstice.

Professor Irwin said: “This is the first study to match a quantitative model with imaging data to explain why Uranus changes color during its orbit.”

“In this way, we demonstrated that Uranus is greener at the solstice due to a decrease in methane abundance in the polar regions but also an increase in the thickness of the brightly scattered methane ice particles.”

Dr Heidi Hamill, from the Association of Universities for Research in Astronomy (AURA), who has spent decades studying Neptune and Uranus but was not involved in the study, said: “The misperception of Neptune’s color, as well as Uranus’s unusual color changes, have confounded us for decades. This comprehensive study should finally put both issues to rest.

Future exploration and continuous research

Building on the legacy of Voyager in the 1980s, ice giants Uranus and Neptune remain a tantalizing destination for future robotic explorers.

Professor Lee Fletcher, a planetary scientist from the University of Leicester and co-author of the new study, said: “The mission to explore the Uranian system – from its strange seasonal atmosphere, to its diverse collection of rings and moons – is a high-profile one.” Priority for space agencies in the coming decades.

However, even a long-lived planetary explorer orbiting Uranus would only capture a brief snapshot of a Uranian year.

“Ground-based studies like this, which show how Uranus's appearance and color have changed over the decades in response to the strangest seasons in the solar system, will be vital in putting this future mission's discoveries into their broader context,” Professor Fletcher added.

Reference: “Modelling the seasonal cycle of color and size of Uranus, and comparison with Neptune” by Patrick J. J. Irwin, Jack Dobinson, Arjuna James, Nicholas A. Tenby, Amy A. Simon, Lee N. Fletcher, Michael T. Roman, Glenn S. Orton, Michael H. Wong, Daniel Toledo, Santiago Pérez Hoyos, and Julie Beck, September 12, 2023, Monthly Notices of the Royal Astronomical Society.
doi: 10.1093/mnras/stad3761