For more than 30 years, the public has believed that Neptune has a bright dark blue hue, significantly different in color from neighbor Uranus.
But a new study debunks this by finally revealing what the two ice giants really look like – and they are much more similar in color than previously thought.
Neptune is actually a pale blue-green or “cyan,” similar to Uranus and much brighter than the famous deep blue in images from the Voyager 2 spacecraft.
The study's new images provide the best approximation yet of what we would see with the naked eye if we could somehow travel to these planets.
At a distance of more than 30 million miles, they are the two most distant known major planets in our solar system.
Neptune is known for its rich blue and Uranus for its green – but the two ice giants are actually much more similar in color than commonly thought. Neptune is actually not as deep blue as often thought and has a color much more similar to Uranus – pale blue-green or “cyan”.
Uranus and Neptune – basic facts
URANUS
– Discovered: 1781
– Average distance from the Sun: 1.8 billion miles
– Diameter: 31,000 miles
– Number of moons: 27
NEPTUNE
– Discovered: 1846
– Average distance from the Sun: 2.79 billion miles
– Diameter: 34,503 miles
– Number of moons: 14
The study was led by Professor Patrick Irwin from the Department of Physics at the University of Oxford.
“We thought it would be useful to point out what the true colors of these planets actually are,” he told Web.
“None of us will ever see these planets from an orbiting spacecraft, and the planets are very difficult to observe with ground-based telescopes, so very, very few people have insight into what these planets actually look like.”
Uranus and Neptune, the seventh and eighth planets in our solar system, are the only two ice giants in the outer solar system.
They consist primarily of a hot, dense fluid of icy materials – water, methane and ammonia – over a small rocky core.
It was NASA's Voyager 2 spacecraft, launched in 1977, that took photos of Uranus and Neptune during flybys of both planets – in 1986 and 1989, respectively.
Although the spacecraft was a monumental success, it actually gave rise to the modern misconception of what the two planets look like.
That's because Voyager 2 captured multiple images with different color filters that had to be combined into composites.
The thing is that the images were not always precisely balanced to achieve a “true” color composition and – especially in the case of Neptune – were often made “too blue”.
Not entirely true: In 1989, NASA's Voyager 2 spacecraft delivered the first close-up images of Neptune. In fact, Neptune has a paler green-blue hue and is closer to Uranus
Voyager 2 is pictured here on August 4, 1977 at the Kennedy Space Center in Florida before its launch 16 days later
In addition, the contrast of Voyager 2's early Neptune images was greatly increased to make the planet's features such as clouds, bands and winds more visible.
“In our work we show that these features tend to look 'washed out' and indistinct when you combine the images together to produce something close to 'real' colour,” Professor Irwin told Web.
“The Voyager team also saw this and therefore decided to combine the images in a way that better communicates the scientifically interesting features.”
“This was mentioned when the images were published, but the distinction has been lost over time, so most people (including planetary scientists) now think that Neptune is dark blue.”
To reveal the true colors, Professor Irwin and colleagues used data from the Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile.
Using data from the instruments, the team was able to rebalance the composite color images captured by the Voyager 2 camera and also by the Hubble Space Telescope's Wide Field Camera 3 (WFC3).
This revealed that Uranus and Neptune actually have a fairly similar greenish blue or “cyan” hue – commonly described as the color of shallow water over a sandy beach.
The main difference is that Neptune has a slight hint of additional blue due to a thinner layer of haze on this planet, but nothing like we previously thought.
Professor Irwin said the Voyager team “did the right thing” in the late 1980s and did not claim that the early Voyager images were misleading.
“However, we should never overlook the fact that some of these images have not necessarily been processed with color fidelity,” he told Web.
Although Uranus looks similar to how it was perceived in the original Voyager 2 snapshots, the new study provided interesting insights about the seventh planet.
The study answers the long-standing mystery of why Uranus' color changes slightly during its 84-year orbit around the sun.
Uranus as seen by the Hubble Space Telescope's Wide Field Camera 3 (WFC3) from 2015 to 2022. During this sequence, its north pole, which is a paler green color, swings downward toward the Sun and Earth
The Earth's axis is tilted at about 23 degrees. But Uranus tilts about 98 degrees – giving the appearance that the planet is spinning on its side
This image from the team's work shows radiation spectra from Uranus and Neptune, revealing their true color
Uranus is unique in the solar system because its axis is nearly parallel to its orbit.
While Earth's axis is tilted at about 23 degrees, Uranus is tilted at about 98 degrees – giving the impression that the planet is spinning on its side.
Measurements have already shown that Uranus appears slightly greener at its solstices (i.e. summer and winter), when one of the planet's poles points towards the sun.
But during the equinox – when the sun is above its equator – it has a slightly bluer hue.
Researchers found that the polar regions of Uranus are more reflective at green wavelengths than at blue wavelengths.
This is because methane, which absorbs greenery, is about half as abundant near the poles as it is at the equator.
The ice giants Uranus and Neptune remain a “tantalizing target” for future robotic researchers and build on the legacy of Voyager 2, the research team says.
“A mission to explore the uranium system – from its bizarre seasonal atmosphere to its diverse collection of rings and moons – is a high priority for space agencies in the coming decades,” said co-author Leigh Fletcher of the University of Leicester.
“Studies like this, which show how Uranus’s appearance and color have changed over the decades in response to the solar system’s strangest seasons, will be critical to putting this future mission’s discoveries into broader context .”
The new study was published in the Monthly Notices of the Royal Astronomical Society.
HOW DOES URANUS’ MAGNETIC FIELD COMPARE TO THAT OF EARTH?
A study analyzing data collected more than 30 years ago by the Voyager 2 spacecraft has found that Uranus' global magnetosphere is unrelated to Earth's, which is known to be nearly aligned with our planet's spin axis.
Pictured is a false-color view of Uranus taken by Hubble
According to researchers at the Georgia Institute of Technology, this alignment would result in behavior significantly different from what is observed on Earth.
Uranus lies on its side and rotates so that its magnetic field is tilted 60 degrees from its axis.
This causes the magnetic field to “wobble” asymmetrically in relation to the solar wind.
This causes the magnetic field to “wobble” asymmetrically in relation to the solar wind.
When the magnetosphere is open, it allows the solar wind to flow in.
But when it closes, it forms a protective shield against these particles.
The researchers suspect that solar wind reconnection occurs upstream of Uranus' magnetosphere at different latitudes, causing the magnetic flux to close in different parts.