Neptune and Uranus are similar colours, study reveals

While many people will think of Neptune as being a rich blue colour and Uranus more green, the planets are much closer in colour than typically thought, a new study has found.

So what colours are they? The two ice giants are a similar shade of greenish blue, Professor Patrick Irwin from the University of Oxford and his team found.

Why was it thought that Neptune and Uranus were different colours?

Experts suggest the idea that both planets were different colours arose because images of them captured in the 20th century – including by Nasa’s Voyager 2 mission, the only spacecraft to fly past these worlds – recorded images in separate colours.

Uranus and Neptune are closer in colour than first thought (Image: Patrick Irwin/University of Oxford/NASA/JPL-Caltech/PA Wire)

The single-colour images were later recombined to create composite colour images which were not always accurately balanced to achieve a true colour image.

These composites were often made too blue, particularly in the case of Neptune.

Early images of Neptune from Voyager 2 were strongly contrast-enhanced to better reveal the clouds, bands and winds that shape what we have come to think the planet looks like, scientists say.

Prof Irwin said: “Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ colour, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue.

“Even though the artificially saturated colour was known at the time amongst planetary scientists – and the images were released with captions explaining it – that distinction had become lost over time.”

He added: “Applying our model to the original data, we have been able to reconstitute the most accurate representation yet of the colour of both Neptune and Uranus.”

How the study revealed that Uranus and Neptune are similar colours

In the new study, the researchers used data from Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope.

In both instruments, each pixel is a continuous spectrum of colours, meaning observations from them can be processed to determine the true apparent colour of Uranus and Neptune.

The researchers used the data to re-balance the composite colour images recorded by the Voyager 2 camera and also by the Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

This revealed that Uranus and Neptune are a rather similar shade of greenish blue.

Having said this, the study also found that Neptune has a slight hint of additional blue which the model reveals to be due to a thinner haze layer on that planet.

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

This is because of how thick certain gases are at the planet’s north and south poles, and how they appear when these poles are closest to the sun, according to the findings published in Monthly Notices of the Royal Astronomical Society.

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

“In this way, we have demonstrated that Uranus is greener at the solstice due to the polar regions having reduced methane abundance and an increased thickness of brightly scattering methane ice particles.”

Dr Heidi Hammel, of 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 colour, as well as the unusual colour changes of Uranus, have bedevilled us for decades.

“This comprehensive study should finally put both issues to rest.”