The giant planets

A light-blue planet, with an oval dark spot in the centre edged by a white cloud, on a black background. — Neptune photographed by the Voyager 2 probe on 20 August 1989; in the centre, the Great Dark Spot, edged with white clouds. NASA / JPL / Voyager-ISS / Aster Cowart, Public domain, source

Neptune, the most distant of the planets, is known as a deep-blue ball.¹

It is not that colour.

The story of this error is one of the most instructive on this site, because at no point did anyone lie.

The 1989 image

The Voyager 2 probe is the only human-made craft to have flown past Uranus and Neptune, in the 1980s.²

Its camera did not produce colour photographs directly, as a phone does. It photographed through separate filters — each shot recording only a single colour. These images were then recombined to obtain a colour view.³

The images of Uranus were published in a form close to true colour.⁴

Those of Neptune, on the other hand, were not. In this composite image, the contrast was strongly enhanced to bring out the clouds, bands and winds — which made Neptune artificially too blue. The researcher who reopened the case puts it this way: the images of Neptune were “stretched and enhanced”.⁵

No one lied

This is the crux of the story.

Planetary scientists at the time knew this perfectly well. It was not a secret, nor an error, nor a manipulation: it was a deliberate technical choice, intended to bring out details that would otherwise have been invisible.

What is more, the images were released with captions that explained it.

So what happened?

The caption got lost along the way.

The researcher is precise on this point: the artificially saturated colour was known among planetary scientists, the images were released with captions explaining it, and that distinction had become lost over time.⁶

The image, meanwhile, went on its way: into newspapers, textbooks, posters, children’s books, onto screens. After a few years, all that remained was the blue ball.

In 2024, a team led from the University of Oxford used instruments in which each point of the image contains a complete spectrum of colours. It rebalanced the Voyager 2 images and those of the Hubble telescope.⁷

The result: Uranus and Neptune are a similar shade, a greenish blue. Neptune leans only slightly more towards blue — because of a thinner haze layer — and it is much paler than in the image that stuck in people’s minds.⁸⁹

What to take away

This story lends itself easily to misuse.

It is not “we were lied to”. The processing was documented, announced, explained. Those who present it as proof that space images are faked have left out half the story.

What happened is more interesting, and much more common.

An image travels without its caption.

This holds for all images, everywhere. A photograph is easily copied, shared and reused. The text that goes with it, by contrast, is cumbersome: it gets shortened, moved, forgotten.

This mechanism appears elsewhere on this site, in other forms: a history book from 1960, refuted by historians from the 1970s onwards and still cited today outside its discipline; a figure on biodiversity, copied for twenty years without its original nuance; an archaeological discovery corrected by those who led the excavations, while the old version carried on its way.

Each time, it is not the fact that gets lost. It is the precision that went with it.

What this changes for us

We draw a rule from this for our own work, and would rather state it openly.

It is not enough to write an accurate caption under each image. An accurate caption offers no protection, since it becomes detached.

The information must therefore travel with the image, and not alongside it: which instrument, which date, which processing.

On this page, the header image is the Voyager 2 one, with its date and its credit. The version rebalanced in 2024 can be seen in the University of Oxford press release listed in the bibliography.

A planet found with a pencil

In 1846, a man calculated where a planet that no one had ever seen ought to be. A telescope was pointed at the spot indicated. It was there.¹⁰

It was Neptune, the only planet in the solar system discovered by calculation before being discovered by observation.

It all started with unexplained irregularities in the orbit of Uranus. Something was pulling on it.

The French astronomer Urbain Le Verrier calculated where this something must be, and sent its position to Berlin. As soon as he received the letter, Johann Galle pointed his telescope and quickly found the planet.¹¹

What the usual account leaves out

It is often said that another mathematician, the Englishman John Couch Adams, had reached the same result at the same time, and that he was unjustly forgotten.

The historians who reopened the case present things differently. Adams had not given one position, but several, which kept shifting — enough to launch a search, not to point a telescope.¹²

On one side of the Channel, an observer was told: point here. On the other: search over there.

That is not the same thing.

A dispute, an admission, and a theft

After the discovery, the Franco-British rivalry nearly turned into an incident. A compromise eventually granted joint credit to the two men.

What followed reads like a novel.

The Greenwich Observatory’s archival papers on this affair had disappeared. They remained untraceable for decades, before resurfacing at the end of the twentieth century.¹³

After examining them, historians concluded that Adams does not deserve the same credit as Le Verrier.¹⁴

Three caveats

This case is too good to be told without reservations.

One. The doubt about Adams does not date from the recovered archives: researchers had already raised it earlier. The documents confirmed an old question; they did not give rise to it.

Two. Le Verrier is not a spotless hero. His calculations, too, rested on bold assumptions. He was right, and he was also lucky; to write the one without the other would be to manufacture a legend.

Three, and most important. Science does not only correct what it says about the facts of nature. It also corrects its own history — and much more slowly. It took a very long time for this case to be reopened.

What the giant planets are

Jupiter, Saturn, Uranus and Neptune have no solid surface: beneath their clouds, gases swirl above a core.¹⁵

Jupiter and Saturn are gas giants, made mostly of hydrogen and helium. Jupiter, eleven times wider than the Earth, is more than twice as massive as all the other planets combined. Saturn is the only planet whose average density is lower than that of water.¹⁶

Uranus and Neptune are ice giants. At least 80% of their mass is a hot, dense fluid of so-called “icy” materials — water, methane and ammonia — above a small rocky core. Uranus’s axis is so tilted that it seems to spin lying on its side. Neptune takes about 165 years to go round the Sun.¹⁷

All of them have rings. Saturn’s, made of billions of small chunks of ice and rock, are by far the most spectacular. Jupiter has only a faint system of dusty rings; Uranus has 13, Neptune at least five main ones.¹⁸

Note 1

both worlds are in fact a similar shade of greenish blue, despite the commonly-held belief that Neptune is a deep azure

Université d’Oxford, “New images reveal what Neptune and Uranus really look like”, communiqué du 5 January 2024