When the New Horizons spacecraft captured those amazing images of Pluto during its close approach, its colour seemed to be a mix of browny red, light yellows and blues and greys.
Pluto’s red colour is thought to be a result of the Sun breaking up hydrocarbons on the surface into compounds called tholins.
Find out more about the colours of the Solar System planets
However, in 2018 NASA released newly-processed versions of the New Horizons image, presenting what NASA called the most accurate representation of Pluto’s colour, according to how the human eye would see it.
This image revealed Pluto to be a muddle of dark and light greys and browns.
But what colour is Pluto and how has the dwarf planet appeared to astronomers and observatories over the years?
Pluto’s discovery
“Some are born great, some achieve greatness, and some have greatness thrust upon them,” wrote William Shakespeare in ‘Twelfth Night’.
Little Pluto falls firmly into the latter camp.
This tiny dwarf planet is a relic of the Sun’s protoplanetary disc: billions of years ago, when matter in orbit around the Sun was busy clumping together to form the planets, Pluto formed far out in the region that would eventually become known as the Kuiper Belt.
And there Pluto sat quietly, undetected, for billions of years… until its existence was first hypothesised in 1909 by Percival Lovell, based upon his observation of perturbations in the orbit of Uranus.
Pluto’s existence was then confirmed in 1930 observations by Clyde Tombaugh, at which point it was declared the Solar System’s ninth planet.
There was a problem, though: the more astronomers studied Pluto – which isn’t easy to do, given its tiny size and its immense distance from Earth – the more it kept ‘shrinking’.
Lowell’s original estimate of Pluto’s mass put it at around seven times that of Earth; following Tombaugh’s observations, that estimate was reduced to roughly the same mass as Earth.
By 1948, Pluto was thought to have around 10% the mass of Earth, and by 1976 that estimate had been downgraded to 1%.
By 2006 it was believed to be just 0.2% as massive as our planet and, following the discovery of Eris and the International Astronomy Union’s adoption of a new definition of ‘planet’, Pluto was downgraded to ‘dwarf planet’ status.
All this confusion was largely a result of the aforesaid difficulties in observing Pluto directly.
We didn’t even have a convincingly planet-like image of it until the 1990s, courtesy of the then newly-launched Hubble Space Telescope.
In direct observations by ground-based telescopes, Pluto just looks like a star.
New Horizons gets a view of Pluto’s colour
And so in 2006, NASA dispatched its New Horizons probe, which made its closest fly-by of Pluto in 2015, and sent back what are still the best images of the dwarf planet and its five moons that we have.
These images revealed much of the surface of Pluto to be covered in a vast, heart-shaped glacier of nitrogen ice, while the planet’s atmosphere – composed mostly of nitrogen, methane and carbon dioxide – appeared blue.
So the most accurate answer to the question “What colour is Pluto?” is, “It depends where you’re looking from”.
If you could stand on the surface of Uranus or Neptune and peer at Pluto through a telescope, it would look blue, but if you stood on Pluto itself, the ground beneath your feet would either be white, if you were standing on the aforesaid glacier.
Pluto’s colour could range from black, grey or brown to orange, depending on the exact constitution of the rocks beneath your feet.
Although that, sadly, is something we still don’t know much about, as no human-made spacecraft has ever landed on Pluto, and there are currently no plans to send one.