Uranus and Neptune are the most distant planets in the solar system (seventh and eighth), orbiting respectively 2.9 billion and 4.5 billion kilometers around our star.
Two icy and little explored worlds - only one probe, Voyager 2, flew over them in the late 1980s.
Since then, thanks to terrestrial and space observatories, astronomers have learned to better understand these giants, both of which are made up of gas.
They were able to observe great similarities in diameter (50,000 km each, i.e. five times more than the Earth), temperature (about -200°C), mass and composition of their atmospheres.
The presence of methane, a gas that absorbs infrared radiation, gives the two sisters this same bluish color, contrasting with the warm colors of Jupiter and Saturn, the two other gaseous planets of the solar system.
But at visible wavelengths, Neptune's blue appears brighter than Uranus', a difference astronomers struggled to explain until new research found a single cause.
Published Tuesday in the Journal of Geophysical Research (JGR), they suggest the existence of an existing haze layer on both planets, but thicker on Uranus.
Which "whitens" the appearance of his complexion, explains to AFP Patrick Irwin, planetary scientist at the British University of Oxford, lead author of the study.
To reach this conclusion, he and his team combined old data collected by Voyager 2 with more recent data from the Hubble and Gemini North telescopes (Hawaii).
And developed a model describing the different atmospheric layers of the ice giants, over a wide range of wavelengths (ultraviolet, visible and near infrared) - previous research focused on specific wavelengths.
One of these layers, containing "photochemical" haze particles, was found to be twice as thick on Uranus.
These particles absorbing the Sun's ultraviolet rays, they would be the cause of the lowest UV reflectivity observed on the planet.
They would also explain this paler blue visible to the human eye, "given that these particles are reflected on a visible spectrum close to white", details the study.
As Neptune's atmosphere is renewed more quickly, it would be more effective in driving out the layer of haze that is causing Uranus' pallor.
It is also in these atmospheric mists that the key to the mystery of the dark spots often visible on Neptune, more rarely on Uranus, could reside, advances Professor Irwin.
© 2022 AFP