“That change was unexpected,” explained Michael Roman, an astronomer at the British University of Leicester, responsible for the study and quoted in a statement from the European Southern Observatory (ESO).
Scientists know relatively little about Neptune, the eighth planet in the solar system.
It is the furthest planet from the Sun, which makes its annual orbit last the equivalent of 165 Earth years.
Previous-generation telescopes were unable to tease out the pale light from this all-blue stellar object, enveloped in a thick layer of ammonia, water ice, and solid methane.
The Voyager 2 probe managed in 1989 to send the first net images of Neptune. And now it is permanently scrutinized by the Hubble Space Telescope and the ESO Large Telescope in Chile.
Neptune’s atmosphere is very dynamic, recalls the study, published by the Planetary Science Journal. The winds are the most violent known in the solar system: more than 2,000 km/h.
The data indicated that normally with the arrival of the austral summer in 2005, the planet would begin to warm up, explained Michael Roman.
But the temperature in the southern part of the planet has fallen by an average of 8 degrees Celsius between 2003 and 2018. The average temperature of the planet is -200 degrees Celsius, which makes measurements difficult.
Only thanks to the sensitivity of infrared images from large telescopes is it possible to see Neptune clearly, explained Leigh Fletcher, co-author of the study and an astronomer at the University of Leicester.
“This technology has only been available for twenty years,” he explained.
The observations have also detected another phenomenon, the sudden heating of Neptune’s south pole, of about 11 degrees Celsius between 2018 and 2020.
There is no clear explanation for these phenomena, which could be due to an evolution of chemistry in the stratosphere, or to solar cycles.
The impact of solar variations is also discussed among experts to explain climate change on Earth.