With seasons each lasting 40 years, the planet Neptune should be spared from abrupt changes in climate, and yet it has experienced a marked cooling since the start of its austral summer 17 years ago, according to a study on Monday.
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“This change was unexpected,” noted Michael Roman, an astronomer at the British University of Leicester, responsible for the study and quoted in a press release from the European Southern Observatory (ESO).
Scientists still know relatively little about Neptune, the eighth planet in the solar system, for which a year stretches about 165 Earth years.
The farthest planet from our star, at about thirty times the Earth-Sun distance, it glowed too faintly to be well studied by older telescopes.
It was not until the visit of the Voyager 2 probe in 1989 that the first clear images of this other blue planet were obtained. Since then, it has been scrutinized in particular thanks to the Hubble Space Telescope and the ESO Very Large Telescope (VLT) in Chile.
This ice giant, with a probably rocky heart enveloped in a mixture of water, ammonia and methane in the solid state, has a very dynamic atmosphere, recalls the study published in The Planetary Science Journal. With the strongest winds recorded on a planet in the solar system, at more than 2,000 km / h. And a cloud system capable of changing its appearance in a few days.
But we had so far little information on the temperature variations of its atmosphere, a mixture of hydrogen, helium and hydrocarbons. “Since we have been observing Neptune since the start of its southern summer, we expected temperatures to be slowly increasing, not getting colder,” explained Michael Roman.
While its austral summer began in 2005 in its southern hemisphere, the star’s average temperature dropped by around 8 degrees Celsius between 2003 and 2018, according to the study’s observations. A figure to compare with its average temperature of -200 degrees Celsius. And which accounts for the difficulty of measuring it from Earth.
“This kind of study is only possible because of the sensitivity of the infrared images of large telescopes like the VLT, which can observe Neptune clearly, and these means have only been available for twenty years”, explained Prof. Leigh Fletcher, co-author of the study and an astronomer at the University of Leicester.
Observations have revealed another phenomenon, the sudden warming of the planet’s south pole, by some 11 degrees Celsius in 2018 and 2020.
The measurements were made by studying the infrared radiation emitted from the planet’s stratosphere, a layer of the atmosphere just above that covering the surface.
Scientists do not understand the cause of these temperature changes. They assume changes in the chemistry of the stratosphere, random phenomena or even a link with the solar cycle.