On Neptune, the 40-year summer experiences a cold snap

Given its remoteness from our star, about thirty times the distance between Earth and the Sun, scientists still know relatively little about Neptune, where this cooling is called “unexpected.”

With seasons each lasting 40 years, the planet Neptune should be safe from sudden climate changes, yet it has experienced marked cooling since the start of its southern summer 17 years ago, according to a study released Monday april).

«This change was unexpected“, remarked 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 solar system’s eighth planet, for which a year spans about 165 Earth years.

The farthest planet from our star, about thirty times the distance between Earth and the Sun, was shining too faintly to be well studied by telescopes of previous generations. 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 ESO’s Very Large Telescope (VLT) in Chile.

This ice giant, with a probably rocky heart wrapped in a mixture of water, ammonia and methane in a 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 until now we had little information about the temperature variations of its atmosphere, a mixture of hydrogen, helium and hydrocarbons. “Since we have been observing Neptune since the beginning of its austral summer, we expected temperatures to slowly rise, not cool. “, explained Michael Roman.

Changes in the chemistry of the stratosphere

While its austral summer began in 2005 in its southern hemisphere, the star’s average temperature fell 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 what explains the difficulty of measuring it from Earth. “This type of study is only possible thanks to the sensitivity of the infrared images of large telescopes such as the VLT, which can observe Neptune clearly, and these means have only been available for twenty years.explained Professor Leigh Fletcher, a co-author of the study and an astronomer at the University of Leicester.

The observations revealed another phenomenon, the abrupt warming of the planet’s south pole, of about 11 degrees Celsius in 2018 and 2020. The measurements were made by studying the infrared radiation emitted by the planet’s stratosphere, a layer of the atmosphere located just above the Earth’s surface. that covers the surface. Scientists do not understand the cause of these temperature changes. They suppose changes in the chemistry of the stratosphere, random phenomena or even a link with the solar cycle.

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