And they can’t explain that right now.

Like the Earth, Neptune – as it revolves around the sun – has different seasons. But where our planet only needs 1 year to complete a circle around the sun, it takes Neptune 165 years. It also means that the seasons on the planet last much longer: about 40 years. It has been summer in the southern hemisphere of the gas giant since 2005. And researchers have been keeping a close eye on the planet ever since; they are curious to see how temperatures change in response to the start of summer. Of course, they had ideas about that prior to their observations. “Since we observed Neptune during its early southern summer, we expect temperatures to gradually rise rather than fall,” said study researcher Michael Roman.


But surprisingly, that didn’t happen; images from several ground-based telescopes indicate that most of Neptune has gradually become cooler over the past 20 years; between 2003 and 2018, the gas giant’s average temperature dropped by 8 degrees Celsius. “This change was unexpected,” says Roman.

Warming of the South Pole

But something else happened: the surprising drop in global temperature was followed by a strong warming of the South Pole. That warming manifested itself between 2018 and 2020; in a few years, temperatures in this region rose by as much as 11 degrees Celsius. And that too has surprised researchers; never before have they seen Neptune’s pole heat up so quickly.


As mentioned, the surprising temperature changes have been observed thanks to recordings from several large ground telescopes, such as the Very Large Telescope, the Gemini South Telescope (both in Chile), and the Keck and Gemini North Telescope (both in Hawaii). “This kind of research is only possible with sensitive infrared images from large telescopes such as the VLT, which can clearly see Neptune, and these have only been available for about 20 years,” explains researcher Leigh Fletcher.

Here you see thermal images of Neptune, taken between 2006 and 2020. The images clearly show that the planet is gradually cooling. And in 2018 the South Pole will start to warm up considerably. Images: ESO/M. Roman, NAOJ/Subaru/COMICS.

The researchers analyzed the images that the various ground telescopes had taken of Neptune over the past 20 years. They also used images from the Spitzer space telescope. They specifically looked at the infrared light emitted by Neptune’s stratosphere. This enabled the researchers to determine the planet’s temperature and how it changed over time. And the results of that research have surprised scientists in several ways. “Our data covers less than half of a Neptune season, so no one expected to see big and rapid changes,” said researcher Glenn Orton.

Neptune is a rather mysterious planet located in the outer reaches of our solar system; it is the eighth and thus the most distant planet from the sun. The planet – like Uranus – can be counted among the ice giants: that means that it is larger than rocky planets (such as Earth and Mars), but smaller than gas giants (such as Saturn and Jupiter). The average temperature of the planet is about -220 degrees Celsius.

The fact that rapid and large temperature changes have now been observed is startling and difficult to explain at the moment. The temperature changes may be traced to changes in the stratosphere or random weather patterns. It could also have to do with the solar cycle (an average 11-year cycle that the sun goes through and that is characterized by a period of increased and a period of decreased solar activity). To gain more clarity about the cause of the remarkable temperature changes, researchers are looking forward to the commissioning of the Extremely Large Telescope† This should be able to observe the temperature changes in even more detail. The recently launched James Webb telescope could also shed new light on the matter by revealing the temperature and composition of the atmosphere (and changes in it).