ESO telescope captures surprising changes in Neptune’s temperatures

An international team of astronomers discovered a startling decrease in Neptune’s overall temperature followed by a huge increase in its south pole.

The team that tracked Neptune’s atmospheric temperatures over a 17-year period was surprised.

“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study published today in The Planetary Science Journal. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”

Because Neptune is located approximately 4.5 billion kilometers from Earth and is extremely cold, with an average temperature of around –220°C, monitoring its temperature from Earth is not easy.

The astronomers determined the temperature of Neptune using thermal cameras, which detect infrared light emitted by astronomical objects. For its investigation, the scientists integrated all available photos of Neptune taken by ground-based telescopes during the last two decades. They examined infrared light emitted by the stratospheric, a layer of Neptune’s atmosphere. This enabled the scientists to piece together information on Neptune’s temperature and its variations throughout the course of a portion of its southern summer.

Neptune, like Earth, has seasons as it rounds the Sun. However, a Neptune season lasts about 40 years, with one Neptune year lasting 165 Earth years. Since 2005, summer has been in Neptune’s southern hemisphere, and astronomers were curious to observe how temperatures increase during the southern summer solstice.

Astronomers looked at almost 100 thermal-infrared photos of Neptune that were taken over 17 years. They could figure out the planet’s general temperature patterns in more depth than ever before. Despite the arrival of southern summer, these statistics indicated that the majority of the world had gradually cooled during the preceding two decades.

The globally averaged temperature of Neptune dropped by 8 °C between 2003 and 2018. In a matter of months between 2018 and 2020, the astronomers were then astounded to observe the sudden warming of Neptune’s south pole when temperatures soared 11 degrees Celsius. Neptune’s heated polar vortex has been known for many years, no other planet has ever experienced such rapid polar warming.

Due to the surprising nature of Neptune’s temperature changes, astronomers do not yet know what caused them. They could be caused by variations in the chemistry of Neptune’s stratosphere, by random weather patterns, or even by the solar cycle. Additional observations will be required over the next few years to elucidate the causes of these variations.

“I believe that Neptune intrigues many of us since we know so little about it,” Roman explains.

“All of this speaks to a more intricate picture of Neptune’s atmosphere and its evolution over time.”

Around a third of all photos collected were taken using the VLT Imager and Spectrometer for Mid-Infrared (VISIR) instrument on ESO’s VLT in Chile’s Atacama Desert. Additionally, the team analyzed data from NASA’s Spitzer Space Telescope and photos collected with the Gemini South telescope in Chile, as well as the Subaru Telescope, the Keck Telescope, and the Gemini North telescope, all of which are located in Hawaii.