Neptune, a very cold planet with average temperature reaching around minus 220 degrees Celsius, has seen rapid and unexpected temperature variations in the last two decades, scientists have found.Using ground-based telescopes, including the European Southern Observatory's Very Large Telescope (ESO's VLT), an international team of astronomers tracked Neptune's atmospheric temperatures over a 17-year period.They found a surprising drop in Neptune's global temperatures followed by a dramatic warming at its south pole."This change was unexpected," said Michael Roman, a postdoctoral research associate at the University of Leicester, UK."Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder," he added.
Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts around 40 years, with one Neptune year lasting 165 Earth years. It has been summer in Neptune's southern hemisphere since 2005.Despite the onset of southern summer, the data published in The Planetary Science Journal, showed that most of the planet had gradually cooled over the last two decades. The globally averaged temperature of Neptune dropped by 8 degrees Celsius between 2003 and 2018.The astronomers were also surprised to discover a dramatic warming of Neptune's south pole during the last two years of their observations, when temperatures rapidly rose 11 degrees Celsius between 2018 and 2020.Although Neptune's warm polar vortex has been known for many years, such rapid polar warming has never been previously observed on the planet.While the astronomers do not know yet what could have caused such unexpected temperature variations, they believe it could be due to changes in Neptune's stratospheric chemistry, or random weather patterns, or even the solar cycle.
As Neptune is roughly 4.5 billion km away and is very cold, measuring its temperature from Earth is no easy task.To piece together overall trends in the planet's temperature the astronomers looked at nearly 100 thermal-infrared images of Neptune, captured over a 17-year period.Around one third of all the images taken came from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on ESO's VLT in Chile's Atacama Desert. The team also used data from NASA's Spitzer Space Telescope and images taken with the Gemini South telescope in Chile, as well as with the Subaru Telescope, the Keck Telescope, and the Gemini North telescope, all in Hawaii.The astronomers investigated infrared light emitted from a layer of Neptune's atmosphere called the stratosphere. This allowed the team to build up a picture of Neptune's temperature and its variations during part of its southern summer.