Free Astronomy Magazine September-October 2022

29 SEPTEMBER-OCTOBER 2022 ASTRO PUBLISHING followed by a dramatic warming at its south pole. “This change was unexpected,” says Michael Roman, a postdoctoral re- search associate at the University of Leicester, UK, and lead author of the study published in The Planetary Sci- ence Journal . “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.” Like Earth, Neptune experiences sea- sons 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- time in Neptune’s southern hemi- sphere since 2005, and the as- tronomers were ea-ger to see how temperatures were changing follow- ing the southern summer solstice. Astronomers looked at nearly 100 thermal-infrared images of Nep- tune, captured over a 17-year pe- riod, to piece together overall trends in the planet’s temperature in greater detail than ever before. These data showed that, despite the onset of southern summer, most of the planet had gradually cooled over the last two decades. The glob- ally averaged temperature of Nep- tune dropped by 8 °C between 2003 and 2018. The astronomers were then sur- prised to discover a dramatic warm- ing of Neptune’s south pole during the last two years of their observa- tions, when temperatures rapidly rose 11 °C 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. “Our data cover less than half of a Neptune season, so no one was ex- pecting to see large and rapid changes,” says co-author Glenn Orton, senior research scientist at Caltech’s Jet Propulsion Laboratory (JPL) in the US. The astronomers measured Nep- tune’s temperature using thermal cameras that work by measuring the infrared light emitted from as- tronomical objects. For their analysis the team combined all existing im- ages of Neptune gathered over the last two decades by ground-based telescopes. They investigated in- frared 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 dur- ing part of its southern summer. Because Neptune is roughly 4.5 bil- lion kilometres away and is very cold, the planet’s average tempera- ture reaching around –220 °C, meas-