A new study led by the University of Leicester and NASA’s Jet Propulsion Laboratory (JPL) has revealed a more complete picture of trends in Neptune’s temperatures than ever before. The scientists combined all existing thermal infrared images of Neptune gathered from multiple observatories over almost two decades. They found a significant decline in the planet’s thermal brightness, suggesting that globally-averaged temperatures in Neptune’s stratosphere have dropped by about eight degrees Celsius from 2003 to 2018.
“This change was unexpected. Since we have been observing Neptune during its early southern summer, we would expect temperatures to be slowly growing warmer, not colder,” said study lead author Dr. Michael Roman, a postdoctoral fellow at the University of Leicester.
Neptune has an axial tilt, so it also experiences different seasons, just like the Earth. However, since it is located at a greater distance from the Sun, Neptune takes over 165 years to complete an orbit around the Sun. This causes its seasons to change slowly, lasting more than 40 Earth-years each. “Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” said study co-author Glenn Orton, a senior research scientist at JPL.
Further satellite observations of Neptune’s south pole revealed that, after the cooling observed from 2003 to 2018, Neptune’s polar stratosphere warmed by 11°C between 2018 and 2020. Such unexpected polar warming has never been observed on this planet before and challenges scientists’ understanding of Neptune’s atmospheric variability.
“Temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools. But random variability in weather patterns or even a response to the 11-year solar activity cycle may also have an effect,” explained Dr. Roman.
Scientists argued that the 11-year solar cycle – characterized by periodic variations in the Sun’s activity and sunspots – could affect Neptune’s brightness and thus its stratospheric temperatures. However, follow-up observations of Neptune’s temperatures and cloud patterns are needed in the coming years in order to further assess any possible connections between these parameters.
Initiating observations later this year with the newly developed James Webb Space Telescope (JWST) will be crucial to answer these questions. “The exquisite sensitivity of the space telescope’s mid-infrared instrument, MIRI, will provide unprecedented new maps of the chemistry and temperatures in Neptune’s atmosphere, helping to better identify the nature of these recent changes,” said co-author Leigh Fletcher, a professor of Planetary Science at the University of Leicester.
The study is published in the Planetary Science Journal.