Neptune is cooling down and scientists don't know why

Neptune is orbiting 30 times farther away from the sun than Earth
with one year lasting 165 Earth years. The ice giant's seasons, too,
last much longer than those on Earth - more than 40 Earth-years
each.
As the planet moved into its southern summer over the past two
decades, astronomers observed its average global temperatures
plummet by a staggering 14 degrees Fahrenheit (8 degrees Celsius).
"This change was unexpected," Michael Roman, a postdoctoral
research associate at the University of Leicester and lead author
on the new paper (https://bit.ly/3vk8yqk), said in a statement.
"Since we have been observing Neptune during its early southern
summer, we would expect temperatures to be slowly growing
warmer, not colder."
The team analyzed observations in the heat-carrying infrared part
of the light spectrum obtained from 2003 to 2018 by some of the
world's best telescopes including the European Southern
Observatory's (ESO) Very Large Telescope in Chile, the Keck
and Subaru Telescopes in Hawai'i, and NASA's Spitzer Space
Telescope.
The observed cooling, however, wasn't uniform, the researchers said
in the statement. 
Measurements of Neptune's stratosphere, the second lowest layer
of the planet's atmosphere, revealed a warming trend above Neptune's
south pole. This data set, which only contained data from a two-year
period between 2018 and 2020, revealed a speedy warming of about
20 degrees F (11 degrees C). The scientists said that such polar
warming has never been observed on Neptune before.
However, observations of the distant Neptune have only been
possible for the past few decades and scientists know very little
about the natural passing of the seasons on the planet.
"Our data cover less than half of a Neptune season," Glenn Orton,
Senior Research Scientist at JPL and co-author on the study said
in the statement. "So no one was expecting to see large and rapid
changes."
Scientists don't know yet what drives the unexpected temperature
fluctuations, but think it could have something to do with the
11-year cycle of the sun's activity, the periodic ebb and flow of
sunspot generation by the star. 
"The temperature variations may be related to seasonal changes
in Neptune's atmospheric chemistry, which can alter how effectively
the atmosphere cools," Roman said. "But random variability in
weather patterns or even a response to the 11-year solar activity
cycle may also have an effect."
Previous studies suggested there might be a link between the amount
of sunspots and Neptune's brightness. The new study also found some
evidence of a possible connection between the solar cycle, the
brightness of clouds in Neptune's atmosphere and the temperature
of its stratosphere. 
Scientists hope that future observations will shed more light on the
distant planet's mysteries. The James Webb Space Telescope, the
most powerful space observatory ever built, is expected to make
major leaps in the understanding of Neptune, as well as of the other
solar system ice giant, Uranus.