(CNN)The most distant planet in our solar system has presented a new mystery.
Astronomers observing Neptune for the past 17 years with multiple ground-based telescopes tracked a surprising drop in the ice giant's global temperatures, which was then followed by a dramatic warming trend at the planet's south pole.
Neptune, which orbits the sun at a distance of 2.8 billion miles (4.5 billion kilometers), experiences seasons like Earth does -- they just last much longer. One year on Neptune lasts for about 165 Earth years, so a single season can last around 40 years. It's been summertime in Neptune's southern hemisphere since 2005.
Astronomers decided to track the planet's atmospheric temperatures once the southern summer solstice occurred that year.
Nearly 100 thermal images of Neptune taken since then showed that much of Neptune gradually cooled, dropping by 14 degrees Fahrenheit (8 degrees Celsius) between 2003 and 2018.
A study on the phenomenon published Monday in the Planetary Science Journal.
"This change was unexpected," said lead study author Michael Roman, a postdoctoral research associate at the University of Leicester, in a statement. "Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder."
Then, a dramatic warming event occurred at Neptune's south pole between 2018 and 2020 and temperatures rose by 20 degrees Fahrenheit (11 degrees Celsius). This warm polar vortex completely reversed any cooling that occurred before.
This kind of polar warming has never been seen on Neptune until now.
"Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes," said study coauthor Glenn Orton, senior research scientist at NASA's Jet Propulsion Laboratory, in a statement.
Images were taken using the European Southern Observatory's Very Large Telescope and Gemini South telescope in Chile, along with Hawaii's Subaru Telescope, Keck Telescope and the Gemini North telescope, as well as data from NASA's now-retired Spitzer Space Telescope. The infrared light emitted by Neptune's stratosphere, or the atmospheric band above the active weather layer, helped astronomers detect the temperature fluctuations.
Frosty Neptune has an average of negative 340 degrees Fahrenheit (negative 220 degrees Celsius), and astronomers still don't know what caused these temperature shifts.
For now, they have considered that the unexpected changes could be due to a number of factors.
"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."
More observations will be needed to truly explore these possibilities. The James Webb Space Telescope will observe Uranus and Neptune later this year. The space observatory's mid-infrared instrument can map the chemistry and temperatures in Neptune's atmosphere and could identify what caused the shift.
Neptune is more than 30 times as far from the sun as Earth, and it's the only planet in our solar system that isn't visible to the naked eye from Earth. So far, only NASA's Voyager 2 spacecraft has flown by Neptune up close, which occurred in 1989.
"I think Neptune is itself very intriguing to many of us because we still know so little about it," Roman said. "This all points towards a more complicated picture of Neptune's atmosphere and how it changes with time."