Neptune is already an ice giant, but it might be having a cold snap

The ice giant's atmosphere is even chillier than we thought.
a blue planet with light patches showing interesting patterns in the clouds and atmosphere
Global temperatures on Neptune might fluctuate more rapidly than previously thought, according to a new study. NASA/JPL

Neptune has always been known as an icy blue behemoth. The eighth most distant planet of the solar system is a dark, cold place—but now an international group of space scientists have found that temperatures in Neptune’s thick atmosphere might actually be even cooler than previously thought. 

Thermal-infrared images taken of Neptune over the past two decades reveal that thermal brightness dimmed in 2003, suggesting that the planet’s average temperatures in the stratosphere, the atmospheric layer that sits above the area where weather brews, dropped by about 8 degrees Celsius (14 degrees Fahrenheit) over that timeframe. The results were published on April 11 in Planetary Science Journal

Like Earth, the smallest of the gas giants sits on an axial tilt which means that it has four seasons. Since it’s much further from the sun, Neptune takes longer to make a complete orbit—about 165 Earth years. This also means a season on Neptune is much slower—imagine experiencing winter for more than 40 Earth years. But the research team from University of Leicester and NASA’s Jet Propulsion Laboratory (JPL) observed the chill trend between 2003 and 2018, which is a surprisingly quick temperature change given Neptune’s long seasons. 

“This change was unexpected,” Michael Roman, a researcher at the University of Leicester and lead author of the paper, said in the university’s press release. “Since we have been observing Neptune during its early southern summer, we would expect temperatures to be slowly growing warmer, not colder.”

a graph of brightness of thermal images of the planet neptune between 2003 and 2020. it shows that the brightness tended to get darker from 2003 to 2018 but then starts to get brighter
Observed changes in Neptune’s thermal-infrared brightness, a measure of temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness from Neptune’s stratosphere with time for all existing images taken by ground-based telescopes. Brighter images are interpreted as warmer. The south pole appears to have become dramatically warmer in just the past few years. Michael Roman/NASA/JPL/Voyager-ISS/Justin Cowart

The study is the largest survey to be conducted so far of Neptune’s curious atmospheric temperatures. The team drew upon more than 95 thermal-infrared observations collected by telescopes in Hawaii and Chile from 2003 to 2020. Brighter thermal images mean warmer temperatures, while darker means cooler temperatures—and the planet’s brightness noticeably darkened over time. “Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” Glenn Orton, senior research scientist at JPL and co-author on the study, also noted in the press release. 

The data collected in different pockets of the planet also hints that temperatures can drastically fluctuate. The south pole, for instance, instead experienced a warming period in the stratosphere by about 11 degrees Celsius, or 20 degrees Fahrenheit, between 2018 and 2020. This level of polar warming has never been observed on Neptune before, the researchers note. While the team has yet to pin down the cause of this ebb and flow, Roman explains one reason could be that the “temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools.” Variability in weather patterns could be at play, he adds. 

[Related: Strange, dark, and hot ice could explain Uranus and Neptune’s wonky magnetic fields]

Neptune’s atmosphere and global temperatures might also be susceptible to the 11-year solar cycle. This is when the sun’s magnetic field flips poles, causing changes in activity on the surface, like sunspots. Solar activity usually calms down when the oscillation approaches the 11-year mark, also known as the solar min, while sunspots peak and activity ramps up halfway through the period during the solar max. Previous studies have suggested that Neptune’s visible brightness is tied to the solar cycle. Now, this study supplies further evidence that solar activity is linked with bright cloud patterns and fluctuating temperatures.  

The James Webb Space Telescope (JWST) is set to observe weather, chemistry, and circulation patterns on both of the far-flung ice giants, Uranus and Neptune, later this year. The research team is hoping atmospheric information the telescope gleans will shed even more light on the icy planet’s mysteries. 

“The atmosphere appears more complicated than we had naively assumed, which, unsurprisingly, seems to be a general lesson that nature teaches scientists again and again,” Roman told Reuters. “I think Neptune is very intriguing to many of us because we still know so little about it.”