On Neptune, the night is dark and full of terrors. The planet’s northern hemisphere is currently enduring one of its 40-year-long winters, while the south is basking in a relatively ‘mild’ summer season (the planet’s temperature averages out at about-220C). But strange weather patterns recorded during a 17-year project to track Neptune’s atmospheric temperatures suggest that, as a global average, winter is indeed coming for Neptune.
An international team of astronomers used ground-based telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT) to track the planet’s atmosphere over 17 years. Combining nearly 100 thermal infrared images gathered of the frosty planet over that time, they’ve pieced together the trajectory of the planet’s temperatures and found something surprising.
A new study reported in The Planetary Science Journal says that despite Neptune being at the apex of its long summer season (it’s been summertime there since 2005), global average temperatures on the planet have dropped by 8C since 2003.
Perhaps even more surprising is that despite this frigid global shift, the planet’s south pole experienced a significant temperature rise of 11C between 2018-2020 alone. 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 expecting to see large and rapid changes,” says co-author Glenn Orton, senior research scientist at Caltech’s Jet Propulsion Laboratory (JPL) in the US.
“This change was unexpected,” adds Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”
The astronomers measured Neptune’s temperature using thermal cameras which work by measuring the infrared light emitted from an object. When it comes to Neptune, that’s no easy feat, because its surface temperatures are so cold and because it’s so far away.
“This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so,” says co-author Leigh Fletcher, a professor at the University of Leicester.
If you want to know just how dramatic these temperature changes are, climate scientists all agree that even 2 degrees of warming on Earth over the more than 150 years since the Industrial Revolution could have vicious knock-on effects for life on Earth. Fortunately, Neptune does not harbour living creatures.
But what could be causing these unusual temperature patterns? Thus far, scientists are stumped. It’s possible they could be linked to changes in Neptune’s stratospheric chemistry, random weather patterns, or the solar cycle. But decoding the secrets of these wild extremes of temperature will take further investigation.
Future ground-based telescopes like ESO’s Extremely Large Telescope (ELT) could observe temperature changes like these in greater detail, while the NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere.