Summer on Neptune is Surprisingly Chilly

Astronomers were surprised to learn that Neptune is getting colder, despite currently experiencing early summer.

A team led by space scientists at the University of Leicester, UK, combined all available thermal infrared data on the eighth planet from half a dozen different observatories stretching back almost 20 years. The dataset includes images from some of the biggest telescopes in the world, including the European Southern Observatory’s Very Large Telescope and the W.M. Keck Observatory. They also used spectral data from NASA’s Spitzer Space Telescope.

The team concluded in the Planetary Science Journal that the globally averaged temperature in Neptune’s stratosphere — the region above the planet’s active weather layer — has plummeted by 8ºC (14ºF) since 2003.

“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” says team member Glenn Orton (NASA’s JPL). While the seasons change every three months on Earth, Neptune takes 165 years to orbit the Sun and so each season lasts more than four decades.

There was another surprise in store. One region of Neptune is bucking the overall cooling trend. Recent observations with the Gemini North and Subaru observatories, both on Mauna Kea in Hawai’i, revealed that the stratosphere over the south Neptunian pole warmed by 11ºC (20ºF) between 2019 and 2020. It’s the first time astronomers have observed polar warming on Neptune.

Atmospheric physicist Karen Aplin (University of Bristol, UK), who was not involved in the research, thinks the findings stack up. “Their approach seems rigorous and, importantly, different results obtained in different ways from different telescopes lead to roughly consistent conclusions,” she says.

The challenge now is understanding what’s causing these unexpected changes. “[They] may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools,” says team member Michael Roman (University of Leicester, UK). “Random variability in weather patterns or even a response to the 11-year solar activity cycle may also have an effect,” he adds.

The team found a tentative correlation between solar activity, stratospheric temperatures, and the number of bright clouds seen on Neptune.

We won’t have to wait too long for a deeper view, thanks to the recently launched James Webb Space Telescope. It’s scheduled to look at Neptune later this year, observations that are being led by team member Leigh Fletcher (also University of Leicester).

“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,” Fletcher says.

Yet even Webb’s observations are no replacement for studying a planet up close. “In the longer term, a mission to the ice giants would provide enormous insight into the atmospheric processes causing this sort of variability,” says Aplin. After all, we’ve only been to Neptune once, and that was a fleeting flyby with Voyager 2 back in August 1989.

Getting to grips with the atmospheric physics of frigid worlds like Neptune becomes particularly relevant when put into a wider context. “Understanding the origin, evolution and behavior of ice giants in the solar system has become more important as exoplanet ice giants have been discovered,” Aplin says.

Clearly, there’s still a lot to learn about the solar system’s outermost planet.

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