Neptune’s temperatures are fluctuating, and no one knows why

Something strange is happening on Neptune. Researchers have studied 17 years of data from the planet, collected using telescopes including the European Southern Observatory’s Very Large Telescope, NASA’s Spitzer Space Telescope, the Gemini South and North telescopes, the Subaru Telescope, and the Keck Telescope. And they’ve found surprising swings in the planet’s temperatures which they can’t explain.

This composite shows thermal images of Neptune taken between 2006 and 2020. The first three images (2006, 2009, 2018) were taken with the VISIR instrument on ESO’s Very Large Telescope while the 2020 image was captured by the COMICS instrument on the Subaru Telescope (VISIR wasn’t in operation in mid-late 2020 because of the pandemic). After the planet’s gradual cooling, the south pole appears to have become dramatically warmer in the past few years, as shown by a bright spot at the bottom of Neptune in the images from 2018 and 2020. ESO/M. Roman, NAOJ/Subaru/COMICS

Neptune does have seasons, as it rotates and moves around the sun. However, its seasons are much slower than those on Earth, lasting around 40 years. In the southern hemisphere, it has been summer since 2005, so researchers were surprised by the apparent significant variation in temperatures they saw.

“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” said co-author Glenn Orton, senior research scientist at NASA’s Jet Propulsion Laboratory, in a statement.

In a series of observations taken between 2003 and 2018, the global average temperature of the planet dropped by eight degrees Celsius (46 degrees Fahrenheit), and then around the planet’s southern pole, the temperatures rose quickly by 11 degrees Celsius (52 degrees Fahrenheit) between 2018 and 2020. While Neptune is known to host vortices, or areas of atmospheric turbulence, including one at the planet’s southern pole which affects temperatures there, that isn’t enough to explain how temperatures rose so fast. And the overall global cooling is strange too.

“This change was unexpected,” said lead author of the study, Michael Roman of the University of Leicester, U.K. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”

The astronomers were puzzled by their findings and have no clear answer as to what could have caused them. Some of the theories suggested by the European Southern Observatory are that the temperatures changes could be due to something happening in the chemistry of the planet’s atmosphere, or due to a weather phenomenon, or even perhaps due to some kind of effects from the sun.

To learn more, astronomers will need to take more readings of Neptune using powerful upcoming telescopes like the Extremely Large Telescope or the James Webb Space Telescope.

“I think Neptune is itself very intriguing to many of us because we still know so little about it,” says Roman. “This all points towards a more complicated picture of Neptune’s atmosphere and how it changes with time.”

The research is published today in The Planetary Science Journal.