Surprising Changes in Neptune’s Temperatures Detected by Astronomers

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Thermal Images of Neptune

Over a 17-year period, an international team of astronomers monitored Neptune’s atmospheric temperatures. They discovered that Neptune’s global temperatures dropped unexpectedly, followed by a dramatic warming at its south pole. Credit: ESO/M. Novel

Using ground-based telescopes, including the European Southern Observatory’s Thermal Images of Neptune

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 was not 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. Credit: ESO/M. Roman, NAOJ/Subaru/COMICS

Astronomers looked at nearly 100 thermal-infrared images of Neptune, captured over a 17-year period, to piece together overall trends in the planet’s temperature in greater detail than ever before.

These data showed that, despite the onset of southern summer, most of the planet had gradually cooled over the last two decades. The globally averaged temperature of Neptune dropped by 8°C between 2003 and 2018.

The astronomers were then surprised to discover a dramatic warming of Neptune’s south pole during the last two years of their observations, when temperatures rapidly rose 11 °C between 2018 and 2020. Although Neptune’s warm polar vortex has been known for many years, such rapid polar warming has never been previously observed on the planet.

Neptune From VLT and Hubble

The image of the planet Neptune on the left was obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE instrument on ESO’s Very Large Telescope. The image on the right is a comparable image from the NASA/ESA Hubble Space Telescope. Note that the two images were not taken at the same time so do not show identical surface features. Credit: ESO/P. Weilbacher (AIP)/NASA, ESA, and MH Wong and J. Tollefson (UC Berkeley)

“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 (Observed Changes in Neptune's Thermal-Infrared Brightness

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. Corresponding thermal-infrared images (top) at wavelengths of ~12 µm show Neptune’s appearance in 2006, 2009, 2018 (observed by the European Southern Observatory’s Very Large Telescope’s VISIR instrument), and 2020 (observed by Subaru’s COMICS instrument). The south pole appears to have become dramatically warmer in just the past few years. Credit: Michael Roman/NASA/JPL/Voyager-ISS/Justin Cowart

Because Neptune is roughly 4.5 billion kilometers away and is very cold, the planet’s average temperature reaches around –220°C, measuring its temperature from Earth is no easy task. “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.

Around one-third of all the images taken came from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on ESO’s VLT in Chile’s Atacama Desert. Because of the telescope’s mirror size and altitude, it has a very high resolution and data quality, offering the clearest images of Neptune. The team also used data from
The evolution of thermal images taken from Neptune using the VLT’s VISIR instrument. The images, taken between 2006 and 2021, show Neptune gradually cooling down, before a dramatic heating of its south pole in the last few years. Credit: ESO/M. Novel

Because Neptune’s temperature variations were so unexpected, the astronomers do not know yet what could have caused them. They could be due to changes in Neptune’s stratospheric chemistry, or random weather patterns, or even the solar cycle. More observations will be needed over the coming years to explore the reasons for these fluctuations. 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 DOI: 10.3847/PSJ/ac5aa4

The team is composed of MT Roman and LN Fletcher (School of Physics and Astronomy, University of Leicester, UK), GS Orton (Jet Propulsion Laboratory/California Institute of Technology, California, USA), TK Greathouse (Southwest Research Institute, San Antonio , TX, USA), JI Moses (Space Science Institute, Boulder, CO, USA), N. Rowe-Gurney (Department of Physics and Astronomy, Howard University, Washington DC, USA; Astrochemistry Laboratory, NASA/GSFC, Greenbelt, MD , USA; Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, USA), PGJ Irwin (

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