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Neptune is Cooler than Astronomers Thought


Using data from ESO’s Very Large Telescope (VLT), NASA’s Spitzer Space Telescope, the Subaru Telescope, the Keck Telescope, the Gemini South and North telescopes, astronomers have revealed a more complete picture of trends in Neptune’s temperatures than ever before.

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. Image credit: ESO / M. Roman / NAOJ / Subaru Telescope / COMICS.

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. Image credit: ESO / M. Roman / NAOJ / Subaru Telescope / COMICS.

Despite being the most distant giant planet in the Solar System, Neptune possesses an extremely dynamic atmosphere.

Like Earth, the ice giant experiences seasons as it orbits the Sun. However, a Neptune season lasts around 40 years, with one Neptune year lasting 165 Earth years.

It has been summertime in Neptune’s southern hemisphere since 2005, and University of Leicester astronomer Michael Roman and his colleagues were eager to see how temperatures were changing following the southern summer solstice.

They looked at nearly 100 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 degrees Celsius 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 degrees Celsius 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 giant planet.

“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” said Dr. Glenn Orton, an astronomer at NASA’s Jet Propulsion Laboratory.

For their study, the researchers combined all existing images of Neptune gathered over the last two decades by ground-based telescopes.

They investigated infrared light emitted from a layer of Neptune’s atmosphere called the stratosphere.

This allowed the team to build up a picture of Neptune’s temperature and its variations during part of its southern summer.

Because Neptune is roughly 4.5 billion km away and is very cold, the planet’s average temperature reaching around minus 220 degrees Celsius, measuring its temperature from Earth is no easy task.

“This type of study is only possible with sensitive infrared images from large telescopes like VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so,” said Professor Leigh Fletcher, an astronomer at the University of Leicester.

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.

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

The findings were published in the Planetary Science Journal.

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Michael T. Roman et al. 2022. Subseasonal Variation in Neptune’s Mid-infrared Emission. Planet. Sci. J 3, 78; doi: 10.3847/PSJ/ac5aa4



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