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Neptune just underwent an unexplained temperature change

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Astronomers who have observed Neptune over the past 17 years with multiple ground-based telescopes have tracked a startling drop in the icy giant’s global temperatures, which was then followed by a dramatic warming trend at the planet’s south pole.

Neptune, which orbits the sun at a distance of 2.8 billion miles (4.5 billion kilometers), experiences seasons like Earth does – they just last much longer. A year on Neptune lasts about 165 Earth years, so a single season can last about 40 years. It has been summer in Neptune’s southern hemisphere since 2005.

Astronomers decided to track the planet’s atmospheric temperatures once the southern summer solstice occurred that year.

Nearly 100 thermal images of Neptune taken since then showed that much of Neptune had gradually cooled, dropping 14 degrees Fahrenheit (8 degrees Celsius) between 2003 and 2018.

A study on the phenomenon published Monday in the Planetary Science Journal.

“This change was unexpected,” said the study’s lead author, Michael Roman, a postdoctoral research associate at the University of Leicester, in a statement. “Since we observed Neptune at the start of the southern summer, we expected temperatures to be slowly warming, not colder.”

Then, a dramatic warming event occurred at Neptune’s south pole between 2018 and 2020 and temperatures rose by 20 degrees Fahrenheit (11 degrees Celsius). This warm polar vortex completely reversed any cooling that had occurred before.

This type of polar warming has never been observed on Neptune until now.

Increasing brightness can be seen at Neptune's south pole between 2018 and 2020, indicating a warming trend.

“Our data covers less than half of a Neptune season, so no one expected to see large, rapid changes,” said study co-author Glenn Orton, principal investigator at the Jet Propulsion Laboratory at the NASA, in a statement.

Images were taken using the European Southern Observatory’s Very Large Telescope and Gemini South Telescope in Chile, as well as the Subaru Telescope in Hawaii, the Keck Telescope and the Gemini North Telescope, along with data from the NASA’s now-retired Spitzer Space Telescope. Infrared light emitted from Neptune’s stratosphere, or the atmospheric band above the active weather layer, has helped astronomers detect temperature fluctuations.

Frosty Neptune has an average of minus 340 degrees Fahrenheit (negative 220 degrees Celsius), and astronomers still don’t know what caused these temperature changes.

For now, they considered that the unexpected changes could be due to a number of factors.

“Temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which may alter the cooling efficiency of the atmosphere,” Roman said. “But random variability in weather patterns or even a response to the 11-year solar activity cycle can also have an effect.”

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Further observations will be needed to truly explore these possibilities. The James Webb Space Telescope will observe Uranus and Neptune later this year. The space observatory’s mid-infrared instrument can map the chemistry and temperatures in Neptune’s atmosphere and could pinpoint the cause of the change.

Neptune is more than 30 times farther from the sun than Earth, and it is the only planet in our solar system that is not visible to the naked eye from Earth. So far, only NASA’s Voyager 2 spacecraft has flown close to Neptune, which happened in 1989.

“I think Neptune itself is very intriguing to a lot of us because we still know so little about it,” Roman said. “All of this points to a more complicated picture of Neptune’s atmosphere and how it changes over time.”