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Astronomers perplexed by plummeting temperatures in Neptune's atmosphere

An image of Neptune captured by Voyager 2 in 1989. New infrared images of the planet has revealed some surprising temperature changes in its atmosphere over the last two decades. (Image credit: NASA/JPL)
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Astronomers have discovered a perplexing trend in Neptune's atmosphere: Ever since the planet's southern hemisphere summer began almost two decades ago, atmospheric temperatures in this region have plummeted, and scientists aren't sure why.

Neptune is the most distant planet in the solar system, around 30 times farther from the sun than Earth is. Just like every other planet orbiting the sun, Neptune has four distinct seasons: spring, summer, fall and winter. However, because Neptune takes around 165 years to orbit the sun, each of these seasons lasts around 40 years. Neptune's southern hemisphere has been experiencing summer, the period when it is tilted toward the sun, since 2005.

In a new study, researchers compiled infrared images of Neptune taken by a variety of ground and space based telescopes between 2003 and 2020. The team initially expected that temperatures in Neptune's southern hemisphere would increase as it entered summer. However, the images revealed that atmospheric temperatures in the southern hemisphere had dropped by 14.4 degrees Fahrenheit (8 degrees Celsius) between 2003 and 2018.

"This change was unexpected," lead author Michael Roman, an astronomer at the University of Leicester in the U.K., said in a statement (opens in new tab). "Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder."

Related: 'Diamond rain' on Uranus and Neptune seems likely 

Moreover, in the last two years of the study, temperatures around Neptune's south pole rose by 19.8 F (11 C) between 2018 and 2020. The researchers were puzzled by rapid and intense temperature change and cannot explain why this hotspot is bucking the overall trend in the southern hemisphere.   

"Our data cover less than half of a Neptune season," co-author Glenn Orton, a planetary scientist at NASA's Jet Propulsion Laboratory in California, said in the statement, "so no one was expecting to see large and rapid changes."

Infrared images of Neptune taken in 2006, 2009, 2018 and 2020. There has been an overall decrease in the temperate (brightness) in the southern hemisphere, except for a hotspot near the southern pole which began in 2018. (Image credit: ESO/M. Roman, NAOJ/Subaru/COMICS)
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This is not the first time that Neptune's atmospheric temperatures have perplexed scientists. In 1989, NASA's Voyager 2 probe passed by Uranus and Neptune on its way out of the solar system and found that Neptune was warmer than its closest neighbor despite being farther away from the sun. Scientists have since discovered that this is likely due to gravitational differences between the two planets, Live Science previously reported

The researchers don't yet know what is causing the newly detected temperature fluctuations in Neptune's atmosphere, but they offered some potential explanations.

One possible reason is a change in atmospheric chemistry. Neptune's atmosphere is made mainly of hydrogen, as well as helium and methane. The methane gives Neptune and neighboring Uranus their blue color. However, Neptune's striking hues are more intense than those of Uranus, which likely means that another unidentified chemical lurks in Neptune's atmosphere, according to NASA (opens in new tab). This mysterious compound or changes in the abundance of other elements could be responsible for these temperature changes, according to the statement. 

Extreme weather could also affect temperatures. Neptune has the strongest winds in the solar system; they can reach up to 1,200 mph (1,931 km/h), according to NASA. These winds push gusts of frozen methane through the planet's atmosphere, potentially affecting the temperature. Neptune also has frequent and massive storms. In 1989, Voyager 2 detected a massive storm near the planet's south pole. At its largest, the storm, known as the Great Dark Spot, was bigger than Earth, and  disappeared in 1994. 

The temperature changes may also result from the solar cycle, the researchers said. Every 11 years, the sun's magnetic field flips, altering levels of solar radiation, which scientists can measure by counting sunspots. There is a loose correlation between the temperature changes and the number of sunspots on the sun over time, but the relationship between the two is not strong enough to conclusively support this idea, according to the new study. 

Continued monitoring by ground-based telescopes and future surveys using NASA's new James Webb Space Telescope could shed light on this phenomenon, according to the statement. But for now, it remains a mystery.  

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

The study was published April 11 in The Planetary Science Journal (opens in new tab).

Originally published on Live Science.

Harry is a U.K.-based staff writer at Live Science. He studied Marine Biology at the University of Exeter (Penryn campus) and after graduating started his own blog site "Marine Madness," which he continues to run with other ocean enthusiasts. He is also interested in evolution, climate change, robots, space exploration, environmental conservation and anything that's been fossilized. When not at work he can be found watching sci-fi films, playing old Pokemon games or running (probably slower than he'd like).