Located 2.8 billion miles away from the Sun, Neptune is understandably chilly. But a new study shows that temperatures on the eighth planet from the Sun might be even colder—and more variable—than scientists originally thought. Thanks to new technology only available in the past 20 years, researchers are getting more insight into this icy giant and its temperatures.
A group of international researchers discovered that between 2003 and 2018, the planet cooled by 14 degrees Fahrenheit (8 degrees Celsius). They were then even more surprised to see that between 2018 and 2020, Neptune's south pole warmed up dramatically—by 20 degrees Fahrenheit (11 degrees Celsius) to be exact.
To understand what makes these discoveries so interesting, we need to know a bit more about Neptune. One of the coldest planets in the solar system, Neptune's average temperature is a chilling -373°F (-207°C). And while Neptune, like Earth, has distinct seasons, they move much more slowly. While on Earth we cycle through our seasons in 365 days, it takes Neptune about 165 years to complete one orbit around the Sun. This means that a single season can last 40 years.
Since 2005, it has been summertime in the planet's southern hemisphere and it was these temperatures that the researchers were interested in tracking. In order to learn more about its summer temperatures, the researchers pored through 17 years of data taken from telescopes around the world. By looking at thermal images, they were able to determine these drastic changes in temperatures.
“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study published in The Planetary Science Journal. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”
As for the rapid warming of the south pole, while the researchers were aware of a warm polar vortex, such a sudden rise in temperatures had not previously been observed. “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, a senior research scientist at Caltech's Jet Propulsion Laboratory (JPL) in the U.S.
Due to its distant location, Neptune is still quite mysterious. Thanks to advances in technology, researchers have only recently been able to grapple with the planet's temperatures. “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.
Most of the images in the study came from the European Southern Observatory's Very Large Telescope (VLT) located in Chile's Atacama Desert. The incredible size and altitude of the telescope's mirror allow it to take high-resolution images of Neptune. And researchers are hopeful that other cutting-edge instruments, like the James Webb Telescope, will allow them to get even more information about the distant planet so that they can better understand its temperature fluctuations.