Neptune is cooler than you thought

An international team of astronomers used ground-based telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track Neptune’s atmospheric temperatures over a 17-year period. They found a startling drop in Neptune’s global temperatures followed by dramatic warming at its south pole.

“This change was unexpected,” says Michael Roman, postdoctoral research associate at the University of Leicester, UK, and lead author of the study published today in The Journal of Planetary Science. “Since we observed Neptune at the start of the southern summer, we expected temperatures to be slowly warming, not colder.”

Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts about 40 years, with a Neptune year lasting 165 Earth years. It’s been summer in Neptune’s southern hemisphere since 2005, and astronomers were eager to see how temperatures changed after the southern summer solstice.

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

These data showed that, despite the onset of the austral summer, most of the planet had gradually cooled over the past two decades. Neptune’s global average temperature dropped by 8°C between 2003 and 2018.

Astronomers were then surprised to discover a dramatic warming of Neptune’s south pole in the last two years of their observations, when temperatures rapidly increased by 11°C between 2018 and 2020. Although Neptune’s warm polar vortex is known for many years, such polar velocity warming has never before been observed on the planet.

“Our data spans less than half of a Neptune season, so no one expected to see big, fast changes,” says co-author Glenn Orton, principal investigator at Caltech’s Jet Propulsion Laboratory (JPL) in the United States.

Astronomers measured Neptune’s temperature using thermal imaging cameras that work by measuring infrared light emitted by astronomical objects. For their analysis, the team combined all existing images of Neptune collected over the past two decades by ground-based telescopes. They studied the infrared light emitted by a layer of Neptune’s atmosphere called the stratosphere. This allowed the team to get an idea of ​​Neptune’s temperature and its variations during part of its southern summer.

Because Neptune is about 4.5 billion kilometers away and very cold, with the planet’s average temperature reaching 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 clearly observe Neptune, and these have only been available for about 20 years,” says co-author Leigh Fletcher, a professor at the University of Leicester.

About a third of all images taken came from the mid-infrared VLT imager and spectrometer (VISIT) on ESO’s VLT in the Atacama Desert in Chile. Due to the size and altitude of the telescope mirror, it has a very high resolution and data quality, delivering the clearest images of Neptune. The team also used data from NASA’s Spitzer Space Telescope and images taken with the Gemini South Telescope in Chile, as well as the Subaru Telescope, Keck Telescope and Gemini North Telescope, all in Hawaii.

Because Neptune’s temperature swings were so unexpected, astronomers don’t yet know what caused them. They could be due to changes in Neptune’s stratospheric chemistry, or random weather conditions, or even the solar cycle. Further observations will be needed over the next few 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 more detail, while NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere.

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

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