Scientists had expected the planet to slowly grow warmer, not colder
The average atmospheric temperature of Neptune fell by 8 degrees Celsius (°C) during 2003-2020, according to a new study.
The period covers the first half of the summer on the planet that started in 2005. Each season on the Neptune lasts 40 years.
The atmospheric temperature of the eighth planet of the solar system fell to -117°C from -109°C between 2003 and 2018, according to the analysis published in The Planetary Science Journal.
Researchers from the University of Leicester, United Kingdom and NASA observed the planet’s temperature from 2003-2020 to understand how its seasons evolve with time.
The researchers relied on a fleet of ground-based telescopes – Chile’s Very Large Telescope and Gemini South telescope, Hawaii’s Subaru Telescope, Keck Telescope, and the Gemini North telescope as well as a space telescope named Spitzer Space Telescope to study Neptune.
The telescopes were equipped with thermal cameras with infrared eyes. The cameras calculate temperature by measuring the infrared light emitted from objects in the stratosphere — a layer of the planet’s atmosphere.
“We expected temperatures to be slowly rise, not dip,” Michael Roman, a postdoctoral research associate at the University of Leicester and lead author of the study, told Down To Earth.
The researchers, however, have not ruled out the possibility that Neptune experienced a similar drop in previous summers.
Roman said:
Reliable observations of the sorts we used only go back to 2003. The conditions before then are unknown. There have been suggestions of cooler temperatures before this, but these older data are sparse and not particularly reliable.
The analysis threw up some more surprising results: Between 2018 and 2020, temperatures in the planet’s southern pole rose by 11°C. As a result, the stratospheric temperature showed a slight uptick, Roman added.
The rapid changes observed in the two years are surprisingly swift for a seasonal response, the researchers noted.
The unexpected reversal of the trend in the polar region suggests some interesting atmospheric dynamics are at play, said Heidi B Hammel, the executive vice-president of the Association of Universities for Research in Astronomy.
The researchers provided a few theories for the factors driving Neptune’s fluctuating temperatures.
Seasonal changes in Neptune’s atmospheric chemistry could have a role to play, they surmised.
Alternatively, the solar cycle – variation in the Sun’s activity every 11 years – might also be involved, the researchers wrote. Previous studies also suggested that the solar cycle might affect Neptune’s visible brightness, according to the report.
Research in the future should focus on observing temperature and cloud patterns, the study stressed.
The researchers have their hopes pinned on the James Webb Space Telescope launched in December 2021 to provide answers to this mystery.
“The new results give a taste of what we can expect to see with the superb infrared sensitivity of the James Webb Space Telescope when it begins science operations later this year,” Hammel said.
But truly understanding the planet requires an orbiter mission, she said. NASA’s Voyager 2 was the only spacecraft that paid a visit to this icy planet in 1989.
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