Frigid and far-flung Neptune, our solar system’s outermost planet, is adding to its reputation as an enigmatic world, with astronomers puzzled by a surprising drop in its atmospheric temperatures during the past two decades.
- More than 95 thermal-infrared images were used to analyze Neptune’s atmospheric temperatures
- Researchers expected to find rising temperatures, but found the opposite
- They believe Neptune may offer lessons about planets beyond our solar system, called exoplanets
The scientists focused on Neptune’s stratosphere — the atmosphere’s relatively stable region above the turbulent weather layer.
They had expected to find it heating up on the side facing Earth as Neptune’s southern hemisphere summer, which lasts four decades, got underway.
Instead, they found temperatures declining significantly.
Neptune’s stratosphere temperature fell as much as 8 degrees Celsius to minus 117C over the 17 years studied.
In contrast, temperatures in Neptune’s troposphere — the even colder weather layer — showed no significant variability while reaching as low as minus 223C.
Most comprehensive study to date
The scientists’ assessments are based on more than 95 thermal-infrared images, taken between 2003 and 2020 using ground-based telescopes mainly in Hawaii and Chile.
It is the most comprehensive study to date of Neptune’s atmospheric temperatures.
“The atmosphere appears more complicated than we had naively assumed, which, unsurprisingly, seems to be a general lesson that nature teaches scientists again and again,” said Michael Roman, a postdoctoral researcher at the University of Leicester in England and lead author of the study, published in the Planetary Science Journal.
Neptune is among the least-explored of the solar system’s eight planets, with its great distance making it difficult to study from Earth.
NASA’s Voyager 2 is the only spacecraft to have made a close-up visit, flying past Neptune in 1989.
The temperature changes were unevenly distributed, with regional variations. The southern tropics cooled, then warmed, then cooled again.
Mid-latitude temperatures initially remained constant before falling gradually.
South pole temperatures initially dropped only slightly before warming dramatically between 2018 and 2020.
“I suspect the overall temperature drop may most likely be due to changes in the atmospheric chemistry, which responds to changing seasonal sunlight and, in turn, alters how effectively the atmosphere cools,” Dr Roman said.
What Neptune could teach us
Neptune’s average diameter is about 49,250 kilometers, making it four times wider than Earth.
It orbits more than 30 times as far away from the Sun as Earth at an average distance of about 4.5 billion kilometers, needing about 165 Earth years to complete a single orbit around the Sun — a Neptunian year.
Neptune and its neighbor Uranus are classified as ice giants, as opposed to the gas giants Jupiter and Saturn.
Neptune, which like those other planets lacks a solid surface, possesses an extremely dynamic atmosphere mainly of hydrogen and helium, with a small amount of methane, atop a mantle mostly of slushy ammonia and water and a solid core.
Neptune also boasts the strongest winds of any planet.
The ice giant may offer lessons about planets beyond our solar system, called exoplanets, said study co-author Glenn Orton, a planetary scientist at Caltech’s Jet Propulsion Laboratory.
“The close relationship that Neptune may share with a large segment of the population of exoplanets means that it may be an exoplanet in our backyard,” Dr Orton said.
“Probably on the colder end of that spectrum, but still a model for the things we might expect to see in the meteorology of various exoplanets.”