WASHINGTON — Frozen and distant Neptune, the outer planet of our solar system, is adding to its reputation as a mysterious world, as it has baffled astronomers by the sudden drop in atmospheric temperatures over the past two decades.
Focusing on Neptune’s stratosphere – the region of relatively stable atmospheric above the turbulent layer – the researchers expected to find warming in the visible part of the planet with the onset of Southern Hemisphere summer, a four-decade season. Instead, they find temperatures drop dramatically.
The study was based on more than 95 infrared thermal images – every one ever taken – spanning from 2003 to 2020 using ground-based telescopes in Hawaii and Chile, most of which are the European Southern Observatory’s Very Large Telescope. It is the most comprehensive assessment to date of Neptune’s atmospheric temperatures.
said Michael Roman, a postdoctoral researcher at the University of Leicester in England and lead author of a study book published on Monday in Planetary Science Journal.
Neptune’s stratospheric temperature has dropped by 14°F (8°C) to 179°F (minus 117°C) over the 17 years studied. In contrast, temperatures in Neptune’s troposphere – the coldest weather layer – didn’t show much change while reaching minus 370 F (minus 223 C).
Neptune is among the eight least explored planets in the solar system, due to its large distance that makes it difficult to study from Earth. NASA’s Voyager 2 spacecraft is the only spacecraft to make a close visit, flying close to Neptune in 1989.
“I think Neptune is interesting to many of us because we still know so little about it,” Roman said.
Temperature changes were unevenly distributed, with regional differences. The southern tropics cooled, then warmed, and then cooled again. Temperatures in the mid-latitudes initially remained constant before gradually decreasing. Antarctic temperatures initially decreased only slightly before warming dramatically between 2018 and 2020.
“I suspect that the overall decrease in temperature is probably due to changes in atmospheric chemistry, which respond to changing seasonal sunlight, thus altering how effectively the atmosphere cools,” Roman said.
Neptune has an average diameter of about 30,600 miles (49,250 km), making it four times larger than Earth. It orbits more than 30 times as far from the Sun as Earth at an average distance of about 2.8 billion miles (4.5 billion km), and it takes about 165 Earth years to complete one orbit around the Sun—the year of Neptune.
Most of the time, the dwarf planet Pluto orbits at a farther distance, but its elliptical orbit sometimes brings it closer to the Sun than Neptune.
Neptune and neighboring Uranus are classified as ice giants, unlike gas giants Jupiter and Saturn. Neptune, like those other planets that lack a solid surface, has a very dynamic atmosphere mainly of hydrogen and helium, with a small amount of methane, over a mantle mostly of dissolved ammonia and water and a solid core. Neptune boasts stronger winds than any other planet.
Neptune may offer lessons about planets outside our solar system, called exoplanets, study co-author Glenn Orton, a planetary scientist at the California Institute of Technology’s Jet Propulsion Laboratory, said.
The close relationship that Neptune may share with a large segment of the population of exoplanets, said Orton, means that it may be ‘an exoplanet in our backyard’—perhaps at the cooler end of that spectrum, but it’s still typical of things that might We expect to see it in meteorology of different exoplanets.”
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