观察海王星的科学家称这颗行星正在变冷

Researchers say new observations of Neptune suggest the planet is getting colder.

An international team of astronomers used ground-based telescopes in Hawaii and Chile to estimate Neptune's atmospheric temperatures. The team examined nearly 100 infrared images captured by the telescopes from 2003 to 2020. The data showed that Neptune experienced a surprising drop in atmospheric temperatures over the past 17 years.

Considered an "ice giant," Neptune is the outermost planet in our solar system. It is about four times wider than Earth. Neptune orbits more than 30 times as far away from the sun as Earth does. This means it needs about 165 Earth years to complete a single orbit around the sun.

Neptune is among the least explored planets. The American space agency NASA's Voyager 2 is the only spacecraft to have passed close to Neptune, which it did in 1989.

The latest examinations of Neptune's atmospheric temperatures are the most detailed ever collected. The team recently reported its findings in the publication Planetary Science Journal.

The researchers centered on Neptune's stratosphere, an upper part of the atmosphere just above the planet's weather layer. Neptune's stratosphere temperature fell to minus 117 Celsius, a drop of eight degrees, during the 17-year period. By comparison, temperatures in Neptune's troposphere – the even-colder weather layer – showed no major changes. The troposphere reached temperatures as low as minus 223 Celsius.

"This change was unexpected," said Michael Roman of the stratospheric readings. He is a postdoctoral researcher at the University of Leicester in Britain and the lead writer of the study. "I think Neptune is very intriguing to many of us because we still know so little about it," Roman added.

He noted that the Neptune observations led researchers to believe that conditions on the planet were more complex than first thought. Roman said the finding "seems to be a general lesson that nature teaches scientists again and again."

The astronomers collected Neptune temperatures using cameras equipped to measure infrared light from space objects, the European Southern Observatory (ESO) said in a statement. Most of the team's observations were made with the ESO's Very Large Telescope, which is based in northern Chile.

The temperature changed over time and unevenly. The area of the planet known as the southern tropics cooled, then warmed, then cooled again. In the middle areas, temperatures remained similar before starting to fall over time. Temperatures at the south pole showed only minor drops during most of the period, the researchers reported. But then, the area warmed quickly and sharply. From 2018 to 2020, the temperature at the south pole rose by 11 degrees.

"I suspect the overall temperature drop may most likely be due to changes in the atmospheric chemistry, which responds to changing seasonal sunlight," Roman said.

Neptune may offer lessons about planets beyond our solar system, called exoplanets, said study co-author Glenn Orton. He is a planetary scientist at NASA's Jet Propulsion Laboratory in California.

"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,'" Orton said. He added that Neptune could become "a model for the things we might expect to see in the meteorology" of exoplanets studied in the future.

I'm Bryan Lynn.

Reuters and the European Southern Observatory reported on this story. Bryan Lynn adapted the reports for VOA Learning English.

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giant – n. extremely large

layer – n. the amount of a substance covering a surface

intriguing – adj. very interesting

tropics – n. the hottest part of the world

meteorology – n. the science of planetary atmosphere and weather

segment – n. one of the parts something can be divided into