Temperatures at its south pole are about 18 degrees warmer than elsewhere on the planet -- not much for a planet with an average temperature colder than 320 degrees below zero.
The apparent reason is that the south pole has been in the summer sunlight for about 40 years.
Neptune is nearly 2.8 billion miles away from the sun. A Neptunian year -- the time it takes to orbit the sun -- is equivalent to about 165 Earth years.
One result of that has been to expose Neptune's southern pole to the sun for nearly 40 years, warming it up. Because it is so far away, Neptune gets only 1/900th of the sunlight that Earth receives, but it still appears to have had a significant impact.
An array of scientific organizations announced the findings Tuesday, including the government-funded CNRS research body in France and ESO, the European Organization for Astronomical Research in the Southern Hemisphere, whose telescope in Chile was used.
"Neptune's south pole is currently tilted toward the sun, just like the Earth's south pole is tilted toward the sun during summer in the southern hemisphere," astronomer Glenn Orton, lead author of the findings, said in an ESO news release.
"On Neptune, the Antarctic summer lasts 40 years instead of a few months, and a lot of solar energy input during that time can make big temperature differences between the regions in continual sunlight and those with day-night variations," he said.
The heat has, in turn, defrosted methane normally trapped as ice in the planet's atmosphere, releasing it as gas, the CNRS said. It said there is eight times more methane over the south pole than in the rest of the planet's atmosphere.
The abundance of gaseous methane in Neptune's stratosphere had gone unexplained until now, it added.
Despite its distance from sun, "the atmosphere of Neptune is a scene of great activity" -- perhaps more so than those of Jupiter or Saturn, even though they are closer, CNRS said.
