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View of Neptune from the Voyager 2 probe
Photo: Kevin M. Gill / JPL-Caltech / NASA
Climate change has made it clear to mankind how vulnerable the earth is.
But other planets are apparently also subject to dramatic climatic changes - such as Neptune.
On the surface of the outermost planet in our solar system, the temperature has changed unusually sharply in a short period of time.
This is shown by the evaluation of observation data.
However, things are going in the opposite direction here than on Earth: because while we are threatened with global warming, Neptune has cooled down by eight degrees globally.
At Neptune's South Pole, on the other hand, an international team of researchers recorded a different trend - here the temperature rose by eleven degrees, contrary to this trend.
Why is that?
So far, there is no explanation for these temperature changes, but they could be influenced by the activity of the sun, the scientists write in the "Planetary Science Journal".
The astronomers have known for a long time that it is quite cold on the ice giant Neptune, which is almost four times the diameter of Earth, with an average surface temperature of minus 220 degrees Celsius.
"But the observed changes came as a complete surprise to us," says Michael Roman from the British University of Leicester.
The temperatures were actually measured at the beginning of summer in Neptune's southern hemisphere.
Therefore, one expected a slow warming of the planet, not a cooling.
Neptune orbits the sun at a distance of about 4.5 billion kilometers.
For comparison: the earth is 150 million kilometers away from the sun.
The eighth planet is correspondingly cooler.
Seasonal fluctuations in Neptune's temperature are also not surprising, because like Earth, Neptune's axis of rotation is tilted against its orbital plane.
It is this tilt -- Earth's 23.5 degrees and Neptune's 28.3 degrees -- and not the change in distance from the Sun, that gives rise to seasons.
Observations in the infrared range
If the north pole of the earth points towards the sun, it is summer in the northern hemisphere - and so is Neptune.
Since the planet's orbital period around the sun is 165 Earth years, the seasons there last correspondingly longer, namely around 40 years each.
In 2005, at the beginning of the observation series, summer was just beginning in the southern hemisphere.
"But our data covers just half of the season, so we didn't expect big changes," says Glenn Orton of the Jet Propulsion Laboratory in the US.
For their analysis, the researchers used observations in the infrared range - because infrared radiation is thermal radiation: the warmer a celestial body is, the more infrared light it emits.
The data from the years 2003 to 2020 come from a number of large telescopes on Earth, including the Very Large Telescope of the European Southern Observatory ESO in Chile, the large Keck telescope in Hawaii and also NASA's Spitzer Space Telescope.
And although summer was just beginning, Neptune cooled down by eight degrees globally from 2003 to 2018.
Even more surprising, however, was a sudden temperature increase of 11 degrees at the planet's south pole between 2018 and 2020. While it has long been known that there is an atmospheric vortex at Neptune's south pole that is warmer than the rest of the planet, such a rapid increase in temperature astronomers had never observed there
Now the planetary researchers are puzzling over what could be the cause of the surprising changes in temperature.
Are they random weather phenomena that are independent of the seasons?
Or does the chemistry in the planet's atmosphere change seasonally, increasing temperature fluctuations?
Another possibility would be an influence of solar activity.
In fact, the data show a slight correlation between the temperature of Neptune's atmosphere and the number of sunspots.
With further observations over the coming years, the researchers hope to track down the cause of the temperature changes.
joe/dpa