The warming of the planet is one of the greatest challenges facing humanity. But temperatures are not only changing on Earth. Astronomers have made a surprising discovery on Neptune.
The eighth planet Neptune is a gas planet like Jupiter and Saturn. It is bitterly cold on it, for Neptune receives little heat from the Sun as it orbits at a great distance from it. With temperatures between -201°C and -218°C, Neptune is one of the coldest places in the solar system. Astronomers have now made an amazing discovery.
For 17 years, an international team of astronomers has been studying Neptune with ground-based telescopes. Among other things, this captured Very Large Telescope (VLT) of the European Southern Observatory (ESO) the temperatures in the atmosphere of the gas planet. They discovered a surprising drop in global temperatures and dramatic warming at Neptune’s south pole.
“This change was unexpected,” says Michael Roman, Research Associate at the University of Leicester, UK. “Since we observed Neptune during its southern early summer, we expected temperatures to increase slowly, not decrease.”
Temperatures on Neptune are falling
Spring, summer, fall, and winter—like Earth, Neptune experiences seasons as it orbits the Sun. However, the duration of a season differs significantly from Earth. Neptune stays in a summer or winter time for 40 years. A Neptune year lasts 165 Earth years.
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Daylight saving time has been in effect in Neptune’s southern hemisphere since 2005. The astronomers studied how temperatures change after the southern summer solstice. They evaluated 100 thermal images from 17 years.
The result shows that despite the onset of southern summer, most of the planet has gradually cooled over the past two decades. Neptune’s global mean temperature dropped by 8°C between 2003 and 2018. But the astronomers made another amazing discovery at the same time.
Amazing Warming From Neptune’s South Pole
The team was surprised to discover dramatic warming of Neptune’s south pole in the last two years of their analysis. Here, temperatures rose by 11 °C between 2018 and 2020. Even the planet’s warm polar winds do not justify this rapid warming of the pole. Such rapid warming of the pole has never been observed on Neptune, according to the scientists.
“Our data covers less than half of a Neptune season, so nobody expected to see large and rapid changes,” says co-author Glenn Orton, principal investigator at Caltech’s Jet Propulsion Laboratory (JPL) in the US.
Analysis with infrared light
The measurements were obtained using thermal imaging cameras that analyze the infrared light emitted by astronomical objects. For the study, the scientists looked at infrared light emitted by a layer of Neptune’s atmosphere, the stratosphere. In this way, the team got a picture of Neptune’s temperature and how it varied during part of its southern summer. In general, measuring the temperature is a challenge due to the distance (4.5 billion kilometers).
“This type of study is only possible with sensitive infrared images from large telescopes like the VLT, which can observe Neptune precisely, and these have only been available for about 20 years,” says co-author Leigh Fletcher, a professor at the University of Leicester.
The VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on the VLT delivered very clear images ESO in the Chilean Atacama Desert. The size of the mirror and the altitude of the telescope provide very high resolution and data quality.
Neptune: cause of the temperature fluctuations unclear
The temperature fluctuations of Neptune are unexpected for the researchers, so they cannot yet name the reason. One cause may be the changes in Neptune’s stratospheric chemistry. But the astronomers also do not rule out random weather patterns or the solar cycle.
In the future, telescopes such as ESO’s Extremely Large Telescope (ELT) will observe temperature fluctuations even more precisely. That James Webb Space Telescope is intended to provide new maps of the chemistry and temperature in Neptune’s atmosphere.
“I think Neptune itself is very fascinating to many of us because we know so little about it,” says Roman. “All of this points to a complex makeup of Neptune’s atmosphere and how it changes over time.”