A dark spot on Neptune. It has been observed from Earth

Observations of Neptune, the farthest “full-fledged” planet of the Solar System, are not easy – due to the huge distance from the Earth and the Sun, noticing details on the surface of the ice giant requires accurate measuring equipment. Only now scientists have been able to observe a dark spot in its atmosphere for the first time with a ground-based telescope.

An international team of researchers used the Very Large Telescope (VLT) at the Paranal Observatory in Chile, part of the European Southern Observatory (ESO), to observe Neptune. Their target was a dark spot first spotted by the Hubble Space Telescope in 2018. These structures cease to exist after some time, which is why their study is difficult. For the first time this phenomenon was immortalized by the Voyager 2 probe, but the spot disappeared after a few years.

The researchers decided to analyze the stain using the MUSE spectrograph. Thanks to it, it was possible to separate the sunlight reflected by Neptune and its sunspot into component colors corresponding to different wavelengths of light, thus obtaining a spectrum that allowed scientists to carefully analyze the sunspot.

– This is an astonishing increase in humanity’s ability to observe outer space. Initially, we could only detect these spots by sending space probes such as Voyager. Then we got the ability to do it remotely with the Hubble Space Telescope. Finally, the technology has advanced enough that we can do it from the surface of the Earth, said study co-author Michael Wong of the University of California, Berkeley. “This could end my career as a Hubble observer,” he added jokingly.

With the spectrum from MUSE, scientists were able to more accurately pinpoint the height of the spot. What’s more, a surprising result was also obtained: it turned out that right next to the dark spot there was also a bright one. This is an extremely rare type of cloud, immortalized for the first time in the history of observation – it has not been noticed even from space.

Neptune is also a target of study NASA. In August, astronomers described a link between the changing abundance of clouds in the ice giant’s atmosphere and the sun’s activity cycle. When the Sun is more active, its ultraviolet radiation floods the planets intensely. As the researchers noted, after the peak of the solar cycle, more clouds appear on Neptune.

The relationship is surprising to planetary scientists because Neptune receives only 0.1 percent of the sunlight that reaches Earth and is very far from the Sun. Probably for this reason, cloud formation is slightly delayed in time compared to the maximum activity of our star – they appear two years later.