Neptune, the eighth and final planet from the sun, is known for its trails of wispy white clouds made up of crystals of frozen methane.
Strong winds whip these clouds across the ice giant at speeds of more than 1,200 mph – the fastest recorded anywhere in the solar system.
But a new study shows they have now all but vanished, in a development that briefly baffled scientists.
Experts have since discovered that the clouds disappear and reappear according to where the sun is in its 11-year cycle – when its magnetic field flips.
They discovered this after studying images from the Hubble Space Telescope dating back to 1994.
It was in 1989 that NASA’s Voyager 2 spacecraft provided the first close-up images of linear, bright clouds – reminiscent of cirrus clouds on Earth – high in Neptune’s atmosphere. Pictured, a view of Neptune from Voyager 2, 1998
For the first time in nearly three decades of observations, clouds seen on Neptune have all but vanished. This sequence of Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune
Neptune: The most distant planet in our solar system
Dark, cold, and whipped by supersonic winds, ice giant Neptune is the eighth and most distant planet in our solar system.
More than 30 times as far from the Sun as Earth, Neptune is the only planet in our solar system not visible to the naked eye and the first predicted by mathematics before its discovery. In 2011 Neptune completed its first 165-year orbit since its discovery in 1846.
NASA’s Voyager 2 is the only spacecraft to have visited Neptune up close. It flew past in 1989 on its way out of the solar system.
A new study describing the findings – led by astronomers at the University of California, Berkeley – has been published in the journal Icarus.
‘I was surprised by how quickly clouds disappeared on Neptune,’ said Imke de Pater, emeritus professor of astronomy at UC Berkeley.
‘We essentially saw cloud activity drop within a few months.’
Neptune, the fourth largest planet in our solar system, is an ice giant – a huge planet made up of a thick soup of water, methane and ammonia, which scientists refer to as ‘ices’.
Above this, in its upper atmosphere, are the planets distinctive swirling clouds, which reflect all the colours of the spectrum in the sun’s light, making them white.
It was in 1989 that NASA’s Voyager 2 spacecraft provided the first close-up images of these bright clouds – reminiscent of cirrus clouds on Earth – high in Neptune’s atmosphere.
Wrapped in teal- and cobalt-colored bands of clouds, the planet looked like a blue-hued sibling to Jupiter and Saturn, the blue indicating the presence of its methane.
To monitor the evolution of Neptune’s clouds, researchers analysed images from Hubble.
They also studied data from California’s Lick Observatory between 2018 and 2019 and the Keck Observatory in Hawaii from 1994 to 2022.
They found an abundance of clouds normally seen at the icy giant’s mid-latitudes started to fade in 2019 – and since then they haven’t gone back to how they were.
From late 2019 onwards, only the south pole showed cloud activity.
‘Even now, four years later, the most recent images we took this past June still show the clouds haven’t returned to their former levels,’ said Erandi Chavez at Harvard’s Center for Astrophysics in Cambridge, Massachusetts.
A nearly-30-year-long set of observations shows that the number of clouds grows increasingly following a peak in the solar cycle – where the sun’s level of activity rhythmically rises and falls over an 11-year period. The sun’s level of ultraviolet radiation is plotted in the vertical axis. The 11-year cycle is plotted along the bottom from 1994 to 2022. The Hubble observations along the top, clearly show a correlation between cloud abundance and solar peak of activity
Pictured, images from Keck Observatory (top two rows) and Hubble (bottom row) that display Neptune’s characteristic appearance throughout the three decades worth of data
‘This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged.’
Data also revealed a connection between Neptune’s disappearing clouds and the solar cycle – the period when the sun’s magnetic field flips every 11 years, causing levels of solar radiation to fluctuate.
This was surprising because Neptune is the farthest planet from the sun and doesn’t receive much sunlight – around only 1/900th of the sunlight we get on Earth.
The team found that two years after the solar cycle’s peak, an increasing number of clouds appear on Neptune.
It’s thought the sun’s UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds.
The team further found a positive correlation between the number of clouds and the ice giant’s brightness from the sunlight reflecting off it.
When the planet’s reflectivity reached its lowest level ever observed in 2020, most of the clouds went away.
The study heavily suggests Neptune’s global cloudy weather is driven by solar activity and not the planet’s four seasons, which each last approximately 40 years.
The Hubble telescope was launched on April 24, 1990, via the space shuttle Discovery from Kennedy Space Centre in Florida
More than 30 times as far from the sun as Earth, Neptune is the only planet in our solar system not visible to the naked eye
The data revealed an intriguing pattern between changes in Neptune’s cloud cover and the solar cycle – the period when the sun’s magnetic field flips every 11 years, causing levels of solar radiation to fluctuate
‘Our data provide the strongest evidence to date that the discrete cloud coverage appears correlated with the solar cycle,’ the team say in their paper.
Further observations of Neptune are also needed to see how long the current near-absence of clouds will last, they add.
This may help deepen understanding not only of Neptune but also of exoplanets – planets outside of our solar system.
This is because exoplanets are thought to have Neptune-like qualities, such as a rocky core surrounded by a thick atmosphere of hydrogen and helium.
One third of planets circling red dwarf stars in our galaxy could be in the ‘habitable zone’ – and may host alien life, study claims
Finding life on other planets has long been one of the biggest quests for astronomers.
Now, a new study suggests the Milky Way galaxy has hundreds of millions of promising targets to probe for signs of life outside our solar system.
Using NASA’s Kepler telescope, researchers studied a small sample of planets that orbit red dwarfs – low mass stars that are common in our galaxy.
They found that a third of the planets – equating to hundreds of millions in the Milky Way overall – likely have the right conditions for hosting life.
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