Why Neptune’s clouds have VANISHED: Ice giant’s overcast conditions are linked to the sun’s 11-year cycle, scientists discover
Neptune, the eighth and last planet from the sun, is known for its trails of wispy white clouds made up of crystals of frozen methane.
Strong winds blow these clouds over the icy giant at speeds in excess of 2,000 km/h – the fastest recorded anywhere in the solar system.
But a new study shows they’ve all but disappeared now, in a development that briefly baffled scientists.
Experts have since discovered that the clouds disappear and reappear depending on where the sun is in its 11-year cycle.
They discovered this after studying images from the Hubble Space Telescope from 1994.
It was in 1989 that NASA’s Voyager 2 spacecraft captured the first close-up images of linear, bright clouds — reminiscent of Earth’s cirrus clouds — high in Neptune’s atmosphere. Pictured is a view of Neptune from Voyager 2, 1998
For the first time in nearly three decades of observations, clouds on Neptune have all but disappeared. This sequence of images from the Hubble Space Telescope shows the increase and decrease in the amount of cloud cover on Neptune
A new study describing the findings – led by astronomers at the University of California, Berkeley – is published in the journal Icarus.
“I was surprised how quickly clouds on Neptune cleared,” says Imke de Pater, professor emeritus of astronomy at UC Berkeley.
“We saw cloud activity essentially decline within a few months.”
Neptune, the fourth-largest planet in our solar system, is an icy giant—a massive planet made up of a thick soup of water, methane, and ammonia that scientists call “ice.”
Above it, in the upper atmosphere, are the planet’s signature swirling clouds, which reflect all the colors of the spectrum in the sunlight, turning them white.
It was in 1989 that NASA’s Voyager 2 spacecraft captured the first close-up images of these bright clouds — reminiscent of Earth’s cirrus clouds — high in Neptune’s atmosphere.
Wrapped in teal and cobalt cloud bands, the planet resembled a blue-hued sibling of Jupiter and Saturn, with the blue indicating the presence of its methane.
To track the evolution of Neptune’s clouds, researchers analyzed images from Hubble.
They also studied data from the Lick Observatory in California between 2018 and 2019 and the Keck Observatory in Hawaii from 1994 to 2022.
They found that a plethora of clouds normally seen in the icy giant’s mid-latitudes began to fade in 2019 — and they haven’t gone back to the way they were since.
As of late 2019, only the South Pole showed cloud activity.
“Even now, four years later, the most recent images we took last June still show that the clouds have not returned to their former levels,” said Erandi Chavez of Harvard’s Center for Astrophysics in Cambridge, Massachusetts.
A nearly 30-year series of observations shows that the number of clouds continues to increase after a peak in the solar cycle – where the sun’s activity level rises and falls rhythmically over an 11-year period. The level of ultraviolet radiation from the sun is plotted on 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 the solar activity peak
Pictured, images from Keck Observatory (top two rows) and Hubble (bottom row) showing Neptune’s characteristic appearance over the three decades of data
“This is extremely exciting and unexpected, especially since the previous period of low clouds on Neptune was not nearly as dramatic and long-lasting.”
Data also revealed a link between Neptune’s vanishing clouds and the solar cycle — the period when the sun’s magnetic field flips every 11 years, causing solar radiation levels to fluctuate.
This was surprising because Neptune is the farthest planet from the sun and doesn’t receive much sunlight – about only 1/900th of the sunlight we get on Earth.
The team found that more and more clouds appear on Neptune two years after the peak of the solar cycle.
It is thought that the sun’s UV rays, when strong enough, can trigger a photochemical reaction that produces Neptune’s clouds.
The team also found a positive correlation between the number of clouds and the brightness of the ice giant due to the sunlight reflecting off it.
When the planet’s reflectivity reached an all-time low in 2020, most of the clouds disappeared.
The study strongly suggests that Neptune’s global cloudy weather is driven by solar activity and not by the planet’s four seasons, which each last about 40 years.
The Hubble telescope was launched on April 24, 1990 via the space shuttle Discovery from the Kennedy Space Center in Florida.
More than 30 times farther from the sun than 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 solar radiation levels to fluctuate.
“Our data provide the strongest evidence to date that the discrete cloud cover appears to be correlated with the solar cycle,” the team says 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 can help to deepen the understanding not only of Neptune, but also of exoplanets – planets beyond our solar system.
This is because exoplanets are thought to have Neptune-like properties, such a rocky core surrounded by a thick atmosphere of hydrogen and helium.