Neptune, the eighth and last planet from the sun, is known for its streaks of wispy white clouds composed of frozen methane crystals.
Strong winds whip these clouds through the icy giant at speeds of over 1,200 mph – the fastest ever recorded in the solar system.
But a new study shows they’ve now all but disappeared, in a development that briefly baffled scientists.
Experts have since discovered that clouds disappear and reappear based on the position of the sun in its 11-year cycle.
They discovered it after studying Hubble Space Telescope images from 1994.
It was in 1989 that NASA’s Voyager 2 spacecraft provided the first close-up images of bright, linear clouds – reminiscent of cirrus clouds on Earth – 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, the clouds seen on Neptune have all but disappeared. This sequence of images from the Hubble Space Telescope chronicles the increase and decrease in the amount of cloud cover on Neptune
A new study outlining the findings – led by astronomers at the University of California, Berkeley – has been published in the journal Icarus.
“I was surprised by how quickly the clouds disappeared on Neptune,” said Imke de Pater, professor emeritus of astronomy at UC Berkeley.
“We’ve basically seen cloud activity drop in a matter of 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 call “ice”.
Above, in its upper atmosphere, are the planet’s distinctive swirling clouds, which reflect all the colors of the sunlight spectrum, turning them white.
It was in 1989 that NASA’s Voyager 2 spacecraft provided the first close-up images of these brilliant clouds – reminiscent of cirrus clouds on Earth – high in Neptune’s atmosphere.
Shrouded in bands of teal and cobalt-colored clouds, the planet looked like a blue sister to Jupiter and Saturn, with blue indicating the presence of its methane.
To follow the evolution of Neptune’s clouds, the 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 an abundance of clouds normally seen in the icy giant’s mid-latitudes began to fade in 2019 – and since then they haven’t returned to where they were.
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 previous levels,” Erandi Chavez of the Harvard Center for Astrophysics told Cambridge, MA.
A series of observations spanning almost 30 years show that the number of clouds increases more and more 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 at the bottom from 1994 to 2022. Hubble observations at the top clearly show a correlation between cloud abundance and peak solar activity
Pictured are images from the Keck Observatory (top two rows) and Hubble (bottom row) that display Neptune’s characteristic appearance throughout the three decades of data
“This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not as dramatic and prolonged.”
The data also revealed a link between the disappearance of Neptune’s clouds and the solar cycle – the period during which the sun’s magnetic field reverses 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 – about 1/900th of the sunlight we receive on Earth.
The team found that two years after the peak of the solar cycle, an increasing number of clouds are appearing on Neptune.
It is believed that the sun’s UV rays, when strong enough, can trigger a photochemical reaction that produces Neptune’s clouds.
The team further found a positive correlation between the number of clouds and the brightness of the ice giant from the sunlight reflecting off it.
When the planet’s reflectivity reached its lowest level on record in 2020, most of the clouds disappeared.
The study strongly suggests that Neptune’s overall cloudy weather is determined 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 Space Shuttle Discovery from 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 low-key cloud cover appears to be correlated with the solar cycle,” the team states in their paper.
More observations of Neptune are also needed to see how long the current near cloudlessness will last, they add.
This can help deepen understanding not only of Neptune but also of exoplanets – planets outside of our solar system.
Indeed, exoplanets are thought to have similar qualities to Neptune, such as a rocky core surrounded by a thick atmosphere of hydrogen and helium.
