All of Neptune's clouds have vanished, and scientists think the sun is to blame.

The disappearance of the wispy, white strands of frozen methane that usually streak Neptune's azure face has been linked to changes in ultraviolet (UV) radiation as the sun's activity climbs to an 11-year maximum.

The clouds around the solar system's eighth and most distant planet — located roughly 2.8 billion miles (4.5 billion kilometers) from the sun — began fading in 2019 and were gone without a trace by 2020. The researchers who made the discovery will publish their findings in the Nov. 1 edition of the journal Icarus.

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"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," Erandi Chavez, a graduate student at the Harvard-Smithsonian Center for Astrophysics who led the study as an undergraduate at the University of California, Berkeley, said in a statement. "This is extremely exciting and unexpected, especially since Neptune's previous period of low cloud activity was not nearly as dramatic and prolonged."

The sun's activity rises and falls in 11-year cycles, but recently, the sun has been far more active than expected, with sunspot appearances nearly double the number predicted by the National Oceanic and Atmospheric Administration's Space Weather Prediction Center. Scientists anticipate that the sun's activity, initially thought to reach its maximum in 2025, could peak as soon as the end of this year.

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Astronomers have pegged the loss of cloud cover to the effects of sunlight hitting Neptune's atmosphere. By analyzing data collected by the Hubble Space Telescope, the Keck Observatory in Hawaii, and the Lick Observatory in California, astronomers observed that, following a two-year delay, peaks in the sun's activity generate more cloud cover over Neptune, while lows cause it to dissipate.

What, exactly, is causing the change isn't known for certain. The most likely explanation is that UV light from the sun starts chemical reactions in Neptune's upper atmosphere, ultimately creating the clouds.

"It's fascinating to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away from us," study co-author Carlos Alvarez, a staff astronomer at the Keck Observatory, said in the statement. "Advances in technology and observations have enabled us to constrain Neptune's atmospheric models, which are key to understanding the correlation between the ice giant's climate and the solar cycle."