Surprising link between Neptune's clouds and solar cycles

Astronomers have unravelled a fascinating link between the ever-shifting cloud cover of Neptune, our solar system's farthest major planet, and the natural 11-year solar cycle. For this discovery, the team utilized three decades of data gathered by NASA's Hubble Space Telescope and the W. M. Keck Observatory in Hawaii, as well as data from the Lick Observatory in California.

Despite receiving a mere 0.1% of the solar intensity that Earth experiences, Neptune's global cloud weather appears to be orchestrated by solar dynamics rather than the planet's four seasons, each of which extends approximately 40 years.

Neptune's current cloud coverage remains minimal, except for sporadic formations hovering above its southern pole. A team of astronomers led by the University of California (UC) Berkeley saw a significant reduction in the cloud abundance typically seen at the icy giant's mid-latitudes, a trend that started in 2019.

To track the evolution of Neptune's appearance, the team, headed by Erandi Chavez, delved into Keck Observatory's image archives from 2002 to 2022. Their quest also encompassed Hubble Space Telescope's data starting in 1994 and data from California's Lick Observatory spanning 2018 to 2019. Chavez, a graduate student at the Center for Astrophysics | Harvard-Smithsonian (CfA) in Cambridge, Massachusetts, led the study when she was an undergraduate astronomy student at UC Berkeley. 

Complementing the Keck observations were the images obtained through the Twilight Zone program and Hubble's Outer Planet Atmospheres Legacy (OPAL) program. These observations unveiled a captivating pattern linking Neptune's cloud cover alterations with the solar cycle - a period during which the Sun's magnetic field undergoes an 11-year oscillation akin to a tangled yarn ball. This magnetic dance translates into increased sunspots and solar flare activity. As the cycle unfolds, solar turbulence peaks, only to give way to magnetic field reversal before settling into a new cycle.

The team found that two years post the solar cycle's peak, also called Solar Maximum, Neptune experiences an upsurge in cloud formations. They also found a link between the number of clouds and the ice giant's brightness from the sunlight reflecting off it.

The fluctuations in Neptune's brightness, attributed to solar influences, seem to mirror the cyclical appearance and disappearance of clouds on the planet. The researchers found a two-year lag between the solar cycle peak and cloud abundance, which they attribute to photochemical reactions occurring high in Neptune's upper atmosphere, which take time to culminate into cloud formations.

Although the study unveils a compelling connection between Neptune's disappearing clouds and solar cycle, there remain complexities to unravel. Continued observations of the planet will be required to see how long the current near-absence of clouds will last.

"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. 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," said Carlos Alvarez, staff astronomer at Keck Observatory and co-author of the study published in the journal Icarus.