Neptune is the most distant planet in the Solar System, an ice-giant that boasts an active and chaotic atmosphere. Using archival near-infrared observations from the NASA/ESA Hubble Space Telescope, the W.M. Keck Observatory and the Lick Observatory, a team of astronomers led by the University of California, Berkeley documented the evolution of Neptune’s cloud activity from 1994 to 2022. They calculated the fraction of Neptune’s disk that contained clouds, as well as the average brightness of both cloud features and cloud-free background over the planet’s disk. They observed cloud activity and brightness maxima during 2002 and 2015, and minima during 2007 and 2020, the latter of which is particularly deep.

“I was surprised by how quickly clouds disappeared on Neptune,” said University of California, Berkeley’s Professor Imke de Pater.

“We essentially saw cloud activity drop within a few months.”

“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, a graduate student at the Harvard-Smithsonian’s Center for Astrophysics.

“This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged.”

To monitor the evolution of Neptune’s appearance, the astronomers analyzed Keck images taken from 2002 to 2022, Hubble archival observations beginning in 1994, and data from the Lick Observatory from 2018 to 2019.

The images reveal an intriguing pattern between seasonal changes in Neptune’s cloud cover and the solar cycle — the period when the Sun’s magnetic field flips every 11 years as it becomes more tangled like a ball of yarn.

This is evident in the increasing number of sunspots and increasing solar flare activity.

As the cycle progresses, the Sun’s tempestuous behavior builds to a maximum, until the magnetic field beaks down and reverses polarity.

Then the Sun settles back down to a minimum, only to start another cycle.

When it’s stormy weather on the Sun, more intense ultraviolet (UV) radiation floods the Solar System.

The researchers found that two years after the solar cycle’s peak, an increasing number of clouds appear on Neptune.

They further found a positive correlation between the number of clouds and the ice giant’s brightness from the sunlight reflecting off it.

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle,” Professor de Pater said.

“Our findings support the theory that the Sun’s UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s 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,” said Dr. Carlos Alvarez, an astronomer at Keck Observatory.

“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.”

A paper on the findings appears online in the journal Icarus.

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Erandi Chavez et al. 2023. Evolution of Neptune at near-infrared wavelengths from 1994 through 2022. Icarus 404: 115667; doi: 10.1016/j.icarus.2023.115667