The connection between Neptune and solar activity is confusing to planetary scientists, as Neptune is the farthest major planet in our solar system and receives sunlight with an intensity of about 0.1% that of Earth. However, Neptune’s global cloudy weather seems to be driven by solar activity rather than the planet’s four seasons, each lasting about 40 years.

The cloud cover currently visible on Neptune is extremely low, except for some clouds hovering over the giant planet’s south pole. A team of astronomers led by the University of California at Berkeley (UC) discovered that the large amount of clouds normally seen in the mid-latitudes of the ice giant began to disappear in 2019.

“I was surprised how quickly clouds disappeared on Neptune,” said Imke de Pater, emeritus professor of astronomy at UC Berkeley and senior author of the study. “We actually saw a drop in cloud activity over the course of several months,” he said.

Center for Astrophysics | He led the research when he was an astronomy student at Harvard-Smithsonian University (CfA) in Cambridge, Massachusetts, University of California, Berkeley. “This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not that dramatic or lengthy.”

To observe the evolution of Neptune’s appearance, Chavez and his team analyzed Keck Observatory images from 2002 to 2022, archival Hubble Space Telescope observations starting in 1994, and data from the Lick Observatory in California in 2018-2019.

In recent years, Keck’s observations have been supported by images taken by the Twilight Zone program and the Hubble Outer Planet Atmospheres Legacy (OPAL) program.

The images reveal the intriguing pattern between seasonal changes in Neptune’s cloud cover and the Sun’s solar cycle, where the Sun’s magnetic field changes every 11 years as it becomes more tortuous, like a ball of thread. This is evident in the increase in the number of sunspots and the increased activity of solar flares. As the cycle progresses, the Sun’s turbulent behavior maximizes until the magnetic field weakens and reverses the poles. The sun then goes down to minimum again only to start another cycle.

This sequence of Hubble Space Telescope images shows the cloud cover on Neptune growing and shrinking. This nearly 30-year series of observations shows that cloudiness gradually increases after the peak of the Sun’s solar cycle, where the level of activity rises and falls rhythmically over an 11-year period. The level of the Sun’s ultraviolet radiation is plotted along the vertical axis. Below is an 11-year cycle from 1994 to 2022. The Hubble observations above clearly show the relationship between high cloud abundance and peak solar activity. Chemical changes are caused by photochemistry, which takes place high in Neptune’s upper atmosphere and takes time to form clouds.Credit: NASA, ESA, LASP, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley)

When the sun is stormy, more intense ultraviolet (UV) radiation fills the Solar System. The team found that two years after the peak of the solar cycle, more and more clouds appeared on Neptune. Additionally, the team found a positive correlation between the amount of cloud and the brightness of the ice giant from sunlight reflected from it.

“This surprising data gives us the strongest evidence yet that Neptune’s cloudiness is related to the solar cycle,” said De Pater. “Our findings support the theory that the Sun’s UV rays, if strong enough, can trigger the photochemical reaction that creates Neptune’s clouds.”

By looking at 2.5 cycles of cloud activity recorded during 29 years of observations of Neptune, scientists discovered a link between the solar cycle and cloudy weather on Neptune. During this time, the planet’s reflectivity increased in 2002 and then decreased in 2007. Neptune brightened again in 2015, then dropped to its lowest level ever seen in 2020 when most of the clouds parted.

Neptune’s changes in brightness caused by the Sun seem to increase and decrease relatively synchronously with the arrival and departure of the planet’s clouds. However, there is a two-year lag between the peak of the solar cycle and the abundance of clouds seen on Neptune. The chemical changes are caused by photochemistry, which takes place high in Neptune’s upper atmosphere and takes time to form clouds.

“It’s exciting to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away,” said Carlos Alvarez, astronomer at the Keck Observatory and co-author of the study. “Advances in technology and observations have allowed us to constrain Neptune’s atmospheric patterns, which is key to understanding the relationship between the ice giant’s climate and the solar cycle.”

However, more work is needed. For example, increased ultraviolet sunlight can create more clouds and fog, while also darkening them, reducing Neptune’s overall brightness. Storms rising from deep in the atmosphere on Neptune affect cloud cover, but are not associated with photochemically produced clouds and thus can complicate studies of correlation with the solar cycle. Continued observations of Neptune are also needed to see how long the current near cloudlessness will last.

The research team continues to monitor Neptune’s cloud activity. “We saw more clouds in Keck’s latest images taken while NASA’s James Webb Space Telescope was observing the planet; “These clouds are particularly visible at northern latitudes and high altitudes, as expected due to the observed increase in solar UV flux over the past 2 years.”

Combined data from Hubble, the Webb Space Telescope, Keck Observatory, and Leake Observatory will enable further research into the physics and chemistry that led to Neptune’s dynamic appearance, helping astronomers not only deepen their understanding of Neptune but also it could be. Many planets outside our solar system are also thought to have properties similar to Neptune. Source