Disappearance of Neptune’s clouds may be linked to solar cycle, study says

Astronomers are intrigued by a mystery surrounding Neptune and now believe they’ve finally cracked the ice planet’s secret. The ghostly, elongated-looking clouds disappeared across much of the planet four years ago. Today, only a speck hangs over Neptune’s south pole.

Thanks to an analysis of nearly three decades of observations of the planet belonging to the solar system – captured by three space telescopes -, scientists have determined that the lower incidence of clouds may indicate a synchrony with the solar cycle, according to a recent study published in Icarus newspaper.

“These remarkable data provide us with the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle,” said study senior author Imke de Pater, professor emeritus of astronomy at the University of California, Berkeley, in a press release. “Our findings support the theory that the Sun’s (ultraviolet) rays, when strong enough, can trigger a photochemical reaction that produces Neptune’s clouds.”

During the solar cycle, the level of activity in the sun’s dynamic magnetic fields waxes and wanes. The magnetic field changes every 11 years as it becomes more tangled like a ball of yarn, according to NASA. When there is heightened activity on the sun, more intense ultraviolet radiation bombards the solar system.

Using data from NASA’s Hubble Space Telescope, the WM Keck Observatory in Hawaii, and the Lick Observatory in California, scientists observed 2.5 cycles of cloud activity during the 29-year period of observations of Neptune—during which the reflectivity of planet brightened in 2002 and dimmed in 2007. Neptune brightened again in 2015, before dimming in 2020 to the lowest level ever seen. That’s when most of the cloud cover disappeared.

“Even now, four years later, the most recent images we obtained last June still show that the clouds have not returned to their previous levels,” said study lead author Erandi Chavez, a doctoral student at the Center for Astrophysics, Harvard & Smithsonian, in a statement.

The findings are “extremely exciting and unexpected, especially since the previous period of low cloud activity on Neptune was not as dramatic and prolonged,” Chávez added.

A surprising correlation

The authors also found that two years after the peak of the cycle, more clouds appeared on Neptune – and the more clouds there were, the brighter Neptune was due to sunlight reflecting off of it. This connection was “surprising to planetary scientists because Neptune is the most distant planet in our solar system and receives sunlight at about 0.1% of the intensity that Earth does,” according to NASA. The findings also contradict the idea that clouds are affected by Neptune’s four seasons, which each last about 40 years.

“This is a very interesting paper and good old-fashioned detailed detective work,” Patrick Irwin, a professor of planetary physics at the University of Oxford, who was not involved in the study, said by email. “This new paper covers a longer time period than previous studies and shows a convincing correlation of observed cloud cover with solar UV brightness.”

But there is a two-year lag between the peak of the solar cycle and the increase in cloud abundance on Neptune. The authors think this gap can be explained by photochemistry occurring high in the planet’s upper atmosphere, which takes time to produce clouds.

The link between increased sun brightness and cloud formation could be due to the generation of ionized molecules that can act as cloud condensation nuclei and help initiate condensation, Irwin said.

“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 study co-author Carlos Alvarez, an astronomer on the Keck Observatory team, in a statement. “Advances in technology and observations have allowed us to narrow down atmospheric models of Neptune, which are essential for understanding the correlation between the ice giant’s climate and the solar cycle.”

The research team is still monitoring Neptune’s cloud activity, as more ultraviolet light could also darken the planet’s clouds, dimming their overall brightness, the authors said.

Furthermore, Neptune’s storms that arise from the deep atmosphere influence the planet’s cloud cover — but are unrelated to clouds formed in the upper atmosphere. This variable may interfere with studies that analyze correlations between photochemically produced clouds and the solar cycle. More research could also suggest how long Neptune’s near-cloudlessness might last.

These searches, in turn, could not only expand astronomers’ knowledge of Neptune, but also help researchers better understand the many exoplanets outside the solar system believed to have similar characteristics to the ice giant, according to NASA.

The study also “underscores the need to continue monitoring the planets in the solar system,” Irwin said. “Only by observing these planets at regular intervals is it possible to build a reliable, long-term dataset to investigate these periodic variations.”