Neptune’s Clouds Have Disappeared, and the Sun Might Be Responsible

As the outermost planet in the solar system, Neptune is more than 30 times farther from the sun than the Earth is, and it takes a staggering 165 years to circle our star. From the outskirts of the sun’s orbit, Neptune bathes in only 0.1 percent of the intensity of sunlight that we get on Earth.

Yet despite the distance between them, the sun still holds sway over the far-off planet. In a paper recently published in the journal Icarus, researchers theorize that the sun is the reason why Neptune’s clouds have recently disappeared. The paper suggests the planet’s cloud coverage could be tied to the ultraviolet rays the sun emits through the solar system.

“That UV emission from the sun could dictate Neptune’s cloud structure is akin to an orchestra conductor giving directions to a lone violin player 2.8 billion miles away,” Grant Tremblay, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who did not contribute to the research, tells the New York Times’ Robin George Andrews. “It’s another illustration that our sun truly is the lord of the solar system, even to its most distant reaches.”

Neptune's Disappearing Clouds Linked to the Solar Cycle

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Neptune’s clouds have been signature features in images of the planet for decades. But now, the world is essentially devoid of them. For the new study, researchers investigated whether the sun had anything to do with this shift on Neptune. They tracked changes in the planet’s clouds between 1994 and 2022, using observations from the Hubble Space Telescope, Keck Observatory and Lick Observatory, according to a statement from NASA.

The team found that cloud activity on Neptune peaked in both 2002 and 2015 and reached its lowest levels in 2007 and 2020. Clouds were almost totally gone in 2020—the lowest coverage ever observed—after they started fading around the planet’s mid-latitudes in 2019.

“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, first author of the new study and a researcher at the Harvard-Smithsonian Center for Astrophysics, says in NASA’s statement. “This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged.”

These changes in cloud coverage appear to be tied to the solar cycle, or the steady oscillation between periods of high and low solar activity every 11 years. Solar activity includes phenomena like solar flares, coronal mass ejections and high-speed solar wind, according to the National Oceanic and Atmospheric Administration. When the solar cycle peaks, the sun’s magnetic field flips as well.

During Neptune’s two cloudiest years, the sun had hit its peak in activity only two years prior, which involves higher emissions of ultraviolet radiation, writes New Scientist’s Leah Crane. And in low-cloud years like 2020, solar activity was near its minimum.

The number of clouds over Neptune was also tied to the brightness of sunlight reflecting off the planet.

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the sun’s cycle,” Imke de Pater, a co-author of the study and an astronomer at the University of California, Berkeley, says in NASA’s statement.

Still, the research did not reveal why Neptune’s current cloudless state is more dramatic than any other known period, per the New York Times.

Hubble chronicles #Neptune's cloud cover over three decades & finds that the number of clouds grows increasingly following a peak in the solar cycle.

Learn more https://t.co/pqVIK0bJ75

@NASA @esa E. Chavez (UC Berkeley), I. de Pater (UC Berkeley) pic.twitter.com/lpbA0aJ0pu

— HUBBLE (@HUBBLE_space) August 17, 2023

As for why this pattern occurs, perhaps the strongest UV rays from the sun set off a chemical reaction in Neptune’s atmosphere that leads to cloud formation, per NASA. This might account for the two-year gap between the solar maximum and the peak of cloud coverage on Neptune—the chemical reactions may just take some time to occur.

This trend remains only a correlation, however, and more research is needed to prove solar activity is causing the changes in Neptune’s cloud coverage, writes New Scientist.

Currently, the sun is building to a peak in solar activity, which NASA has forecasted will occur in July 2025 (though other research has moved that date earlier). Watching Neptune to see if its clouds respond as predicted could give scientists more evidence to bolster this new theory.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.