Each planet of the solar system has its own look. Earth has aquamarine oceans. Jupiter has panchromatic tempests. Saturn has glimmering rings. And Neptune has ghostly clouds — at least, it used to. For the first time in three decades, the electric-blue orb is almost completely cloud-free, and astronomers are spooked.
Neptune’s cloud cover has been known to ebb and flow. But since October 2019, only one patch of wispy white has been present, drifting around the planet’s south pole.
“It was the first time anybody had ever seen this,” said Imke de Pater, an astronomer at the University of California, Berkeley. “There’s just nothing there. What’s going on?”
To crack the case of the vanishing clouds, scientists spooled through 30 years of near-infrared images of Neptune made with ground-based observatories and the Hubble Space Telescope. In a study published in June in the journal Icarus, Dr. de Pater and her colleagues named the prime suspect in this cloud cleansing: the sun.
Neptune, a frigid planet decorated with supersonic storms, has been visited by only one spacecraft, Voyager 2, which zipped by in 1989. Consequently, little is definitively known about the planet, including the nature of its eccentric hydrocarbon clouds.
Until another robotic envoy pops over to greet Neptune, astronomers must rely on telescopes to decode its secrets. Interested in the nearly-naked status of the ice giant, a team led by Erandi Chavez, a graduate student now at the Harvard-Smithsonian Center for Astrophysics, set to work.
The researchers combined images taken by Hubble, the Keck Observatory in Hawaii and the Lick Observatory in California to create a 29-year photo album of Neptune going back to 1994. Then, they compared it with the sun’s cycles.
The sun goes through cycles of hyperactivity and tranquility, lasting from eight to 14 years, that are driven by the repeated inversion of the sun’s magnetic field. These cycles seem to rise and fall in sync with Neptune’s cloud cover. In 2002 and 2015, for example, Neptune showcased myriad clouds, hot on the heels of peak solar activity on both occasions. It’s thought that the bombardment of ultraviolet light triggers an alchemic, cloud-making reaction in the planet’s ethereal skies.
Conversely, during the sun’s nadir, Neptune’s vaporous veil fades away — although it isn’t known why the current dearth of clouds is so extreme compared with previous cycles.
It has been suggested that these two extremely distant celestial objects may be improbably linked in this manner. But this study offers the strongest evidence yet that Neptune’s cloudy couture can be attributed to solar flare, hinting at the ice giant’s mysterious dynamism.
“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,” said Grant Tremblay, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not involved with the work. “It’s another illustration that our sun truly is the lord of the solar system, even to its most distant reaches.”
Heidi Hammel, a planetary astronomer and vice president for science at the Association of Universities for Research in Astronomy who was not involved with the study, also offered praise for the findings.
“One of the things I’ve always enjoyed about studying Neptune is that it never looks the same,” she said. “And this study is helping us better understand why that is.”
But a handful of solar cycles is neither sufficient to understand the mechanism that creates these clouds, nor can it confirm that the correlation between the two represents causation. Scientists are eagerly anticipating the next solar maximum, forecast for 2025, curious to see if the planet’s clouds bloom soon after.
“With outer planets like Neptune and Uranus, you have to play the long game,” Dr. Hammel said.