Neptune's vanishing clouds linked to Sun's solar cycle

“Even four years later, the images we took this past June showed the clouds haven’t returned to their former levels. This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged,” said Erandi Chavez, a graduate student at Harvard University’s Center for Astrophysics who led the study as an undergraduate astronomy student at UC Berkeley. 

On further analysis, an unexpected correlation between Neptune’s clouds and our host star’s solar cycle came into the spotlight.

According to the statement, it was quite unexpected to discover this link given that the icy giant is the outermost planet from the Sun, receiving only a tiny fraction of the sunlight as compared to the Earth. For reference, Neptune is located at a huge distance of 2.8 billion miles (4.5 billion kilometers) from the star.

After a thorough examination of data gathered from the W. M. Keck Observatory in Hawaii spanning from 1994 to 2022, along with space imagery acquired through the Hubble Telescope, the scientists arrived at this conclusion.

The solar cycle leading to this disappearing cloud act

The fluctuating cloud density on Neptune is synchronized with the peak of the solar cycle, a phase during which the Sun's magnetic fields reverse every 11 years. This cycle governs the output of solar radiation, which has the ability to impact the solar system's planets.

The researchers established a clear connection between the prevalence of clouds on Neptune and the specific phase within the Sun's solar cycle. This correlation between the solar cycle and Neptune's cloud distribution was deduced from 2.5 solar cycles' worth of cloud activity data collected over a span of 29 years.

In these peak periods, the Sun emits highly intense ultraviolet (UV) radiation, which subsequently leads to heightened cloud cover on Neptune approximately two years afterward. Subsequent to that peak, the clouds began their dissipation.

The team also noticed a “positive correlation” between the number of clouds and the brightness of the planet from sunlight reflecting off it. 

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle. Our findings support the theory that the Sun’s UV rays, when strong enough, maybe triggering a photochemical reaction that produces Neptune’s clouds,” said de Pater. 

The findings have been reported in the journal Icarus. 

Study Abstract:

Using archival near-infrared observations from the Keck and Lick Observatories and the Hubble Space Telescope, we document the evolution of Neptune’s cloud activity from 1994 to 2022. We calculate 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. We observe cloud activity and brightness maxima during 2002 and 2015, and minima during 2007 and 2020, the latter of which is particularly deep. Neptune’s lack of cloud activity in 2020 is characterized by a near-total loss of clouds at mid-latitudes and continued activity at the South Pole. We find that the periodic variations in Neptune’s disk-averaged brightness in the near-infrared H (), K (), FWCH4P15 (893 nm), F953N (955 nm), FWCH4P15 (965 nm), and F845M (845 nm) bands are dominated by discrete cloud activity, rather than changes in the background haze. The clear positive correlation we find between cloud activity and Solar Lyman-Alpha (121.56 nm) irradiance lends support to the theory that the periodicity in Neptune’s cloud activity results from photochemical cloud/haze production triggered by Solar ultraviolet emissions.