Neptune, the final planet in our solar system, is renowned for its wispy white clouds made of frozen methane crystals. These clouds are whipped across the ice giant by strong winds at speeds exceeding 1,200 mph – the fastest recorded in the solar system. However, a recent study has revealed that these clouds have nearly vanished, leaving scientists puzzled. Through analyzing images from the Hubble Space Telescope dating back to 1994, experts have determined that the clouds disappear and reappear based on the sun’s 11-year cycle and magnetic field flips. In 1989, NASA’s Voyager 2 spacecraft provided the first up-close images of linear, bright clouds reminiscent of cirrus clouds on Earth in Neptune’s atmosphere.
As the most distant planet in our solar system, Neptune is a cold and dark ice giant with supersonic winds. It is 30 times farther from the Sun than Earth and is not visible to the naked eye. Prior to its discovery, Neptune was mathematically predicted to exist. It completed its first 165-year orbit in 2011 since being spotted in 1846. Voyager 2 is the only spacecraft to have visited Neptune directly, passing by in 1989 on its journey out of the solar system.
The University of California, Berkeley, astronomers led a study describing these findings, which has been published in the journal Icarus. Imke de Pater, an emeritus professor of astronomy at UC Berkeley, expressed surprise at the rapid disappearance of clouds on Neptune. The fourth largest planet in our solar system is composed of a mixture of water, methane, and ammonia, which scientists refer to as “ices.” Above this layer, distinctive swirling clouds reflect all colors of the spectrum from the sun, appearing white. Voyager 2’s images in 1989 provided the first close-up view of these bright clouds in Neptune’s atmosphere, resembling cirrus clouds on Earth.
Researchers examined images from the Hubble Space Telescope, data from California’s Lick Observatory between 2018 and 2019, and data from the Keck Observatory in Hawaii from 1994 to 2022 to monitor the evolution of Neptune’s clouds. They observed that an abundance of clouds typically seen at the planet’s mid-latitudes began to fade in 2019 and have not returned since. Since late 2019, only the south pole has displayed cloud activity. According to Erandi Chavez at Harvard’s Center for Astrophysics, even the most recent images taken in June still show a lack of clouds compared to previous levels.
The study reveals a connection between the disappearance of Neptune’s clouds and the solar cycle, which occurs every 11 years when the sun’s magnetic field flips, causing fluctuations in solar radiation. This connection is surprising considering Neptune’s distance from the sun and limited sunlight exposure. Two years after the solar cycle’s peak, an increasing number of clouds appear on Neptune due to the sun’s strong UV rays potentially triggering a photochemical reaction. The data also demonstrate a positive correlation between the number of clouds and the planet’s brightness from sunlight reflection. When Neptune’s reflectivity reached its lowest level ever observed in 2020, most of the clouds vanished.
These findings suggest that solar activity, rather than the planet’s four seasons, largely drives Neptune’s global weather. Further observations are needed to determine the duration of the current cloud absence and deepen understanding of not only Neptune but also exoplanets, which share similar qualities to Neptune.
In a separate study using NASA’s Kepler telescope, researchers examined planets orbiting red dwarfs in the Milky Way galaxy. They discovered that a third of these planets, amounting to hundreds of millions in total, likely possess conditions suitable for hosting life. The search for life on other planets has long been a goal for astronomers, and this study provides promising targets for future exploration beyond our solar system.
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