An unusual storm on Neptune is seen as several white streaks in this view from the Hubble Space Telescope. Credit: E. Molter et al./Icarus

Planetary science

Epic storm roils a tranquil region of Neptune

Amateur astronomers help to track a disturbance in the icy planet’s atmosphere.

A storm wider than Mars tore across Neptune’s equator in 2017 — the first tempest observed in the giant planet’s midsection, which tends to sport a cloudless sky.

Astronomers have observed several colossal dark spots between Neptune’s equator and its poles. These blemishes, such as the Earth-sized Great Dark Spot seen in 1989, are storms that propel atmospheric gases upwards, producing bright clouds.

In June of 2017, Edward Molter at the University of California, Berkeley, and his colleagues spotted a roughly 8,500-kilometre-wide storm on Neptune. Unlike previous systems, this one was at the planet’s equator and lacked a dark spot. The scientists, along with amateur astronomers that they recruited, imaged the storm's massive cloud for about seven months until it dissipated.

Photo analyses and computer models indicated that the cloudy storm drifted eastward at more than 200 metres per second, roughly three times faster than a category-5 hurricane on Earth. The researchers conclude that the cloud formed either from rising gases or from moist gases that became trapped at Neptune’s equator and drifted east with the planet’s rotation.