A storm is coming

Since Pluto’s status-change from fully-fledged planet to dwarf planet, Neptune holds the title of outermost planet in the Solar System. This new image of the planet was made during Hubble’s Outer Planet Atmosphere Legacy (OPAL) programme, under which it has observed the four outermost planets, including Neptune, on a yearly basis since 2014.

The observations of Neptune carried out in September and November 2018 show the first evidence of a huge storm brewing, with the discovery of a new northern Great Dark Spot (visible here to the upper left of the planet’s disc, partially overlapping a large patch of white). This new dark storm is of a similar size and shape to the storm discovered in 1989 by the Voyager 2 space probe.

While the future evolution of the storm will be tracked through the continued yearly Hubble observations and also by ground-based telescopes, older OPAL observations from Hubble show that its appearance was preceded by increased cloud activity throughout the region. There are hints of the storm forming in images from as early as 2015. This slow origin process indicates that the storm developed deep within Neptune's atmosphere, pulling up dark material from its depths, and only became visible once the top of it reached higher altitudes.