Seasons and weather events are not an exclusive feature of Earth. Many objects in our Solar System experience changes as they move around the Sun and while seasons last for months on Earth, on other planets they can last decades. Space probes and the Hubble Space Telescope have kept an eye on the planets and the telescope has recently discovered some new features of Uranus and Neptune.
In observations collected over the last few years, Hubble has expanded our knowledge of Uranus’ polar vortex and discovered a new dark storm on Neptune. These interplanetary weather forecasts are part of the Outer Planet Atmospheres Legacy (OPAL), a long-term project that hopes to understand the decades-long changes on these two ice giant planets.
Uranus’ polar vortex looks like a vast bright cloud that covers most of the planet’s north pole. Planetary scientists believe that this unique feature formed as a result of the extreme tilt of the planet. Due to a colossal impact, the planet rotates almost on its side. And for this reason, during Uranus' summer, the Sun shines almost directly over the north pole, without ever setting. The planet is now getting into mid-summer and the polar cap is becoming more prominent, a strong indicator of seasonal variation.
Uranus also sports a compact methane-ice cloud, visible near the edge of the polar cap, and a thin band of clouds just north of the equator. It is unclear how such features persist given the strong winds present on both Uranus and Neptune.
Further out, we have Neptune and its incredible dark storm. The new one, seen by Hubble in September 2018, is the sixth and latest dark vortex seen on the planet. Two were discovered by the Voyager 2 probe and the rest were spotted by the keen eye of the space telescope. The latest one is 11,000 kilometers (6,800 miles) across and it is accompanied by bright white companion clouds. These are formed by methane freezing and being pushed upwards by the dark vortex.
Both planets are considered ice giants. They have thick atmospheres and no solid surface. Their interiors are likely to be a hydrogen/helium mantle around a water-rich (and maybe even rocky) core. The methane present in their atmosphere scatters the sunlight back into space and gives the planets their characteristic blue/green hues.