Hubble image of Uranus (November 2014 and November 2022)

Planetary oddball Uranus rolls around the Sun on its side as it follows its 84-year orbit, rather than spinning in a more ’vertical’ position as Earth does. Its weirdly tilted ‘horizontal’ rotation axis is angled just eight degrees off the plane of the planet’s orbit. One recent theory proposes that Uranus once had a massive moon that gravitationally destabilised it and then crashed into it. Other possibilities include giant impacts during the formation of the planets, or even giant planets exerting resonant torques on each other over time. The consequences of Uranus’s tilt are that for stretches of time lasting up to 42 years, parts of one hemisphere are completely without sunlight. When the Voyager 2 spacecraft visited during the 1980s, the planet’s south pole was pointed almost directly at the Sun. Hubble’s latest view shows the northern pole now tipping toward the Sun.

[LEFT] - This is a Hubble view of Uranus taken in 2014, seven years after the northern spring equinox when the Sun was shining directly over the planet’s equator, and shows one of the first images from the OPAL programme. Multiple storms with methane ice-crystal clouds appear at mid-northern latitudes above the planet’s cyan-tinted lower atmosphere. Hubble photographed the ring system edge-on in 2007, but the rings are seen starting to open up seven years later in this view. At this time, the planet had multiple small storms and even some faint cloud bands.

[RIGHT] - As seen in 2022, Uranus’s north pole shows a thickened photochemical haze that looks similar to the smog over cities. Several little storms can be seen near the edge of the polar haze boundary. Hubble has been tracking the size and brightness of the north polar cap and it continues to get brighter year after year. Astronomers are disentangling multiple effects — from atmospheric circulation, particle properties, and chemical processes — that control how the atmospheric polar cap changes with the seasons. At the Uranian equinox in 2007, neither pole was particularly bright. As the northern summer solstice approaches in 2028 the cap may grow brighter still, and will be aimed directly toward Earth, allowing good views of the rings and the north pole; the ring system will then appear face-on. This image was taken on 10 November 2022.

[Image description: Two views of the tipped planet Uranus appear side-by-side for comparison.]

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