The gas giants Jupiter and Saturn have garnered more attention over the years because they’re closer, and let’s face it, they look cool. Uranus is easy to overlook out there in the outer solar system, but there are some newly released images from NASA and the ESA that might make you notice it again. The images are a composite of data from Hubble and the Voyager 2 probe showing aurora activity in the planet’s atmosphere.
Uranus is the third largest planet in the solar system after Jupiter and Saturn. It’s quite a bit smaller than Saturn, actually, with a diameter of 15,759 miles (25,362 km). Saturn is more than twice as large, but you could still fit 63 Earths inside Uranus. The Planet appears as a uniform blue-gray globe from a distance, but there are some subtle pattern in the clouds when viewed in certain wavelengths of light. It also has a ring system — it’s no match for the majestic rings of Saturn, but it’s got Jupiter beat in that department. In addition, Uranus has the distinction of rotating with an axial tilt of 97 degrees — almost parallel to the plane of the solar system. Astronomers hypothesize it was struck by a smaller planet in the distant past that tipped it over on its side.
The above images show bright auroras glowing in the clouds of Uranus, a phenomenon that was only confirmed in 2011. Astronomers had previously seen auroras on other gas giants like Jupiter and Saturn, but never Uranus. Auroras are caused by streams of charged particles like electrons picked up by the solar wind or from a planet’s own ionosphere. They are channeled into the upper atmosphere by the planet’s magnetic field, where they interact with gas molecules like oxygen and nitrogen. The ionized gas then gives off light, which we can observe.
The blue disk of Uranus in the above images comes from the snapshots taken by Voyager 2. It executed a flyby of the planet in 1986 on its way to the edge of the solar system. This is the only mission to get so close to Uranus, so these images are still among the best we have. The auroras and the ring system parts of the image come from Hubble, based on data acquired in 2014. Those observations used the ultraviolet capabilities of the Space Telescope Imaging Spectrograph (STIS) instrument on the space telescope.
The team that captured the auroras tracked the interplanetary shocks resulting from powerful bursts of solar wind. When the time came for those gusts to reach Uranus, Hubble was ready and watching. This analysis led to evidence that these huge auroras actually rotate with the planet. The team also re-discovered Uranus’ magnetic poles, which were “lost” following Voyager 2’s visit because of uncertainty in the measurements.