Discovered by Sir William Herschel in March 1781, gas giant Uranus is the penultimate planet of the Solar System and currently well placed for observation in the constellation of Pisces. Despite being four times the diameter of Earth, its immense distance from the Sun (2,870 million kilometres) means that most visual observers consider discerning its tiny 3.7-arcsecond, magnitude +6, blue-green disc in backyard telescopes is achievement enough.
The face of Uranus is experiencing some epic storms. It turns out that enormous cloud systems are sweeping across the planet and have now become so bright that, for the first time ever, they can be spotted by amateur astronomers.
“The weather on Uranus is incredibly active,” said Imke de Pater, one of the researchers, in a news release.
In fact, the researchers examining Uranus detected eight large storms on the planet’s northern hemisphere on Aug. 5 and 6. One in particular was the brightest storm ever seen on Uranus; in fact, it accounted for 30 percent of all light reflected by the rest of the planet at the wavelength of 2.2 microns.
“This type of activity would have been expected in 2007, when Uranus’s once-every-42-year equinox occurred and the sun shined directly on the equator,” noted co-investigator Heidi Hammel of the Association of Universities for Research in Astronomy. “But we predicted that such activity would have died down by now. Why we see these incredible storms now is beyond anybody’s guess.”
In all, de Pater, Hammel and their team detected eight large storms on Uranus’s northern hemisphere when observing the planet with the Keck Observatory on Aug. 5 and 6. One was the brightest storm ever seen on Uranus at 2.2 microns, a wavelength that senses clouds just below the tropopause, the lower boundary of the stratosphere – where the pressure ranges from about 300 to 500 mbar, or half the pressure at Earth’s surface.
When amateur astronomers heard about the activity, they turned their telescopes on the planet and were amazed to see a bright blotch on the surface of a normally boring blue dot.
French amateur astronomer Marc Delcroix processed the amateur images and confirmed the discovery of a bright spot on an image by French amateur Régis De-Bénedictis, then in others taken by fellow amateurs in September and October. He had his own chance on Oct. 3 and 4 to photograph it with the Pic du Midi one-meter telescope, where on the second night, “I caught the feature when it was transiting, and I thought, ‘Yes, I got it!'” said Delcroix.
“I was thrilled to see such activity on Uranus. Getting details on Mars, Jupiter or Saturn is now routine, but seeing detail on Uranus and Neptune is the new frontier for us amateurs and I did not want to miss that.
“I was so happy to confirm myself these first amateur images on this bright storm on Uranus, feeling I was living a very special moment for planetary amateur astronomy.”, said Marc.
“The colors and morphology of this cloud complex suggests that the storm may be tied to a vortex in the deeper atmosphere similar to two large cloud complexes seen during the equinox,”
Such vortices could be anchored much deeper in the atmosphere and extend over large vertical distances, as inferred from similar vortices on Jupiter, including its Great Red Spot.
What is causing the storms now is still unknown, but scientist continues to watch the Uranian weather to see what will happen next. Results from studies were presented at the Division for Planetary Sciences meeting of the American Astronomical Society in Tucson, Arizona. Plans for publication and whether the research was peer-reviewed were not disclosed in press releases concerning the findings.