A bright spot that appeared on Uranus has caused a flurry of excitement among amateur astronomers, but what has happened to it is a puzzle.

Uranus imaged by Voyager 2, the only spacecraft to visit it. Its cloud tops are bland, revealing no detail. However, storms have been seen to blow up. Image credit: NASA

The spot, assumed to be a storm in the ice giant’s clouds, was revealed in images taken with a 60-inch telescope at the historic observatory at Mount Wilson, California, on October 22nd.

Photographer Blake Estes posted on Instagram to show the spot he had imaged with Richard Bell on the southern edge of the planet’s bright polar region. It was said to have been detected with amateur telescopes too.

Appeals for further observations were made, including by Mike Foulkes, Director of the Saturn, Uranus and Neptune Section of the British Astronomical Association.

Mike wanted to know whether the feature was still visible or had been short-lived. He told me that similar bright features had imaged in 2016 by larger amateur instruments, generally using infrared (IR) filters.

One potential problem is that imaging the planet is very challenging for amateurs, and over-processing with software afterwards can introduce artefacts.

Mike said that the few recent images he had seen showed the bright region in the northern hemisphere plus a darker band to the south. Of the spot itself, observations of November 9th and November 18th possibly showed something, but nothing appeared on others taken on October 20th and November 15th. Mike was planning to examine the images more closely.

When I received the BAA alert, I recalled having seen a tweet from a Hubble Space Telescope account at the weekend, reporting that the orbiting observatory was observing Uranus with its Wide Field Camera 3 for Dr Amy Simon, who is Senior Scientist for Planetary Atmospheres Research at NASA’s Goddard Space Flight Center. So I emailed her to ask what Hubble might have seen.

Dr Simon said that the Space Telescope team want to produce a press release of their own in due course, so are not making the latest images of Uranus available to the public at the moment.

However, she told me that they had imaged Uranus with Hubble through two full rotations, adding: “I can say that we do see storms, though nothing as big as the initial report.” (Uranus rotates once every 17h 14m).

So whatever the bright spot was on Uranus on October 22nd, it sounds like it had diminished substantially by the time Hubble came to observe the planet a month later.

Storms observed in August 2014 in infrared light by the 10-meter Keck telescope on Mauna Kea, Hawaii. The white spot was the brightest feature ever recorded on the planet, while the cloud rotating into view at lower-right grew big enough to be seen at visible wavelengths by amateur astronomers with big telescopes. Image credits: Imke de Pater (UC Berkeley) & Keck Observatory

Uranus is an unusual planet in that its axis is tilted so much that it virtually rolls around the Sun on its side. For half its 84-year orbit, the northern hemisphere is directed towards the Sun before the southern hemisphere is turned towards the sunlight. But at two points in its orbit, known as the equinoxes, the Sun shines equally on both hemispheres. Uranus, which is known to be a windy planet, was last at an equinox in 2017.

At these equinoxes, storms appear to become more frequent in Uranus’s disturbed atmosphere, in contrast to the calm conditions and bland cloud tops observed by NASA’s Voyager 2 space probe when it flew past the ice giant in January 1986.

Stormy weather has been observed a number of times in recent years, however, with a notably bright one appearing in 2014, seven years after the last equinox.

Dr Simon told me: “We’ve seen some sustained storm activity well past equinox,” adding “in a sense that was no surprise, because on any planet, we often see storms well after equinox, and there can be a lag between when an area heats up and when you see storms.”

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