By Paul Rincon
Science reporter, BBC News, Preston
The rings around the planet Uranus may have been spotted nearly 190 years prior to the accepted date for their discovery, according to a theory.
Uranus' rings could have first been seen in the 18th Century
According to the orthodox view, the rings around Uranus were detected during an experiment in 1977.
Now, a scientist has re-evaluated a claim made in 1797 by astronomer Sir William Herschel that he saw rings around the seventh planet.
The claim had previously been dismissed as a mistake.
The new idea was presented at the Royal Astronomical Society's National Astronomy Meeting in Preston, UK.
Dr Stuart Eves, who works for Surrey Satellite Technology Limited, first came up the idea when he was given a framed page from an encyclopaedia published in 1815 for his birthday.
The page shows an orrery - a mechanical device detailing the relative positions and motions of planets and moons.
Made by the craftsman William Pearson, it showed the planet Uranus, with its spin axis in the correct plane, with six smaller objects spinning around it.
It was unlikely that these objects were moons. Although two Uranus satellites were found the 18th Century, the sixth moon of Uranus was not found until 1985, after Nasa's Voyager probe flew past the planet.
After researching the subject, Dr Eves found that the Pearson orrery in the encyclopaedia page was based on observations made by Sir William Herschel, who discovered the seventh planet in 1781.
'A ring suspected'
When Dr Eves tracked down Herschel's notes detailing his observations of Uranus, he found the following passage: "February 22, 1789: A ring was suspected".
Herschel even drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case. Herschel's notes were published in a Royal Society journal in 1797.
Dr Eves told BBC News: "I was thinking, 'could he have got all of that right'? He has one ring, rather than multiple rings as there are at Saturn; it is relatively close to the planet and it's about the right size.
"The opening angle is about right. Astronomical software indicates that it may have been slightly more open as viewed from Earth on the dates Herschel was observing," he added.
"But there are reasons for thinking that the ring plane moves about a bit, and he has the major axis of the ring plane in the right direction. I started to add up all the statistics and I said: I reckon he had a point.
"[Herschel] is not just superimposing a saturnian-style ring system on Uranus. I think it is compelling from a psychological point of view, because he really didn't have much to compare it with at the time."
Other astronomers have dismissed the possibility that Herschel discovered rings around Uranus. They claim that it would have been far too faint for him to have seen from the ground, using contemporary telescopes.
The ring was officially discovered in 1977 during an occultation experiment. One question some will be asking is why no one saw the ring in the intervening years.
Stuart Eves thinks there may be a few reasons for this. Firstly, there are only a few windows of opportunity during which the rings present themselves to Earth.
The Cassini-Huygens mission has also observed darkening of the rings of Saturn. This may be due to dust accumulating on the icy material in the rings.
If this process is happening on Saturn, Dr Eves argues, it could be happening on Uranus. The seventh planet's rings may have been brighter in 1787, allowing Herschel to spot them from Earth. Several other effects could also cause variability in the rings, including loss of material from them.
Another factor may lie with the Earth's atmosphere. As the industrial revolution proceeded apace, light pollution and smog may have prevented subsequent observers from seeing the planet's rings.
More speculatively, a cold snap called the Maunder Minimum, which lasted from 1645 to 1715 and saw temperatures that were on average five degrees lower than today, might have removed water vapour from the atmosphere, locking it up as ice.
If the climate was still relatively cold by the time Herschel made his observations, less water vapour may have made skies clearer and therefore more suitable for astronomy.