Comet ISON's recent outburst and increase in brightness was caused by a chunk of the comet breaking off, according to new analysis by German astronomers who say the evidence is in the form of two faint 'wings' that are flanking the comet's coma.

The wings flaring from the coma of Comet ISON, as seen on 16 November. Image: Wendelstein Observstory of the LMU/MPS.

The much-heralded ISON had been flattering to deceive on its run up to its closest approach of the Sun, called perihelion, on 28 November. Then observers began to report that ISON had jumped in brightness by almost three magnitudes in the space of a few days to the point that, by 14 November, it was a faint naked eye object. Now scientists at the Max Planck Institute for Solar System Research and the Wendelstein Observatory belonging to the Ludwig-Maximilians-University in Munich suggest that the brightening was caused by a fragment of the comet, at least 50 metres across, breaking away from ISON's nucleus. While the fragment cannot be individually distinguished by telescopes, they say the comet must have grown its own mini-tail which is now interacting with the main coma and tail of ISON, creating two faint wing-like structures called arclets emanating from the head of ISON. These wings are not visible to the naked eye or in ordinary images - the glare of the comet as a whole is too great - but they are apparent in images that have been processed using mathematical algorithms to pick out spatial changes in the light intensity of the comet.

News that the comet has partially fragmented will come as a bit of a scare to observers hopeful that ISON will develop into a spectacular comet, but for now they can relax. "The outburst by no means indicates the complete disruption of the original nucleus," the Max Planck Institute's Dr Hermann Böhnhardt tells Astronomy Now. "Our calculations imply that ISON lost only one fragment, or very few at the most."

Comets are not thought to be the most sturdy of objects, but more like loosely-held together aggregates of dirty rubble and ice. However, no two comets are exactly alike and some are more solid than others. The loss of a fragment could have been an inevitable result of the comet's fragile interior structure, but equally it could have been an unexpected event caused by heating from the Sun leading to a particularly large outburst of water vapour and dust from beneath the surface, consequently blowing a chunk off.

Wings have been seen before in other comets that have suffered break ups - around 50 examples at last count, says Böhnhardt. One well known example was the fragmentation of the comet 73P/Schwassmann-Wachmann, part of which was observed by the Hubble Space Telescope in 2006.

However, Matthew Knight of the Lowell Observatory in the United States is not fully convinced by the analysis. Although he accepts that it is possible a fragment has broken off, he points to several factors that do not mesh well with that explanation. Had a chunk broken off one would expect the arclets to be lopsided, not symmetrical, he says, while jets of outgassing material have been reported from the comet and these could be the source of the arclets, with the solar wind streamlining them into the beginnings of an ion tail - a comet's straight, secondary tail.

If a fragment really has broken off, then how long it will survive depends on its size. At the lower end of the scale, 50 metres, the fragment will fizzle away quite quickly, while a larger fragment could follow ISON all the way around its close encounter with the Sun. ISON could be in for a rough ride over the next week or two, as the Sun's heat and gravity really begins to crank up.

"According to past experience, comets that have lost a fragment tend to do this again," says Böhnhardt. If ISON does survive, the hope is that it will still make a fantastic sight during the first half of December. For more ISON updates, keep visiting our website and for more on comets download our free iPad app.