Comet Holmes (17P/Holmes) in the constellation of Perseus, optical image. The comet is the green dot just above centre left. This image was obtained on 1 November 2007. The comet is visible as sunlight reflects off its coma, a dusty cloud surrounding the comet. By late-November the coma had swollen to a diameter of over 1.4 million kilometres (larger than the Sun), due to an outburst that took place in late October. The central comet is only 3.4 kilometres across, and it is thought that the coma expansion was caused when a fragment broke away from the comet and disintegrated. Comet Holmes is a short-period comet, with an orbit of only 6.9 years. It was discovered in 1892 by the British astronomer Edwin Holmes.