On November 9, 2007 the coma, the thin dissipating dust ball around the comet, was found to be the largest object in the solar system, with a diameter greater than that of the Sun. (Though by Solar System standards, the mass of the comet is minuscule.)
The comet remained visible in February 2008 though it had become a challenging target at about magnitude +5 in the constellation Perseus. It had expanded to greater than 2 degrees of arc as seen from the Earth, and thus had very little surface brightness.
Comet 17P/Holmes was discovered by Edwin Holmes on November 6, 1892 while he was conducting regular observations of the Andromeda Galaxy (M31). Its discovery in 1892 was made because of and during magnitude changes similar to the 2007 outburst. 17P/Holmes brightened to an approximate magnitude of 4 or 5 before fading from visibility over a period of several weeks.
The comet's discovery was confirmed by Edward Walter Maunder (Royal Observatory, Greenwich, England), William Henry Maw (England), and Kidd (Bramley, England) and independent discoveries were made by Thomas David Anderson (Edinburgh, Scotland) on November 8 and by Mike Brown , (Wilkes) , (USA) and by John Ewen Davidson (Mackay, Queensland, Australia) on November 9.
The first calculations of the elliptical orbits of 17P/Holmes were done independently by Heinrich Kreutz and George Mary Searle. Additional orbits eventually established the perihelion date as June 13 and the orbital period as 6.9 years. These calculations proved that the comet was not a return of 3D/Biela.
The 1899 and 1906 appearances were observed, but the comet was lost after 1906 until recovered on July 16, 1964 by Elizabeth Roemer (US Naval Observatory, Flagstaff, Arizona, USA). Aided by the computer predictions of Brian G. Marsden, the comet has been observed on every subsequent return.
Between October 23–24, 2007, Comet Holmes grew much brighter, going from about magnitude 17 to about magnitude 2.8 in just 42 hours. The first person reportedly to notice a change was J. A. Henríquez Santana on Tenerife in the Canary Islands; minutes later, Ramón Naves in Barcelona noticed the comet at magnitude 7.3. It became easily visible to the naked eye as a bright yellow "star" in Perseus, and by October 25 17P/Holmes appeared as the third brightest "star" in that constellation.
While large telescopes showed fine-scale cometary details, naked-eye observations gave a view similar to that of a star until October 26. After that date, 17P/Holmes began to appear more comet-like to naked-eye observers. During the comet's outburst, its orbit took it to near opposition with respect to Earth, and since comet tails point away from the Sun, Earth observers were looking nearly straight down along the tail of 17/P Holmes, making the comet appear as a bright sphere.
Based on orbital computations and luminosity before the 2007 outburst, the comet's nucleus was estimated at 3.4 km. In late October 2007 the coma's diameter increased from 3.3 arcminutes to over 13 arcminutes, about half the diameter that the Moon subtends in the sky. At a distance of around 2 AU, this means that the true diameter of the coma swelled to over 1 million km, or about 70% of the diameter of the Sun. By comparison, the Moon is 380,000 km from Earth. Therefore, during the 2007 outburst of Comet Holmes the coma was a sphere wider than the diameter of the Moon's orbit around Earth. On 2007 November 9, the coma had dispersed to an area larger than the sun, briefly giving it the largest extended atmosphere in the solar system.
The cause of the outburst is not definitely known. The huge cloud of gas and dust may have resulted from a collision with a meteoroid, or, more probably, from a build-up of gas inside the comet's nucleus which eventually broke through the surface.