Formerly, the Sun was the largest object in the Solar System. Now, comet 17P/Holmes holds that distinction.
Spectacular outbursting comet 17P/Holmes exploded in size and brightness on October 24. It continues to expand and is now the largest single object in the Solar system, being bigger than the Sun (see Figure). The diameter of the tenuous dust atmosphere of the comet was measured at 1.4 million kilometers (0.9 million miles) on 2007 November 9 by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy. They used observations from a wide-field camera on the Canada-France-Hawaii Telescope (CFHT), one of the few professional instruments still capable of capturing the whole comet in one image. The present eruption of comet Holmes was first reported on October 24 and has continued at a steady 0.5 km/sec (1100 mph) ever since. The comet is an unprecedented half a million times brighter than before the eruption began. This amazing eruption of the comet is produced by dust ejected from a tiny solid nucleus made of ice and rock, only 3.6 km (roughly 2.2 miles) in diameter.
Caption: (Left) Image of comet Holmes from the 3.6-meter Canada-France-Hawaii telescope on Mauna Kea showing the 1.4 million km diameter coma. The white ''star'' near the center of the coma is in fact the dust-shrouded nucleus. (Right) the Sun and planet Saturn shown at the same scale for comparison. (Sun and Saturn images courtesy of NASA's SOHO and Voyager projects).
The new image also shows the growth of a tail on comet Holmes (the fuzzy region to the lower right in the comet picture), caused by the pressure of sunlight acting on dust grains in the coma. Over the next few weeks and months, the coma and tail are expected to expand even more while the comet will fade as the dust disperses. Comet Holmes showed a double outburst in November 1892 and January 1893. It is not known if the present activity in the comet will follow the pattern from 1892, but continued observations from Mauna Kea are planned to watch for a second outburst. Most comets show small fluctuations in brightness and some have distinct outbursts. The huge event on-going in comet Holmes is unprecedented, however.
The orbit period of comet Holmes is about 6 years, putting it in the class of Jupiter Family Comets whose orbits are strongly influenced by Jupiter. These objects are thought to have spent most of the last 4.5 billion years orbiting the Sun beyond Neptune, in a region known as the Kuiper Belt. Holmes probably was deflected into its present orbit within the last few thousand years and is losing mass as it evaporates in the heat of the Sun. In another few thousand years it is likely either to hit the Sun or a planet, be ejected from the Solar system, or simply die by running out of gas.