Summary
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. Other astronomers involved in the UH program
to study the comet include Bin Yang, Nuno Peixinho and David Jewitt.
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.