Comet 17P/Holmes is now the largest object in the solar system.
Since beginning its unexpected and unexplained expansion last month, the comet has increased its apparent brightness nearly one million times, and swollen to a diameter of 1.4 million kilometers, greater than that of the Sun.
Calling it the largest object in the solar system is a bit misleading, since nearly all of that enormous 1.4 million kilometer diameter is composed of the comet’s coma, the cloud of tiny particles that have been ejected from 17P/Holmes’s comparatively tiny solid nucleus. The nucleus of 17P/Holmes, probably composed of rock and frozen gas, is estimated to be less than 4km in diameter. Below, courtesy of Space.com, is an image comparing the size of the Holmes coma to the Sun, with Saturn shown to scale in the lower right corner as a reference.
This isn’t the first time 17P/Holmes has suddenly erupted and swelled to many times its former size and apparent brightness. The comet was discovered during such an eruption, in November 1892, when British amateur astronomer Edwin Holmes noticed it while observing the Andromeda Galaxy. The comet was named for Holmes. “17P” indicates it was the 17th comet to be catalogued, and is a periodic comet, defined as one having an orbital period of less than 200 years. The orbital period for Holmes has been calculated at about 6.9 years.
Despite its dramatically increased apparent brightness, and the huge diameter of its coma, 17P/Holmes is still dwarfed by the largest non-physical structure known to exist in the solar system: the ego of Bill O’Reilly. A scale comparison is below.
As you can see, if Bill O’Reilly’s head was increased in physical size commensurate to that of his ego, it would have a diameter several times the size of the coma of 17P/Holmes, and, as illustrated by the bottom-right inset, would, like the comet, be many trillions of times larger than O’Reilly’s own, unremarkable penis.