The Sun is no longer the largest object in the Solar System: That honour has fallen temporarily to a previously innocuous comet. The comet called 17P Holmes, shot to prominence in late October when its brightness suddenly increased roughly a million-fold.
Since then, both its size and its profile have grown - earlier this month astronomers at the University of Hawaii Institute for Astronomy declared that its diameter had outstripped that of our Sun.
Many ancient cultures interpreted comets as portents of doom. Should we be worried? Are not comets supposed to be small? It seems ridiculous that the titanic Sun could be dwarfed by a comet.
Although huge in diameter, 17P Holmes 's gravitational field is negligible in comparison. "Its just a few snowflakes per cubic meter," Roche says - a far cry from the superdense, raging nuclear inferno of the Sun.. So how is this comet holding together? It is not really.
The ice is falling away from the comet's core, and as the coma gets bigger it also gets more dispersed. Eventually it will get so big and spread out that it won't even be discernible as belonging to the comet anymore. Is 17P Holmes dominating the sky?
Not exactly dominating, although it is visible as a fuzzy 'star' in the northeastern skies, and should continue to be large and bright for weeks, if not months.
Holmes Comet has been seen to burst in brightness before. In November 1892 and January 1893 it displayed a 'double burst' - although the Hawaii astronomers describe the current ongoing burst as "unprecedented."
It is very difficult to say what triggered this outburst, Roche admits. "Comets are tumbling through space, flexing and rolling," he says. "They are undergoing lots of stresses and strains, and they are very porous - they are more like Swiss cheese than a solid ice cube, so bits can easily crack and flake off." It is also possible that an interaction with the Sun's 'weather' - a stream of radiation flowing from the Sun - could have triggered the comet to bloom in brightness.
And it is showing signs of developing a tail, as many comets do when when their comas begin to be buffetted backward by the Sun's rays. How long will it last? Also difficult to say. The Hawaii team estimates that it is still expanding at a staggering 0.5 kilometres every second. But Roche points out that, the more it grows, the more its mass dwindles as its ice drifts off into space or gets left behind. "It is shedding mass all the time," he says. "It may just fade away and become a normal, unspectacular comet again." Some porous, rocky bodies in the Solar System are thought to be the rocky corpses of comets that have lost all their mass. Others, such as Shoemaker-Levy 9, are ripped apart when they stray too close to other huge bodies such as Jupiter. And some comets just die a mundane death, Roche says: "Every now and then, they just fall apart, almost as if they are dying of old age."
Excerpts from: "Sun no longer the biggest body in the Solar System" by Michael Hopkin
Courtesy: The Hindu, Science & Technology page, December 13, 2007
