Published online 16 November 2007 | Nature | doi:10.1038/news.2007.261

News: Briefing

Sun loses crown as biggest body in the Solar System

Comet Holmes is now bigger than the Sun. So are we doomed or is there a more mundane explanation?

Comet Holmes: bigger than the SunJohn Lanoue

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.

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Aren't comets supposed to be small?

It seems ridiculous that the titanic Sun could be dwarfed by a comet. Comets are typically referred to as 'dirty snowballs' tumbling through space, and for good reason. Most comets that swing by Earth are from the Oort Cloud — a distant band of dust and ice at the limit of the Solar System — and so are composed of ice and dry ice (frozen carbon dioxide) bound up with dust and small chunks of rubble. They are usually just a few kilometres wide, and zoom past planets on their travels.

The rocky core of 17P Holmes, called its nucleus, is thought to be only 3.6 kilometres across — although because it is 240 million kilometres away, not even the Hubble Space Telescope can see it in enough detail to measure this accurately.

So in what way is 17P Holmes bigger than the Sun?

Though its core is small, this comet has a huge 'coma' — a spherical cloud of icy particles surrounding it. Estimates by Rachel Stevenson and her colleagues at the Hawaii observatory now estimate the diameter of the coma to be 1.4 million kilometres — just edging past the Sun, with a diameter of 1.39 million kilometres.

How did that happen?

The answer may well be linked to the comet's mysterious boost in brightness, suggests comet expert Paul Roche of Cardiff University, UK. Many comets are coated with a sooty layer of carbon, which absorbs light. But in Holmes's case, this may have fractured and split, releasing the highly reflective ice inside. This release of ice would have boosted both the comet's brightness and its size, as the icy particles float out from the rocky nucleus.

Why hasn't the Solar System's gravity gone crazy?

Relax, the Sun is still by far the most massive body in the Solar System (it contains more than 99.8% of all the system's mass). Although huge in diameter, 17P Holmes's gravitational field is negligible in comparison. "It’s just a few snowflakes per cubic metre," Roche says — a far cry from the super-dense, raging nuclear inferno of the Sun.

So how is this comet holding together?

It isn't 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.

Has this sort of thing happened before?

Yes, but generally only with the largest comets, such as Hale-Bopp, which grew even bigger than 17P Holmes when it passed close to the Sun in 1997. According to the European Southern Observatory, it reached more than 2 million kilometres across. Comet Halley also broke the 1.3-million-kilometre mark in 1988, they say.

Holmes Comet itself 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".

Why is Holmes doing this now?

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It's very difficult to say what triggered this outburst, Roche admits. "Comets are tumbling through space, flexing and rolling," he says. "They're undergoing lots of stresses and strains, and they're very porous — they're 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 now showing signs of developing a tail, as many comets do when their comas begin to be buffeted backwards by the Sun's rays.

How long will it last?

Also difficult to say. The Hawaii team estimates that it's 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's 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're dying of old age." 

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