Published online 23 August 2004 | Nature | doi:10.1038/news040823-3


Sedna 'has invisible moon'

Distant planetoid may have pitch-black partner.

Does the cold, distant Sedna have an invisible partner?Does the cold, distant Sedna have an invisible partner?© Caltech

It is truly a far-reaching debate: does Sedna, the most distant object ever discovered in our Solar System, have a moon? And if it does, why haven't we seen it yet?

A group of British astronomers has set out to shed light on the problem, and are claiming that Sedna has a moon, but it is so sooty that it reflects almost no light at all.

Writing in this month's issue of The Observatory1, Chandra Wickramasinghe of Cardiff University, UK, and colleagues argue that Sedna's putative partner is an entirely new type of object. More like an extinct comet than a rocky planetary body, they claim that the object could be made of the tarry carbon left behind when the ice that traditionally forms a comet melts away.

The resulting 'frozen smoke' is so black because much of it is empty space, Wickramasinghe's team claims. With such a pitted structure, the moon would absorb more than 99% of all the light that hit it, the researchers calculate.

Turning it over

So why, if we cannot see it, do experts think there is anything there at all? The answer lies in Sedna's unusually slow rotation. The planetoid turns on its axis just once every 20 Earth days, leading astronomers to suspect that it is being slowed by the gravity of a partner.

Sedna was spotted in November 2003 by Michael Brown and colleagues at the California Institute of Technology in Pasadena. Wickramasinghe's team used Brown's telescope data to examine Sedna's rotation and weigh up the likely force exerted on it by any mysterious moon.

The dark body would be not much bigger than Pluto's moon, Charon, which is around 1,170 kilometres across, Wickramasinghe reckons. It would be pitch-black, with a "fluffy, feathery surface" he says, meaning that it would give out hardly a glimmer when light hit it.

No such object has definitely been found in the Solar System. But Wickramasinghe thinks that if there is one, there may well be hundreds, lurking beyond Neptune and Pluto. "If Sedna has captured such an object, it must be from a significant population in the outer reaches of the Solar System," he told

Such talk may well prove premature. We do not yet know whether Sedna even has a moon, let alone one as exotic as Wickramasinghe suggests. But there may be a way to find out: although a dark comet would be virtually invisible to optical telescopes, it might emit infrared radiation. Wickramasinghe hopes that the world's infrared telescopes will now be trained on the edge of the Solar System to see whether the blackness is really teeming with invisible worlds. 

  • References

    1. Wickramasinghe J. T., Wickramasinghe N. C. & Napier W. M.. Observatory, 124. 300 - 302 (2004).