Astronomers Predict that Pluto Has a Ring

Until recently, the only ring in the Solar System was Saturn’s. But in 1960s and 70s, astronomers discovered rings around Uranus and Neptune. Meanwhile, the Voyager 1spacecraft sent back images of Jupiter’s ring.

To be sure, these rings are much less impressive than Saturn’s but the implications are clear: rings seem much more common than astronomers once thought. Perhaps they are even the norm.

And that raises an interesting question: could Pluto possibly have a ring?

The observational evidence is that Pluto does not have a ring. The best images are from the Hubble Space Telescope and they show nothing.

But today, Pryscilla Maria Pires dos Santos and pals at UNESP-São Paulo State University in Brazil say that Pluto ought to have a ring after all, but one that is too faint for Hubble to spot.

Their conclusion comes from modelling the way that micrometeorite impacts on Pluto’s satellites, Nix and Hydra, ought to send dust into orbit about the dwarf planet.

This dust inevitably spirals into Pluto and its satellites because of its interaction with the solar wind. In this way, the dust is removed from orbit.

But that doesn’t mean it can’t form a ring. The important question is whether the dust can be replaced as quickly as it is removed.

Pires dos Santos and co calculate that the dust initially forms a ring about 16,000 km wide, encompassing the orbits of both Nix and Hydra. However, the solar wind then removes about 50 per cent of the dust within a year.

However, that still leaves enough to form a ring, albeit it an extremely faint one. “A tenuous ring…can be maintained by the dust particles released from the surfaces of Nix and Hydra,” say Pires dos Santos and co.

They calculate that its transparency (or optical depth) has a value of 10^-11. By comparison, the main ring of Uranus has a transparency of between 0.5 and 2.5.

Hubble ought to be able to to see a ring around Pluto with a transparency of about 10^-5 so it’s no surprise that it hasn’t seen the ring that the Brazilian team predict. There’s no way to see such a ring directly from Earth.

Fortunately, there is a way to settle the matter.

The New Horizons spacecraft is currently on its way to Pluto, equipped not with a camera capable of seeing the ring but with a dust counter that should do the trick instead. If this probe finds itself in the lightest of dust clouds when it arrives on 14 July 2015, we’ll finally know for sure.

Ref: arxiv.org/abs/1108.0712: Small Particles In Pluto’s Environment: Effects Of The Solar Radiation Pressure