Pluto Has Moons

That distant world called Pluto has surprised astronomers again, yielding up yet another moon.  Pluto’s largest moon is Charon and was discovered in 1978.  Two more — Nix and Hydra — were found in 2005. The new one, called P4 (for now), is quite small, somewhere between 13 to 34 kilometers across, and small enough that it was probably missed in earlier images of the system taken by Hubble Space Telescope. This latest HST image was taken as part of  a search for ring material around the distant dwarf planet, in support of the New Horizons mission, which is en route to Pluto.

So, how would Pluto, itself a small world like many others in the outer solar system, get moons?  The current thinking is that a collision between Pluto and another world early in the history of the solar system would have flung material out into orbit. Eventually, the pieces and parts would have coalesced back together, forming the family of moons we see today.

When I read this story, the first things I wondered were “Why search for rings around Pluto?”  and “Where would the material for Plutonian ringlets come from?”  A long-ago collision would have provided material for rings, but by now, that material would have been cleared away or coalesced into moons, such as Nix, Hydra, P4 (and maybe even Charon?).  To maintain a ring system, you need a constant source of material being tossed out to space.  At Pluto, that source may well be material “chipped away” from the icy surface by the impacts of tiny micrometeoroids.  That would provide chips of ice to form a faint, thin ring. If it exists, it hasn’t yet been detected. But, HST would be the best instrument we have at this time to find the ring.  Once New Horizons gets there, it may well “see” the ring, if it exists.

I like it when HST finds things like this. It’s a continuing reminder that the venerable telescope has a lot of life in it yet; and will keep surprising astronomers with new finds.