Pluto’s moon system – how cool can it be?

One of the most valuable results New Horizons was able to deliver in its historic Pluto’s flyby was more information and precious imagery of its moon system -which is far more interesting than anybody had imagined. A lot has been written in these last months about it – here I just want to take stock and offer some links / images.


First of all, some details regarding its origin. Differently from other cases, scientists believe that the moon system itself was formed by a collision between Pluto and another similar body (possibly another KUO) in the early days of the Solar System – the fragments would eventually become its satellites.

The moons themselves are five:  Charon (known since 1978, about half the size of Pluto and tidally locked), Nix and Hydra (discovered in 2005 by Hubble Space Telescope) Kerberos (found in 2011) and finally Styx (the last one to be identified -only in 2012, when scanning the areas for dangers to the New Horizons’ flyby).

Apart from Charon, these moons are tiny. A lot of descriptive details are available at New Horizons’ NASA page, but this one is a good summary.


Hydra, Pluto’s outermost known moon, orbits Pluto every 38 days at a distance of approximately 40,200 miles (64,700 km), while Nix orbits every 25 days at a distance of 30,260 miles (48,700 km). Each moon is probably between 25-95 miles (approximately 40- 150 kilometers) in diameter, but scientists won’t know their sizes more precisely until New Horizons obtains close-up pictures of both of them in July. Pluto’s two other small moons, Styx and Kerberos, are still smaller and too faint to be seen by New Horizons at its current range to Pluto; they will become visible in the months to come.”

Even more interesting is the way the moons itself orbit around Pluto. First of all, Styx, Nix and Hydra are linked by a gravitational resonance – like in the case of Jupiter’s moons Ganymede, Europa and Io -which helps maintain their orbit.  “The resonant relationship between Nix, Styx and Hydra makes their orbits more regular and predictable, which prevents them from crashing into one another. This is one reason why tiny Pluto is able to have so many moons.” (Hamilton & Showalter, University of Maryland). 

images (1)

However, due to the presence of the tidally locked couple Pluto- Charon, they present a chaotic rather rotation, which is quite difficult to predict -not even keeping the same side facing their parent planet. “If you lived on Nix, you would not know if the sun is coming up tomorrow; it is that extreme.You’d have days where the sun rises in the east and sets in the north.” (For the full explanation of their research, have a look at this).

A couple of videos will show what it does happen in practice.

For a more scientific coverage of Pluto’s moons, and the article itself, this is the reference:  M. R. Showalter & D. P. Hamilton (2015)  Resonant interactions and chaotic rotation of Pluto’s small moons Nature 522, 45–49 (04 June 2015), available online at Nature.

Leave a Reply

%d bloggers like this: