New Revelations About Pluto

Few interesting results about the dwarf planet Pluto are published in Nature six weeks ahead of NASA’s New Horizons first fly-by. New Horizons Mission will zoom past Pluto on July 14^th within 12,500 km of planet’s surface. It is expected to unravel more details about Pluto. Besides the popular Earth- Moon binary planet system in our Solar System, another pair that falls in same category is the Pluto-Charon system. With a mass of 11% of the Pluto, Charon its brighter moon, orbits the binary planet system centre of mass at a distance of 17,500 km every 6.4days The images of the Hubble Space Telescope (HST) revealed presence of four circumbinary satellites whose masses roughly 0.001% of Pluto. These are the inner most and least massive of moons Styx, others are Nix, Kerberos and Hydra. These three bodies are so closely packed that it hardly leaves room for any stable satellite to exist between their orbits. The new finding essentially sheds light on satellites and planet formation and how they remain stable in their orbits for billions of years.

The Plutoid system helps us to understand how every object has gravitational sphere of influence that prevents other objects from orbiting nearby. Hence larger the size of object greater will be its sphere of influence. When the sphere of influences of neighbouring objects overlap, it is impossible to place any object on the stable orbits between them. So smaller objects like interplanetary dust might orbit in their intervening spaces but larger objects cannot.

Till now scientists had a different hypothesis regarding the formation of satellites or planets around a star. It was thought that a satellite or planet start as a small seed in a disk or a ring surrounding the star (or planet) at the centre. These seeds start growing by agglomeration of other small solid objects in its path. Eventually these growing bodies would experience the gravitational pull of other adjacent objects. Continued growth would result in overpacking whereby sphere of influence of many objects would overlap. As the gravitational forces increases, the orbital motions become chaotic prompting the merger of nearby objects. This hypothesis is strikingly different from the Pluto-Charon system wherein it is conceived that proto-Pluto and proto-Charon might have collided to form a binary planet surrounded by expanding ring of debris. Pre-moons might have survived the collision and new moons might have grown from small particles in the debris. The four moons that have resulted from this process are found to have an orbital periods in the ratio of (3:4:5:6) to that of Charon.

Astronomers Showalter and Hamilton have worked out the orbital period, orientation and the ellipticity of each orbit. Styx, Nix and Hydra are locked together as what is referred as three-body resonance system a phenomenon wherein timings of their orbits are linked coordinating and stabilizing their movements (preventing them from crashing into each other). Perhaps, this is the reason why a dwarf planet like Pluto has so many moons. A similar three-body resonance system is found among Jupiter’s moons- Lo, Europa and Ganymede. Nix and Hydra have bright surfaces similar to Charon and reflects 40% of light that hits it Kerberos is as dark as coal. Thus this raises questions about formation of the heterogenous satellite system. Binary planets constitute two bodies of similar planetary mass orbiting their common barycentre. The mutual motions of this binary create an asymmetric gravitational field inducing wobbles in the orbits of the outer most moons. Nix and Hydra rotate chaotically, meaning they don’t keep the same side facing Charon-Pluto driven by the Pluto-Charon binary system. Implying that there might be days when sunrises in the east and sets in the north for the satellites of Pluto.

 

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