Research work of the Department of Physics
Why do moons of Pluto have a chaotic rotation? UA found out the answer
O investigador Alexandre Correia
In June this year Nature has announced that four out of the five moons of Pluto have a chaotic rotation. A research led by scientists Mark Showalter (SETI Institute) and Doug Hamilton (University of Maryland), United States, showed that the rotation of moons Nix, Hydra, Kerberos and Styx, unlike what happens to the moons of most planets that were already known, is not constant and varies unforeseeably over time. However, the research did not explain the reason for this unexpected phenomenon. Two months later explanation came from the Department of Physics (DFis), of the University of Aveiro (UA), and was made available to the world in the last issue of "Astronomy & Astrophysics Letters".

The work of the international team led by DFis explains the reason for the strange rotation of Pluto’s four moons by applying the laws of Physics. “In order to understand this problem”, says Alexandre Correia, coordinator of the research and specialist on solar systems, extrasolar planets and planetary physics, “it is necessary to consider two factors that distinguish these four moons from all others”, namely from Charon, the other moon of Pluto that has a regular rotation.

Thus, “as [the four moons in question] are small-sized bodies less than 50 kilometers across, they look more like potato-shaped asteroids than spherical bodies like the Moon and the Earth”. They have therefore, says Alexandre Correia, one axis longer than the others.

Another factor that makes Nix, Hydra, Kerberos and Styx unique within the solar system is that, unlike the four small moons, Pluto’s biggest moon, Charon, “is almost as big as Pluto". Therefore, technically, “the Pluto-Charon system must be classified as a binary system [system with two similar-sized bodies orbiting around a common centre of mass] and not as a planet-moon system”.

UA’s article published in Astronomy & Astrophysics Letters may be obtained here.