Two of Pluto's moons, Nix and Hydra, wobble unpredictably, according to a comprehensive analysis of data from NASA's Hubble Space Telescope.


The moons wobble because they are embedded in a gravitational field that shifts constantly, researchers said. This shift is created by the double planet system of Pluto and Charon as they whirl about each other.


Pluto and Charon are called a double planet because they share a common centre of gravity located in the space between the bodies.


Their variable gravitational field sends the smaller moons tumbling erratically. The effect is strengthened by the football-like, rather than spherical, shape of the moons. Scientists believe it's likely Pluto's other two moons, Kerberos and Styx, are in a similar situation.


"Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system," said Mark Showalter of the SETI Institute in Mountain View, California, who conducted the study with Doug Hamilton of the University of Maryland at College Park.


"Our research provides important new constraints on the sequence of events that led to the formation of the system," Showalter said.