<P> The Neptune--Pluto system lies in a 3: 2 orbital resonance . C.J. Cohen and E.C. Hubbard at the Naval Surface Warfare Center Dahlgren Division discovered this in 1965 . Although the resonance itself will remain stable in the short term, it becomes impossible to predict the position of Pluto with any degree of accuracy, as the uncertainty in the position grows by a factor e with each Lyapunov time, which for Pluto is 10--20 million years into the future . Thus, on the time scale of hundreds of millions of years Pluto's orbital phase becomes impossible to determine, even if Pluto's orbit appears to be perfectly stable on 10 MYR time scales (Ito and Tanikawa 2002, MNRAS). </P> <P> Jupiter's moon Io has an orbital period of 1.769 days, nearly half that of the next satellite Europa (3.551 days). They are in a 2: 1 orbit / orbit resonance . This particular resonance has important consequences because Europa's gravity perturbs the orbit of Io . As Io moves closer to Jupiter and then further away in the course of an orbit, it experiences significant tidal stresses resulting in active volcanoes . Europa is also in a 2: 1 resonance with the next satellite Ganymede . </P> <P> The planet Mercury is especially susceptible to Jupiter's influence because of a small celestial coincidence: Mercury's perihelion, the point where it gets closest to the Sun, precesses at a rate of about 1.5 degrees every 1000 years, and Jupiter's perihelion precesses only a little slower . At one point, the two may fall into sync, at which time Jupiter's constant gravitational tugs could accumulate and pull Mercury off course . This could eject it from the Solar System altogether or send it on a collision course with Venus, the Sun, or Earth with 1--2% probability, 3--4 billion years into the future . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (November 2013) </Td> </Tr> </Table>

Planets closer to the sun have unstable orbits