Some years ago, astronomers at the Naval Observatory constructed a computer called the 'Digital Orrery' which calculated the positions of the outer planets every 40 days or so, for the next billion years.
Even though the orbits of Pluto and Neptune intersect, they are locked in a gravitational resonance condition which seems to prevent them coming closer than a few 100 million miles of one another. The above computer simulation of thousands of Pluto orbits is made relative to a frame or reference that orbits with Neptune. It shows that although Pluto's orbit is complex, it can never get very close to Neptune itself because of the detailed ways in which energy is transferred between the planets, gravitationally.
On January 21, 1979 Pluto passed inside the orbit of Neptune, and on September 5, 1989 it was at its closest distance to the Sun. Then in March, 1999 it will cross Neptune's orbit once again and resume its title as the farthest planet in the solar system. This situation will reoccur in the year 2236 and happens every 247 years.
Probably not...at least not for the next 750 million years or so. Using a special-made computer called a 'Digital Orrery' the celestial mechanics of the solar system were simulated at 40 day intervals over the next billion years or so. Pluto was found to be in a 'chaotic' orbit, but one that would not take Pluto closer than tens of millions of miles from Neptune. This line of investigation has also turned up the troubling prediction that Mars or Mercury could be ejected from the solar system because of their elliptical orbits and the cumulative effects of chaotic perturbations added up over billions of years. Also, by simply shifting the start position of Mars by a kilometer, the location of Pluto in its orbit was changed by one half of an orbit, which shows how sensitive the dynamics of the solar system are to slight changes in its initial conditions. Astronomers now worry that solar systems as benign as ours for life, may be rare, if planetary systems are so precariously balanced.
This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
Return to Ask the Astronomer