Expert: Tom Whiting - 5/10/2010

Question
I know that earth is a stable planet due to the presence of it's nearby moon. But i was wondering if mars is a stable planet. And i want to know what do they mean when they say a planet is stable ( is it the climate, positioning, speed, angle, precession...)
Thank you in advance.

Answer
Hi Gabriel,
Stability requires a time frame to be stated.  Over 10,000 years (past and future) all major planets are stable. But if you're thinking of 100 million years, then there is no perfect stability, even for the Earth.  Astronomers today talk less of stability and more of chaos theory and probabilities of what is called secular (long term) resonance. Even a 15 meter change in the Earth's orbital position would mean that we cannot determine with any accuracy where the Earth is in it's orbit some 100 million years later, see
http://www.fortunecity.com/emachines/e11/86/solarsys.html

Turns out that over the long run, the planet Mercury is the most dangerous body in the Solar System. There is a fine article about this by Gregory Laughlin, Professor of Astronomy at U. of C. Santa Cruz, in the April 2010 issue of Sky & Telescope Magazine, entitled "Hanging in the Balance", pgs 26-33.  In that article, he explains that Henri Poincare back in the late 1800's proved that a definitive trajectory for the Solar System cannot be computed indefinitely far ahead, and that any fate of a planet's orbit must be expressed in the language of probabilities.
(This led to the modern day chaos theory and the famous 'butterfly effect').
He goes on to explain where secular resonance on Mercury's orbit, extending the eccentricity over hundreds of millions of years, could spell catastrophic results with a probability of 1%
before the sun goes Red Giant in a few billion years.
So the word stable is out, and chaos theory and probabilities, are in. But in any case, turns out that the entire Solar System is very "stable" for at least the next 50 million years or so, except for some very low probability... extra-solar system intruder.
Clear Skies,
Tom Whiting
Erie, PA USA