I am studying for a project the dynamics of multi-planetary system in resonance. I have two questions about this domain.
In many articles they speak about 'mean motion resonance', what is the difference with a classic orbital resonance?
How can we define that two exoplanets are in resonance? Scientific articles are using the ratio of the orbital periods being related by an integer ratio, but in a book (exoplanet handbook-Michael Perryman) I saw "mean motion resonance: definition is via the librating or oscillating behaviour of some linear combination of resonant arguments". What is the difference between this or the ratio of periods to define the resonance?
Also, if you have any recommandation of books/articles about this topic, I would be glad to hear it.