  The dynamical interactions that occur in newly formed planetary systems may
reflect the conditions occurring in the protoplanetary disk out of which they
formed. With this in mind, we explore the attainment and maintenance of orbital
resonances by migrating planets in the terrestrial mass range. Migration time
scales varying between millions of years and thousands of years are considered.
In the former case, for which the migration time is comparable to the lifetime
of the protoplanetary gas disk, a 2:1 resonance may be formed. In the latter,
relatively rapid migration regime commensurabilities of high degree such as 8:7
or 11:10 may be formed. However, in any one large-scale migration several
different commensurabilities may be formed sequentially, each being associated
with significant orbital evolution. We also use a simple analytic theory to
develop conditions for first order commensurabilities to be formed. These
depend on the degree of the commensurability, the imposed migration and
circularization rates, and the planet mass ratios. These conditions are found
to be consistent with the results of our simulations.
