The properties of the orbits (Fig. 6.8) may be examined for observational proof of orbital migration. These properties also serve as constraints in choosing the migration model. There is not a regular distribution of orbits, but rather an accumulation at short distances from the star (Fig. 6.7).
Planets with very short periods (4 days) have circular orbits whereas planets with periods of 7-12 days (known as 'borderline' planets) display a dispersion in eccentricity (Fig. 6.8, top). The tidal mechanism suggested to circularize orbits is most effective with stars that rotate slowly. It may be noted that this mechanism may be tested by considering the borderline planets. Those associated with young, rapidly rotating stars (for example, those in the Pleiades cluster), should have greater eccentricities than those associated with older stars that are rotating slowly (for example, those in the Hyades cluster).
One of the consequences of migration is heating of the planet, which may be subject to evaporation, losing its gaseous atmosphere, and retaining only its core of ice or rock. A Jupiter-type planet may thus evolve into a planet similar to Neptune. The first example of this mechanism has perhaps been observed in the planet HD 209458b, where detection of an extended hydrogen exosphere, observed in the Lyman-a line, enable us to estimate an extremely high rate of evaporative loss of the atmosphere (1010 g/sec). This rate is such that in a few million years it may lead to a giant planet without an atmosphere.
Systems with several planets are favourable places to search for signs of migration (see Sect. 7.3). Several of these systems include planets whose orbits are in mean-motion resonance. Modelling shows, however, that if the migration is sufficiently slow, the orbits of the planets that are trapped in resonance display a specific configuration of the angle between the periapses of the planetary orbits. The two systems in a 2:1 resonance and whose orbits are known with sufficient accuracy, GJ 876 and HD 82943, satisfy these conditions. The 55-Cancri system, with a 3:1 resonance, also displays a configuration of this type. All the resonant systems known so far are thus the result of a migration mechanism.
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