outer planet migrates outwards. If the inner planet stabilizes, the outer planet also stabilizes, and its presence may be sought to test this mechanism.
Yet another proposed mechanism is the capture of an 'orphan planet', i.e., one that is not bound to a star, by a protostar. The planet, in an almost parabolic orbit is subsequently braked by tidal effects.
Whatever the migration mechanism, one question that still remains largely in suspense is the end of this migration. What stops the planet's decay and prevents it from falling onto the star? In fact, the distribution of orbits (Fig. 6.7) clearly shows an accumulation of planets around 0.05 AU, and a gap at about 0.3 AU. This distribution, which is not caused by any observational bias, is statistically significant. It cannot be explained if planets end their orbital contraction by falling into the star. Some braking mechanisms have been proposed, based on interactions with the star or properties of the disk, but they require specific configurations (see Sect. 6.2.5).
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