Planetary mid life crisis

Last post, I mentioned how the current IAU definition of a planet causes many of the classical planets to only attain planethood late in the solar system’s history. Saying that a planet has to clear its neighborhood implies that a planet has only one neighborhood. Interpreting this law in the context of planetary migration is tricky.

For example, an abbreviated timescale is shown below.



The red area to the left is the 30-50 million year time that it took the terrestrial planets to form. As is shown, moon rocks can be found that are only slightly younger than this. The orange band is the age of the Late Heavy bombardment, which created the large lunar impact basins. This is hypothesized to have been caused by Neptune and Uranus changing their orbits. Neptune, in particular, would have ended up in a new neighborhood, and had to clear it, scattering stuff all over the solar system.

Under the IAU definition, it is unclear if Neptune ceased to be a planet during that time, and if so, when it regained planethood. Similarly, if the objects were scattered into the path of other planets (which seems to be the case), did they also lose planetary status for this period?

Many exoplanets show even more extreme examples of planetary migration. So if we want a consistent definition for all systems, it is unclear how this will apply.

Note that in the time scale above, the LHB happens long after the planets are formed. Planetary processes were well and truly underway on all terrestrial planets at the time of this bombardment, and to suddenly toss in a period of non-planetness makes no sense whatsoever.