Home→ FAQ/Tutorial → What is type I, type II, and type III exoplanet migration?
Planetary migration refers to the physical motion of a planet relative to the host or parent star, involving changes in the size and other parameters of the orbit. It is important to remember that it is a theoretcial concept and has never been observed in our solar system or in any other exoplanet system. However, it has been invoked to be common in exoplanet systems because the currently popular theory of planet formation fails to explain certain observational facts such as the hot Jupiters that are found very close to the host star. Even then, migration cannot explain the pile-up of hot Jupiters close to the host star so further theory has to be invented to fix that (a halting mechanism for migration, which would otherwise cause a planet to plunge into the star instead of stay in orbit close to it). A satisfactory halting mechanism has not been found so the migration theory doesn't even fix the problem it was supposed to fix. I am not saying that migration is not possible. What I am saying is that the arguments to invoke it are currently circular. It is invoked to fix a theory of planet formation that does not work in the first place. However, in doing so, the argument is made that the planet formation theory (that it is trying to fix) does not allow planets to form close to the star. But the theory doesn't work so any of the physical assumptions and hand-fudging involved could be wrong, so it is circular logic.
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Curiously, most scientists are very defensive about the core accretion theory of planet formation, and the necessary migration (and the as yet undiscovered halting mechanism). However, since the majority of scientists also believe in upholding these theories of planet formation and migration that don't work, the theories don't even need to be defended very rigourously because the scientists have nobody to answer to except their peers. As a result, the scientific method, scientific rigor and any resemblance of checks and balances has evaporated, leaving only circular and hand-waving arguments.
The Wikipedia article on planet migration is a reflection of the view of the majority, and as such assumes the posture of conceited hand-waving and circular arguments as I have described above. Please read the article and count the number of times the phrase “believe to have” (or similar) is used. The article starts by saying that the standard planet formation theory cannot make close-in planets so we must make them further out and migrate them inward towards the star. The next sentence says that “it has also become clear that” terrestrial planets may be subject to rapid migration as well but not a single citation or justification is given for the “it has also become clear that.” Nobody has flagged this as needing a citation either. The paragraph ends with “Planetary migration is the most likely explanation for hot Jupiters...” That's it? Yes, that's the entire justification for planetary migration. Next, the middle of the article is all about the disk from which planets are supposed to have formed from, and it is all theoretical. The end of the article has a whole section on why migration is necessary to explain objects in the outer parts of our solar system. Essentially, the accepted theory of the fomation of the solar system cannot account for many of the properties of the objects in the outer solar system. Every detail of the history of the formation of the solar system is stated as fact. However, we know that the standard theory of planet formation does not work. Therefore, every detail of the history of formation of the solar system cannot be stated as fact.
Anyway, you still may need to know about the different types of (theoretical) planetary migration (you may have an exam on it or something), so here they are:
Type I Migration
In type I migration the mass of the planet is small enough that the basic structure of the embryonic disk (i.e., the protoplanetary disk) is not affected by the migration of the planet. (In jargon, the perturbations, in the form of spiral waves created by the planet, are linear.) Generally, a mass of about 10 that of Earth is taken as the threshold (upper limit).
Type II Migration
In type II migration, the motion of the planet through the protoplanetary disk does affect its structure and a gap in the disk around the planet is created as a result of migration.
There is also a type III migration, which is even more speculative.
Type III Migration
In type III migration is like type I migration in that it does not affect the`basic structure of the protoplanetary disk, but type III migration is faster. The difference with type I migration lies in the type of disk-planet interaction. In type I migration the interaction is associated with spiral wave disturbances, but in type III migration the interaction is associated with corotational flows in the disk.
Whatever the form of the migration, exchange of angular momentum between the disk and the planet occurs.
Learn more about exoplanets with Exoplanets and Alien Solar Systems.
File under: Planet migration theory. Three types of planetary migration in the protoplanetary debris disk. Reasons why planet migration is required.