Home→ Exoplanets FAQ/Tutorial → Core-accretion theory of planet formation
The core-accretion theory of planet formation is the currently accepted theory of planet formation, whether in our solar system's history or in a star-planet system elsewhere in the Universe. (The word “accretion” refers to the process of a “core” seed gathering more and more material to itself resulting in growth.) The basic idea is as follows:
1. A star in its early life develops a flattened disk of material that is rotating with it in the star's mid plane. The disk is sometimes known as a debris diskbecause aside from abundant Hydrogen and Helium it contains all kinds of other material, some of which is called “dust” (of varying chemical compositions).
2. The dust is initially microscopic but eventually collisions and aggregation are supposed to produce centimeter-sized grains. That this can be done is still being studied in the laboratory and theoretically. There seem to be some conditions under which it is possible.
3. The dust particles are supposed to grow by further “sticking together.&rduo;
4. Statisically, a few bodies are supposed to grow so much that their self-gravity then overwhelms any matter in the neighborhood at which point very rapid growth is supposed to take place. This can only happen if a body reaches hundreds of kilometers in size, and at this stage it is called a planetesimal.
5. Various pathways are possible after the previous stage and theoretical work is still underway to work out the details of how terrestial planets and gas giants (for example) are formed. The simulations are complex and involve making assumptions, and have many knobs and dials that can be tweaked.
There is a major problem with the above scenario, however. That is, the entire theory is based on a physical impossibility. It is not physically possible to go from step 3 to 4 because lumps of matter that have reached a meter or tens to hundreds of meters in size do not stick together. The same theory that creates them has them crashing into each other at very high speeds. The same theory that creates them smashes them apart before they ever reach planetismal size. Their self-gravity is not enough to make them stick together even if they were not traveling at high relative speeds, and the forces that stick dust grains together don't operate on rocks and boulders.
Note that although debris disks have been directly observed, there is not a single case of direct observation of a planet that is unambiguously in the process of formation or migration. There is no smoking gun. The evidence in support of core accretion is all theoretical, and it could be flawed in a number of ways. Observing a planet in a debris disk does not prove that it was formed in the disk, and it does not indicate the mechanism of formation. In other words, there is no direct proof that planets form in the disk, it is an inference made simply from the coexistence of the planet and the disk. If you catch someone at a crime scene, you have to establish many things to prove that the person committed the crime. The same applies to a more recent case (Kraus and Ireland 2011, LkCa 15: A Young Planet Caught at Formation?). The question mark at the end of the title was conveniently omitted in a flurry of popular news stories. The paper discussed caveats pertaining to the fact the observed “blob” could just be a “random” chunk of matter that has nothing to do with planet formation. The authors themselves stated that further observations should be made to make conclusive deductions, but the spin in popular news stories (even those on dedicated astronomy websites) did not reflect this bottom line.
The core-accretion theory of planet formation is founded on a physical impossibility so it was bound to eventually conflict with observations, and indeed there are several observational facts that directly show that something is wrong with the theory. However, most planetary scientists are still hanging on to the theory, hoping that somebody will find a way to magically go from step 3 to step 4. Even if that were possible (and it is not), there would still be some observations that conflict with the theory or cannot be explained by it. At the moment the core-accretion theory, with its growing conflicts with observations, is being propped by increasingly more complex and contrived explanations and patch-ups (mostly of the hand-waving variety), but this will not be sustainable for much longer.
If you are interested in more on this topic, please read the section on exoplanet migration, and then the more detailed section on planet formation theory. The latter explains in more detailed the conflicts between observation and theory, backed up by references to the scientific literature, and it also has a synopsis of how scientists at the frontier of this field respond to the conflicts.
Learn more about exoplanets and alternative theories and ideas that have been proposed for planet formation, with Exoplanets and Alien Solar Systems.
File under: Currently accepted theory of planet formation. Core accretion theory summarized.