The current theory of planet formation, known as the “core accretion theory” is increasingly running into direct conflicts with observations. This was only a question of time because it is based on a physical impossibility: nobody has figured out a way to
grow planetesimals from tiny dust grains in the nascent disk because gravity does not kick in until the seeds have reached a sufficiently large size and the turbulent environment expected works in the opposite sense to that desired (i.e., it smashes apart colliding debris instead of promoting growth). Thus, planet-formation models require the researcher to put in certain things by hand, which is, yes, fudging. This should be a big clue that the theory may be completely wrong. It is an incredible stretch to refer to a theory as even satisfactory, let alone successful, if this amount of hand-tweaking has to be done. You will rarely read in popular news articles that the planetesimals have to be made by sticking dust grains together by hand, or worse, simply assuming sufficiently large seeds already exist. In the latter case there is nothing gained because the problem is still there.
Aside from this fundamental difficulty there are additional problems and the paper I discuss here describes one of them. The paper is entitled The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters (Science 330 (2010), 653-655), by A. W. Howard et al. This paper reveals one of the most fundamental problems with the current core accretion scenario in addition to the “no sticking” problem. The paper examines the properties of a sample of exoplanets that have an orbital period of less than 50 days and addresses how the number of exoplanets found in various mass ranges compares with the prediction of the core accretion scenario. The authors studied exoplanets around 166 stars and found that, compared to the theoretical prediction, there is a statistically significant overdensity of planets in the mass range of 5 to 30 Earth masses (with orbital periods less than 50 days). The core accretion scenario actually predicts a “planet desert” in this regime, but no such deficit of planets is found. The severe conflict between the prediction and reality is heavily underplayed in the paper. The abstract states, “This region of the parameter space is in fact well populated, implying such models need substantial revision.” In other words, the paper does not state that the current models could be completely wrong, or that the current models are based on an impossible premise, requiring the need for a paradigm shift. The paper never really clarifies what is meant by “substantial revision” and stops at that. The observational result reported in the paper is robust to selection effects because the “completeness correction” that was applied to account for planets that might have been missed by observational bias increases the number of planets in a given mass range. In other words any observational bias would work in the direction of reducing the disagreement between predicted and observed planet numbers, not increasing it. The paper does not of course mention anywhere that the models that are referred to require the sticking together of boulders by hand.
Now, the reason why the core accretion scenario predicts a planet desert for the mass range of around 5 to 30 Earth masses and an orbital period of less than 50 days is very simple. A growing planet in this regime undergoes rapid evolution in only one of two ways. Either interactions with the environment result in energy loss that makes the planet rapidly spiral inwards towards its host star, or rapid runaway accretion of gaseous material grows the planet too quickly into a giant. Since planets with masses of 5 to 30 Earth masses are short-lived in the model, a snapshot survey should not find many of them. It is quite remarkable that even with a foundation that forces the sticking together of boulder-sized and mountain-sized chunks by hand, even with all the additional knobs and dials that are available to tweak, the standard model of planet formation fails in a very fundamental way. The spiraling into the host star and the runaway growth scenarios are both controlled by basic physics, and neither scenario can be made to go away by the knobs and dials in the model. This should be a big clue that something is very wrong with the current paradigm, rather than indicating that there is a problem with the details of the model. What is interesting is that both problems of planet migration and runaway accretion were known before observational verification, but the problems were never confronted with real data until recently.
