Theoretical Consequences
The discovery was advocated as a piece of evidence for the popular solar nebula accretion model, where planets are formed from the accretion of smaller objects. In this model, giant planet embryos grow large enough to acquire large envelopes of hydrogen and helium. However, opponents of this model emphasize that only one example of such a dense planet is not proof. In fact, such a huge core is difficult to explain even by the core accretion model.
One possibility is that because the planet orbits so close to its star, it is — unlike Jupiter — ineffective in cleansing the planetary system of rocky bodies. Instead, a heavy rain of heavier elements on the planet may have helped create the large core.
Careful radial velocity measurements have made it possible to detect the Rossiter-McLaughlin effect, the shifting in photospheric spectral lines caused by the planet occulting a part of the rotating stellar surface. This effect allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. In the case of HD 149026 b, the alignment was measured to be +11° ± 14°. This in turn suggests that the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle would have suggested a violent interplay with other protoplanets. As of August 2008 this is still the canonical measurement.
Read more about this topic: HD 149026 B
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