Today , many extrasolar planets have been detected .
Some of them exhibit properties quite different from the planets in our solar system and they have eluded attempts to explain their formation .
One such case is HD 149026 b .
It was discovered by .
A transit-determined orbital inclination results in a total mass of 114 \mathrm { M _ { \earth } } .
The unusually small radius can be explained by a condensible element core with an inferred mass of 67 \mathrm { M _ { \earth } } for the best fitting theoretical model .

In the core accretion model , giant planets are assumed to form around a growing core of condensible materials .
With increasing core mass , the amount of gravitationally bound envelope mass increases .
This continues up to the so-called critical core mass – the largest core allowing a hydrostatic envelope .
For larger cores , the lack of static solutions forces a dynamic evolution of the protoplanet in the process accreting large amounts of gas or ejecting the envelope .
This would prevent the formation of HD 149026 b .

By studying all possible hydrostatic equilibria we could show that HD 149026 b can remain hydrostatic up to the inferred heavy core .
This is possible if it is formed in-situ in a relatively low-pressure nebula .
This formation process is confirmed by fluid-dynamic calculations using the environmental conditions as determined by the hydrostatic models .

We present a quantitative in-situ formation scenario for the massive core planet HD 149026 b .
Furthermore we predict a wide range of possible core masses for close-in planets like HD 149026 b .
This is different from migration where typical critical core masses should be expected .