Forming Neptunes and mini-Neptunes within 1 AU

Migration then assembly: Formation of Neptune mass planets inside 1~AU
http://arxiv.org/abs/1105.2050

Quote :

We demonstrate that the observed distribution of 'Hot Neptune'/'Super-Earth' systems is well reproduced by a model in which planet assembly occurs in situ, with no significant migration. This is achieved only if the amount of mass in rocky material is 50--100 M_{\oplus} interior to 1 AU, so significant radial migration of material is likely still required, but it must occur earlier than the final assembly stages. The model not only reproduces the general distribution of mass versus period, but also the detailed statistics of multiple planet systems in the sample.
We furthermore demonstrate that cores of this size are also likely to meet the criterion to gravitationally capture gas from the nebula, although accretion is rapidly limited by the opening of gaps in the gas disk. If the mass growth is limited by this tidal truncation, then the scenario sketched here naturally produces Neptune-mass objects with substantial components of both rock and gas, as is observed.
The quantitative expectations of this scenario are that most planets in the `Hot Neptune/Super-Earth' class inhabit multiple-planet systems, and that they divide naturally into gas-rich (Hot Neptune) and gas-poor (Super-Earth) classes at fixed period. The dividing mass ranges from 3 M_{\oplus} at 10 day orbital periods to 10 M_{\oplus} at 100 day orbital periods. For orbital periods $< 10$ days, the division is less clear because a gas atmosphere may be significantly eroded by stellar radiation.

More on the subject of mini-Neptunes, contemplating planets down to a few Earth masses...

Formation and Structure of Low Density Exo-Neptunes

Some of the hot, Neptune-sized planet candidates could be substantially less massive than Neptune...

That is not good information because density all planets down to 1 earth radius will be as low as 1g/cm3 or even less 0.5 g/cm3.Like admin(sirius) mentioned in past low amount of the light material(h,he,water)during accretion in alpha centauri system may be more favourable than we ever expected.We will see in future but at present we have only 1 multiplanet system with small sizes, low density minineptunes.Massess all planet candidates are not measured yet. I Hope formation low density planet with small sizes is not existing in all extrasolar planetary systems

tommi59 wrote:

I Hope formation low density planet with small sizes is not existing in all extrasolar system system

I'm guessing that the planetary systems that have these short-period mini-Neptunes probably had a different formation history than planetary systems with small, truly terrestrial planets like ours. If so, we just need to keep looking. No doubt the mini-Neptune systems are just easier to find.