Super Earths: failed cores of giant planets?

Generally, Super Earths are defined as exoplanets with masses up to earth's mass. It’s good to note that this definition doesn’t tell anything about other physical quantities of the planet like: Chemical composition, orbital properties, planetary radius temperature, etc. Discovery of the first member of this family is as old as discovery of exoplanets but it takes almost 13 more years for finding the next one! The first super earth(s) observed in 1991, two planets orbiting around a pulsar. They have masses about 4 earth's mass. The first Super Earth around main sequence star was observed in 2005 and since then the members of this family rapidly growing in time. Based on data given in “Exoplanet Data Explorer” we know 17 Super Earths today while 13 of these discovered in 2009 and 2010. Figure 1 shows the mass and semi major axis of Super Earths; Colors indicate the date of discovery.

    Planets can be classified in three major categories: gas giants, icy and rocky planets. Planet formation scenarios based on the core-accretion mechanism seek to understand the different origins of these objects.

    Gas giants, made essentially of hydrogen and helium, are formed through a runaway gas accretion phase onto a heavy-element core, which makes them grow quickly to large masses in the Jupiter and Saturn range.[1]

    Alternatively, rocky planets form in the inner regions of the disk as truly terrestrial planets like the ones in our solar system, and are therefore made essentially of silicates and iron. In this case, the timescale for their formation is however much longer than for gas giant cores formed in the outer regions.[1]

    Icy-rocky bodies, on the other hand, might have two different origins. On the one hand, they may form beyond the snow line as failed gas giant cores, accumulating a large amount of ices but failing for some reason (e.g. disk dissipation) to acquire a massive H/He envelope. Subsequent inward migration could then bring them on close-in orbits such as those of the presently-known objects.[1] There is another scenario was claimed by Boss(2006) which shoes that these type of planets also could have been formed by competing mechanism in disk instability, coupled with photo evaporative loss of their gaseous envelopes by an strong source of UV radiation like a hot star.[2] Based on his simulation of these processes around an M dwarf star he concludes that Jupiter-like planets could be formed in the 10 AU and then may migrate inward to form presently observed systems. But this is the case when the parent star located in the low mass star forming region. If the parent star is born in high mass region then it probably will have luminous neighbors in the future. They can evaporate the volatile gasses in the proto-planetary disk while the planets are forming. As stated in the Boss’s paper: “The key factor for whether a gaseous proto-planet becomes a gas giant or an ice giant is the critical orbital radius r_e outside of which photo evaporation can remove the disk gas and hence the proto-planetary envelope gas.”[2]

    As final part it’s important to note that there is short period Super Earth observed in 2005 (G1876; Rivera et al. 2005). Presence of other two gas giants outside the G1786’s orbit implies that the Super Earth formed interior to the gas giants. Formation of such planets could not be explained in the context of scenarios explained before!