# Formation of a planet with a mercury core

Building on this question about the magnetosphere for a planet with a substantial mercury core, what is the proto-planetary environment required to form a planet with a mercury core? Specifically, I'm looking for initial conditions to the protoplanetary disk in terms of elemental composition, not the process by which planets form from that disk. Assume that a star does form and is stable enough to support planet formation.

Details about crust composition/formation and details of life on such a planet are out of scope. Also, if this planet ends up being made partly of gold, that would be interesting too. Speculate if you please, but it's not required.

• The Earth is partly made of gold. May 2 '17 at 13:12

Mercury is an example of a volatile, which, for our purposes, means that it exists as a solid only at very low temperatures. One thesis (Funk (2015)) classifies it as "moderately volatile" (Table 1.1), also noting that its 50% condensation temperature, $$T_{c}$$1, is less than that of sulfur. It is, in fact, around 250 K. That is really, really, really low. For a graphical comparison, see Figure 2 of Albarede (2009).
Therefore, you need a really cold planetary disk. HL Tauri appears to be a possibility. Carrasco-Gonzalez et al. (2016) found a temperature of $$\sim70\text{-}140\text{ K}$$ at 10 AU. Given that temperatures in disks roughly obey a power law2 of $$T(r)=T(r_o)\left(\frac{r}{r_o}\right)^{-q}$$ and $$q=0.5\text{-}0.6$$, then if we take the lower temperature bound, we find that $$T(1\text{ AU})$$, for instance, can be $$221\text{ K}$$, which would seem to work. There still might not be enough mercury for a whole core to form, but one can hope.
1 $$T_c$$ is the temperature at which 50% of all the mercury is condensed.
2 In general, disks around Sun-like stars follow a power law of $$T(r)\propto\left(\frac{r}{1\text{ AU}}\right)^{-q}\left(\frac{M_*}{M_{\odot}}\right)$$ where $$M_*$$ is the mass of the star and $$q>0$$. The proportionality constant is probably somewhere in the range of $$200\text{-}300\text{ K}$$; these slides give it as $$280\text{ K}$$.