For this question, the word 'metals' does not refer to elements [heavier than hydrogen and helium][1]. It instead refers to metals [as defined by the periodic table of elements][2].

After researching the [abundance of chemical elements][3], it is very clear to me that hydrogen and helium (and occasionally oxygen) far outrank all of the other elements.

In terms of mass fraction in the Milky Way:  
Hydrogen = 73.9%  
Helium = 24.0%  
Oxygen = 1.04%  
**Which equals 98.94%**


The following table shows the cosmological origin of each element:
[![enter image description here][4]][4]

In the scope of the universe:
## Just how likely or unlikely to form is a planet composed almost entirely of metals?

For this question, let's define "composed almost entirely of metals" as:

 - Elemental composition by _volume_, not mass: More than 90% total by volume.
 - Metals are defined in this question as belonging to either the [transition metals][5] or the [post-transition metals][6] groups.
 - All other elements are not considered metals for this question. This include the [alkali metals][7], the [alkaline earth metals][8], or the [metalloids][9] groups.

Now, iron is the biggest contender here in terms of metals due to its abundance. But let's not forget about all of the others! [(here's a question involving a planet made of iron)][10].

I would love to further ask what the chance of life forming on such a planet would be, but of course that would make this question [too broad][11].


  [1]: https://en.wikipedia.org/wiki/Metallicity
  [2]: https://en.wikipedia.org/wiki/Metal
  [3]: https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
  [4]: https://i.sstatic.net/AZ54h.jpg
  [5]: https://en.wikipedia.org/wiki/Transition_metal
  [6]: https://en.wikipedia.org/wiki/Post-transition_metal
  [7]: https://en.wikipedia.org/wiki/Alkali_metal
  [8]: https://en.wikipedia.org/wiki/Alkaline_earth_metal
  [9]: https://en.wikipedia.org/wiki/Metalloid
  [10]: https://worldbuilding.stackexchange.com/questions/27277/a-planet-made-of-iron
  [11]: https://worldbuilding.stackexchange.com/help/on-topic