[Pseudoscience-is-malarkey (OP)]

My question here is inspired by the findings that 16 Cygni Bb is shockingly high in lithium. Astrophysicists are constantly going back to the drawing boards about planetary limitations and such.

[Kryptid]

Magnesium is already the 8th most abundant element on Earth: https://web.archive.org/web/20110927064201/http://www.gly.uga.edu/railsback/Fundamentals/ElementalAbundanceTableP.pdf

In order for your question to be answered more exactly, you'd need to specify what kind of quantities you are talking about.

[evan_au]

Magnesium is believed to be distributed into space primarily by the explosion of massive stars.
- So if you had a star that is massive enough to burn three Helium-4 nuclei to Magnesium-24...
- But not massive enough to burn Magnesium-24 to Silicon-28...
- Then, when this massive star exploded, it would distribute Magnesium-rich material into the interstellar medium
- From which magnesium-rich planets could form (in theory, if stars of this mass were the major contributor to the material in the subsequent protoplanetary disk).

https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Universe
https://en.wikipedia.org/wiki/Nuclear_binding_energy#Nuclear_binding_energy_curve

[Pseudoscience-is-malarkey (OP)]

Magnesium is already the 8th most abundant element on Earth

I didn't know that.