Forming Dense Metal Planets like Mercury is Probably Pretty Difficult and Rare in the Universe

MESSENGER image of Mercury from its third flyby (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The planet Mercury, the closet planet to our Sun, is something of an exercise in extremes. Its days last longer than its years and at any given time, its sun-facing side is scorching hot while its dark side is freezing cold. It is also one of the least understood planets in our Solar System. While it is a terrestrial (i.e. rocky) planet like Earth, Venus and Mars, it has a significantly higher iron-to-rock ratio than the others.
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