What substances were found in the innermost regions (within about the inner 0.3 auau) of the solar system before planets began to form?

Hydrogen and helium compounds.

Explanation:

We know that the solar System was formed around 4.6 billion years ago, due to the gravitational collapse of a giant interstellar molecular cloud.

This cloud  is a type of interstellar cloud and its density and size permit the formation of molecules, most commonly molecular hydrogen.

Therefore the principal substances were found before planets began to form are hydrogen and helium compounds, besides Rocks, metals, most of them in gaseous form.

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