WMG / Zone of the Enders

Aumaan doesn't actually have the power to destroy the solar system
Nohman was just power-tripping. Aware that in spite of his enormous power as the pilot of Anubis, that he would soon die from Metatron poisoning, his narcissism and his madness manifested (like Kuja's from FFIX) into an Omnicidal Maniac urge to destroy everything. The attack would probably have ripped Mars a new one, though.

Metatron is eezo, and Nohman was Indoctrinated

Anubis was probably enhanced by Lost Reapertech, leading to Nohman's indoctrination. (Which makes Jehuty's defeat of it all the more impressive.) So when he saw that ADA was a sentient AI... "The Cycle Must Continue."

Metal Gear is a prequel series to ZOE

Going off the design of the latest metal gear from The Phantom Pain, this doesn't seem that far fetched. Add in all of the cameos ZOE get in Metal Gear (including a statue of Jehuty next to other metal gear prototypes on the desk of the man who designed the first Metal Gear) and this actually seems likely.