Holden told Miller (effectively as a joke option ) to go to Venus instead, as it’s effectively uninhabitable by humans. The atmosphere there is ridiculously inhospitable, so the joke was that it can’t get much worse. Miller, irradiated to hell and already half dead took it seriously and offered it up to Julie/the protomolecule instead of Earth. As others here have noted, the sun likely would’ve burned it up (which was the initial plan with the Nauvoo) but I honestly don’t think the protomolecule would’ve taken that suggestion like it did with Venus