John Tarduno at Rochester University [h/t Science Daily] has constrained how young planets might or might not run a dynamo in their core.
Earth’s present core is separated between a whitehot inner layer and a churning outer layer; and it is this which raises up our magnetic shield against our ever-heating sun. Tarduno says that Earth got our shield only 565 million years back - less than four billion years after Earth's formation.
Tarduno further assumes an earlier dynamo. For that he proposes that the Theia impact, besides throwing up our moon, also had mocked up this dynamo, via a shock of magnesium salts sinking to the bottom. But this dynamo had frozen out earlier. [QUESTION 3/15 - Mantle dynamo? NO ANSWER 4/9 - How would we know?]
This 565 mya point is in the Vendian era I believe. Earth had life during the Vendian but it was miserable life, mostly oceanic. Jellyfish and sponges were about as good as it got. Lichens on land.
This means that planet hunters cannot count on a dynamo to protect young Hadean / Venuslike planets. By "young" we include anything around a star 1.5 solar masses and up, like Procyon; they go nova before four billion.
Science-fiction worldbuilders are pretty much stuck, as well. Those tundra planets are likely irradiated as well. Terraformed Hadean planets might be okay, since they're living during a bombardment... until the next big one hits.
TARDUNO 7/25/22: He's finetuned 565 Mya to when the old core failed. 550 Mya is when the new core started up again. The rest is history. More exactly palaeontology.
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