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Earth is protected by a magnetic field. Not only does it shield life from solar radiation, it also guides animals to their migratory grounds. But here's the thing, according to Universe Today:

The core probably accounts to 42% of Mercury’s volume, while Earth is just 17%.

So let's say we have found an alternate Earth in an alternate universe where Earth's core makes up 42% of its volume rather than the 17% ours has. Would this larger volume of core give Earth a far stronger magnetic field?

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First off, Mercury is much smaller than Earth (2400km vs 6300km radius), if you compare the cores without taking into account the rest of the planet you'll find that they are of similar size:

2400 * 42% = 1008km
6300 * 17% = 1071km

I think what that means is that the rest of Mercury simply eroded away.

But let's say we have a planet with radius of 6300km and core radius of 2600km (42% of the total radius). What would the effects be?

That depends on what we mean by core. Here is what the Earth's structure looks like: enter image description here

The coresponding densities can be found here: http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/earthstruct.html

Density grows with depth and you want the total radius to stay the same, so depending on which layers you grow you'll get different effects. Growing the denser layers will give you an increase in average density, therefore you'll end up with higher gravity. More iron in the core will also lead to stronger magnetic field.

Those are the two most obvious effects that I can think off.

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    $\begingroup$ That's 42% and 17% by volume, not by radius/diameter. $\endgroup$
    – Alexander
    Commented Jun 15, 2017 at 23:26
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    $\begingroup$ It's a little more complicated than simply size; geomagnetic fields are generated by the currents within the outer core (the inner core is solid and doesn't contribute to the geomagnetic field). It's possible for a smaller core to produce a more intense magnetic field if the material is more fluid and flows faster. This would require a higher core temperature, and with the core's heat arising by radioactive decay, it would require a greater ratio of radioactive elements. $\endgroup$
    – Pak
    Commented Jun 16, 2017 at 18:11
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Like ventsyv stated, the gravity on this planet would be much greater. However, If animals developed with stronger bones to stand this gravity, its magnetic field would indeed be much stronger as well. This would lead to the planet's ability to harbor a much thicker atmosphere, which would in turn protect life there from solar-radiation and space debris much easier. Hope this helped

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