Are strong planetary magnetic fields in the universe as common as we believe?

 Are strong planetary magnetic fields in the universe as common as we believe? 

The magnetic field is important for lifeforms on planets. It protects the planet's atmosphere against plasma flow from the star. If the planet doesn't have a magnetic field. That ion flow blows the atmosphere and gasses out from around the planet. 

How did Mars lose its magnetic field? That can help SETI and life search programs eliminate planetary candidates that cannot host any lifeforms. In the history of the Red Planet, it lost its lithosphere at least once. The asteroid impact sends lots of material into space. 

In that process, the Mars planet's core's rotation speed decreased. The reason for that is Mars ' mantle cooled down. The planet's core or nucleus acts like a generator. And if the rotation speed is low. 

The planet loses its magnetic field. This is the reason why the planet's size must be right. Things like gravitational fields and elements that form planets are, of course, important. But if the planet's size is too small its mantle loses its temperature faster than a large planet's mantle. 

"Did Mars ever look like this? (Credit: A.R. Bhattarai, reproduced with kind permission / Strange Map #1034 / Frank Jacobs)" (Bighink.com/How Mars lost its magnetic field — and then its oceans)

The planet's core must rotate at a different speed than the planet's shell. 

The reason for that is the small planet's shell is larger compared with the planet's volume than the large planet.  And that causes a situation, where the planet radiates infrared radiation stronger than the large planet. And that causes a decrease in temperature in the mantle. That thing decreases the rotation speed of the planet's core. And that thing decreases the power of the magnetic field.      

Magnetic field formation requires the metallic core in the planet's nucleus. Another thing that is required is that there are magnetic materials like iron in the planet's shell. If there is no magnetic material in the shell or outer mantle the magnetic field cannot form. And without that magnetic field, the planet loses its atmosphere. 

Then the core and shell must rotate at different speeds. If the rotation speed of the shell is the same as the planet's core there is no magnetic field. 

So maybe strong magnetic fields are not as common in the universe as we thought. A small planet's nucleus shines its energy faster if it has a large size. If the nucleus fills the entire planet there is a possibility that there is no mantle between it and the planet's rocky shell.        

https://bigthink.com/hard-science/mars-lost-oceans-magnetic-field/