Earthed

How much mass must a planet have to hold an atmosphere and a magnetosphere to hold it?

Mars has .111 the mass and .377 the gravity of Earth. What is the Goldilocks zone for a planet? How close in size and mass to Earth does another planet have to be that we could colonize it?

Public Comments

1. Mars used to have an atmosphere.
   
   now it doesn't.
   
   but it's not lighter.
   
   
   
   the difference is that early on, it's molten core generated a magnetic field, that deflected most of the solar radiation.
   
   when the core cooled, and that radiation was no longer deflected, then it excited the atmosphere, and blew it away.
   
   
   
   so, maybe the question ought to be worded, "How small can a planet be and have a sufficiently strong magnetic field to provide shielding? Or, alternatively, how large must a planet be to retain an atmosphere w/o help from a magnetic field?
   
   
   
   and then, of course, there's always the question of how close to the sun would/could the planet be in either case? OH, and how large/bright could/would the sun be as well?