[danparsonson]

If you're willing to invoke 'alien technology' as an explanation then no-one could really argue differently because we don't know what aliens are capable of. A few things that come to mind:

a) That seems like an awful lot of trouble to go to when you could surely directly harvest the energy of a star much more easily, and use that to drive whatever machinery you wanted to build.

b) How would one ignite an atmosphere and once having done so, keep it burning? Would it not all burn up in a short time? Say a few weeks at most?

c) You can't fool a spectrum - the light coming from such an object would indicate its chemical composition; if it looks like primarily H and He then that's what it is. Planetary atmospheres tend to be low in those elements because they get blown away over time.

d) As another commenter points out, the magnetic field of a planet is tiny in comparison to that required for the effect the paper is postulating - Earth's magnetic field strength for example is less than 1 gauss, whereas the scientists here are suggesting fields of a few thousands to a million gauss. Still not that high for a stellar object, but very high for a planet.

Of course I'd love for us to find evidence of aliens - but I don't think this is it.