I've been putting the finishing touches on a paper that describes an investigation into whether planets that orbit very close to their parent stars can visibly alter or disturb the radiation output of those stars. Actually, `visible' is a bit of a misnomer, since the study looks into the X-ray photons pouring out from the million degree plasma surrounding stars, known as the corona. This tenuous, super hot, mix of raw nuclei, electrons and ions is the result of as-of-yet not fully understood energy transfer from the much cooler (a mere few thousand degrees) visible surface of stars. Magnetic fields almost certainly play a big role in this heating, and a star like the Sun has a doozy of a magnetic field. So too do gas giant planets. Jupiter has a roughly averaged magnetic field strength more than ten times that of the Earth, in certain regions it can be thousands of times stronger. Put a gas giant planet, like a hot Jupiter, in close proximity to a star - say where it orbits every couple of days, and you might expect the two magnetic fields to get into some interesting tangles.
What does this have to do with life and worlds like the Earth ? Our magnetic field makes a big difference; apart from providing some amount of protection for surface life from cosmic radiation streaming in (by deflecting particles with electrical charge) it has - over the lifetime of the planet - helped slow the rate at which our atmosphere leaks out into the cosmos. This deflecting power decreases the impact of high energy cosmic particles that would otherwise smack into the upper atmosphere and gradually erode it. However, we don't completely understand the details of the massive circulations, or dynamos, deep in the Earth that generate the magnetic field.
For terrestrial-type exoplanets it'd be incredibly useful to either be able to measure magnetic fields (and put another pin on the map to see if they're more or less Earth-like), or to make believable predictions about what their magnetic fields might be.
Which gets back to the study of X-rays from stellar coronas. After some careful analysis it looks like there is a surprisingly strong relationship between the amount of energy pouring out and the size of the hot Jupiters whizzing around the closest to their stars. There could be all sorts of nasty hidden biases and effects that are really responsible, but in the best tradition of sticking ones scientific neck out I explore what the implications are if we're actually witnessing the clash of planetary and stellar magnetic fields. Lo and behold, the effect seen is remarkably similar to what might be expected if the deep dynamos of these planets obey a set of rules that seem to work well for planets in our own solar system.
So...there may indeed be some universal laws governing how strong planetary magnetic fields are, and we might be able to probe the deep circulations in planetary cores that generate these fields by exploiting the fact that some planets are causing a ruckus in their star's hairy outer reaches. It's early days, but studying the interiors of exoplanets may not be impossible.