Recent discoveries of water-rich , sub-Neptunian to Neptunian-massed exoplanets with short-period orbits present a new parameter space for the study of exoplanetary dynamos .
We explore the geometry of the dynamo source region within this parameter space using 1-D interior structure models .
We model planets with 4 chemically distinct layers that consist of ( 1 ) an iron core , ( 2 ) a silicate layer , ( 3 ) an H _ { 2 } O layer , and ( 4 ) an H/He envelope .
By varying the total planetary mass in the range of 1 - 19 M _ { \oplus } , the mass fraction of the H/He envelope between 0.1 - 5.1 % , and the equilibrium temperature between 100 - 1000 K , a survey of the parameter space for potential dynamo source region geometries is conducted .
We find that due to the nature of the phase diagram of water at pressure and temperature conditions of planetary interiors , two different dynamo source region geometries are obtainable .
Specifically , we find that smaller planets and planets with thicker H/He envelopes are likely to be in the regime of a thick-shelled dynamo .
Massive planets and planets with thin H/He envelopes are likely to be in the regime of a thin-shelled dynamo .
Also , small variations of these parameters can produce large interior structure differences .
This implies the potential to constrain these parameters based on observations of a planet ’ s magnetic field signature .