<P> In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet . It is the first (indirect) detection of a magnetic field on an exoplanet . The magnetic field is estimated to be about one tenth as strong as Jupiter's . </P> <P> Interaction between a close - in planet's magnetic field and a star can produce spots on the star in a similar way to how the Galilean moons produce aurorae on Jupiter . Auroral radio emissions could be detected with radio telescopes such as LOFAR . The radio emissions could enable determination of the rotation rate of a planet which is difficult to detect otherwise . </P> <P> Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions . Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles . Forms of magnesium oxide such as MgSi O could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths . </P> <P> Hot Jupiters have been observed to have a larger radius than expected . This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand . The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet . This theory matches the observation that stellar activity is correlated with inflated planetary radii . </P>

The majority of known extrasolar planets are giants like jupiter and saturn