Electric Resistance May Make Hot Jupiters Puffy

One of the surprises coming from the discoveries of the class of exoplanets known as “Hot Jupiters” is that they are puffed up beyond what would be expected from their temperature alone. The interpretation of these inflated radii is that extra energy must be being deposited in the regions of the atmosphere with large amounts of circulation. This extra energy would be deposited as heat, causing the atmosphere to expand. But from where was this extra energy coming? New research is suggesting that ionized winds passing through magnetic fields may create this process.

Magnetic fields on Jovian type planets is no new news. Our own Jupiter has the strongest in the solar system with a strength 14 times greater than Earth’s. The large magnetosphere created by this extends as far as 7 million kilometers towards the Sun and is stretched nearly the distance to Saturn’s orbit. The interaction of charged solar particles with such an immense field creates gigantic aurora, similar to those on Earth.

Hints of magnetic fields on extra solar planets have been discovered as well. In 2004, a team lead by Evgenya Shkolnik, of the University of British Columbia reported detection of the effects of a planet’s magnetic field on its parent star by observing the extra energy this magnetic field returned to its parent star. The interaction excited transitions in the familiar Calcium H & K lines that were locked in phase with the planet’s orbit. Follow-up observations including other Hot Jupiters confirmed the presence of planetary magnetic fields acting on their parent stars although none have yet suggested just how strong these fields might be.

The new research, linking magnetic fields with planetary radius, was first started in February of 2010 by a team led by Rosalba Perna of the University of Colorado in Boulder. In it, they demonstrated that the interaction of winds in the atmospheres of these planets could experience significant drag as they passed through the magnetic field lines due to their partially ionized nature. In May, Batygin & Stevenson of the California Institute of Technology suggested that this friction may induce heating sufficient to puff the planet up. Perna’s team picked up from the hypothetical basis and put Batygin’s & Stevenson’s idea to the test of a simulation. The simulation used a range of field strengths but found that for Hot Jupiters with strengths greater than 10 Gauss, were sufficient to explain the increased size.

But is this field strength truly plausible? Many astronomers seem to think so and the literature is filled with expectations of large magnetic fields for these planets although nothing seems to suggest that field strength has ever been measured on any planets outside our solar system to support this. Jupiter’s magnetic field strength ranges from 4.2 – 14 Gauss, putting the value of 10 Gauss in the possible range. However, work by Sanchez-Lavega of the University of the Basque Country in Spain, has suggested that as planets become tidally locked their magnetic field strengths decrease. For Hot Jupiters, he suggests that older planets of this type may have their magnetic fields reduced to a measly 1 Gauss. This may suggest an explanation for why experiments designed to search for fields on extrasolar planets through their radio emissions have failed.

Regardless, future simulations will undoubtedly take place and additional observations may help constrain the plausibility of this electromagnetic swelling.