Day 2/5, Session 2: Hot Jupiters and Magnetic Fields


Our first talk was an overview of giant planet atmospheric dynamics by Adam Showman.

Giant planet atmospheres are rotationally dominated, resulting in east-west jets. Jupiter and Saturn’s upper atmospheres exhibit a large number of zonal bands, whereas Uranus and Neptune have  only very few thick bands.  The size and shape of the jets depend on the radius of the planet compared to the characteristic scales at which vortices form. For Hot Jupiter exoplanets the characteristic scales of the vortices are comparable to the size of the planet.

As for Jupiter’s jets, it is unclear whether they are shallow or deep. The upcoming Juno mission hopes to answer this question.

The second part of the talk then discussed observations. Measured phase curves show that different hot Jupiters have a range of temperature differences between their day and night sides. Observations also hinted at a blueshifted atmospheric line in the HD 209458b transmission spectrum, maybe due to km/s atmospheric winds.

The second speaker was Fran Bagenal, who talked about planetary magnetic fields. A key component of this talk was the discussion of  the mechanism for transporting plasma from Io (released from volcanism) into Jupiter’s orbit, which leads to an expansion of the magnetosphere.  She showed a nice movie of observations of the plasma torus around Jupiter at different wavelengths, where you can see the wobble due to the 10 degree misalignment between the magnetic pole and the rotation axis.

One interesting observation is that magnetic fields are very diverse within our solar system. Mercury’s field is so small that the planet and the magnetosphere together will fit within the Earth. Earth and its magnetic field will then fit inside Jupiter. Venus and Mars have no magnetic field, although Mars likely used to. She also mentioned the MAVEN mission, which will examine the planet’s upper atmosphere.

We were also shown beautiful pictures of the aurorae around Jupiter and Saturn.

Our third speaker was Emily Rauscher, who discussed some of the observable effects of magnetic drag in planetary atmospheres. In short, for stronger magnetic fields, the ohmic heating increases, which can be a contributory factor in radius inflation. Also, the magnetic drag lowers the wind speeds in the atmosphere. This is observable in the phase curve of a transiting exoplanet.  Fast winds mean that the hottest point is offset from the substellar point, and magnetic drag will decrease this offset.

Our final speaker, Andrew Youdin, continued on the theme of inflating hot Jupiters by discussing two theories: ohmic dissipation and the mechanical greenhouse.

In the theory of ohmic dissipation, surface winds induce currents, which then dissipate and get converted into heat. He also pointed out that the high speed winds required for this are damped by large magnetic fields, which is a potential flaw for this scenario as an explanation of radius inflation.

In the mechanical greenhouse model, the outer radiative zones of hot Jupiters are turbulent due to the intense heating from the star. This then drives eddies downwards into the interior of the planet, acting in reverse to convection.

Feature image: NASA, Hubble website 


About Author

I am undertaking an observational astronomy PhD focusing on the characterisation of planetary atmospheres, working with Dr. David Sing and Prof. Frédéric Pont. I am specialising in using transmission spectroscopy, applied to the hot Jupiter HD 189733b with HST. I am interested in obtaining a more complete picture for the few well-studied exoplanets, comparing observations in different regimes of planetary atmospheres, from secondary eclipse and transmission observations at different wavelengths, both from the ground and from space. (More)