Saturday, December 8, 2012

Keeping Hot Jupiters Inflated

Hot Jupiters are a class of extrasolar planets with similar characteristics as Jupiter but have high surface temperatures as they orbit very close to their parent stars. While Jupiter takes almost 12 years to orbit the Sun, many of these hot Jupiters take only a few days to orbit their parent stars. The discovery of a number of hot Jupiters with up to twice the radius of Jupiter is puzzling because as these planets age, they are expected to cool and contract to around the radius of Jupiter within the age of several million years. It seems that some process is at work to stall the contraction of these planets or to re-inflate them. A number of models such as intense stellar irradiation and tidal heating have been proposed to explain the observed radii of these inflated hot Jupiters. Nevertheless, these models are inadequate to fully account for the large radii of these planets.

Figure: Size comparison of WASP-17b (right) with Jupiter (left). Although WASP-17b is less than half the mass of Jupiter, it has an inflated radius that is about twice Jupiter’s radius.

A mechanism known as Ohmic heating was proposed by Batygin & Stevenson (2010) to explain the larger-than-expected radii of these hot Jupiters. Ohmic heating occurs when strong stellar irradiation partially ionizes the planet’s atmosphere and drives a surface wind which blows across the planet’s magnetic field. This induces a current which travels inwards into the deeper regions of the planet where the current is deposited as heat. Wu & Lithwick (2012) further proposed that Ohmic heating can stall the cooling contraction of hot Jupiters but cannot significantly re-inflate the radii of hot Jupiters that have already contracted.

For a hot Jupiter starting off in a state of high entropy where it has not previously cooled and contracted significantly, Ohmic heating can allow the planet to persist in a state of perpetual youth by keeping the planet inflated for billions of years. In the same radiation environment, a less massive planet can be kept inflated at a larger planetary radius than a more massive planet. With Ohmic heating, the radii at which contraction is stalled is consistent with the observed radii of most inflated hot Jupiters.

Inward orbital migration can transport a Jovian planet into a close-in orbit around its parent star a few million to a few billion years after the planet’s formation. For such a hot Jupiter, it is expected to have previously cooled and contracted significantly before being subjected to strong stellar irradiation and Ohmic heating. In this case, Ohmic heating becomes inefficient as it is unable to penetrate beyond the shallower layers of the planet since higher pressures are reached at shallower depths within the planet. As a result, the more the planet has contracted, the more inefficient Ohmic heating is in re-inflating the planet.

1. Konstantin Batygin and David J. Stevenson (2010), “Inflating Hot Jupiters With Ohmic Dissipation”, arXiv:1002.3650 [astro-ph.EP]
2. Yanqin Wu and Yoram Lithwick (2012), “Ohmic Heating Suspends, not Reverses, the Cooling Contraction of Hot Jupiters”, arXiv:1202.0026 [astro-ph.EP]