Monthly Archives: December 2015

Hubble study of water in hot-Jupiter atmospheres

NASA have put out a press release regarding the largest-ever study of hot-Jupiter atmospheres by the Hubble Space Telescope and the Spitzer Space Telescope. Of the ten planets studied, six are WASP discoveries.

Clear to cloudy hot Jupiters (annotated)

The results, published in Nature, report that hot Jupiters are a diverse group that have atmospheres ranging from clear to cloudy. Strong water absorption lines are seen when the planets have a clear atmosphere, but less so when the atmospheres are dominated by clouds and hazes.

hubble_water

Planets such as WASP-17b and WASP-19b have clear atmospheres and show the strongest water features, whereas planets such as WASP-12b and WASP-31b are more cloudy.

The NASA press release has so far resulted in articles on over 110 news websites worldwide. The paper was lead-authored by David Sing of the University of Exeter.

Magnetospheres of hot Jupiters

If a hot Jupiter has a magnetic field of a few Gauss it would be surrounded by a magnetosphere that would carve out a hole in the stellar wind of the host star. Since the planet orbits rapidly, this would lead to a “bow shock” where the magnetosphere ploughs through the stellar wind.

In a new paper, Richard Alexander, of the University of Leicester, and co-authors, report computer simulations of this effect for several hot Jupiters, including WASP-12b and WASP-18b.

Hot Jupiter magnetospheres

In the colour-coded figure (see scale on the right) the blue and red show the density of the stellar wind. A low-density (black) magnetosphere surrounds each planet (white dots).

Since these planets orbit edge on to us, the bow shock would absorb ultra-violet light from the star, and so produce a characteristic light-curve with a broad dip preceding the transit.

Hot Jupiter magnetospheric light curves

This magnetospheric bow-shock is a possible alternative explanation for the UV absorption observed in WASP-12, which has previously been attributed to material being lost from the planet owing to Roche-lobe overflow. Alexander et al suggest that WASP-18 is a critical test of these models, since the much higher gravity of the massive planet WASP-18b means that there should not be any Roche-lobe overflow.