A map of HD 189733 b using eclipse mapping


An outline of the paper: A TWO-DIMENSIONAL INFRARED MAP OF THE EXTRASOLAR PLANET HD 189733B by Majeau, Agol & Cowan.

Transiting systems offer two ways to map the temperature distribution on a planet’s atmosphere: an easy one and a hard one.

The easy one was used in the seminal study by Knutson et al. on HD 189733 b. The phase curve of the planet can be inverted to yield a distribution of temperature with longitude in the atmosphere of the planet.

Spitzer phase curve at 8 micron for HD 189733 b from Knutson et al. (2007) and inferred temperature map. Only the longitude temperature distribution is measured, the equator-to-pole contrast is just a guess.

Now Majeau, Agol & Cowan have used the hard way on the same planet. The distribution of temperature on the planet slightly modifies the shape of the occultation around ingress and egress. The star “scans” the planet at the start and end of the occultation, and a broad-brush temperature map of the planet can be recovered if the data is precise enough. This is much more delicate than the phase-curve method, but there are some advantages. The signal is concentrated over a short time, alleviating the effect of instrumental systematics and stellar variability. As a further bonus, if the occultation happens at a high enough latitude on the star, the latitudinal (equator-to-pole) temperature map can also be reconstructed.

"Eclipse mapping": as the planet disappear behind the star, it can be mapped slice by slice.Unless the transit is equatorial, the ingress and egress can be recouped to obtain a 2-D map. Figure 1 of the paper.

The authors have used 7 occultation events measured at 8-micron with the Spitzer space telescope to reconstruct a temperature map of HD 189733 b. They confirm the eastward displacement of the hottest point found by Knutson et al. They also find that, as expected, the planet is cooler near the pole than near the equator.

Temperature map of '189 at 8 microns from "eclipse mapping". This time the equator-to-pole contrast is measured as well. From Figure 2 of the paper.

There may be some time before this method can be applied to any other object than the very favourable ‘189 system, and we may hit the fundamental limit of stellar variability before reaching Earth-size planets. Also, the orbit needs to be perfectly circular for the method to work well, otherwise the delay introduced in the occultation by eccentricity will be partly degenerate with the temperature map.

Nevertheless, applying this method to exoplanets sounded like a very long shot when it was first proposed, and it is a sign of the vivacity of the field to see it applied so successfully only a few years later.

Feature Image: Figure 2 of the paper.


About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.