Mapping new worlds
Posted: 10 September 2010
Mapping the surface of an exoplanet, or even the cloud tops of a turbulent gaseous hot jupiter, is possible even if we cannot capture direct images of the planets themselves, revealed Dr Nick Cowan of Northwestern University, USA, at this week’s Exoclimes conference at the University of Exeter.
Although we can’t resolve the planet directly, it is possibly to separate the combined light of a star and its planet, and this leads to all kinds of possibilities using methods known as eclipse mapping and reflected light mapping.
‘Exocartography’ may reveal details about the surfaces of exoplanets, such as information about cloud cover and oceans. Image: ESO.
“Eclipse mapping and reflected light curves are probing two very different regimes,” Cowan told Astronomy Now. Eclipse mapping can provide a measure of thermal brightness. Imagine a planet orbiting its star, disappearing around the back of the star in an eclipse as seen from our point of view. If the hottest region on the planet is not directly beneath the planet’s sun (see http://www.astronomynow.com/news/n1009/08hotj/), but has been shifted to one half of the dayside by atmospheric circulation or the rotation of the planet (the same way that on Earth it can be hotter at 2pm than at midday), we can picture a scenario where 50 percent of the planet has moved behind the star, but we still see more than 50 percent of the planet’s heat. A case in point is the example of the exoplanet HD 189733b, which displays a 40 second discrepancy between the timing of its eclipse and the disappearance of its hot spot. Consequently this allows us to make crude maps of the planet’s heat distribution, from which we can make guesses about atmospheric circulation and any day/night differences in temperature.
Reflected light, on the other hand, depends on the albedo (reflectivity) of features on the planet. For instance, the glint of blue oceans, or highly reflective stretches of ice, could be seen in reflected light. Ultimately, each method is useful for characterising the habitability of a planet in different ways.
“If your definition of habitability is liquid water, then you should work in reflected light because between glint and seeing blue oceans rotating out of view you can figure out if there is liquid water,” says Cowan. “On the other hand, if you’re definition of habitability is a certain temperature range then you have to look at thermal eclipse mapping, because that’s the only way you can learn about the climate.”
Things get more interesting when we consider mapping Earth-like planets. Oceans, deserts, forests and clouds all display distinctive infrared signatures, as proven by the fly-by of Earth by the Deep Impact/EPOCh (Extrasolar Planet Observations and Characterisation) mission, which imaged our planet the same way that we might try to map an exoplanet. The spacecraft saw the reflected glint of our oceans, and a blue hue to our planet, but here we have to be careful, says Cowan: the blue comes from Rayleigh scattering of light in our atmosphere (the reason the sky looks blue) rather than from the reflected light of oceans.
More speculatively, eclipse mapping or reflected light maps could reveal volcanoes on exoplanets, which have been in the news recently (see yesterday's related news story here). On Mars for example, many of the largest volcanoes (Olympus Mons, Arsia Mons, Pavonis Mons and Ascraeus Mons) are found in the same region of the red planet, and when they were active in the past they would have made that corner of Mars appear very different to the rest of the planet. Similarly, any planets with highly active volcanoes in a specific part of the planet could create some variability in thermal maps and reflected light (from their volcanic ash clouds) that might be noticeable as the planet rotates, says Cowan.