As scientists improve on our ability to detect extrasolar planets and launch new observatories like the Kepler, the collection of exoplanets continues to rise. But, so far, we haven't been able to say a whole lot about them, other than their apparent mass and distance from the host star. That's slowly beginning to change, as today's issue of Nature contains a report of the first observation of phases in an extrasolar planet. And although this discovery could be expected, other research suggests that the techniques involved may ultimately help future instruments identify oceans on distant planets.

The recent discovery was made using the European Space Agency's orbiting CoRoT observatory, which couples a small telescope to a wide-field camera. That camera can observe thousands of stars at once, and CoRoT began picking up transiting planets very shortly after it became operational back in 2007. The new data is based on continual observation of one of its own discoveries, CoRoT-1b, which belongs to a class of planets called "hot Jupiters." These are gas giants that orbit close in to their host star, and tend to be one of the easiest things to detect. In this case, researchers performed 55 days of near-continuous observations, enough to follow 36 orbits of CoRoT-1b. (For anyone concerned that we were wasting time staring at something we already knew was there, rest assured that CoRoT was able to keep a digital eye on 12,000 or so other stars at the same time.)

Filtering out the noise and averaging out the data from multiple transits, the researchers were able to extract a general trend in the light coming from CoRoT-1b as it completed each orbit cycle: the light tended to peak shortly before the planet was eclipsed while traveling behind its host star, and reached a minimum as it came closest to transiting before the star. The trend could be explained fairly simply, as it's one well recognized from planets within the solar system, or our own moon. They were seeing the phases of CoRoT-1b. Earth's moon waxes and wanes as a different fraction of the portion facing us is lit directly by sunlight. The same behavior provided a nice fit for the data on CoRoT-1b.

Knowing this and something about the residual emissions that occurred as less and less light was reflected, the authors were able to draw some reasonable inferences about the albedo, or reflectivity, of the planet's surface. This didn't involve the sort of chemistry that might get astrobiologists excited, as the atmosphere appears to contain things like vaporized titanium and vanadium oxides, but measuring the albedo is a major step forward when it comes to describing extrasolar planets.

The significance of this advance is actually driven home by another paper, wihch will eventually appear in a future release of The Astrophysical Journal (there's an arXiv version available). The authors took advantage of NASA's Deep Impact probe, which travelled a decent way across the solar system to stage an informative collision with comet Tempel1. While off in the distance, it took the opportunity to image earth; it's obviously far closer than an exoplanet, but its imaging instruments are nowhere near as sophisticated as dedicated telescopes. As such, Deep Impact gathered a reasonable approximation of the sort of data that might be produced by future space-based observatories.

That data revealed that the rotation of the earth produced regular variations in the albedo, variations that accounted for up to 30 percent of the total light reflected from the planet. By separating out the wavelengths most affected by these changes, the authors were able to make a map of the planet, one that roughly corresponds to the distribution of land and ocean across the planet's surface. Combined, the two results suggest that we are edging closer to knowing how to learn a lot more about extrasolar planets.

Nature, 2009. DOI: 10.1038/nature08045

Listing image by Leiden Observatory