I'll be writing quite a bit about the annual meeting of the Division of Planetary Sciences (DPS) of the American Astronomical Society this week; see this post for an introduction.
One of the first sessions at DPS was on results from the EPOXI mission, which is what they are calling the Deep Impact extended mission. There are two components to the extended mission: Deep Impact is on a long cruise toward a second cometary encounter, with Hartley 2 in 2010; but in the meantime, astronomers are using Deep Impact's defocused high-resolution imager to study known transiting exoplanets, and also to study Earth as though it were an exoplanet. What's a transiting exoplanet? It's a planet that orbits another star, which has been detected from Earth by the way it dims the light from its star as it crosses in front of the star on its orbit. In order for a planet to be detected this way, we have to be looking at the star system edge-on, so it's only possible for us to detect such exoplanets around the small fraction of stars whose planetary disks just happen to be oriented that way.
David Grinspoon presented the results of a similar experiment being performed by Venus Express. At Venus Express' distance from Earth, its highest-resolution VIRTIS instrument can't even resolve Earth -- it's always much smaller than one pixel -- which is actually a better analogue to future studies of extrasolar planets. If VIRTIS can't see Earth as a disk, it does get excellent data on Earth's spectrum, which is fun to study from a distance because of all of the different sources of variability: the spectrum changes as Earth rotates, bringing more continents or more oceans into view; it changes as weather patterns increase or decrease cloud cover and change their distribution; it changes with the seasons, as more or less ice covers the northern hemisphere (the southern hemisphere, having much less continent, shows much less seasonal ice cover variability); and so on. Again, the presentation didn't contain any real surprises; more than anything else it demonstrated that VIRTIS has captured a valuable, real-world data set that will be useful in developing models of what might be causing spectral variability on distant worlds that we will someday study with powerful telescopes.