AAS DPS 2008: Pale blue dot
Sometime soon, astronomers are going to announce an Earth-sized extrasolar planet in the habitable zone of its star. And not too long after that, telescopes are going to be able to actually get an image of these things – albeit as a crude, unresolved, disc-averaged blob within a single pixel of the telescope's camera. There are modelers who spend their time thinking about what that blob should look like, and whether its spectroscopic signals would represent something Earth-like. But the models are quirky and need some ground-truthing. They need some Earth-truthing.
And so two current missions, ESA's Venus Express and NASA's EPOXI, a secondary mission for the old Deep Impact spacecraft, are turning their cameras around and taking some fuzzy pictures of Earth. Scientists on the two teams gave talks about the observations over the weekend here. “The crudeness of our observation is really our strength,” says David Grinspoon, astrobiology curator at the Denver Museum of Nature and Science and also a scientist on Venus Express.
This is not first time that Earth has been imaged from afar. In 1990, Voyager 1 took a snapshot of Earth from 4 billion miles away. We are the speck in the middle of the blue circle, pictured here. Carl Sagan used the picture as the inspiration for his book “Pale Blue Dot”. But the Voyager pic and others were one-off, snapshot sort of things. Grinspoon says this is the first time anyone has tried to do good, disc-averaged imaging and spectroscopy on Earth from afar, over time. How does that little blob, and its spectroscopic signature, change with the seasons? With a day, as oceans spin past?
You might think it trivial to calculate this, by just averaging the sum-total of observations from fancy satellites. Turns out that it's very difficult. Ocean and continental signatures are very different; sunlight glints off of oceans in weird ways; atmospheric edge effects are problematic. “Our models are not doing that well right now,” says Tim Livengood of the EPOXI mission. “We have so much information about the Earth, that you have to thin it down to a practical level.”
Both groups, in their 'bad' pictures, found something called the “red edge”, a sudden shelf of brightness in the infrared part of the spectrum caused by plant life. Grinspoon says that a team of Venusian astronomers would thus conclude that Earth indeed has the signature of life.
Could a renegade, retrograde ice ball signal a new population of distant bodies in the solar system? Brett Gladman of the University of British Columbia in Vancouver was on hand at a press briefing here to describe his discovery earlier this year of a 50-kilometre chunk of ice that doesn't fit with known populations. It was discovered with a Hawaiian telescope, as a part of a 
Well, I've arrived at the Division for Planetary Sciences meeting of the American Astronomical Society here at Cornell University in Ithaca, New York. And lemme tell you, Ithaca is GORGES. (For those that don't get the town slogan, just navigate the hilly campus for a bit: most traverses of campus involve a suspension bridge or two.)