Distant Mirrors (and Cribsheets)

Speaking of astrobiology (as I was in my last post), I just discovered this May article from SEED magazine. It discusses what sorts of signs of life a distant planet might display, using as an example the only planet that we know with certainty harbors life - Earth. It turns out they have done the experiments with several departing planetary exploration spacecraft, pointing their instruments back at Earth to examine our variations in brightness and color in various wavelength bands.

When they do this, they see a planet with tremendous variation in color and reflectance, thanks to our oceans, clouds, and land areas. These variations could allow an observer with similar data to determine that we have areas of water and non-water, as well as how fast our planet is rotating. If they observed a wide range of spectral bands, they would find Earth to be quite bright in some long wavelengths (radio, TV, radar, etc.). With spectroscopic analysis, they would find the chemically unstable combination of oxygen and methane in our atmosphere (along with other gases), suggesting that some dynamic process is constantly replenishing these gases (that would be life, in our case).

This gives scientists ideas for how they will examine exoplanets as our near-future instruments become good enough to discover and examine smaller and more Earth-like planets, especially any that lie within the habitable zone of their stars. Even if the planet is initially represented by only one color pixel, the spectrum of that pixel, and changes in its brightness over time may give clues about its atmosphere, rotation, and the presence of large areas of water or other liquids.

Habitable zone? If you need help with that, I recommend cribsheet #14 from SEED, a quick single page review of exoplanets, how we find them (over 300 so far, most very massive and Jupiter-like), and how we will be able to study these distant worlds in the near future. SEED has useful cribsheets on a lot of other subjects too. Check out plate tectonics, nuclear power, and light.