Finding Life in the Solar System
Mike Brown's penultimate lecture in his Caltech/Coursera Solar System course concerns what he calls "the best experiment to find life in the Solar System" yet. On it's trip to Jupiter, the Galileo spacecraft flew by a couple of planets to get a slingshot boost on its way, taking some data in the process. The data consisted of infrared spectra, imagery at multiple filtered wavelengths, and radio data.
Some of the data was highly suggestive: oxygen in the atmosphere, presence of liquid water, very abundant methane in the atmosphere, and some peculiar features in the wavelength filtered data. All of the above were suggestive of the presence of life, but hardly conclusive.
You have perhaps guessed the identity of the planet in question: Earth. The data, analyzed by Carl Sagan and colleagues, also included imagery with a resolution of roughly 1 kilometer. No definitive signs of human construction were found at that scale.
The only definitive evidence of life, and in fact semi-intelligent life, was found in the radio signals from Earth, which in addition to natural ionospheric, solar and other features included some with regularities not known in any natural phenomena: radio and television shows.