Having made the discovery that planets orbit distant stars, astronomers have made remarkable progress in learning about those planets, given the unimaginable distances that separate the Earth from distant stellar systems. Already, scientists have learned how to glean such data as the number, size, and orbital radius of planets orbiting distant stars. The next step is to learn more about the surfaces of those planets, including looking for clues as to whether they include life. For those of you who just love scanning for life forms, a new technique has recently been tested that promises to be useful in the future.
Detecting light from a planet directly is extremely difficult, as the feeble reflected light of planets tends to be swamped by the brightness of the parent star. However, the light reflected from a planet exhibits polarization not found in the light from the star the planet orbits, and so the planet’s light can theoretically be extracted from the overwhelming light of the main star, and analyzed. With large enough instruments and sensitive measurements, such polarized planetary light has the potential to reveal the characteristics of a planet’s atmosphere, possibly revealing the telltale signs of life.
In the first proof-of-concept test of this idea, astronomers with the ESO have tested the Earth’s own light as reflected from the surface of the Moon. The Earth is bright enough, and the moon reflective enough, that the planet’s light can illuminate the dark part of the Moon’s surface, sometimes bright enough to be visible to the naked eye. By observing the reflected light, the astronomers were able to deduce that the Earth’s atmosphere was partly cloudy, that the Earth had oceans, and that vegetation was present on the surface – all of which, of course, are true.
No current telescopes are able to make the same sort of observations of exoplanets, but the astronomers hope that their test shows that it may be possible in the future to examine light from other solar systems for evidence of life.