Class M planet detected orbiting Sun
ESA’s Venus Express probe is being put to some very creative use, looking for life on Earth. Yes, on Earth.
How might we detect living worlds orbiting other stars? This is a pressing question, because it will soon be practical to image terrestrial planets in other star systems. Extrasolar planets have been imaged, already. Local sunlight passing through extrasolar planet atmospheres has been split into spectra, and chemical elements identified, already. It is only a matter of time before planet-hunting astronomers are able to locate a terrestrial world orbiting another star, image it, and collect enough light out of its atmosphere to identify chemical biomarkers for life.
Current planet-finder missions such as the ESA’s COROT and NASA’s Kepler – not to mention a bevvy of future missions now being planned – will push the limits of vision further and further, with the hopes of spotting other life-bearing worlds.
How would one recognize a life-bearing world? Look for chemicals that have no business being there. Earth is a great example, because plants here continuously regenerate molecular oxygen (O2), which would otherwise bleed away over a few hundred thousand to a few million years if all plants died tomorrow. Molecular oxygen is corrosive and reactive, unstable in the presence of sunlight, organic matter and many types of rocks. Without chemical factories constantly pumping out new oxygen our planet couldn’t support more than a trace concentration of this reactive gas. If we find an alien planet and look at its spectra and find a clear O2 signal, we’ve most likely found life.
Astronomers will probably do just this – find alien life-bearing worlds – within the next few years. But they won’t do it by traveling there, or by shaking tentacles with visitors in motherships. They’ll do it by reading light spectra.
This is where Venus Express comes in. At its current distance from Earth and at its built-in resolution, Earth is just one pixel to its cameras. The probe can’t image continents, much less cities. But it can read the signature of light coming out of our atmosphere, and that signature records life: O2, ozone (O3), methane (CH4), and nitrogen oxides (N2O) that could not be easily explained if Earth were lifeless.
The image shows Earth as it looks to us (a globe), and Earth as it looks to Venus Express (a spectrum). In the VE spectra are clear signs of life… most notably O2 and ozone peaks, and giant H2O peaks provided by our world-ocean. If we see this, or something like this, coming out of an extrasolar world, the search for life elsewhere in the cosmos will have succeeded.
Just you wait and see.