Have you ever looked up at the night sky and wondered if maybe, somewhere out there, someone might be looking back at you? Well, I’m here to tell you the answer to that question is yes. Or at least there are aliens out there who are trying very hard to find us. I even have video evidence to prove it!
For us Earthlings, it’s pretty obvious that there’s life on this planet. How could you possibly miss it? But for aliens observing Earth from a distance—perhaps a very great distance—the most obvious biosignatures are frustratingly difficult to detect.
In the early 1990’s, Carl Sagan wrote a famous paper about this problem. One of NASA’s own space probes, which was heading out to Jupiter, briefly turned all its instruments back on Earth. Based on that data alone, without any prior knowledge about this planet, you could probably figure out there’s life on Earth. Probably.
This more recent paper published in The Astrophysical Journal follows up on Sagan’s work. Assuming the aliens are smart (a big assumption, based on what the video evidence shows us), they should be looking for a planet with both an oxidizing gas AND a reducing gas in its atmosphere.
Oxidizing and reducing agents should react with each other relatively quickly, removing each other from the planet’s atmosphere. So in order to have those two things coexisting long term, some exotic process (like biological activity) must be constantly replenishing them.
A spectroscopic analysis of Earth’s atmosphere would reveal a whole lot of the chemicals in our air, but not all of them. Apparently some spectral signatures are so strong they cover up others, which I think is an important thing to know. But oxygen (an oxidizing gas) should still be detectable in the visible light part of the spectrum, and methane (a reducing gas) should show up in visible and infrared.
But still, it sounds like difficult work, teasing the signatures of oxygen and methane out of all the other spectral signatures you’d get from Earth’s atmosphere. This could be why the aliens are having such a hard time finding us, and also why we are having such a hard time finding them.