Over the past several years, astronomers have detected a handful of exoplanets that seem like promising candidates for supporting some kind of life. Maybe even something intelligent.
This means that for the first time, we can do more than just randomly listen for signals from alien civilizations, we can actually start targeted searches for radio signals coming from the planetary systems where they might be living. And if there is advanced intelligent life in these planetary systems, it seems likely that we'd be able to spot it, because just like humans — theoretically, at least — aliens would likely start by colonizing multiple planets in their home system. If they do that, they'd need interplanetay communications, and that's the type of thing we might be able to spot with radio telescopes from Earth.
For a few months back in 2011, the SETI Institute used the the Green Bank radio telescope in West Virginia to systematically survey 86 potentially habitable exoplanets discovered by the Kepler telesope, looking for narrow-band radio transmissions that would be fairly unambiguous evidence of engineered technology. The researchers ultimately filtered their data down to 52 candidate signals, but, as they bluntly put it, "no signals of extraterrestrial origin were found."
Yeah, it's not the result that anyone was hoping for, but cheer up, the galaxy is a big place and there's a huge number of planets out there. The fact that we didn't find anything after this one tiny little targeted search shouldn't be especially discouraging. That said, SETI suggests several reasons why nothing was discovered this time around, and why it's possible that there may be lots of life out there that we just can't detect with the sensors that we currently have available:
Ultimately, experiments such as the one described here seek to firmly determine the number of other intelligent, communicative civilizations outside of Earth. However, in placing limits on the presence of intelligent life in the galaxy, we must very carefully qualify our limits with respect to the limitations of our experiment. In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise. Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate. The search for extraterrestrial intelligence is still in its infancy, and there is much parameter space left to explore.
It's not too hard to imagine that a civilization more advanced than ours may have dumped radio communications for something a lot more futuristic, like quantum entanglement communication or subspace. If this is the case, we can search the galaxy with radio telescopes until we turn blue in the face and we'll never, ever spot them. The simple fact is that any extraterrestrial intelligence out there is highly likely to be stupendously more advanced than we are, which puts us at a significant disadvantage when it comes to communicating with them. Our one saving grace might be that even though we're relative dummies when it comes to technology, aliens would know that we're dummies, so all we have to do is be just as smart as dummies should be, and we'll be okay.
So why haven't we heard from aliens yet? There are lots of potential reasons. Maybe we're not looking in the right places or listening in the right ways, as the researchers suggest. Or maybe aliens aren't out there at all. Or maybe, it's just going to take a little bit more searching, and tomorrow, we'll find a signal. That's what I tell my self when I read the news every morning, because sooner or later, I'm going to get to write an article about how we've detected the first definitive evidence for extraterrestrial intelligence. And while we're all hoping for sooner, the important thing is to keep looking, because even if finding evidence for aliens takes a long, long time, the implications of success are so mind blowing that it's absolutely worthwhile. As the SETI researchers put it:
"Are we alone as technologically-capable intelligent beings?" is among the most profound questions we can ask as scientists.