SETI and the Targeted Search For ET Civilizations


In what is its most targeted search to date, the SETI Institute has scanned 86 potentially habitable solar systems for signs of radio signals. Needless to say, the search came up short (otherwise the headline of this article would have been dramatically different), but the initiative is finally offering some quantitative data about the rate at which we can expect to find radio-emitting intelligent life on Earth-like planets — a rate that’s proving to be disturbingly low.

Indeed, by the end of its survey, SETI calculated that less than one-percent of all potentially habitable exoplanets are likely to host intelligent life. That means less than one in a million stars in the Milky Way currently host radio-emitting civilizations that we can detect.

A narrow-band search

The SETI researchers, a team that included Jill Tarter and scientists at the University of California, Berkeley, reached this conclusion after scanning 86 different stars using the Green Bank Telescope in West Virginia. These stars were chosen because earlier Kepler data indicated they host potentially habitable planets — Earth-like planets that sit inside their sun’s habitable zone.

As for the radio bands searched, SETI looked for signals in the 1-2 GHz range, a band that’s used here on Earth for such things as cell phones and television transmissions. SETI also constrained the search to radio emissions less than 5Hz of the spectrum; nothing in nature is known to produce such narrow band signals.

Each of the 86 stars — the majority of which are more than 1,000 light-years away — were surveyed for five minutes. Because of the extreme distances involved, the only signals that could have been detected were ones that were intentionally aimed in our direction — which would be a deliberate effort by ETIs to signal their presence (what’s referred to as Active SETI, or METI (Messages to ETIs)).

“No signals of extraterrestrial origin were found.” noted the researchers in the study.”[I]n the simplest terms this result indicates that fewer than 1% of transiting exoplanet systems are radio loud in narrow-band emission between 1-2 GHz.”

Wanted: Alternative signatures

Despite the nul result, SETI remains hopeful for the future. Scanning potentially habitable solar systems is a fantastic idea, and it’s likely the first of many such targeted searches. At the same time, however, SETI will have to expand upon its list of candidate signatures.

It has been proposed, for example, that SETI look for signs of Kardashev scale civilizations, and take a more Dysonian approach to their searches. Others have suggested that SETI look for laser pulses.

Indeed, the current strategy — that of looking for radio-emitting civilizations — is exceedingly limited. Even assuming we could detect signals from a radio-capable civilization within a radius of 1,000 light-years, the odds that it would be contemporaneous with us is mind-bogglingly low (the time it takes for radio signals to reach us notwithstanding).

And as we are discovering by virtue of our own technological development, the window of opportunity to detect a radio-transmitting civilization is quite short. Looking to the future, it’s more than reasonable to suggest that alternative signatures — whether they be transmitted deliberately or not — be considered.

This is something SETI is very aware of, and the researchers said so much in their paper:

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.

The paper is set to appear in the Astrophysical Journal and can be found here.

I suppose this is the natural outreach of the Kepler planetary searches; to see if there are radio signals coming from some of these planets. But as Terence McKenna once said, “To search expectantly for a radio signal from an extraterrestrial source is probably as culture-bound a presumption as to search the galaxy for a good Italian restaurant.

Words of wisdom. I think it’s a mistake to believe that civilizations will use radio to broadcast out into the Universe. Convergent theories of evolution aside, it’s not a proven fact that other intelligences would follow the same evolutionary path as humans and thus invent similar communication techniques.

Of course, time will tell.

SETI Conducts First Ever Targeted Search For Intelligent Life On Earth-Like Planets

Hat tip to the Daily Grail.

13 responses

  1. If the only signals the equipment can detect are those aimed at us deliberately, how does that say anything at all about the number of radio-using civilizations? “I’m looking for oranges, and I’ve found 86 apples, so statistically that indicates oranges are rare.” Yes, but only if you’re looking in a grove of apple trees.

    All this really means is that, at a distance of a thousand light years, catching a signal by accident is much more likely than finding one deliberately aimed this way.

  2. Good point Robert. My question is that how can we assume radio communications are hallmarks of technical cultures, and not just unique to us?

  3. Everyone has to follow the laws of physics, and radio is a useful tool. I can’t imagine a technical civilization that doesn’t use it in some form.

  4. Hmm..another good point. Laws of physics not withstanding, I’m inclined to think that radio broadcasting isn’t as ubiquitous as believed. But I like your looking for oranges in an apple grove analogy.

  5. Thanks.

    The point others have made (to go by our example) that the time window of open broadcasting is fairly narrow is valid, but it would only apply to others who are NOT aiming signals at us. Someone who is would keep it up for a long time (if they could).

    Since we now have the capability to image exoplanets directly and get their spectra (and we have), many recent papers discuss what to look for that would indicate life.

    (Note to those who advocate not revealing ourselves to aliens because they MIGHT be hostile: if we can take the spectra of planets at interstellar distances, so can others. How would you hide that from them?)

  6. (Note to those who advocate not revealing ourselves to aliens because they MIGHT be hostile: if we can take the spectra of planets at interstellar distances, so can others. How would you hide that from them?)

    Food for thought and one I didn’t think of right away.
    Mainstream media channels such as Nat Geo and History Channel had specials over the past few years proposing that aggressive nomadic interstellar alien groups using the above method to detect Earth-type planets in order to plunder them for resources. I think that’s stretching credibility quite a bit, but it goes to show that the issue is in the mainstream now because of Stephan Hawking’s influence.

    1. I’d never have guessed Hawking was a closet xenophobe, but ya never know…

      Besides, there is nothing here that an interstellar civilization couldn’t make more cheaply at home.

  7. Really, who’da thunk it?

    Hawking’s ETI speech:

    And this from a person who is now more machine than man, a true Transhuman!

    It goes to show even if humans merge with machines, old prejudices will die hard.

  8. I agree.

    Seems to me that talking about aliens in the abstract is fine, but anything that might be evidence they exist must discredited at all costs.

    1. It’s a real plaeruse to find someone who can think like that

  9. Very true.

    That’s because the mainstream ( politics, media, science, religion ) believe the public will panic if real evidence of ETIs were found. It’s better to keep them at nice, safe and very far interstellar distances.

  10. True. It goes back to that War of the Worlds radio broadcast Orson Welles did back in the 30s. Yes, some people panicked, but that was because 1: they didn’t hear all of it and realize it was a dramatization, and 2: the Martians were INVADING and KILLING PEOPLE. Who wouldn’t at least be worried if they didn’t know #1?

    But 80 years later, people are more likely to rush TO the scene of an alien landing. I know I would if it was reasonably close.

    1. Yup, I know I would!

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