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Hunting ETI on the Ecliptic

Imagine a team of astronomers from a distant extraterrestrial civilization. Anxious to find blue and green living planets like their own, they study various methods of planetary detection and put them to work on small, relatively nearby stars. Detecting planetary transits, they refine their techniques until they trace the signature of a planet much like home.

Now assume that, despite the presence of their own version of skeptics like myself (some of us think that sending deliberate signals to the stars is premature without further, wider discussion), they decide to encode information about themselves into a message to be sent by a repeating beacon. Naturally, they turn to those stars around which they’ve found planets that look to be not only the right size, but in the right position, within the habitable zone where water could exist on the surface.

Fanciful? You bet, especially in the idea that a nearby extraterrestrial civilization would be more or less at the same state of technological development as ourselves. But maybe they’re not. Maybe they made this detection aeons ago, and have been signaling ever since in the hope that a civilization would arise. Without knowing more about what’s out there, we can’t rule scenarios like this out. And if you take a step in the other direction, you can see that there are SETI applications here on Earth. Hunting for other civilizations might depend upon their being a bit like us.

After all, we’ve always tried to figure out where to search, which is why so much attention has historically been paid to factors like the most obvious frequency for transmissions. Now Richard Conn Henry (Johns Hopkins) looks in at our Solar System from the outside, telling the recent American Astronomical Society meeting that our SETI search should focus on the swath of sky within which an alien civilization might be able to detect the Earth as it makes its own transit across the face of our Sun.

Henry is talking about the ecliptic, which is the plane of the Earth’s orbit around the Sun. As we see things from Earth, the Sun seems to move along this circle each year. Taking up just three percent of the sky, the ecliptic usefully constrains the area that observatories like the Allen Telescope Array (ATA) would have to examine. Henry puts the matter concisely:

“If those civilizations are out there — and we don’t know that they are — those that inhabit star systems that lie close to the plane of the Earth’s orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable. Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life.”

Image: The plane of the ecliptic is illustrated in this Clementine star tracker camera image which reveals (from right to left) the Moon lit by Earthshine, the Sun’s corona rising over the Moon’s dark limb, and the planets Saturn, Mars, and Mercury. The ecliptic plane is defined as the imaginary plane containing the Earth’s orbit around the Sun. In the course of a year, the Sun’s apparent path through the sky lies in this plane. The planetary bodies of our solar system all tend to lie near this plane, since they were formed from the Sun’s spinning, flattened, proto-planetary disk. The snapshot above nicely captures a momentary line-up looking out along this fundamental plane of our solar system. Credit: NASA/The Clementine Project.

Yes, and making smart decisions about how you allocate your observing time could spell the difference between a hit and a miss. If you look at the ecliptic in relation to the galactic plane, the two circles intersect in the region of Taurus and again in Sagittarius, the implication being that these are the most likely regions for finding an extraterrestrial civilization. Assuming long life to civilizations, a factor Henry goes on to investigate:

“These models are nothing but pure speculation. But hey … it is educational to explore possibilities. We have no idea how many — if any — other civilizations there are in our galaxy. One critical factor is how long a civilization — for example, our own — remains in existence. If, as we dearly hope, the answer is many millions of years, then even if civilizations are fairly rare, those in our ecliptic plane will have learned of our existence. They will know that life exists on Earth and they will have the patience to beam easily detectable radio (or optical) signals in our direction, if necessary, for millions of years in the hope, now realized, that a technological civilization will appear on Earth.”

Henry joins Seth Shostak (SETI Institute) and Steve Kilston (Henry Foundation) in using the Allen Telescope Array to perform this search. Ultimately, the entire ecliptic will be investigated through the ATA, but the intersection of ecliptic and galactic planes does present an area of unusual interest. We’ve long known that this area could be interesting, but the ATA’s hundreds of dishes and computer processing capabilities offer a significant upgrade to our search methods. Count me a SETI skeptic, at least in terms of finding intelligent life via radio signals, but count me, too, as one who would rejoice at being proven wrong.

Comments on this entry are closed.

  • Ronald June 5, 2008, 11:17

    Seems a rather dumb idea to me to limit oneself to the 3% orso of the ecliptic plane. This assumes that an advanced technological civilization would detect us through the transit method (instead of for instance radial velocity, followed by direct imaging) and limit itself to primary detection through that method.

    Surely an advanced techno civ that got one big hit would get a tremendous incentive (as we would) to go on and use any other method for direct imaging, spectroscopic analysis, and (if desirable) sending messages, and to do so in any direction.

  • philw1776 June 5, 2008, 11:47

    Makes sense to prioritize our initial efforts to “search under the streetlight” given our limited capabilities. Listen for contact signals (since we aren’t sensitive enough to eavesdrop) from those who would have the best chance to know we’re here.

  • Charles Jenkins June 5, 2008, 11:55

    This proposal will not increase the chances of detecting ETI.

    The proposal asks ‘which civilizations will know about us and thus wish to beam a powerful radio signal at us?’ and answers ‘those in the plane of our Ecliptic who can detect us by our effect on the atmosphere of our planet as the Earth transits in front of the Sun as seen by that civilization’. It then says ‘we should thus point our radio telescope at such stars’.

    But any civilization advanced enough to devote the large resources needed to beam a strong radio signal at us – which is the type of civilization the proposal is considering – will be as easily able to detect us if they are in our Ecliptic or not.

    To demonstrate this, consider ourselves. We will be able soon to detect non-transiting earth-like planets with oxygen atmospheres orbiting other stars as easily by direct imaging as by transits. For instance, the proposed TPF mission is designed to do this. It will however take us longer to become rich enough to devote the substantial resources to beam a radio signal at such planets on the off-chance that in centuries any intelligent life on such planets will hear us.

    So civilizations which are not in the plane of our Ecliptic will be able to detect us as easily by direct imaging with their own space telescopes as by the transit method. So we are just as likely to see a radio signal coming from a civilization not in our Ecliptic as we are from one in our Ecliptic.

    The proposed search strategy will not increase the chance of success of a SETI search.

  • philw1776 June 5, 2008, 18:05

    TPF is sorely didtance limited. Transits for ANY civ are easiest to detect from very long distances compared to super TPF scopes.

    They’re STARTING their search where the higest possibility of us being detected and therefore contacted lies. Makes sense.

  • Charles Jenkins June 5, 2008, 21:00

    It has been argued that, from a distance, it is easier to see transits than do imaging and thus it is likely that there will be advanced civilizations (remember it must be an advanced one to devote the resources for a directed radio beacon) able to detect us by transits but not by imaging.

    However, advanced civilizations will have imaging capabilities significantly beyond ours and will be able to detect us by imaging from across the Galaxy. Thus this argument does not hold.

    The class of civilizations in our Galaxy able to detect us by transits but not by imaging does not include those who are able to set up long-term directed radio beacons.

    This proposed search will necessarily target stars further away than a search which selects stars from the entire sky, and will thus miss those civilizations with weak radio beacons. It will be a search program that is worse than an all-sky one.

  • philw1776 June 6, 2008, 20:29

    The laws of physics do not change with technological advancement. Your argument does not hold. It’s always easier to detect transits across much of the galaxy than to do direct imaging.

    I’m in agreement with the Allen Array researchers’ initial strategy. Sadly, I believe the odds of detecting anyone are extremely close to zero.