Centauri Dreams

Beamed propulsion continues to be a particular fascination of mine, which is why I want to start a discussion tomorrow of Jim Benford’s latest paper on beamed sails and how they might be optimized for both performance and cost. Reading through Benford’s work, however, I also came across Chris Wilson’s recent articles in Slate, which discuss Jim and Gregory Benford’s work on interstellar beacons and the SETI ramifications. I want to be sure to point to Wilson’s How to Build a Beacon because I don’t see ‘Benford beacons,’ as they’re increasingly called, discussed much in the media, and Wilson does a fine job at setting the concept in context.

Messages into Deep Time

The two part Slate series (the first article is The Great Silence) considers humanity’s legacy and relates it to the issues raised by SETI. The Arecibo message sent in the direction of the globular cluster M13 in November of 1974 is Wilson’s point of departure. Carl Sagan and Frank Drake set up the famous message in the form of 1679 binary digits that could be decoded into a set of simple pictures showing the image of a human being and other aspects of our existence including a double helix and a graphic of the Solar System. There is rudimentary content here, but the idea that such a fleeting signal would be received is dubious, given the odds on its happening to have a receptive civilization in its path in the first place. As Wilson writes:

Even if we left the Arecibo telescope squealing out its signal until its power ran out and its hardware rusted, there’s virtually no chance that the emanations would get anywhere in particular, and hang around long enough to be seen or heard. The only way we’ll make contact is if we can make a beacon that keeps going for millions or billions of years after we’re gone.

Two of the signals sent to nearby star systems by Alexander Zaitsev from Evpatoria show up as part of the same argument, the point being that for an interstellar beacon to be noticed, it has to transmit for a long period and be energy-efficient as well. By ‘a long time’ I mean potentially eons, because such a beacon might be set up as the final gift to the universe from a dying civilization, and it might not be found for millions of years. Enter the Benford brothers, whose work on cost-optimized beacons we discussed here in articles like A Beacon-Oriented Strategy for SETI (and you can use the search function to pull up other Centauri Dreams stories on this work).

Signature of a Cost-Optimized Beacon

If you’re building such a beacon, you’re naturally going to think in terms of efficiency, as the cost of delivering a powerful signal continuously over a vast period of time would be mind-boggling. An efficient beacon is one that would choose its coverage area carefully to optimize the chances of being heard, and one that would offer short pulses that recur over regular periods. Note what a departure such a signal would be from the kind of signal conventional SETI is optimized to find, its searches running quick sweeps past stars to find continually broadcasting beacons. A Benford beacon’s signature would be intermittent, a brief pulse that would eventually recur.

No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs… But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon’s message arrived. This more sensible approach—a sort of Energy Star specification for SETI—would save enough power to keep the beacon running for millions of years.

If we’re trying to receive such a signal, slow and steady scans of the galactic plane might turn it up in the form of a short narrowband burst that would eventually repeat, which would call for longer ‘dwell times’ — the time devoted to looking at a particular target — and a good deal of patience. Gregory Benford notes that a civilization building such a beacon in our system might locate it at roughly 0.5 AU, allowing for plenty of energy for the beacon’s solar cells. Such a beacon would also face the threat of space debris, given that we’re talking about an artifact that will need to survive for hundreds of millions of years — deep time — and remain functional. Advanced robotics are one way Benford sees to repair damage and keep systems running.

Monument to a Lost Civilization

Benford beacons get around the synchronicity problem that bedevils those obsessed with communication with extraterrestrial civilizations. We have no way of knowing the average lifespan of a technological culture, and it’s possible that such lifetimes are measured in mere hundreds of years or millennia. A long-term beacon keeps sending detectable signals long after the civilization that created it is gone, perhaps as a monument in the fashion of the pyramids. Wilson writes about the Star Trek episode called ‘The Inner Light,’ in which Captain Picard lives out an entire existence on an unknown planet as he experiences an alien mind-probe, only to come to the end of the message and awake to find that a mere 25 minutes have passed.

Would our own species build a Benford beacon if catastrophe loomed? It’s an interesting notion, and it gets around the concerns that some have expressed about METI (Messaging to Extraterrestrial Intelligence), in that a society that builds a ‘funeral pyre’ beacon isn’t even thinking about a response, whether dangerous or otherwise. Wilson suggests a kind of beacon insurance system — build an interstellar beacon that is programmed not to function unless it loses all contact from its creators for an extended period. If it determines the creating culture is extinct, it switches on to send out the valedictory. A civilization dies, but in the remote future, perhaps another detects and decodes its final thoughts. There’s a bleakness in the concept, and yet at the same time a certain degree of grandeur.

To explore Benford beacons at the source, see James Benford et al., “Cost Optimized Interstellar Beacons: METI,” available here, and Gregory Benford et al., “Cost Optimized Interstellar Beacons: SETI,” available here.

The on-again, off-again SETI search at the Allen Telescope Array is back in business as Jill Tarter and team focus in on some of the more interesting worlds uncovered by the Kepler space telescope and follow-up observations. You’ll recall that last April the ATA was in hibernation, having lost its funding from the University of California at Berkeley, which had operated the Hat Creek Observatory in northern California where the ATA is located. It took a public campaign to raise the funds needed for reactivation and new operations, as well as help from the US Air Force in the form of its own assessment of the ATA’s applicability in its space situational awareness studies, which include developing a catalog of orbiting space objects.

The SETI Institute, along with third-party partners and volunteers, has set up SETIstars.org as a fund-raising operation specifically targeting the ATA — it’s important to realize that getting the array back in operation is a first step in the larger process of meeting expenses for continuing work, so you’ll want to check in on SETIstars regularly to see how the campaign is going.

But back to the forthcoming ATA work on Kepler planets. The plan is to work through the Kepler discoveries, taking advantage of the fact that SETI now knows for a fact that the stars in question have planets. Thus Jill Tarter (SETI Institute):

“This is a superb opportunity for SETI observations. For the first time, we can point our telescopes at stars, and know that those stars actually host planetary systems – including at least one that begins to approximate an Earth analog in the habitable zone around its host star. That’s the type of world that might be home to a civilization capable of building radio transmitters.”

The worlds found in the habitable zone (here defined as the zone in which liquid water could exist on the surface) will receive priority, but the people behind the ATA are as aware of the dangers of preconceived notions as any of us, and if sufficient funding is found, all the planetary systems Kepler discovers will be examined across the 1 to 10 GHz terrestrial microwave window. The ATA’s ability to search across tens of millions of channels simultaneously gives it capabilities far beyond those of more common SETI work in limited frequency ranges.

Planets Around Massive Stars

Meanwhile, the Kepler Science Conference continues, with the program available online. And I don’t want to get deep into the Kepler conference without getting to the recent work at Caltech, where astronomers announced the discovery of 18 planets around stars more massive than the Sun. This work involved the Keck Observatory in Hawaii with follow-up work at McDonald and Fairborn Observatories (Texas and Arizona), focusing on ‘retired’ A-class stars more than one and one-half times as massive as the Sun. These so-called ‘retired’ stars are now in the process of becoming sub-giants. The planets were detected by radial velocity methods.

The planets here all have masses similar to Jupiter’s and represent a 50 percent increase in the number of planets known to be orbiting massive stars. What’s particularly interesting here is the wider orbits in which these planets are found. All are at least 0.7 AU from their stars, while a sample of 18 planets around stars like the Sun would turn up at least half of them in close orbits, the result of planetary migration. John Johnson (Caltech), first author of the paper on this work, says the question is whether gas giants around massive stars do not migrate in the first place, or whether they do migrate but are simply destroyed when they plunge into their stars. Also interesting is the fact that the orbits of these planets are primarily circular, while planets around Sun-like stars vary from elliptical to circular.

The authors see the new planets as further evidence supporting the core accretion model of planet formation, in which planets grow through the accumulation of small objects to ‘snowball’ into the resulting world. Stellar mass seems to be a crucial factor, which would not necessarily be the case with the gravitational instability model, in which knots of matter in the circumstellar disk collapse rapidly to form a planet.

From the paper:

[The] observed correlations between stellar properties and giant planet occurrence provide strong constraints for theories of planet formation. Any successful formation mechanism must not only describe the formation of the planets in our Solar System, but must also account for the ways in which planet occurrence varies with stellar mass and chemical composition. The link between planet occurrence and stellar properties may be related to the relationship between stars and their natal circumstellar disks. More massive, metal-rich stars likely had more massive, dust-enriched protoplanetary disks that more efficiently form embryonic solid cores that in turn sweep up gas, resulting in the gas giants detected today.

The paper is Johnson et al., “Retired A stars and their companions VII. Eighteen new Jovian planets,” accepted by the Astrophysical Journal (preprint).