I would like to thank the many Centauri Dreams readers who contributed to the successful Kickstarter campaign to fund a year’s worth of study of KIC 8462852. As I write, there is less than an hour to go, but we have already gone well over the needed $100,000 mark. Congratulations to Tabitha Boyajian, and thanks for all the work she and her colleagues have put into this effort. Now we have a year of observations ahead using the Las Cumbres Observatory Global Telescope Network. The long-term observations will be crucial because we don’t know what to expect in terms of sudden dimming in this star’s light curve.
What a pleasure it is to write for this audience. Readers here have played a large role in pushing this project over the top, and we’ll follow the work on KIC 8462852 closely in coming days. Meanwhile, have a look at Penn State’s Jason Wright discussing ‘Tabby’s Star.’
Speaking of Unusual Stars…
If KIC 8462852 is a star that some believe is becoming less bright over time (the controversy between Bradley Schaefer and Michael Hippke is well documented here), FU Orionis is the opposite, a star whose notable brightening in 1936 is the most extreme such event we have seen around a star of Sun-like size. This is intriguing stuff, because we seem to be looking at a solar system in its infancy, undergoing a process that our own Sun may have experienced on the way toward the formation of the planets in our system. If so, this stellar activity would have changed the chemistry in the surrounding disk, with implications for how planets form.
The 1936 brightening event took FU Orionis, some 1500 light years away in the constellation Orion, from an apparent visual magnitude of 16.5 to 9.6, and the star is now around magnitude 9, with visible light observations showing a slow fade in progress. Stars like this — V1057 Signi is another — are young pre-main sequence stars in a class named after FU Orionis and nicknamed FUors. All exhibit extreme changes in magnitude and spectral type, evidently caused by the star consuming inner disk materials in a sudden feeding spree.
Joel Green (Space Telescope Science Institute) and colleagues wondered when FU Orionis might return to pre-1936 brightness levels, and what effect such brightening events could have on early system formation. The team used infrared data from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and checked it against Spitzer Space Telescope data from 2004. Green presented the results, showing the star’s behavior over the twelve-year interval, at the just concluded meeting of the American Astronomical Society in San Diego.
Image: This artist’s concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 (left) to 2016 (right). Credit: NASA/JPL-Caltech.
The brightening of FU Orionis observed in 1936 was the result of the infant star devouring gas and dust from its surrounding disk, a three-month process that caused it to become 100 times brighter in short order, heating the disk to temperatures up to 7000 K. It’s clear from the most recent study that the star continues to ingest disk material. According to this JPL news release, it has consumed the equivalent of 18 Jupiters in the eighty years since the first outburst.
Even so, the SOFIA measurements confirm that the total amount of visible and infrared light from FU Orionis has decreased by about 13 percent during the twelve years since the Spitzer data were taken. The dimming is occurring at short infrared wavelengths only, an indication that about 13 percent of the hottest disk material has disappeared even as cooler materials have remained intact. Says Green:
“A decrease in the hottest gas means that the star is eating the innermost part of the disk, but the rest of the disk has essentially not changed in the last 12 years. This result is consistent with computer models, but for the first time we are able to confirm the theory with observations… The material falling into the star is like water from a hose that’s slowly being pinched off. Eventually the water will stop.”
Based on this work, we could expect FU Orionis to return to 1936 brightness levels within the next several hundred years. Still unanswered is the question of what set off the rampant activity in the first place. The fact that a protoplanetary disk can experience such sudden activity is striking. Green’s team believes that a similar event in the early Sun’s history could explain why materials close to the Sun have a different composition than those farther out, accounting for the different abundances of certain elements on Mars than on the Earth. The brightening would have had a much larger effect on the inner disk than on more distant materials.
The presentation is Green, “The Evolution of FU Orionis Disks,” American Astronomical Society, AAS Meeting #228, id.#308.05 (abstract).
Before getting into today’s subject, the discovery of an interesting long-period circumbinary planet, I want to make another pitch for Centauri Dreams readers to support the Kickstarter campaign for Tabby’s Star. I’ve written often about this mysterious star whose light curves are anomalous and demand further study. Trying to find out what’s happening around KIC 8462852 means acquiring more data, and the Kickstarter campaign would provide an entire year of observations using the Las Cumbres Observatory Global Telescope Network.
We’re now down to 48 hours and of the $100,000 needed, about three-fourths has been raised. Coming down the homestretch, the remaining $24,000 should be achievable, but it looks to be a dramatic finish. If you haven’t been following the KIC 8462852 story, you can check the archives here, or for a quick overview, see my article A Kickstarter Campaign for KIC 8462852. Whatever you can do to help would be hugely appreciated as we try to learn as much as possible about what Penn State’s Jason Wright has called ‘the most mysterious star in our galaxy.’
Kepler-1647b: Insights into Planet Formation
On to the ongoing meeting of the American Astronomical Society in San Diego, where the discovery of the largest planet yet found around a double-star system was announced. That’s ‘around’ a double-star system rather than ‘in’ one, the planet in question orbiting both stars. Such circumbinary planets are welded to the imagination of Star Wars viewers because of the world Tatooine, the cinematic home of Luke Skywalker, and as in the film, they cast a hypnotic spell on the imagination as we think about what such worlds might look like.
No standing on a planetary surface to watch interesting sunsets here, though. Kepler-1647b is a gas giant, about 3700 light years away in the constellation Cygnus, and at 4.4 billion years old, it’s roughly the same age as the Earth. The planet orbits its eclipsing binary host — two solar mass stars — every 1100 days, making it the longest-period transiting circumbinary planet yet discovered. Despite the lengthy orbital period, three times longer than the Earth’s, Kepler-1647b appears to be in the circumbinary habitable zone for the entire duration of its orbit. Terrestrial moons are theoretically possible, but no evidence for them turns up in the data.
Image: Artist’s impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647 b. Such a double eclipse event is known as a syzygy. Credit: Lynette Cook.
The discovery of this Jupiter-like world is the work of a team led by Veselin Kostov (NASA Goddard). A transit was originally detected back in 2011 but additional data were needed before the circumbinary planet could be confirmed, part of the problem being the length of the planet’s orbital period. As co-author William Welsh (San Diego State University) notes: “…finding circumbinary planets is much harder than finding planets around single stars. The transits are not regularly spaced in time and they can vary in duration and even depth.”
At three years, Kepler-1647b’s orbital period is the longest of any confirmed transiting planet. The gas giant is also the largest circumbinary planet yet found since the first such world, Kepler-16b, was detected through its transits. We now know of 10 transiting circumbinary planets in eight eclipsing binary systems, and what’s intriguing here is that all of these except Kepler-1647b are near the critical orbital distance for dynamical stability. Get any closer to the binary host, in other words, and their orbits would become unstable.
But Kepler-1647b is on a much wider orbit and its large size contrasts with all previously discovered circumbinary planets, which have been Saturn-sized or smaller. Theoretical models had predicted that Jupiter-mass circumbinary worlds should be less common and should orbit at large distances from the central binary. The discovery of a gas giant on a wide orbit is thus consistent with these models. No long-period circumbinary worlds have been detected before this, a fact whose implications for planetary evolution are discussed in the paper:
As important as a new discovery of a CBP [circumbinary planet] is to indulge our basic human curiosity about distant worlds, its main significance is to expand our understanding of the inner workings of planetary systems in the dynamically rich environments of close binary stars. The orbital parameters of CBPs, for example, provide important new insight into the properties of protoplanetary disks and shed light on planetary formation and migration in the dynamically-challenging environments of binary stars. In particular, the observed orbit of Kepler-1647b lends strong support to the models suggesting that CBPs form at large distances from their host binaries and subsequently migrate either as a result of planet-disk interaction, or planet-planet scattering…
Image: The orbit of Kepler 1647b (white dot) around its two suns (red and yellow circles). Kepler-1647 b was observed transiting each of its two suns during a single orbit (days 0 and 4.3). Credit: Kostov et al.
The paper is Kostov et al., “Kepler-1647b: the largest and longest-period Kepler transiting circumbinary planet,” accepted at the Astrophysical Journal (preprint). A news release from the University of Hawaii at Manoa is also available.
Yesterday’s look at organic compounds on Comet 67P/Churyumov-Gerasimenko needs to be augmented today by a just released study of the comet with implications for how all comets evolve. But first, a renewed pointer to the Kickstarter campaign for KIC 8462852, the unusual star whose light curves continue to baffle astronomers. Please consider contributing to the project, which would raise enough money ($100,000) to support a year of observations.
We’re about halfway through the campaign but not yet at the halfway point in funds. Have a look at the information provided on the Kickstarter page, or in my essay A Kickstarter Campaign for KIC 8462852, which also has the relevant links. We know the light curves of ‘Tabby’s Star’ are not periodic, so we need continuous monitoring to gain more data on what may be happening there. If we can raise the funds, the Las Cumbres Observatory Global Telescope Network, already supporting the project, can give us the multi-wavelength observations we need.
A Comet’s Evolution
The rubber-duck shape of Comet 67P/Churyumov-Gerasimenko has long been noted. The ‘neck’ of the comet is what connects the two larger lobes, as is obvious in the image below. As a new study led by Masatoshi Hirabayashi (Purdue) and Daniel Scheeres (University of Colorado) points out, two large cracks appear on the neck connecting the two larger lobes. The team simulated rotation rates for the twin-lobed assembly different from its actual 12-hour spin.
The result: Two cracks similar enough to those on 67P to show just how much stress is imparted. The rotation rate is variable in an object like this one because flybys of the Sun or of Jupiter can produce a gravitational torque. And as also appears in the photo, cometary outgassing is a factor, with compounds like carbon dioxide and ammonia sublimating from the surface. A fast enough spin produced by these factors can cause the two lobes to separate. Seven hours per rotation is what it takes for the head of the ‘duck’ to break off.
Image: Comet 67P’s distinctive shape tells us much about its history. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0.
The researchers used numerical models that examined 1000 instances of 67P ‘clones’ under varying conditions over a 5000 year period. What Hirabayashi and Scheeres have learned is that the breakup and reassembly is an ongoing process as comets respond to these stresses. It’s also one that could last the lifetime of the comet. Says Scheeres:
“The head and body aren’t going to be able to escape from each other. They will begin orbiting each other, and in weeks, days or even hours they will come together again during a slow collision, creating a new comet nucleus configuration.”
As strange as it looks, Comet 67P may not be all that unusual. So far we have imaged seven comets at high resolution, five of which are bi-lobed. The researchers have learned that all of the bi-lobed comets have similar volume ratios between each lobe, an indication that the same cycle of disassembly and reassembly is happening in them as well. In some, there are similarities to what we find in a certain kind of asteroid. From the paper:
…bilobate nuclei observed by spacecraft encounters or ground-based radar have component volume ratios consistent with their nuclei being trapped in a similar cycle to that of 67P’s nucleus. For bilobate nuclei with a volume ratio between their lobes larger than about 0.2, the total energy of these systems will be negative after fission. This means that they are bounded in a similar way to some rubble pile asteroids; however additional sublimation effects could further erode or spin up the individual lobes before re-impact.
The process may be a major factor in cometary evolution, giving us insights into how these objects change over time:
Taking material density to be constant, we computed the volume ratios of the imaged bilobate nuclei of comets 1P/Halley, 8P/Tuttle, 19P/Borrelly, 67P and 100P/Hartley 2; we found that all of these nuclei had a volume ratio higher than 0.2… Observed nuclei with a single component might either be primordial, or have been part of a multi-component object, from which smaller parts are more easily shed.
Window into the Late Heavy Bombardment?
67P/Churyumov-Gerasimenko is a Jupiter-family comet orbiting the Sun every 6.5 years; such periodic comets are thought to originate in the Kuiper Belt, far beyond Neptune’s orbit. We learn that chaotic spin rate changes and the subsequent breaking into parts and reassembling probably caused the breakup of many ancient periodic comets originating at similar distances from the Sun. Enough erosion would have been produced by the continuing reconfiguration of their nuclei to reduce their ability to survive migration into the inner Solar System.
This could explain why comets were not a strong factor in the late heavy bombardment some four billion years ago, when numerous asteroids collided with the early terrestrial planets — two recent papers have made this case. “The reconfiguration cycles of short-period cometary nuclei,” the paper adds, “constitute a new evolutionary process that could affect their ability to survive during migration into the inner solar system.”
ESA’s mission to Comet 67P may, in other words, be giving us insights into the primordial bombardment that reshaped terrestrial worlds. The paper is Scheeres et al., “Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov-Gerasimenko,” Nature 1 June 2016 (abstract).
Heading for the hotel lobby the first night of the Breakthrough Discuss meeting, I thought about a major theme of the Breakthrough Starshot initiative: Making things smaller. Robert Forward wrote about sails hundreds of kilometers in diameter, and vast lenses deep in the Solar System to collimate a laser beam that would drive them. But Breakthrough Starshot is looking at a sail four meters across, carrying a payload more like a smartphone than a cargo ship. That big lens in the outer system? No longer needed if we can power up the sail close to home.
How to Look at Alpha Centauri
Digitization works wonders, and Moore’s Law takes us into ever smaller and more tightly packed realms on silicon chips. The trend affects every aspect of spaceflight and astronomy, as witness ACEsat, a small coronagraph mission with an explicit mission, the search for planets around Alpha Centauri A and B. Ruslan Belikov (NASA Ames), working with Northrop Grumman, led the team that conceived this mission, which was recently submitted for NASA Small Explorer funding. The funding did not come through, but the ACEsat idea persists.
At Breakthrough Discuss, we heard Ruslan Belikov describe the mission. Lead author of a recent paper on the work (“How to directly image a habitable planet around Alpha Centauri with a 30-45cm telescope,” available here), Belikov showed a half-scale model of ACEsat that was small enough to hold in your hand. The mission as currently conceived uses a 35 centimeter by 18 cm optical mirror, fits into a CubeSat, and in the view of its creators, is capable of directly imaging Earth-like planets around Alpha Centauri (see ACEsat: Alpha Centauri and Direct Imaging, Ashley Baldwin’s article from last December, for more).
“Alpha Centauri isn’t the tip of the iceberg,” Belikov told conference attendees during a panel discussion on exoplanet detection. “It is the iceberg. We know that small stars like Barnard’s Star or Proxima Centauri have small habitable zones. But nature has given us Alpha Centauri, a favorable outlier — think about how large its habitable zone appears on the sky in terms of its angular size. We would need a telescope three times larger for other stars.”
Image: Stars in the Alpha Centauri system as compared with the Sun. Credit: David Benbennick. CC BY-SA 3.0.
When asked what surprises we would find around the Alpha Centauri stars, Sara Seager (MIT) suggested that the biggest surprise would be finding a true Earth twin with life on it, and Belikov was quick to agree — “Sara stole my answer,” he said with a laugh. But Seager would go on to talk about future telescopic efforts focusing on just one object, one specialized type of telescope for each type of star. That more specialized approach seems to fit in nicely with Belikov’s ACEsat concept, tuned as it is for not just a specific type of star but a specific stellar system.
Cornell’s Lisa Kaltenegger urged the audience to think about an Earth ‘twin’ being utterly unlike what we might imagine:
“A world like this could meet our conditions for habitability and yet be much different from the Earth. We can imagine a different kind of biota taking over. When you think about an Earth twin, don’t think about the picture you know. Think yellow, think green, whatever color you want. We can look at some of the exotic deep water fish we find in the ocean and realize that they, as strange as they seem, evolved right here. What might wind up evolving around Centauri?”
Kaltenegger’s remarks drew on her work at the Carl Sagan Institute, where she has built a new interdisciplinary research group spanning ten departments. The effort includes a color catalog that examines the differing reflectivity of various planets depending on possible biota. Which gets us to a major point about Breakthrough Starshot. As Kaltenegger mentioned more than once, data return from the nearest stars needs to include more than basic photographic images. Spectroscopic analysis is crucial as we examine planetary atmospheres for biosignatures.
Natalie Batalha (NASA Ames) pointed out that while we all hoped to find planets in the habitable zone of Alpha Centauri, we might consider that a null result there could still leave us with interesting science. We have not resolved the surface features of any main sequence star except for our own Sun. An Alpha Centauri mission gives us the prospect of resolving the surface of three stars, a science windfall that adds to the benefits of the mission.
Image: Exoplanet hunters Sara Seager (left), Lisa Kaltenegger, Natalie Batalha and Debra Fischer during a break at the meeting.
SETI and Its Consequences
The question and answer sessions at Breakthrough Discuss tended to run into our break periods, when many of the issues continued to circulate. A key issue for SETI is that factor in the Drake equation that addresses the lifetime of a technological civilization. Just how long do such entities survive? It’s an issue that lies like morning fog over the SETI landscape as we look at our own problems with the proliferation of powerful weapons systems and ask what happens when small organizations, even individuals, can acquire ever more deadly capabilities.
Here Harvard’s Avi Loeb brought up a point many others had been thinking about, to judge from the ensuing conversations outside. We naturally focus on finding biological life because that is what we are, but the technologies we are discussing remind us off the possibility that life transforms at a certain level of its development into a technological phase that leads to post-biological intelligence. Sara Seager pointed to existing tools like pacemakers and artificial limbs — if life goes post-biological, how do we create a SETI capable of identifying it?
For that matter, what do we do if it’s non-biological, as Denise Herzing (Florida Atlantic University) asked. Researching dolphin communication through the Wild Dolphin Project, Herzing wonders what the signatures of a non-technological species might be. We are only beginning to understand the complexity of dolphin communication. What if a Breakthrough Starshot mission encounters a world with an intelligent, non-technologial species in charge? Is there any way we could identify it?
The SETI session itself, led by Jill Tarter, focused on optical SETI and the new directions it opens to scrutiny. Again, the issue seems timely given the nature of the Starshot initiative, for as we conceive of technologies here on Earth, we then must ask whether equivalent technologies would be visible to us. Breakthrough Starshot envisions using phased laser arrays scaling up to the 100 GW level. As we learn how to create such ‘beamers,’ we have to ask whether the most common SETI signature in the optical might not be transient pulses that are the byproducts of other activities rather than intentional efforts to communicate with us.
Image: Jill Tarter takes the podium to begin the SETI session.
Puzzlingly, Jim Benford’s paper on that very idea was put into the session on using the Sun as a gravitational lens, but the methods Benford is discussing are very much part of the optical SETI landscape. Centauri Dreams readers already know Benford’s findings: Depending on the use of the beamed energy technology, we would indeed be able to detect transient signals from another star with equipment existing today (see Quantifying KIC 8462852 Power Beaming for background). See also “Power Beaming Leakage Radiation as a SETI Observable,” available here).
Moreover, there does exist a communications dimension, as Benford explained:
“SETI messages may be found on power beaming leakage. That leakage is more powerful than any beacons we’ll ever be likely to build, so we should look at transient signals to see if there is a message embedded on them.”
Beamed energy options for an advanced civilization might involve everything from microwave thermal rockets (beaming to orbit) to orbit boosting to interplanetary and even interstellar missions. With each of these missions, you get progressively higher energy usage. And we can certainly go beyond this list of applications into areas like asteroid deflection, which might one day be necessary to adjust a dangerous trajectory using a high-powered laser. Another possibility: Using a beamer to create a thrust (by evaporation and desorption) of a comet’s surface, which might be a technique one day used in terraforming a planet like Mars.
Benford points out that given their visibility over interstellar distances, many of these uses of beamed energy should make us reconsider optical SETI strategy. Rather than focusing on narrow-band transmissions of the kind likely to be found in a beacon, we should look for more powerful beams with a wider bandwidth. Whereas earlier optical SETI work has involved the 1 ? 10GHz microwave band, we might, considering our atmosphere, want to examine bands where lower oxygen and water vapor allow transmission, which means windows at 35GHz, 70 ? 115GHz, 130 ? 170GHz and 200 ? 320GHz.
The caveat is that power beaming is not isotropic but highly directed. The geometry is not always going to favor detection — the fact that we do not see the beam does not mean that it is not there. An interesting suggestion is to revisit the thousands of transients that have been observed and are now stored in the archives of the SETI@home project.
James Guillochon (Harvard) examined rapid travel in the Solar System via lightsails powered by beamed radiation, underlining some of Benford’s points. A lightsail network in the Solar System is the kind of SETI observable Benford and son Dominic have already addressed in their upcoming paper, the point being that leakage around the edges of the sail cannot be avoided. You may recall that Guillochon, working with Avi Loeb, discussed SETI possibilities in 2015 in “SETI via Leakage from Light Sails in Exoplanetary Systems,” available here).
Image: Beautiful weather and a Palo Alto spring made the occasional break a welcome chance to relax.
But enough for today. More SETI discussion tomorrow, after which we move into the Breakthrough Starshot mission itself as described by former NASA Ames director Pete Worden, and the possibilities of the Sun’s gravitational lens as both a mission target and a mission facilitator.
David Kipping and Alex Teachey have a new paper out on the possibility of ‘cloaking’ a planetary signature. The researchers, both at Columbia University, make the case that any civilization anxious to conceal its existence — for whatever reason — would surely become aware that all stars lying along its ecliptic plane would see transits of the home world, just as its own scientists pursued transit studies of planets around other stars. And it turns out there are ways to make sure this signal is masked by adjusting the shape of the planet’s transit light curves.
Now this is a fascinating scenario as presented by the head of the Hunt for Exomoons with Kepler, whose business it is to know about the slightest of variations in light curves because they may contain information about exomoons or rings. Thus Kipping is a natural to look into the artificial manipulation of light curves, a study with definite SETI implications. Because methods like these work in two directions — a civilization that does want to communicate could also alter its light curve in ways that would be unambiguously artificial. Today I want to focus on the latter idea, reserving cloaking methods for tomorrow’s post.
This veers into the METI debate, but that’s not my purpose. Messaging to Extraterrestrial Intelligence is highly controversial, triggering arguments in these pages for the last decade. But let’s hold METI at one remove. The paper, delightfully titled “A Cloaking Device for Transiting Planets,” allows us to imagine how the manipulation of transit signatures could change a distant planet’s visibility. We may well decide not to brighten the Earth’s visibility through intentional transmissions, while understanding that an extraterrestrial culture might choose differently. Knowing what is possible by way of cloaking or enhancing a planetary signature, then, gives us plenty of food for thought for SETI as we consider what might turn up in our data.
Modifying the Light Curve
Here’s the notion in a nutshell, as drawn from the paper.
Image: Figure 1. Illustration (not to scale) of the transit cloaking/broadcasting device. A laser beam (orange) is fired from the night side of inhabited planet (blue) towards a target star whilst the planet appears to transit the star, as seen from the receiver. In the case of the Earth, the planet could be cloaked by generating an inverse transit-like signal of peak power 60 MW. Credit: Kipping and Teachey.
Controlled laser emissions are the key, effectively distorting the shape of a transit light curve. Just how this could be done is something we’ll discuss in the next post. But to begin, let’s think about how visible we are to any advanced civilization studying us. Forget about our radio and television signature, which is infinitesimal, and focus instead on the things we are doing right now to study other stars. I often write about transmission spectroscopy, which is how we search for the constituent molecules in a planetary atmosphere. The transiting planet, as it moves in front of its host star (as seen by our instruments) is bathed in the star’s light, its atmosphere providing a particular ‘overlay’ to the star’s spectrum.
We’ll use the method to look for biosignatures as we refine it for smaller and smaller worlds, but we’ve already seen how successfully we can examine the atmospheres of ‘hot Jupiters’ like HD 189733b. A sufficiently capable civilization able to see our world transiting should be able to pick up the cluster of biosignatures that would identify the Earth as a living world. There have been proposals to look for atmospheric pollutants produced by industrial activity as a part of future SETI practice, and such activity could indeed become visible, though the idea of widespread pollution lasting for millennia seems a stretch. Understanding the damage it caused, surely the culture in question would either solve its pollution problems or else succumb to them.
Let’s not forget the recent flurry of interest in the unusual star KIC 8462852. Here we’re looking at what could well be a natural phenomenon in the form of clouds of comets, but could possibly be evidence of artificial megastructures throwing highly distinctive light curves. We have much work ahead on KIC 8462852, so I only bring it up to suggest that there are many ways an intelligent species might make itself visible whether its intent was to do so or not.
Turning Transits into ‘Broadcasts’
Scientists as diverse as Ronald Bracewell, Richard Carrigan, Michael Papagiannis and Robert Freitas have studied the possibilities of such detections, with Carrigan most prominently identified with the search for Dyson spheres, swarms of energy collectors that surround a star to feed its colossal energies to a growing Type II civilization. And back in 2005, Luc Arnold (now at Aix Marseille Université) suggested that a civilization wanting to be known could resort to deliberate signaling by building a particular kind of geometric megastructure, one that when viewed in a transit would all but shout, through its distinctive light curve, that it was artificial.
Kipping and Teachey are, as you would imagine, well aware of Arnold’s work, and argue that lasers offer far more practical methods. From the paper:
Whilst any number of artificial transit profiles can be created with lasers, one ideally seeks a profile which is both energy efficient and unambiguously artificial. Producing upward spikes in-transit might seem like an obvious suggestion, but star spot crossings produce these forms with complex and information rich signatures (e.g. see Beky et al. 2014). Here, we argue that cloaking the ingress/egress of a transit, but leaving the main transit undistorted, would be a highly effective strategy since no known natural phenomenon is likely to produce such an effect.
Image: Figure 6 from the paper, highlighting broadcasting rather than cloaking. Top: The power profile of a laser array designed to broadcast the Earth. An array of lasers producing a peak power of ? 20 MW for approximately 15 minutes nullifies the transit ingress and egress. Bottom: The resulting light curves as viewed by different broadband optical photometers. The observed impact parameter would be complex infinity, for which a normal light curve fit would be unable to explain and thus indicating the presence of artificial transit manipulation.
A monochromatic laser emission could be spotted in a transit survey (and could be searched for in Kepler data), with follow-up spectroscopy confirming the artificial nature of the transmission. Kipping and Teachey also note what James and Dominic Benford recently pointed out in their paper on SETI efforts at KIC 8462852 — a beamed signal of whatever kind could carry information, so that in addition to identifying the presence of a technological culture, the beam could attempt more detailed communication (see SETI: Power Beaming in Context).
I’ve focused in on broadcasting via transit light curve alteration because, as the paper argues, it is the most efficient use of these techniques as compared to cloaking, which I’ll describe tomorrow. I’m trying to stay out of the METI weeds here — this is not an argument in favor of identifying the Earth to ETI. Rather, it is an argument that other civilizations may use such methods, and thus this paper shows us a signature we should add to our catalog.
And it has this further implication: If a civilization did choose to broadcast its existence through these methods, it would probably choose the shortest period planet in its solar system to carry the message. The choice is obvious, as the paper notes, for this produces “a higher duty cycle of distorted events,” making the detection all the more obvious. “We therefore suggest that any survey in archival data should not be limited to rocky planets in the habitable-zone of their host star.” An excellent reminder not to succumb to easy assumptions!
Tomorrow I’ll return to the Kipping and Teachey paper with a look at cloaking possibilities. The paper is “A Cloaking Device for Transiting Planets,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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