Centauri Dreams
Imagining and Planning Interstellar Exploration
What’s Next for Unusual KIC 8462852?
I want to revisit the paper on KIC 8462852 briefly this morning, as I’m increasingly fascinated with the astrophysics we’re digging into here. The fact that the star, some 1480 light years away, is also a candidate for further SETI investigation makes it all the more intriguing, but all my defaults lean toward natural processes, if highly interesting ones. Let’s think some more about what we could be looking at and why the ‘cometary’ hypothesis seems strongest.
Remember that we’re looking at KIC 8462852 not only because the Kepler instrument took the relevant data, but because the Kepler team took advantage of crowdsourcing to create Planet Hunters, where interested parties could sign up to study the light curves of distant stars on their home computers. KIC 8462852 has been causing ripples since 2011 because while we do seem to be seeing something passing between its light and us, that something is not a planet but a large number of objects in motion around the star. Some of the dips in starlight are extremely deep (up to 22 percent), and they are not periodic.
Here’s how Phil Plait describes the situation:
…it turns out there are lots of these dips in the star’s light. Hundreds. And they don’t seem to be periodic at all. They have odd shapes to them, too. A planet blocking a star’s light will have a generally symmetric dip; the light fades a little, remains steady at that level, then goes back up later. The dip at 800 days in the KIC 8462852 data doesn’t do that; it drops slowly, then rises more rapidly. Another one at 1,500 days has a series of blips up and down inside the main dips. There’s also an apparent change in brightness that seems to go up and down roughly every 20 days for weeks, then disappears completely. It’s likely just random transits, but still. It’s bizarre.
A ragged young debris disk would be the natural conclusion, but arguing against this is the fact that we don’t see the infrared excess that a dusty disk would create. I also got interested in what nearby objects might be doing to this star when I started digging into the paper, which is cited at the end of this piece. Yale postdoc Tabetha Boyajian and colleagues present an image from the UK Infrared telescope (UKIRT) that shows KIC 8462852 along with a second source of similar brightness, as shown in the image below. Notice the ‘extension’ of KIC 8462852 to the left.
Image: UKIRT image for KIC 8462852 and another bright star for comparison, showing that it has a distinct protrusion to the left (east). For reference, the grid lines in the image are 10?
× 10?. Credit: Tabetha Boyajian et al.
A follow-up Keck observation revealed what the UKIRT image suggested, that there is a faint companion star.
Image: Keck AO H-band image for KIC 8462852 showing the companion was detected with a 2? separation and a magnitude difference ?H = 3.8. Credit: Tabetha Boyajian et al.
This gets important as we consider the cometary debris hypothesis. The paper argues that the chance alignment possibility is only about one percent. If the companion is at the same distant as KIC 8462852, which is an F-class star, then we would be looking at an M-class red dwarf, roughly 885 AU distant from its companion. From the paper:
At this separation, the second star cannot currently be physically affecting the behavior of the Kepler target star, though could be affecting bodies in orbit around it via long term perturbations. If such a star is unbound from KIC 8462852, but traveling through the system perpendicular to our line of sight, it would take only 400 years to double its separation if traveling at 10 km sec?1. So, the passage would be relatively short-lived in astronomical terms.
Recall that the paper settles on cometary activity as the most likely natural explanation for the unusual KIC 8462852 light curve. We could be looking at a series of comet fragments seen close to the star as they move on a highly eccentric orbit, a collection of objects that has spread around the orbit and may be continuing to fragment. And as seen yesterday, Boyajian and team make the case that both thermal stress and the presence of super-Earth planets orbiting within 1 AU of the star could account for the tidal disruption that would have produced this scenario.
We’ve often discussed cometary disruptions in these pages, speculating on what the passage of a nearby star might do to comets in the Oort Cloud. As per the images above, it’s a natural speculation that the anomalies of KIC 8462852 are the result of a similar scenario. We have no idea whether the companion star is bound to KIC 8462852, but assume for a moment that it is not. A star passing close enough to this system has the potential for triggering a swarm of infalling comets. If the star is gravitationally bound, then we can invoke the so-called Kozai mechanism, ‘pumping up comet eccentricities,’ as the paper puts it. We can explore this hypothesis by studying the motion of the companion star to confirm its bound or unbound status.
The paper, as we saw yesterday, explores other hypotheses but settles on comet activity as the likeliest, given the data we currently have. The kind of huge collision between planets that would produce this signature would also be rich in infrared because of the sheer amount of dust involved, and we don’t see that. You can see why all this would catch the eye of Jason Wright (Penn State), who studies SETI of the Dysonian kind, involving large structures observed from Earth. Because if we’re looking at cometary chunks, some of these are extraordinarily large.
So what’s next? The paper explains:
First and foremost, long-term photometric monitoring is imperative in order to catch future dipping events. It would be helpful to know whether observations reveal no further dips, or continued dips. If the dips continue, are they periodic? Do they change in size or shape? On one hand, the more dips the more problematic from the lack of IR emission perspective. Likewise, in the comet scenario there could be no further dips; the longer the dips persist in the light curve, the further around the orbit the fragments would have to have spread. The possibility of getting color information for the dips would also help determine the size of the obscuring dust.
Monitoring of KIC 8462852 will continue from the ground thanks to the efforts of the MEarth project, which will begin the effort in the fall of this year, and that’s going to be useful for tracking the variability of the dips. Remember, too, that problem of lack of infrared excess. Those numbers could change if we really are witnessing a recent event. The paper continues:
Several of the proposed scenarios are ruled out by the lack of observed IR excess but the comet scenario requires the least. However, if these are time-dependent phenomenon, there could be a detectable amount of IR emission if the system were observed today. In the comet scenario, the level of emission could vary quite rapidly in the near-IR as clumps pass through pericenter (and so while they are transiting). The WISE observations were made in Q5, so detecting IR-emission from the large impact scenario, assuming the impact occurred in Q8 is also a possibility. We acknowledge that a long-term monitoring in the IR would be demanding on current resources/facilities, but variations detected in the optical monitoring could trigger such effort to observe at the times of the dips.
What a fascinating object! There has been a media flurry about the SETI possibilities, but that doesn’t mean that we shouldn’t investigate KIC 8462852 in SETI as well as astrophysical terms. No serious scientist is jumping to conclusions here other than to say that there is nothing in the laws of physics that would preclude the existence of civilizations more advanced than our own, and nothing that we know of that would keep us from detecting large artifacts. How they could be detected around other stars will be the subject of a forthcoming paper from Jason Wright and colleagues in The Astrophysical Journal, one we’ll obviously discuss here.
The paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint).
KIC 8462852: Cometary Origin of an Unusual Light Curve?
Dysonian SETI operates under the assumption that our search for extraterrestrial civilizations should not stop with radio waves and laser communications. A sufficiently advanced civilization might be visible to us without ever intending to establish a dialogue, observed through its activities around its parent star or within its galaxy. Find an anomalous object difficult to explain through conventional causes and you have a candidate for much closer examination.
Is KIC 8462852 such a star? Writing for The Atlantic, Ross Andersen took a look at the possibilities yesterday (see The Most Mysterious Star in Our Galaxy), noting that this F3-class star puts out a light curve indicating not a planetary transit or two, but a disk of debris. That wouldn’t be cause for particular interest, as we’ve found numerous debris disks around young stars, but by at least one standard KIC 8462852 doesn’t appear to be young. In a paper on this work, Tabetha Boyajian, a Yale University postdoc, and colleagues see it as a main sequence star with no kinematic indication that it belongs to the population of young disk stars.
The age of a star can be a hard thing to calculate, and unfortunately, at 1480 light years, this one is too far away for us to measure its rotation period or gauge its chromospheric activity. [Addendum: My mistake: Jason Wright just pointed out that we do have data on rotation period and chromospheric activity — the problem is that these are not good age indicators for F-class stars].
But the authors also find that there is no excess emission at mid-infrared wavelengths of the kind we would expect from a dusty disk. That makes for an object unusual enough to have caught the eye of a Dysonian SETI specialist like Jason Wright (Penn State), who told Andersen “Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build.” Working on a paper of his own, Wright and his co-authors find the star’s light pattern not inconsistent with a swarm of large structures.
One of the classic Dysonian SETI scenarios would be the discovery of a Dyson sphere, an artificial construction built around the parent star to harvest the maximum energy possible. Such a sphere, although frequently depicted in fiction as a solid object, would more likely exist as a swarm of orbiting objects, and as we imagine these things, a light signature like KIC 8462852’s could be the result. That makes the search for alternative explanations all the more interesting, as we try to understand what natural causes might explain the KIC 8462852 light curve.
Image: This view of Comet Halley’s nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on 13 March 1986. The recent paper on KIC 8462852 discusses a cometary influx as a possible cause of the unusual light curves. Credit: ESO.
We’re fortunate to have four full years of Kepler data on this target, allowing the authors to explore a range of possibilities. A large-scale impact within the system is the first thing that comes to my mind. On that score, think of something on the scale of the event that caused our own Moon to form. The problem here is the time frame. The collision would have had to occur between observations from the WISE observatory and a large dip in flux (nearly 15%) seen in later Kepler observations, because we would expect such an event to trigger a strong infrared excess that was not seen by WISE. Such an excess could be there now, but this would also mean that we chanced upon an impact that occurred within a window of just a few years.
Coincidences happen, so we can’t rule that out. The paper also considers catastrophic collisions in this star’s analogue to our asteroid belt, as well as the possibility that we are seeing the passage of a disintegrating comet through the system. In this scenario, the comet would have passed well within one AU. Add in a few other factors and it might work:
The temperatures of comets at such close proximity to the star (> 410 K) would render them susceptible to thermal stresses. The existence of multiple super-Earth planets orbiting < 1 AU from many main sequence stars also points to the possibility that the comet could have been tidally disrupted in a close encounter with one such planet. It is even possible that the comet came close enough to the star for tidal disruption in the absence of other considerations; e.g., a comet similar to Halley's comet would fall apart by tidal forces on approach to within 3-7 stellar radii (0.02 - 0.05 AU).
And this:
Also, since fragments of the comet family would all have very similar orbits, this mitigates the problem noted in Section 4.4.2 that the detection of multiple transits may require orders of magnitude more clumps to be present in the system. Instead a single orbit is the progenitor of the observed clumps, and that orbit happens to be preferentially aligned for its transit detection. That is, it is not excluded that we have observed all the clumps present in the system.
But can the comet scenario explain details in the light curves of KIC 8462852? The paper notes how much remains to be explored, but concludes that a cometary explanation is the most consistent with the data. Conceivably a field star might have made its way through this system, triggering instabilities in KIC 8462852’s analogue to the Oort Cloud. There is in fact a small nearby star that whether bound to the system or not could be implicated in cometary infall.
So what’s next? Andersen tells us that Boyajian is now working with Jason Wright and Andrew Siemion (UC-Berkeley) on a proposal to study KIC 8462852 at radio frequencies that could implicate the workings of a technological civilization. That could lead to further work at the Very Large Array in New Mexico. All of this is as it should be: The appropriate response to a stellar anomaly is to study it more closely while working through a range of possibilities that might explain it. The fact that we don’t see a light curve like this among any of Kepler’s other 156,000 stars is telling. Whatever is going on here is rare enough to merit serious follow-up.
The paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint).
A Mission to Jupiter’s Trojans
Back in 2011, a four planet system called Kepler-223 made a bit of a splash. Researchers led by Jack Lissauer (NASA Ames) at first believed they were looking at two planets that shared the same orbit around their star, each circling the primary in 9.8 days. These co-orbital planets were believed to be in resonance with the other two planets in the system. If the finding were confirmed, it would indicate that one planet had found a stable orbit in a Lagrange point — the L4 and L5 Lagrange points lie 60° ahead and behind an orbiting body. We call an object sharing an orbit like this a trojan, as shown in the figure below, which depicts the best known trojans in our system, the asteroids associated with Jupiter.
Image: Jupiter’s extensive trojan asteroids, divided into ‘Trojans’ and ‘Greeks’ in a nod to Homer, but all trojans nonetheless. Credit: “InnerSolarSystem-en” by Mdf at English Wikipedia – Transferred from en.wikipedia to Commons. Licensed under Public Domain via Commons.
By sheer coincidence I have been reading Peter Green’s splendid new translation of The Iliad (University of California, 2015), so I pause for a moment on the classical theme in naming conventions for Jupiter’s trojans. The German astronomer Max Wolf was the first to spot one of Jupiter’s trojans in 1906, naming it 588 Achilles. Their number quickly swelled, and we now have over 6000 identified Jovian trojans, with a total population of objects over one kilometer in diameter believed to be about one million. The trojan 617 Patroclus, another Homeric reference, was found in 2006 to be composed of water ice, making the Jupiter trojans interesting sources of volatiles.
The work on Kepler-223 was the first time we thought we had found something as large as a trojan planet, but Lissauer and team soon realized that a different interpretation of the light curve was more likely, one in which one of the two co-orbital possibilities had an orbital period that was twice the original estimate. Too bad, because this was quite a fascinating find. There has been speculation that the Earth itself may have once had a small planet at one of its Lagrange points, the ‘Theia’ impactor whose collision with our planet would have produced the Moon.
We now know that trojans can appear at many places in our Solar System, with seven under study at Mars, nine at Neptune, and 2010 TK7 confirmed as the first known Earth trojan in 2011. But Jupiter’s population remains the most robust, and given the composition of 617 Patroclus, it’s good to see that a mission design to explore the Jupiter trojans is emerging. One of five investigations recently chosen by NASA for further study, the project, called Lucy, comes out of the Southwest Research Institute, with Harold Levison as principal investigator.
“This is a once-in-a-lifetime opportunity,” Levison said of the proposed 11-year mission. “Because the Trojan asteroids are remnants of that primordial material, they hold vital clues to deciphering the history of the solar system. These asteroids are in an area that really is the last population of objects in the solar system to be visited.”
$3 million will go into the concept design studies and analysis involved in developing a mission that would study five of the Jupiter trojans, with a launch some time in 2021. The final trojan encounter would occur in 2032. This SwRI news release discusses a spacecraft package containing remote-sensing instruments to study the physical properties of trojans, with three imaging and mapping instruments including a color imaging and infrared mapping spectrometer, a high-resolution visible imager, and a thermal infrared spectrometer. The name ‘Lucy’ is a reference to the fossil remains of an early hominid dating back over three million years.
Image: Lucy, an SwRI mission proposal to study primitive asteroids orbiting near Jupiter, is one of five science investigations under the NASA Discovery Program up for possible funding. Credit: SwRI.
From the standpoint of naming conventions, we haven’t quite finished with the Jovian trojans, though. It turns out that before the idea of naming these objects after Homeric references had fully stabilized, with ‘Trojans’ on one side (L5 in relation to Jupiter) and ‘Greeks’ on the other (L4), both 617 Patroclus and the even more martial 624 Hektor were assigned positions in the wrong camps. Not a recipe for tranquility for any classicist — it was Hector who finished off Patroclus, an event that led to the return of Achilles to battle and a sea-change in the fortunes of the war around Troy.
Pluto’s Circumbinary Moons
Kepler-47 is an eclipsing binary some 4900 light years from Earth in the direction of the constellation Cygnus. It’s a system containing two transiting circumbinary planets, meaning the planets orbit around the binary pair rather than around one or the other star. That configuration caught the eye of Simon Porter, a postdoc at the Southwest Research Institute, because the configuration is so similar to another circumbinary system, the one involving four small moons around Pluto/Charon. In both cases, we have a binary at the center of the orbit. Porter writes about the configuration in this post from the New Horizons team.
In the case of Pluto, the binary could be considered a binary planet, with Charon the other half of the duo. Both are orbited by a system of four moons, each of them less than 50 kilometers in diameter, the moons orbiting around the system’s center of mass. New Horizons, the gift that keeps on giving, has already sent some striking images of these small moons, but we have even better imagery yet to come as we continue to download data from the craft’s Long Range Reconnaissance Imager (LORRI), the high resolution camera that has given us so so many unforgettable images already. But I’ll open the week with an image not from LORRI but from the Multispectral Visible Imaging Camera, just to provide a sense of context and a bit of awe.
Image: Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles (called tholins) that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible. Credit: NASA/JHUAPL/SwRI.
Blue atmospheric haze in the Kuiper Belt is not something anyone was expecting. SwRI’s Carly Howett offers a read on what we’re seeing:
“That striking blue tint tells us about the size and composition of the haze particles. A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins.”
This JHU/APL news release has more, explaining current thinking that tholins form in the upper atmosphere as ultraviolet light breaks nitrogen and methane molecules apart, allowing them to form increasingly complex negatively and positively charged ions that recombine to form macromolecules. Small particles can grow out of the process, with volatile gases condensing to coat their surfaces before they fall back to the surface, adding to its reddish hue.
But back to the system of moons. The closest to New Horizons during the July encounter was Nix, of which LORRI has delivered three close-ups so far. Have a look at the object as, in the second view, it reveals its ‘potato-like’ aspect — the elongation is lost in the first image because we’re looking down the long axis. What stands out here is the size of that crater. Are we looking at a fragment of an older moon, as Porter speculates, or was Nix just lucky to have survived a shot that could leave a crater of that size on such a small surface? A crescent Nix shows up on the far right, which may yield information on the surface of the diminutive moon.
Image: Pluto’s moon Nix is viewed at three different times during the New Horizons July 2015 flyby. Credit: NASA/JHUAPL/SwRI.
Now have a look at Nix as seen through the Ralph/Multispectral Visible Imaging Camera. Here we’re working with only a quarter of LORRI’s resolution, but we’ve got color now and can discern that most of Nix is white, while that provocative crater and the ejecta it produced show up as reddish. It’s a natural assumption that Nix’s interior is made up of much darker material than the surface. “We don’t actually know what either the dark or the light material is,” writes Porter, “nor will we be able to tell until we download the Nix data from the Ralph-Linear Etalon Imaging Spectral Array (LEISA) composition mapping spectrometer.”
Image: Pluto’s moon Nix is shown in high-resolution black-and-white and lower resolution color. Credit: NASA/JHUAPL/SwRI.
Below is Hydra as seen through LORRI, with the caveat that this moon was on the other side of Pluto during close approach, so we don’t have the same level of resolution we had for Nix. Porter notes a certain similarity in aspect with another object that caught our attention this summer: Comet 67P/Churyumov-Gerasimenko, around which the ESA’s Rosetta spacecraft continues its operations. In both cases, we have the possibility of a low-speed collision which melded two originally separate objects. The images of Styx and Kerberos that we’ll get later in the year, by the way, should be of roughly the same resolution as this image of Hydra.
Image: Pluto’s moon Hydra as seen from NASA’s New Horizons spacecraft, July 14, 2015. Credit: NASA/JHUAPL/SwRI
New Horizons also detected surface water ice on Pluto, with areas showing the most apparent water ice signatures corresponding to areas that appear red in other recent images of Pluto. Figuring out how water ice interacts with the reddish tholins is going to take some work.
Image: Regions with exposed water ice are highlighted in blue in this composite image from New Horizons’ Ralph instrument, combining visible imagery from the Multispectral Visible Imaging Camera (MVIC) with infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA). The strongest signatures of water ice occur along Virgil Fossa, just west of Elliot crater on the left side of the inset image, and also in Viking Terra near the top of the frame. A major outcrop also occurs in Baré Montes towards the right of the image, along with numerous much smaller outcrops, mostly associated with impact craters and valleys between mountains. The scene is approximately 450 kilometers across. Note that all surface feature names are informal. Credit: NASA/JHUAPL/SwRI.
In addition to the sheer thrill of seeing places as tiny as Nix at some level of detail, not to mention the often startling and mesmerizing views of Pluto and Charon themselves, I note the fact that exoplanets have become so common that we can draw analogies from our catalogues to describe what we see in our own system, as Simon Porter did in his description of Pluto’s moons. The world has changed so much in the past twenty years of exoplanet hunting, meaning that our view of ourselves and our place in the universe has been much enriched, and we have a panoply of planetary configurations to draw on as we consider how solar systems are made.
SETI: A Networked Galaxy?
We often conceive of SETI scenarios in which Earth scientists pick up a beacon-like signal from another star, obviously intended to arouse our attention and provide information. But numerous other possibilities exist. Might we, for example, pick up signs of another civilization’s activities, perhaps through intercepting electromagnetic traffic, or their equivalent of planetary radars? Even more interesting, as Brian McConnell speculates below, is the idea of listening in on a galactic network that contains information not just from one civilization but many. As Centauri Dreams readers know, McConnell and Alex Tolley have been developing the ‘spacecoach’ concept of interplanetary travel, discussed in the just published A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2015). It’s a shrewd and workable way to get us deep into the Solar System. Today McConnell turns his attention to a SETI network whose detection could offer a big payoff for a young civilization.
by Brian McConnell
With the revival of SETI funding, it’s interesting to contemplate what we might find if SETI succeeds. One possibility that is especially tantalizing is that first contact would not be with an individual civilization but rather a large scale network of civilizations that is organized not unlike the Internet. This is not a new idea (Timothy Ferris and others have explored this concept) but it is one that should be considered seriously. Assuming that communicative civilizations are commonplace, a big if of course, a decentralized or mesh network will be the most time and energy efficient way for them to organize their communications.
Consider the energy cost of sending a unit of information from one edge of the galaxy to the other (~ 100,000 light years) via direct means versus a peer-to-peer relay system. The savings ratio can be estimated as:
daverage / wgalaxy
If communicative civilizations are separated by an average distance of, say, 1000 light years, the energy cost of sending a unit of information across the galaxy via relay will be about 1/100th that of direct communication. The energy requirement per link drops off by the ratio of (daverage / wgalaxy)2 but as more hops are required with shorter links, the overall energy requirement drops by daverage / wgalaxy. This is an approximation, but it highlights the order of magnitude improvements in economy, and suggests that if communicative civilizations are widespread, energy economics and other considerations will favor this type of arrangement.
Reliability and redundancy are another important feature of a mesh network. When sending information across such great distances, and with such long transit times, a sender may want to protect especially important information against loss or corruption by sending it repeatedly or by sending it via multiple paths between endpoints. This technique can virtually guarantee that information is eventually transmitted even if the network is damaged, even without the use of sophisticated forward error correction codes. This isn’t to say that an extraterrestrial intelligence will copy the TCP/IP protocol, but it’s safe to assume that someone who is sophisticated enough to build an interstellar communication link will probably be familiar with the characteristics and benefits of decentralized mesh networks.
There will also be benefits to receivers, especially newcomers, as contact with one node will be the same as contacting many nodes, since any node in the network can function as a relay for others. The cost of joining the network is also reduced, as a new node need only establish communication with its nearest neighbors, and can relay messages to and receive information from any other site on the network. Such a network would not merely be a communication system, but also a long term repository of knowledge as important information from long dead civilizations could continue to circulate throughout the network in perpetuity.
Image: The Milky Way as seen from the mountains of West Virginia. Could the galaxy be filled with the traffic of networked civilizations? Credit: ForestWander.
Fermi Implications
The existence of such a system might also help explain the Fermi Paradox, as the most energy efficient mode of operation, in terms of detecting new civilizations, will be for each node to concentrate its detection efforts on its immediate neighborhood using a listen and reply strategy. There would be little point in building powerful omnidirectional beacons that are detectable over great distances, as they would cost far more energy to operate, would have to wait millennia for a response, and would be plagued by duty cycle issues. Better for peripheral nodes to listen for microwave leakage from nearby civilizations as they develop early communication technology, and then target those for active communication soon after they are detected. This sort of strategy would be cheap both in terms of energy and the number of radio-telescopes required at each node in the network, and would offer a high probability of success in detecting new nodes just as they become active, while not wasting energy by transmitting in the blind.
An important point to consider here is that an emerging technological civilization would become detectable independently of any intent to attempt interstellar communication. Indeed here on Earth, the vast vast majority of energy expended on electromagnetic signaling has been for purposes other than Active SETI. It seems likely that most technological civilizations would go through a period where they are microwave bright, even if they later go dark due to transitioning to other technology, fear of ETI contact, etc.
As we would just now be detectable to nodes within about 100LY (80LY is probably a better estimate), we would just now expect to be receiving a response from a node within 40-50LY. It’s possible that rapid changes in atmospheric spectra, as Earth has experienced with the sudden increase in carbon dioxide, might also serve as a early tripwire for attempting active communication, but those could be ambiguous signals with natural explanations like volcanism, whereas a sudden spike in monochromatic microwave transmission points definitively to a technological origin. Viewed from the network’s perspective, this decentralized strategy would enable detection of new sites with the least energy expenditure and the shortest possible lag time between detection and active communication, with the added bonus feature that the first nodes to establish contact could relay stored information from nodes far beyond the initial radius of communication. On the other hand, if the average distance between nodes is large, it may be a long time before the nearest nodes are aware of us, and it may be hard for such a network to become established in the first place.
Choice of Encoding Schemes
Another interesting aspect of a long running galactic communication system is that there will be a natural selection of sorts that favors the message encoding schemes that are most likely to be mimicked. The selection pressure in this case will favor an encoding scheme that is broadly comprehensible (easy to understand the basic design pattern) and flexible (able to accommodate many different types of information via that framework). A transmission that is extremely difficult to parse, for example because of strong encryption or sophisticated forward error correction codes, is less likely to be mimicked than one whose basic design pattern is comprehensible to many receivers, even if it is less than optimal in terms of capacity or error resistance. This leads to the fittest message being more likely to replicate (be mimicked in retransmission) than its competitors. This is also an incentive for civilizations wishing to project influence through remote communication to design messages that peer sites will want to and be able to copy.
The point is not to speculate about what would be in such a message, but how it is organized at a low level. To build a mesh network that can handle many data types, you don’t need a very sophisticated message format, even if some of the data types sent within the message are extremely complex or difficult to comprehend. Typically you break a large amount of data, be it a file or communication stream, into smaller predictably organized subunits which are labeled with metadata, which might include:
- a frame or packet number : identifies a message segment’s position within a collection, file, stream, etc
- a collection or file number : to identify a larger grouping of frames, pages, packets, etc
- an author or sender number : to identify the author or sender of a particular segment.
- a receiver number : to identify the intended recipient, if any
- a content identifier : to identify the type of content represented by the frame or packet
- a blob of data, or payload, that is described by the above metadata
While one could design more complex schemes, the above defines the minimal set of metadata needed to describe something like a mesh network or file system with many files, authors and varying file types. What someone decides to convey with such a system is a different matter entirely, but a mesh network in its simplest form consists of a long chain of | meta data | blob of data | meta data | blob of data | segments with obvious repeating structures.
Implications for SETI
While the basic design pattern of a system like this can be rather simple, it will be capable of delivering data that varies widely in content type and “difficulty level”, and also offers a high degree of durability (important message fragments can be resent out of sequence or sent via multiple paths). Some content types such as rasterized or bitmapped images will probably be nearly universally understood due to their utility in astronomy and space photography, while others that are based on advanced math may be unrecognizable to many recipients. It’s not unlike DNA, whose basic encoding scheme has just four letters, yet can encode for something as simple as an isolated protein or as complex as a human being. That’s one of the interesting characteristics of the fittest message — it should be easy to parse at a low level, yet capable of conveying data types representing a wide range of complexity.
All of this suggests that SETI surveys should be concentrating a portion of their observing time on nearby targets. This also suggests that a large scale network will probably need to find us before we can find it, but will also be relatively easy to spot once it does. This doesn’t exclude other possibilities, and indeed SETI should be trying many strategies in parallel, from looking for distant beacons to Bracewell probes.
Should we encounter a network like this, the implications of that would be nothing short of staggering because of the volume and variety of information that could flow through a system like this. It’s possible that much of that communication will be over our heads. On the other hand, the quasi Darwinian selection pressure on message formats may favor those that are broadly comprehensible, or at least contain elements like rasterized photos that virtually any astronomically communicative receiver can understand, including us.
A Comparative Look at Solar Systems
With almost 2000 exoplanets now confirmed, not to mention candidates in the thousands, it’s amazing to recall that it was just twenty years ago that the first planet orbiting a main sequence star beyond the Solar System was found. Continued work on the world revealed that 51 Pegasi b is about half as massive as Jupiter, though 50 percent larger. Orbiting its star in roughly four days, the planet is some fifty light years from Earth. Thus we began to learn not just that exoplanets were out there, but that their environments could be truly extreme — remember that it was just in 1992 that planets were found around the pulsar PSR 1257+12.
Without any evidence other than my imagination, I grew up believing that most stars should have planets, and just assumed that their stellar systems were more or less like our own. There should be a few planets too close to their star for life to exist, and several gas giants out at the outskirts of the system, and somewhere in between there should be a world not so different from Earth. It was a naive view, but not completely implausible, and anyway, we lacked data.
Discovering ‘hot Jupiters’ is one way we began to realize that other configurations could exist, and the number of ‘super-Earths’ has made the same case. Just how ‘normal’ is our Solar System in the first place? A new paper from Rebecca Martin (University of Nevada, Las Vegas) and Mario Livio (Space Telescope Science Institute) tackles the question, comparing what we see in our Solar System to our growing database of exoplanetary information.
Obviously, this is a work in progress, for we’re not only still examining abundant Kepler and K2 data but continuing a robust planet hunt that looks forward to space-based missions like TESS (Transiting Exoplanet Survey Satellite) and PLATO (PLAnetary Transits and Oscillations of stars), surveys of considerable scope that should build our catalog of ‘nearby’ planets. Nonetheless, we can draw some conclusions based on what we see in the image below.
Image (click to enlarge): Model of all the multi-planet systems found by Kepler as of November 2013; our terrestrial planets are shown in grey at the top left for comparison. A new study examines how our solar system compares to the exoplanetary systems we’ve found. Credit: NASA/Kepler/Dan Fabricky.
Our Solar System is composed of a good deal more than planets, of course, which leads to an important caveat, one that Martin and Livio mention early on in the paper. We have two belts orbiting the Sun, the main asteroid belt and the Kuiper Belt. The problem is that although we can see a number of debris and dust belts around other stars, belts with as little mass as ours would not be observable to us around other stars. So a study like this one has to base its findings solely on planets. It can be mentioned, though, that hundreds of debris disk candidates are now in play, and about two-thirds of these are best modeled as two component disks.
That’s a plus for the idea that our Solar System isn’t all that atypical. What about the planets? The authors use a mathematical transformation that allows them to set up a statistical comparison. The low mean eccentricity of planets around our Sun is one area where we differ from other multi-planet systems — our planets move in largely circular orbits — but as the paper notes, our observation methods are biased toward finding high eccentricity planets. Circular orbits work to our benefit, for planets with low eccentricity are more likely to be dynamically stable. Indeed, the terrestrial planets in our system are thought to be stable until that distant time when the Sun becomes a red giant and disrupts the entire inner system.
What about age? The Sun is about half the age of the Milky Way disk, hardly setting up our system as special, and at least one study has found that about 80 percent of existing Earth-like planets were already formed when the Earth came into existence. We also know that terrestrial planets in the habitable zone of their host star appear to be common. The paper notes, for example, the work of Courtney Dressing and David Charbonneau, which uses Kepler data for M-dwarfs and finds an occurrence rate for Earth-sized planets in the habitable zone of 18% to 27%, a conservative estimate that Martin and Livio say could be as high as 50 percent (see How Common Are Potential Habitable Worlds in Our Galaxy?)
If we’re not unusual in terms of age or habitability, we do differ considerably from other systems in two respects. First, we have no planets inside the orbit of Mercury, in contrast to systems with rocky worlds on far closer orbits. Moreover, the Solar System lacks a super-Earth, a category of planet now turning out to be common.
The paper summarizes its findings this way:
We find that the properties of the planets in our solar system are not so significantly special compared to those in exosolar systems to make the solar system extremely rare. The masses and densities are typical, although the lack of a super-Earth-sized planet appears to be somewhat unusual. The orbital locations of our planets seem to be somewhat special but this is most likely due to selection effects and the difficulty in finding planets with a small mass or large orbital period. The mean semi-major axis of observed exoplanets is smaller than the distance of Mercury to the Sun. The relative depletion in mass of the solar system’s terrestrial region may be important. The eccentricities are relatively low compared to observed exoplanets, although the observations are biased toward finding high eccentricity planets. The low eccentricity, however, may be expected for multi-planet systems. Thus, the two characteristics of the solar system that we find to be most special are the lack of super-Earths with orbital periods of days to months and the general lack of planets inside of the orbital radius of Mercury.
So while we’ve had quite a few surprises in the past twenty-five years, going from no exoplanets known to planets around pulsars and then main sequence stars, and moving from those early detections to thousands of candidates, we’re not seeing anything that would peg us as being unique. In terms of habitability, the authors see nothing in the Solar System that would make it especially conducive to life’s formation as opposed to other planetary systems:
If exosolar life happens to be rare it would probably not be because of simple basic physical parameters, but because of more subtle processes that are related to the emergence and evolution of life. Since at the moment we do not know what those might be, we can allow ourselves to be optimistic about the prospects of detecting exosolar life.
That lack of a super-Earth troubles me, though. Systems that have a super-Earth generally have more than one. The authors ask a good question: Does the presence of a super-Earth affect terrestrial planet formation? Several studies have looked at a migrating super-Earth moving slowly through the habitable zone, finding that a terrestrial planet that forms there later will tend to be rich in volatiles. Many observed super-Earths are found in orbits where they were unlikely to have formed, so scenarios of super-Earth migration surely deserve further study.
The paper is Martin and Livio, “The Solar System as an Exoplanetary System,” The Astrophysical Journal Vol. 810, No. 2 (3 September 2015). Full text.
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