by Paul Gilster | Oct 15, 2015 | Astrobiology and SETI |
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).
by Paul Gilster | Oct 14, 2015 | Astrobiology and SETI |
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).
by Paul Gilster | Jun 28, 2024 | Astrobiology and SETI |
Searching for biosignatures in the atmospheres of nearby exoplanets invariably opens up the prospect of folding in a search for technosignatures. Biosignatures seem much more likely given the prospect of detecting even the simplest forms of life elsewhere – no technological civilization needed – but ‘piggybacking’ a technosignature search makes sense. We already use this commensal method to do radio astronomy, where a primary task such as observation of a natural radio source produces a range of data that can be investigated for secondary purposes not related to the original search.
So technosignature investigations can be inexpensive, which also means we can stretch our imaginations in figuring out what kind of signatures a prospective civilization might produce. The odds may be long but we do have one thing going for us. Whereas a potential biosignature will have to be screened against all the abiotic ways it could be produced (and this is going to be a long process), I suspect a technosignature is going to offer fewer options for false positives. I’m thinking of the uproar over Boyajian’s Star (KIC 8462852), where the false positive angles took a limited number of forms.
If we’re doing technosignature screening on the cheap, we can also worry less about what seems at first glance to be the elephant in the room, which is the fact that we have no idea how long a technological society might live. The things that mark us as tool-using technology creators to distant observers have not been apparent for long when weighed against the duration of life itself on our planet. Or maybe I’m being pessimistic. Technosignature hunter Jason Wright at Penn State makes the case that we simply don’t know enough to make statements about technology lifespans.
On this point I want to quote Edward Schwieterman (UC-Riverside) and colleagues from a new paper, acknowledging Wright’s view that this argument fails because the premise is untested. We don’t actually know whether non-technological biosignatures are the predominant way life presents itself. Consider:
In contrast to the constraints of simple life, technological life is not necessarily limited to one planetary or stellar system, and moreover, certain technologies could persist over astronomically significant periods of time. We know neither the upper limit nor the average timescale for the longevity of technological societies (not to mention abandoned or automated technology), given our limited perspective of human history. An observational test is therefore necessary before we outright dismiss the possibility that technospheres are sufficiently common to be detectable in the nearby Universe.
So let’s keep looking, which is what Schwieterman and team are advocating in a paper focusing on terraforming. In previous articles on this site we’ve looked at the prospect of detecting pollutants like chlorofluorocarbons (CFCs), which emerge as byproducts of industrial activity, but like nitrogen dioxide (NO₂) these industrial products seem a transitory target, given that even in our time the processes that produce them are under scrutiny for their harmful effect on the environment. What the new paper proposes is that gases that might be produced in efforts to terraform a planet would be longer lived as an expanding civilization produced new homes for its culture.
Enter the LIFE mission concept (Large Interferometer for Exoplanets), a proposed European Space Agency observatory designed to study the composition of nearby terrestrial exoplanet atmospheres. LIFE is a nulling interferometer working at mid-infrared wavelengths, one that complements NASA’s Habitable Worlds Observatory, according to its creators, by following “a complementary and more versatile approach that probes the intrinsic thermal emission of exoplanets.”
Image: The Large Interferometer for Exoplanets (LIFE), funded by the Swiss National Centre of Competence in Research, is a mission concept that relies on a formation of flying “collector telescopes” with a “combiner spacecraft” at their center to realize a mid-infrared interferometric nulling procedure. This means that the light signal originating from the host star of an observed terrestrial exoplanet is canceled by destructive interference. Credit: ETH Zurich.
In search of biosignatures, LIFE will collect data that can be screened for artificial greenhouse gases, offering high resolutions for studies in the habitable zones of K- and M-class stars in the mid-infrared. The Schwieterman paper analyzes scenarios in which this instrument could detect fluorinated versions of methane, ethane, and propane, in which one or more hydrogen atoms have been replaced by fluorine atoms, along with other gases. The list includes Tetrafluoromethane (CF₄), Hexafluoroethane (C₂F₆), Octafluoropropane (C₃F₈), Sulfur hexafluoride (SF₆) and Nitrogen trifluoride (NF₃). These gases would not be the incidental byproducts of other industrial activity but would represent an intentional terraforming effort, a thought that has consequences.
After all, any attempt to transform a planet the way some people talk about terraforming Mars would of necessity be dealing with long-lasting effects, and terraforming gases like these and others would be likely to persist not just for centuries but for the duration of the creator civilization’s lifespan. Adjusting a planetary atmosphere should present a large and discernable spectral signature precisely in the infrared wavelengths LIFE will specialize in, and it’s noteworthy that gases like those studied here have long lifetimes in an atmosphere and could be replenished.
LIFE will work via direct imaging, but the study also takes in detection through transits by calculating the observing time needed with the James Webb Space Telescope’s instruments as applied to TRAPPIST-1 f. The results make the detection of such gases with our current technologies a clear possibility. As Schwieterman notes, “With an atmosphere like Earth’s, only one out of every million molecules could be one of these gases, and it would be potentially detectable. That gas concentration would also be sufficient to modify the climate.”
Indeed, working with transit detections for TRAPPIST-1 f produces positive results with JWST’s MIRI Low Resolution Spectrometer (LRS) and NIRSpec instrumentation (with “surprisingly few transits”). But while transits are feasible, they’re also more scarce, whereas LIFE’s direct imaging in the infrared takes in numerous nearby stars.
From the paper:
We also calculated the MIR [mid infrared] emitted light spectra for an Earth-twin planet with 1, 10, and 100 ppm of CF₄, C₂F₆, C₃F₈, SF₆, and NF₃… and the corresponding detectability of C₂F₆, C₃F₈, and SF₆ with the LIFE concept mission… We find that in every case, the band-integrated S/Ns were >5σ for outer habitable zone Earths orbiting G2V, K6V, or TRAPPIST-1-like (M8V) stars at 5 and 10 pc and with integration times of 10 and 50 days. Importantly, the threshold for detecting these technosignature molecules with LIFE is more favorable than standard biosignatures such as O₃ and CH₄ at modern Earth concentrations, which can be accurately retrieved… indicating meaningfully terraformed atmospheres could be identified through standard biosignatures searches with no additional overhead.
Image: Qualitative mid-infrared transmission and emission spectra of a hypothetical Earth-like planet whose climate has been modified with artificial greenhouse gases. Credit: Sohail Wasif/UCR.
The choice of TRAPPIST-1 is sensible, given that the system offers seven rocky planet targets aligned in such a way that transit studies are possible. Indeed, this is one of the most highly studied exoplanetary systems available. But the addition of the LIFE mission’s instrumentation shows that direct imaging in the infrared expands the realm of study well beyond transiting worlds. So whereas CFCs are short lived and might flag transient industrial activity, the fluorinated gases discussed in this paper are chemically inert and represent potentially long-lived signatures for a terraforming civilization.
The paper is Schwieterman et al., “Artificial Greenhouse Gases as Exoplanet Technosignatures,” Astrophysical Journal Vol. 969, No. 1 (25 June 2024), 20 (full text).
by Paul Gilster | Oct 13, 2023 | Astrobiology and SETI |
The problem of perspective haunts SETI, and in particular that branch of SETI that has been labeled Dysonian. This discipline, based on Freeman Dyson’s original notion of spheres of power-gathering technology enclosing a star, has given rise to the ongoing search for artifacts in our astronomical data. The fuss over KIC 8462852 (Boyajian’s Star) a few years back involved the possibility that it was orbited by a megastructure of some kind, and thus a demonstration of advanced technology. Jason Wright and team at Penn State have led searches, covered in these pages, for evidence of Dyson spheres in other galaxies. The Dysonian search continues to widen.
I cite a problem of perspective in that we have no real notion of what we might find if we finally locate signs of extraterrestrial builders in our data. It’s so comfortable to be a carbon-based biped, but the entities we’re trying to locate may have other ways of evolving. Clément Vidal, a French philosopher and one of the most creative thinkers that SETI has yet produced, likes to talk not about carbon or silicon but rather ‘substrates.’ Where, in other words, might intelligence eventually land, and is that likely to be a matter of chemistry and biology or simple energy?
Vidal is currently at the UC Berkeley SETI Research Center, though he has deeper roots at the Free University of Brussels, from which he created his remarkable The Beginning and the End (Springer, 2014), along with a string of other publications. Try to come up with a definition of life and you may well emerge with something like this: Matter and energy in cyclical relationship using energy drawn from the environment to increase order in the system. I think that was Vidal’s starting point; it’s drawn from Gerald Feinberg and Robert Shapiro in Life Beyond Earth, Morrow 1980). No DNA there. No water. No carbon. Instead, we’re addressing the basic mechanism at work. In how many ways can it occur?
As Vidal reminded the audience at the recent Interstellar Research Group symposium in Montreal (video here), we are even now, at our paltry 0.72 rank in the Kardashev scale, creating increasingly interesting software that at least mimics intelligence to a rather high order. Making further advances that may exceed human intelligence is conceivably a matter of mere decades. If we consider intelligence embedded in a substrate of some kind, it makes sense that our planet may house fewer biological beings in the distant future than creatures we can call ‘artilects.’
The silicon-based outcome has been explored by thinkers like Martin Rees and Paul Davies in the recent literature. But the ramifications go much further than this. If we consider life as critically embedded in energy flows, the notion of life upon a neutron star swims into the realm of possibility. Frank Drake is one scientist who wrote about such things, as did Robert Forward in his novel Dragon’s Egg (Ballantine, 1980). If the underlying biology is of less importance and matter/energy interactions take precedence, we can further consider concepts like intelligence appearing wherever these interactions are at their most intense. Vidal has explored close binary systems as places where a civilization might mine energy, and for all we know, extract it to support a cognitive existence far removed from our notion of a habitable zone.
What about stars themselves? Greg Matloff has pointed to the low temperatures of red dwarf stars as allowing molecular interactions in which a primitive form of intelligence might emerge. Olaf Stapledon dreamed up civilizations using stellar energies in novel ways in Star Maker (Methuen, 1937) and mused on the emergence of stellar awareness. At Montreal, Vidal presented recent work on how an advanced civilization – in whatever substrate – might deploy a star orbited closely by a neutron star or black hole as a system of propulsion, with the ‘evaporation’ from the host star flowing to the compact companion and being directed by timing the pulsations to coincide with the orbital position of each. Far beyond our technologies, but then we’re at 0.72, as opposed to Kardashev civilizations at the far end of Kardashev II.
We have no idea how likely it is that such entities could emerge, but consider this. Charles Lineweaver’s work at Australian National University shows that the average Earth-like planet in the galaxy is on the order of 1.8 billion years older than Earth. The Serbian astronomer and writer Milan Ćirković has made the further point that this 1.8 billion year head-start is only an average. There must be planets considerably more than 1.8 billion years older than ours, and that makes for quite a few millennia for intelligence to develop and technologies to flourish.
Our first encounter with another civilization, then, is almost certainly going to be with one far older than our own. What, then, might we find one day in our astronomical data? I’ve quoted Vidal on this in the past and want to cite the same passage from The Beginning and the End today:
We need not be overcautious in our astrobiological speculations. Quite the contrary, we must push them to their extreme limits if we want to glimpse what such advanced civilizations could look like. Naturally, such an ambitious search should be balanced with considered conclusions. Furthermore, given our total ignorance of such civilizations, it remains wise to encourage and maintain a wide variety of search strategies. A commitment to observation, to the scientific method, and to the most general scientific theories remains our best touchstone.
The specific speculation Vidal tantalized the crowd with at Montreal is one he calls the ‘spider stellar engine,’ about which a quick word. Two types of ‘spider engines’ get his attention, the ‘redback’ and the ‘black widow.’ I assume Vidal is not necessarily an arachnophile, but rather a man aware of the current astrophysical jargon about extreme objects and pulsar binaries in particular. A redback refers to a rapidly rotating neutron star in tight orbit around a star massing up to 0.6 solar masses. A black widow has a much smaller companion star, and the term spider simply refers to the fact that the pulsar’s gravity draws material away from the larger star.
The larger star in such systems can be, spider-like, completely consumed, a useful marker as we study effects such as accretion disks and mass transfer between the two objects. Here is the energy gradient we are looking for in the question of a basic life definition, one that can be exploited by any beings that want to take advantage of it. A long-lived Kardashev II civilization, having feasted on the host star for its energies, could use what is left of the dwindling star at the end of its life to move to another host. The question for astronomers as well as philosophers is whether such a system would throw an observational signal that is detectable, and the question at this point remains unanswered.
Image; An illustrated view of a black widow pulsar and its stellar companion. The pulsar’s gamma-ray emissions (magenta) strongly heat the facing side of the star (orange). The pulsar is gradually evaporating its partner. ZTF J1406+1222, has the shortest orbital period yet identified, with the pulsar and companion star circling each other every 62 minutes. Credit: NASA Goddard Space Flight Center/Cruz deWilde.
We do have some interesting systems to watch, however. Vidal cites the pulsar PSR B1957+20 as having pulsations between host star and pulsar that match the orbital period, but notes that of course there are other ways of explaining this effect. We may want to include this particular signature as an item to look for in our pulsar work related to SETI, however. Meanwhile, the question of stellar propulsion (I think also of the ‘Shkadov thruster,’ another type of hypothesized stellar engine), explored by Vidal in his Montreal talk, yields precedence to the broader question with which we began. Are our perspectives sufficient to look for the kind of astronomical signatures that might be pointing toward forms of life almost unimaginably beyond our own?
by Paul Gilster | Jun 24, 2020 | Astrobiology and SETI |
Harmonizing with yesterday’s post about a NASA grant to study technosignatures is word from Breakthrough Listen, which has released a catalog of what it calls ‘exotica’ or, to cite the accompanying paper: “an 865 entry collection of 737 distinct targets intended to include “one of everything” in astronomy.” The idea is to produce a general reference work that can guide astronomical surveys and, in the case of Breakthrough, widen the search for technosignatures.
Brian Lacki (UC-Berkeley), who is lead author of the new catalog, notes that it’s not meant to be restricted to SETI, though its uses there may prove interesting. Here are the four categories of exotica the catalog defines:
- ‘Prototypes.’ Here the intent is to list one example, perhaps more, an archetype of every known type of non-transient object in the sky. According to the paper, “We emphasize the inclusion of many types of energetic and extreme objects like neutron stars…, but many quiescent examples are included too.”
- ‘Superlatives.’ These are objects with the most extreme properties (“among the most extreme in at least one major physical property, the record-breakers”), including unusually metallic stars, or the fastest known pulsar, the stars with the highest metal content and those with the lowest, etc. Here the list “includes objects of known subtypes but that are on the tail ends of the distribution of some properties, to better span the range of objects in the Universe.”
- ‘Anomalies.’ The enigmas go here, including objects like KIC 8462852 (Boyajian’s Star), whose odd lightcurve is still under examination and a long way from being explained, and ‘Oumuamua, the interstellar visitor that entered our system in 2017 and is now leaving it. We can also include phenomena that have triggered searches at both radio and optical frequencies — here I think of Fast Radio Bursts (FRBs), but stars with excess infrared radiation would also be on the list.
- A fourth category is a control sample of “sources not expected to produce positive results.”
So what to make of this? It’s apparently introduced as an attempt to jog our preconceptions, at least according to what Lacki says:
“Many discoveries in astronomy were not planned. Sometimes a major new discovery was missed when nobody was looking in the right place, because they believed nothing could be found there. This happened with exoplanets, which might have been detected before the 1990s if astronomers looked for solar systems very different than ours. Are we looking in the wrong places for technosignatures? The Exotica catalog will help us answer that question.”
Lacki’s point is well taken with regard to exoplanets. We quickly learned at the beginning of our exoplanet detections that stellar systems come in a huge variety of configurations, so that our own Solar System can hardly be considered a common template. Every new system studied now seems to drive this point home. As the paper notes, everything from the cosmic microwave background to gamma-ray bursts has been found by scientists who were not explicitly looking for what they discovered, usually because new instruments and telescopes widen our capabilities.
From the paper:
Other discoveries – like the moons of Mars or Cepheid variables in external galaxies – were delayed because no thorough observations were carried out on the targets (Hall 1878; Dick 2013). The pattern persists to this day. Because ultracompact dwarf galaxies have characteristics that fall in the cracks between other galaxies and globular clusters, they were only recognized recently despite being easily visible on images for decades (Sandoval et al. 2015). Of relevance to SETI, hot Jupiters were speculated about in the 1950s (Struve 1952), but they were not discovered until 1995 in part because no one systematically looked for them (for further context, see Mayor & Queloz 2012; Walker 2012; Cenadelli & Bernagozzi 2015). This may have delayed by years the understanding that exoplanets are not extremely rare, one of the factors in the widely-used Drake Equation in SETI relating the number of ETIs to evolutionary probabilities and their lifespan (Drake 1962).
Are there SETI discoveries that could be made if we widen the range of targets? Breakthrough Listen has already upped the pace of both radio and optical SETI, being a 10-year program whose core is a search for artificial radio emission from over 1,000 nearby stars, although a million more stars in the Milky Way are targeted for related study. The organization is clearly not averse to trying new approaches, hence its interest in technosignatures of the kind once suggested by Freeman Dyson, and its intention to expand the parameter space.
Image: This is Figure 1 from the paper. Caption: A cartoon of the three directions of target selection and the relative advantages of Breakthrough Listen’s primary programs observing stars and galaxies (green), a survey of the Breakthrough Listen Exotica Catalog (blue), and some example campaigns. Previous SETI surveys have generally aimed for depth, achieving strong limits for a small number of similar targets, or high-count, achieving modest limits for a large number of similar targets. Other exotica efforts can be high-depth (red) or high-count (gold) campaigns, but observations of the Exotica Catalog will be broad, achieving modest limits on a small number each of a wide variety of targets. Future discoveries may be added to a later version of the catalog (pale blue), or prompt new campaigns that we cannot yet plan for (grey). Credit: Lacki et al.
Andrew Siemion (UC-Berkeley), who leads the Breakthrough Listen science team, notes that the few searches for technosignatures that have taken place have largely focused on stars thought to host planets in their liquid water habitable zone (although exceptions like Penn State’s Glimpsing Heat from AlienTechnologies, working at the galactic scale, are clear exceptions to this). What Siemion wants to do is expand the search. ‘Survey breadth’ — how wide a range of objects is studied in an observing program — is the operative term.
Or we might ask, are there objects we now consider natural that may in fact be artificial? And which natural objects — perhaps Boyajian’s Star, for instance, or some FRBs — mimic the kind of artificial signal that SETI researchers are looking for? Breakthrough Listen will spend 10 percent of its observing time on exotic objects. The 737 objects in the Exotica Catalog are sorted into different levels of priority for observation, with about a dozen considered high priority for SETI. Most entries are considered low priority and slated for observation as time allows.
The paper continues:
There are many reasons to search for technological intelligence in unconventional places. Unearthlike or nonbiological entities will not be constrained to live in Earthly habitats hospitable to lifeforms like us. It is also conceivable that some kinds of seemingly natural phenomena are the result of alien engineering. Yet there are good motivations for observing unusual objects even if ETIs cannot possibly live there. Extreme, energetic objects are more likely to produce unusual signals, particularly transients, that might be confused with artificial signals. Breakthrough Listen has unique instrumentation, and observation of a broad range of objects would benefit the general astronomy community. Finally, there could be unaccounted for systematic errors in our systems that give false positives. Observing exotic objects and empty regions on the sky allow us to constrain these possibilities.
You can find the Breakthrough Listen Exotica Catalog here. The paper is Lacki et al., “One of Everything: The Breakthrough Listen Exotica Catalog,” available in draft version online.
