I’ve mentioned before the irony that we may discover signs of robust extraterrestrial life sooner around a distant exoplanet than right here in our own Solar System. The scenario isn’t terribly implausible: Perhaps we come up empty on Mars, or find ourselves bogged down with ambiguous results. As our rovers dig, we still have Europa, Enceladus and other outer system possibilities, but probably face a wait of decades before we could build and fly the missions needed to identify life.
Meanwhile, the exoplanet hunt continues. While we’ve had many a setback — the Space Interferometry Mission will always stand out in this regard, not to mention the inability to follow through with Terrestrial Planet Finder, Darwin and other high-end concepts — it’s just possible that within the next few decades, a space-based observatory will detect a solid biosignature from an exoplanet’s atmosphere. Even the James Webb Space Telescope should be able to detect the transmission spectrum of an Earth-class planet transiting a dim red dwarf star. Future instruments will be able to take atmospheric characterization down to an Earth 2.0 around a Sun-like star.
Then again, maybe the outer Solar System will prove so enticing that we do decide to make it a priority. We could be seeing this happen right now. Every new piece of evidence from Cassini helps to build the case that Enceladus is an attractive proposition for the life search, the latest news being that the Saturn orbiter has identified 101 distinct geysers erupting on the moon’s surface. We first detected geysers of ice particles and water vapor at Enceladus’ south pole almost a decade ago. Now we have a map of geysers erupting from the so-called ‘tiger stripe’ fractures coincident with surface hot spots.
Image: This two-image mosaic is one of the highest resolution views acquired by Cassini during its imaging survey of the geyser basin capping the southern hemisphere of Saturn’s moon Enceladus. It clearly shows the curvilinear arrangement of geysers, erupting from the fractures. From left to right, the fractures are Alexandria, Cairo, Baghdad, and Damascus. As a result of this survey, 101 geysers were discovered: 100 have been located on one of the tiger stripes , and the three-dimensional configurations of 98 of these geysers have also been determined. The source location of the remaining geyser could not be definitively established. These results, together with those of other Cassini instruments, now strongly suggest that the geysers have their origins in the sea known to exist beneath the ice underlying the south polar terrain. Credit: NASA/JPL-Caltech/SSI.
The reason this is so exciting is that the hot spots that Cassini’s heat-sensing instruments found in the south polar region are only a few tens of meters across. That means they’re too small to be produced by the kind of frictional heating that would be caused by the repeated flexing of Enceladus due to tidal effects from Saturn. Frictional heating could have accounted for the geyser phenomena by turning surface ice into vapor and liquid, but it now appears that we’re dealing with water from the ocean below being exposed by opening and closing of the fractures.
Carolyn Porco (Space Science Institute) is leader of the Cassini imaging team, and lead author of a new paper on the Cassini findings:
“Once we had these results in hand we knew right away that heat was not causing the geysers but vice versa. It also told us the geysers are not a near-surface phenomenon but have much deeper roots.”
The source of the material forming the geysers of Enceladus is thus found to be the sea that exists under the ice shell, a sea that Cassini’s gravity data on the moon has confirmed. This news release from CICLOPS (Cassini Imaging Central Laboratory for Operations) has more, including the results of a second paper in which the authors report that the brightness of the combined geyser plume as viewed by Cassini changes periodically during the moon’s orbit of Saturn. In most respects, the brightness variations track the expected tidal venting cycle.
But not entirely. What would be expected from the opening and closing of the fractures does not predict when the plume begins to brighten, a finding that could implicate the spin rate of Enceladus. Francis Nimmo (UC-Santa Cruz) is lead author on the second paper:
“It’s an interesting puzzle. Possibilities for the mismatch include, among other effects, a delay in the response of the ice shell, which would suggest tides are heating the bulk of the ice at the south pole, or subtle changes in the spin rate of Enceladus.”
That last remark points to the possibility that the liquid water under the Enceladan ice may be global, even if deeper under the south pole region. So we have yet another reason for fascination with a moon whose salty sea, known to contain organic compounds, is spouting geysers and, possibly, reaching the surface on occasion as a liquid. We have a potentially habitable environment under the ice that periodically offers up samples to nearby spacecraft.
Enceladus is too good a target to resist, and it’s worth remembering mission concepts like Life Investigation for Enceladus (LIFE), developed by Peter Tsou. LIFE could launch in the early 2020s, reaching Saturn in 2030 with the help of gravity assists along the way, capturing material from the Enceladus geysers with an aerogel collector like the one NASA used in its Stardust comet mission. With a final gravity assist at Titan, LIFE would then bring its samples back to Earth in 2036.
I’m remembering, too, NASA astrobiologist Chris McKay’s exhortation that the venting of water and organics into space is ‘an open invitation to go there.’ The German Aerospace Center (DLR) has likewise been exploring Enceladus mission concepts, envisioning a lander that would drill through the ice. Enceladus Explorer would use an ice drill probe to melt its way into a water-bearing crevasse to look for microorganisms, on the theory that any life in the plumes would have been destroyed by sudden exposure to space. Thus the need to probe the ocean itself.
So the ideas for sampling Enceladus for life are out there and they’ll doubtless increase as Cassini continues to demonstrate how potent an astrobiological target this moon is. Which concept should we choose, and for that matter, which should we choose between Enceladus and Europa in terms of life-seeking mission destinations for spacecraft that can be flown in the near future? Both have legitimate claims on our attention, and the possibility of plumes on Europa itself (see Water Vapor Detected Above Europa) may change the equation. Will these enticing moons motivate us to reach them before a near-term space telescope finds the first biosignatures around an exoplanet?
The papers are Porco et al., “How the Geysers, Tidal Stresses, and Thermal Emission Across the South Polar Terrain of Enceladus are Related,” The Astronomical Journal Vol. 148, No. 3 (2014), 45 (abstract) and Nimmo et al., “Tidally Modulated Eruptions on Enceladus: Cassini ISS Observations and Models,” The Astronomical Journal Vol. 148, No. 3 (2014) 46 (abstract). On the LIFE mission, see Tsou et al., “LIFE: Life Investigation For Enceladus: A Sample Return Mission Concept in Search for Evidence of Life,” Astrobiology Vol. 2, No. 8 (September 12, 2012). Abstract available.
When I was a boy in ninth grade, I asked our science teacher whether the nearest star was likely to have planets. He loved the question because it gave him the chance to explain to the class that Alpha Centauri was a binary star (we left poor Proxima out of the discussion), and that as a binary, it couldn’t possibly have planets because their orbits would be too disrupted by gravitational effects to survive. That sounded reasonable to me, and I began putting my hopes on places like Tau Ceti and Epsilon Eridani, single stars with no disruptive companion.
Since then we’ve begun finding binary stars with planets and are learning about the diversity of exoplanetary systems, putting Alpha Centauri back into the game. A good thing, too, given the fact that binary stars are common, and keeping them in the planet hunt allows that many more chances to find an Earth 2.0, not to mention all the other interesting kinds of planets including ‘super-Earths’ that we’re locating. But the fact that binary systems can have planets doesn’t mean we can ignore the powerful effects two stars in the same system can have on the objects orbiting them.
Take the case of the interesting HK Tauri system, located some 450 light years from Earth in the constellation Taurus. Here we’re dealing with a young system, the two stars being between one and four million years old, the age range in which planet formation is believed to occur. Their separation is about 58 billion kilometers, which works out to 386 AU. Given the youth of the system, it’s not surprising to find protoplanetary disks here, one of them (HK Tauri B) edge-on and observable in visible or near-infrared wavelengths. The orientation of the disk helps to block the light of the central star, making observations at these wavelengths possible.
The other disk, around HK Tauri A, is best observed in millimeter-wavelength light because we do not see it edge-on and visible light observations are overpowered by the star’s light. Now we have new results from the Atacama Large Millimeter/submillimeter Array (ALMA), which draw on observations of the planet-forming disks in this system. A team led by Eric Jensen (Swarthmore College), able to measure the rotation of the HK Tauri A disk for the first time, has discovered that the two disks are mutually misaligned by at least 60 degrees.
Image: This artist’s impression shows a striking pair of wildly misaligned planet-forming gas discs around both the young stars in the binary system HK Tauri. ALMA observations of this system have provided the clearest picture ever of protoplanetary discs in a double star. The new result demonstrates one possible way to explain why so many exoplanets — unlike the planets in the Solar System — came to have strange, eccentric or inclined orbits. Credit: R. Hurt (NASA/JPL-Caltech/IPAC).
What we’re seeing is that when stellar orbits and protoplanetary disks are not in the same plane, the planets under formation are likely to end up in eccentric, tilted orbits, with the gravitational effects of one star perturbing the disk of the other. Is the disk arrangement we find here a unique case or a common process around binary stars? A good deal of work lies ahead before we can answer that question, and not all oddball exoplanet orbits can be explained by this mechanism. But in at least this case, disk misalignment is a powerful indicator. Says Jensen:
“Our results show that the necessary conditions exist to modify planetary orbits and that these conditions are present at the time of planet formation, apparently due to the formation process of a binary star system. We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”
Image: This picture shows the key velocity data taken with ALMA that helped the astronomers determine that the discs in HK Tauri were misaligned. The red areas represent material moving away from Earth and the blue indicates material moving toward us. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).
The paper on this work notes that the team’s findings are consistent with recent simulations of binary formation that predict such misalignments, especially in systems with separation greater than 100 AU, as we find here. Moreover, we may not always be aware of the companion responsible for a perturbed disk. From the paper:
While it remains to be seen how the protoplanetary disks in a statistical sample of young binary systems are oriented, it is suggestive that in the handful of systems where this measurement has been made, the misalignments are large. If this is a common outcome of the binary formation process, and especially if it extends to lower-mass binary companions (which may easily go undetected) as well, then perturbations by distant companions may account for many of the orbital properties that make the current sample of extrasolar planets so unlike our own solar system.
The paper is Jensen and Akeson, “Misaligned Protoplanetary Disks in a Young Binary System,” published online in Nature 31 July 2014 (abstract).
Be aware of Open Source, a radio show on Boston’s WBUR that last week did a show about exoplanets and the possibility of extraterrestrial life. Earth 2.0 is available online, featuring David Latham (Harvard-Smithsonian Center for Astrophysics), Dimitar Sasselov (Harvard University), Jason Wright (Penn State) and Sarah Rugheimer (a PhD student at Harvard studying exoplanet atmospheres). The discussion ranges through the Kepler mission to the Fermi question and recent studies of exoplanet atmospheres, the latter particularly appropriate to today’s post.
For I want to talk today about ‘Hot Jupiters’ and their atmospheres, and what we can learn about planet formation by studying their composition. Hot Jupiters were a surprise when first discovered, but models of planetary migration seemed to explain them. We would expect a gas giant to form at or beyond the ‘snow line,’ where volatiles like water would form ice grains. As we saw in our discussion of Kepler-421b (see Transiting World at the Snow Line), planetary embryos that become gas giants should coalesce in this low temperature regime, with the resulting worlds richer in ice and water than the drier inner Solar System, which relies on volatile delivery by impacting comets or other objects with a formation history in the outer system.
Planetary migration is a way of getting those ‘hot Jupiters’ where they have been observed to be. We assume gravitational interactions with other young worlds that drive some gas giants into the inner system, taking a planet that has formed in the cold regions beyond the snow line into close proximity to the parent star. It would be reasonable to assume high water content in these worlds, but new work led by Nikku Madhusudhan (University of Cambridge, UK) comes up with a surprisingly different result.
Madhusudhan and team used near-infrared spectra of hot Jupiters observed by the Hubble Space Telescope, whose position in space allows accurate measurement of water in an exoplanetary atmosphere because it is far above contaminating water in the Earth’s own atmosphere. The method is transmission spectroscopy, in which some of the star’s light passes through the atmosphere of a planet in transit across its face as seen from Earth. The spectrum that results tells us much about the molecules in the atmosphere, but the researchers are finding only a small fraction of the water predicted by standard planet formation models.
Madhusudhan calls the result ‘astonishing,’ and adds:
“It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially ‘hot Jupiters’, and investigate how they’re formed.”
Image: This graph compares observations with modeled infrared spectra of three hot-Jupiter-class exoplanets that were spectroscopically observed with the Hubble Space Telescope. The red curve in each case is the best-fit model spectrum for the detection of water vapor absorption in the planetary atmosphere. The blue circles and error bars show the processed and analyzed data from Hubble’s spectroscopic observations. Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI).
The planets in question are HD 189733b, HD 209458b, and WASP-12b, with temperatures ranging from 800 to 2200 degrees Celsius. The water measurement of HD 209458b is the highest-precision measurement of any chemical compound in an exoplanet, and while it does find water, the low abundance creates problems for core accretion scenarios of planet formation beyond the snow line. Is our Solar System unusual in its high water content?
One thing to remember is that exoplanets are, in certain respects, easier for us to measure than some of the worlds in our own system. We know little about the constituents of the planetesimals that formed our own gas giants. The paper explains this seeming paradox while also pointing to an upcoming space mission that can help (internal references omitted for brevity):
Atmospheric elemental abundances of solar-system giant planets have led to important constraints on the origin of the solar system. The observed super-solar enrichments of C, S, N, and inert gases, support the formation of Jupiter by core accretion. However, the oxygen abundance of Jupiter is yet unknown. The upper atmosphere of Jupiter (P < 1 bar) has T < 200 K, causing water to condense and to be confined to the deepest layers (> 10 bar), requiring dedicated probes to measure it. The upcoming Juno mission to Jupiter aims to measure its O abundance, which is important to estimate the amount of water ice that was available in the planetesimals forming Jupiter and the rest of the solar system.
So Juno should be able to give us a better read on Jupiter’s oxygen, thus helping us better understand the kind of planetesimals that formed in our system’s earliest days. As to the measurements of exoplanets vs. planets closer to home:
The O/H and C/O ratios are easier to measure for hot giant exoplanets than they are for solar-system giant planets. The vast majority of extrasolar gas giants known have equilibrium temperatures of ~1000-3000 K, thus hosting gaseous H2O in their atmospheres accessible to spectroscopic observations.
HD 189733b, HD 209458b, and WASP-12b are good choices because they range widely in temperature, with HD 189733b being one of the ‘coolest’ hot Jupiters known, and Wasp 12b one of the hottest. On the matter of equilibrium temperature (Teq), I’m drawing on Sara Seager’s book Exoplanet Atmospheres: Physical Processes (Princeton, 2010), which explains that equilibrium temperature is the temperature attained by an isothermal planet after it has attained complete equilibrium with the radiation from the star it orbits. The Madhusudhan paper adds that these hot Jupiters have the best spectroscopic precision of all the hot Jupiters that have been observed using the transmission spectroscopy technique.
So we have high-quality results that have the researchers looking at various scenarios to explain low water abundances. The paper adds that the Galileo probe reported a low H20 abundance in Jupiter that was explained by saying the probe moved through an unusually dry region. But at least one alternative explanation came in a 2004 paper suggesting that Jupiter may have formed by planetesimals dominated by tar rather than water ice. The Madhusudhan results reawaken such questions and cause us to look anew at formation and migration models for all giant planets.
The paper is Madhusudhan et al., “H2O abundances in the atmospheres of three hot Jupiters,” The Astrophysical Journal Letters Vol. 791, No. 1 (2014) L9 (abstract / preprint). On carbonaceous matter in the formation of Jupiter, see Lodders, “Jupiter Formed with More Tar than Ice,” The Astrophysical Journal Vol. 611, No. 1 (2004), 587 (abstract).
Both the Kepler and Spitzer space telescopes had a role to play in recent work on the planet Kepler-93b, whose size is now known to an uncertainty of a mere 120 kilometers on either side of the planet. What we have here is the most precise measurement of an exoplanet radius yet, a helpful result in the continuing study of ‘super-Earths,’ a kind of world for which we have no analogue in our own Solar System. A third instrument also comes into play, for studies of the planet’s density derived from Keck Observatory data on its mass (about 3.8 times Earth’s mass) and the known radius indicate this is likely an world made of iron and rock.
And that is absolutely the only similarity between Kepler-93b and Earth, for at 0.053 AU, six times closer than Mercury to the Sun, the planet’s surface temperature is estimated to be in the range of 760 degrees Celsius. The planet is 1.481 times the width of Earth. The accuracy of the measurement is the story here, a result so precise that, in the words of Sarah Ballard (University of Washington), lead author of the paper on these findings, “it’s literally like being able to measure the height of a six-foot tall person to within three quarters of an inch — if that person were standing on Jupiter.”
Image: Using data from NASA’s Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist’s conception. The diameter of the exoplanet, dubbed Kepler-93b, is now known with an uncertainty of just one percent. Credit: NASA/JPL-Caltech.
Just how the measurement was made is a story in itself. The Spitzer instrument provided data for seven transits of Kepler-93b between 2010 and 2011, three of them studied with a new observational technique called ‘peak up’ that halved the uncertainty of Spitzer’s own radius measurements. Kepler-93 thus served as a test subject for the new technique, which was developed in 2011 and allows tighter control over how light affects individual pixels in the observatory’s infrared camera. The paper examines all seven light curves in detail.
Meanwhile, we have the Kepler data, which provided light curves as well as the dimming of the star caused by seismic waves in motion in the interior. Now we’re in the realm of asteroseismology, which is a powerful way to probe the makeup of individual stars. Asteroseismic measurements over a long observational baseline can provide useful information about the density of the star (with a precision of 1 percent) as well as its age (within 10%). Such measurements require a long observational baseline at high cadence — cadence refers to the time between observations of the same target — as well has high photometric precision.
When we have both an asteroseismic density measurement of the exoplanet host star as well as a transit light curve, we can improve the precision of our radius measurements. Sara Seager (MIT) and colleagues examined host star densities in relation to planetary orbits and the radius of the star as early as 2003, and later work by a team led by Philip Nutzman (Harvard-Smithsonian CfA) used asteroseismology along with transit light curves to constrain the radius of HD 17156b, highlighting a method that has been found to be relevant to a wide number of recent studies.
From the paper:
The Kepler mission’s long baselines and unprecedented photometric precision make asteroseismic studies of exoplanet hosts possible on large scales… Kepler-93 is a rare example of a sub-solar mass main-sequence dwarf that is bright enough to yield high-quality data for asteroseismology. Intrinsically faint, cool dwarfs show weaker-amplitude oscillations than their more luminous cousins. These targets are scientifically valuable not only as exoplanet hosts, but also as test beds for stellar interior physics in the sub-solar mass regime.
The combination of the Kepler data and Spitzer’s new technique was powerful, and adds luster to the already rich history of Spitzer’s Infrared Array Camera (IRAC) in exoplanetary science. The instrument has been helpful in mapping planetary weather and characterizing super-Earth atmospheres, and has been a major tool in ruling out exoplanet false-positives, because an actual planet will present the same transit depth no matter the wavelength at which it is observed. After losing its coolant in 2009, the telescope, now dubbed ‘warm Spitzer,’ continues to provide key readings that are now enhanced with the development of the ‘peak up’ process.
Kepler-93 is a star of approximately 90 percent of the Sun’s mass and radius, located some 300 light years from Earth. With the Spitzer data corroborating the find and the use of asteroseismology to constrain the result, we wind up with an error bar that is just one percent of the radius of Kepler-93b. A planet thought to be 18,800 kilometers in diameter might be bigger or smaller than that by about 240 kilometers, but no more, an outstanding result for exoplanetary science and a confirmation of the power of asteroseismology in determining stellar radii.
The paper is Ballard et al., “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode,” The Astrophysical Journal Vol. 790, No. 1 (2014), 12 (abstract / preprint). A JPL news release is also available.
Are we alone in the universe? Nick Nielsen muses on the nature of the question, for the answer seems to depend on what we mean by being ‘alone.’ Does a twin of Earth’s ecosystem though without intelligent life suffice, or do we need a true peer civilization? For that matter, are we less alone if peer civilizations are widely spaced in time and space, so that we are unlikely ever to encounter evidence of them? And what of non-peer civilizations? SETI proceeds while we ponder these matters, a search that Nick sees as a priority because of the disproportionate value of an exterrestrial signal. Like Darwin in the Galapagos, we push on, collecting data in a quest that is without end. It’s a prospect Nick finds invigorating, and so do I.
by J. N. Nielsen
One of the great questions of our time is, “Are we alone?” Even though it is, for us, an existential question that touches upon our cosmic loneliness, it is, at the same time, a scientific question, as befits our industrial-technological civilization, driven as it is by progress in scientific knowledge. Because it is a scientific question, it hinges upon empirical evidence, but this empirical evidence must be placed in a theoretical context in order to make it meaningful. (Anecdotal evidence is not going to resolve the question.) Empirical evidence provides the observational content of a theory; formal concepts provide the theoretical framework of a theory. Neither in isolation constitutes a science (with the possible exception of the formal concepts of mathematics), but a given science may place more emphasis upon the empirical or the formal aspects of a theory. I will try to show below how Fermi’s paradox can be approached primarily formally or empirically.
Clarke’s tertium non datur
There is an understandable human desire to answer a question as clear as “Are we alone?” with an equally clear yes-or-no answer, but it is not likely that this will be the case. What we discover as we explore the cosmos is likely to be unfamiliar, unprecedented, and perhaps unclassifiable. Or, at least, the unclassifiable will be part of what is found, along with that which fulfills our expectations. It will be what challenges our expectations, however, that will shape the development of our thought and force us to revise our theoretical frameworks.
The yes-or-no formulation of the question of a cosmic loneliness I have elsewhere called Arthur C. Clarke’s tertium non datur, following Clarke’s well-known line that, “Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”  The logic of this compelling assertion seems undeniable, until one studies logic and one finds that the law of the excluded middle to which Clarke appeals (and which is also known as tertium non datur) is controversial, and that intuitionistic logics do without the law. Making the claim that Clarke makes, then, constitutes a subtle form of Platonism, and a constructivist or an anti-realist will reject this claim. Thus a formal approach to the question “Are we alone?” becomes, in part, a logical question rather than a question of empirical research.
From an empirical point of view, only a little reflection will show that the question “Are we alone?” is not likely to be satisfyingly answered in yes-or-no terms. If we find simple (single-celled) life below the surface of Mars or in the oceans of Europa, will we say that we are no longer alone in the cosmos? Apart from evidence that life can independently emerge in the universe, thus making it all the more likely that a peer civilization exists somewhere in the Milky Way, exobiological bacteria will not satisfy our desire for fellow beings with whom we can communicate as moral equals.
If we find a world of complex life, perhaps even a complex biosphere consisting of multiple diverse biomes, but no sentient, intelligent life, will we say that we are no longer alone? From a biological point of view, a twin of Earth’s ecosystem would mean that Earth is no longer alone, but that still does not rise to the level of finding conscious, communicative beings in the context of a peer civilization. I will admit without hesitation, however, that for some among us such a discovery would carry with it the feeling of cosmic companionship; the feeling of what it means to be alone in the universe is subject to individual variability, and therefore disagreement.
It seems likely to me that most human beings are only going to feel we are not cosmically isolated if we find a peer civilization, that is to say, another civilization roughly technologically equivalent to our own, being the work of biological beings who have converged upon a technology commensurate with our own, or some technology near that level. However, we are not yet prepared to say what a peer civilization is, because we cannot yet say what our own civilization is. We have no science of civilization, and therefore no way to employ scientific concepts to classify, compare, or quantify civilizations.  This does not mean that we have no idea whatsoever what civilization is, or what our civilization in particular is, only that these ideas cannot be called scientific.
The law of trichotomy for exocivilizations
Elsewhere I have discussed what I called the law of trichotomy for exocivilizations, which is the straight-forward observation that another civilization, presumably a peer civilization, must, in relation to our own civilization, appear before our civilization, during the period of our civilization, or after our civilization.  The dichotomy between being alone or not alone in the cosmos, and the trichotomy of another civilization coming before, during, or after our own civilization, are formal ideas based on conceptual distinctions. In other words, they are not ideas based on empirical evidence, and so they derive from the theoretical context employed to interpret empirical evidence.
While the law of trichotomy for exocivilizations is ideally applicable, in practice it runs into relativistic problems. Relativity means the relativity of simultaneity, so that the absolute simultaneity implied by an ideal interpretation of the law of trichotomy (as when we apply the law to real numbers) does not work if the simultaneity in question is the punctiform present . If, however, we allow a little leeway, and grant some temporal “width” to the present, we could define a broad present in which peer civilizations exist simultaneously, but this width would rapidly exceed the age of industrial-technological civilization as we attempt to expand this broadly-defined present in the galaxy (much less the universe). Thus, what we will not find are peer or near-peer civilizations existing simultaneously with our own, unless scientific discoveries force major changes in relativity theory or something like the Alcubierre drive proves to be a practicable form of interstellar transportation.
The act of traveling to the stars in order to seek out peer civilizations involves a lapse of time both on our home planet and on the homeworld of a peer civilization. The kind of temporally-distributed civilization that I described in Stepping Stones Across the Cosmos could constitute one form of temporal relations holding among mutual exocivilizations: the overlapping edges of two or more temporally-distributed civilizations may come into contact, but given that both civilizations are temporally distributed, the home world of these civilizations can never be in direction contact, and any radio communication between them might require hundreds, thousands, or millions of years—periods of time probably well beyond the longevity of our present civilization.
Using formal concepts in the absence of observation
The examples given above of Arthur C. Clarke’s tertium non datur and the law of trichotomy for exocivilizations seem to point to the limitations of formal conceptions in the face of the stubborn facts of empirical observation, but formal concepts can prove to be a powerful tool in the absence of empirical observation, when these observations require technologies that do not yet exist, or which have not been built for institutional or financial reasons.
One of the most obvious ways in which we are now limited in our ability to make empirical observations is that of imaging exoplanets. We know that this technology is possible, and in fact we could today build enormous telescopes in space, such as a radiotelescope on the far side of the moon, shielded from the EM spectrum radiation of Earth, and possibly sufficiently sensitive to detect the passive EM radiation of an early industrial-technological civilization. That we do not do so is not a matter of scientific limitations, and not even a matter of technological limitations. We have the technology now to do this, though there would be many engineering problems to be resolved. The primary reason we do not do so is lack of resources.
Because of our inability at present to see or to visit other worlds, we have no empirical data about life or civilization elsewhere in the universe. It is sometimes said that we have only a single data point for life, and scientific extrapolation from a single data point is unreliable, if not irresponsible.
While a merely formal grasp of life and civilization may seem a pale and ghostly substitute for actual empirical data, in the absence of such empirical data a formal understanding may allow us to extract from our own natural history, and the history of our civilization, not one data point but many data points. If we can take a sufficiently abstract and formal view of our own world, that is to say, if we can rise to the level of generality of our conceptions that attends only to the structure of life and civilization on Earth, we may be able to derive a continuum of historical data points from the single instance of life on Earth and the single instance of human civilization.
Credit: Wikimedia Commons.
Spatio-temporal distribution of life in the universe
Life on Earth taken on the whole constitutes a single data point, but the natural history of life on Earth reveals a continuum of data points. The temporal distribution of the natural history of life on Earth – if this is at all representative of life simpliciter – can be roughly translated into the spatial distribution of life on Earth-like planets in the universe, on the assumption that Earth-like planets are continuously in the process of formation.
In more detail:
1. The universe is about 13.7 billion years old.
2. The Milky Way galaxy may be nearly as old as the universe itself – 13.2 billion years, by one estimate , which means that, in one form or another, the Milky Way has persisted for about 96 percent of the total age of the universe.
3. Population I stars, with higher a metallicity consistent with the formation of planetary systems with small, rocky planets are as much as 10.0 billion years old , or have existed for about 73 percent of the total age of the universe – almost three-quarters of the age of the universe.
4. The Earth formed about 4.54 billion years ago, so it has been around for 33% of the age of the universe, or about a third.
5. Life is thought to have started at Earth about 4.2 to 3.8 billion years ago, so life has been around for 28 percent of the age of the universe, or more than a quarter. Life started at Earth almost as soon as Earth cooled down enough to make life possible. Although life started early, it remained merely single-celled microorganisms for almost two billion years before much more of interest happened.
6. Eukaryotic cells appeared about 2 billion years ago, for a comparative age of 15% of the age of the universe.
7. Complex multicellular life dates from about 580 million years ago (from the Cambrian explosion), so it has been around for 4 percent of the age of the universe.
8. The mammalian adaptive radiation following the extinction of dinosaurs (and thereby giving us lots of animals with fur, warm blood, binocular vision, sometimes color vision, proportionally larger brains necessary to process binocular color vision, and thus a measure of consciousness and sentience) began about 65 million years ago, and thus represents less than a half of one percent of the total age of the universe.
9. Hominids split off from other primates somewhere in the neighborhood of five to seven million years ago, and thereby began the journey that resulted in human beings, which possess a greater encephalization quotient than any other terrestrial species. This period of time represents about half of a thousandth of one percent of the age of the universe. 
10. The earliest forms of civilization emerged about 10,000 years ago, roughly simultaneously starting in the Yellow River Valley in China, the Indus River Valley, Mesopotamia, and what is now Perú (with a few other scattered locations). Industrial-technological civilization – the kind of civilization that can (potentially) build spacecraft and radiotelescopes – is a little more than 200 years old, which is too small of a fraction of one percent to bother calculating. This is the proverbial needle in the cosmic haystack.
We can recalculate these percentages specific to the age of the Earth (rather than to the age of the universe entire), so that 88 percent of the Earth’s age has included life, 44 percent has included eukaryotic cells, 13 percent has included complex multicellular life, 1.4 percent has included mammals of the post-K-Pg extinction event, 1.5 thousandths of a percent has included hominids, and a miniscule fraction of a percent of the total age of Earth has included civilization of any kind whatever.
Given a small, rocky planet in the habitable zone of its star (i.e., given an Earth twin, which recent exoplanet research suggests are fairly common), such a planet is 88 percent likely to have reached the developmental stage of rudimentary life, 44 percent likely to have reached the stage of eukaryotes, 13 percent to have progressed to something like the Cambrian explosion, and a little more than one percent may have produced animal life of a rudimentary degree of sentience and intelligence.  If we take current estimates of Earth twins of 8.8 billion in the Milky Way galaxy alone , only somewhat more than a million would have advanced to the stage corresponding to early hominids on Earth – and these million must be found within the 300 billion star systems in the galaxy.
The data points that we can extract from our own natural history leave us almost completely blind as to our future, and therefore equally blind in regard to civilizations more technologically advanced than our own. We have no experience of the collapse of industrial-technological civilization, so we have no evidence whatsoever that would speak to the longevity of such a civilization. 
Image: Stromatolites in Shark Bay, photograph taken by Paul Harrison. Is this what most habitable planets in the galaxy look like? Credit: Wikimedia Commons.
A universe of stromatolites
Of course, it is misleading to speak of taking an Earth twin at random. The universe is not random.  Like the Earth itself, it exhibits a developmental trajectory (sometimes called “galactic ecology” or “cosmological ecology”), so that any particular age of the universe is going to yield a different percentage of Earth twins among the total population of planets in the universe. Someone versed in astrophysics could give you a better number than I could estimate, and could readily identify the period in the development of the universe when Earth-like planets are likely to reach their greatest number, though we know from our own existence that we have at least passed the minimal threshold.
Despite the fact that my estimates are admittedly misleading and probably inaccurate, as a rough-and-ready approach to what we are likely to see when we have the technology to observe or to visit Earth twins, these percentages give us a little perspective. We are more likely than not to find life. Life itself seems likely to be rather common, but this is only the simplest life. We may live in a universe of stromatolites – i.e., thousands upon thousands of habitable worlds in the Milky Way alone, but inhabited only by rudimentary single-celled life . Maybe a tenth of these worlds will have seas churning with something like the equivalent of trilobites, and possibly one percent will have arrived at the stage of development where many species have relatively large brains, precise vision (something like binocular color vision), and limbs capable of manipulating their environment. In other words, possibly one percent of worlds will have produced species capable of producing civilization. The chance of finding the tiny fraction of a percent of these species that go on to create an industrial-technological civilization (and therefore could be considered a peer civilization to our terrestrial civilization) remains vanishingly small.
In a universe of stromatolites, are we alone or are we not alone? The answer is not immediately apparent, and that is why I said that the tertium non datur form of the question, “Are we alone?” is not likely to be given a satisfying answer.
Image: A caricature of Darwin collecting beetles by fellow young naturalist Albert Way. Credit: The Darwin Project. Credit: The Darwin Project.
A journey to distant worlds
From the above considerations, I consider the search for a peer civilization to be like the proverbial search for a needle in a haystack. But it is still a search that is well worth our while – as well as being worth our investment. If you are personally invested in a search for a particular needle in a haystack, you are likely to continue the search despite the apparently discouraging odds of being successful. We are, as a civilization, existentially invested in the search for a peer civilization, as a response to our cosmic loneliness. For this reason if for no other, the search for a peer civilization is likely to be pursued, if only by a small and dedicated minority.
Far from suggesting that the difficulty of a successful SETI search means that we should abandon the search, I hold that the potentially scientifically disruptive effect of a SETI search that finds an extraterrestrial signal would be so disproportionately valuable that SETI efforts should be an integral part of any astrobiological effort. The more unlikely the result, the greater would be the falsification of existing theories upon a successful result, and therefore the more we would have to learn from such a falsification. This is the process of science. A single, verifiable extraterrestrial signal would give a satisfying answer to the “Are we alone?” question, since a single counter-example is all that is needed.
Anticipating responses that I have encountered previously, I should mention that I do not find this point of view to be in the least depressing or discouraging. A universe of stromatolites, with the occasional more complex biosphere thrown into the mix, strikes me as an exciting and worthwhile object of exploration and scientific curiosity. With so many worlds to explore, it is easy to imagine the re-emergence in history of the gentleman amateur natural historian, which is how Darwin began his career, and some future Darwin collecting the extraterrestrial equivalent of beetles might well make the next major contribution to astrobiology. Darwin wrote, “…it appears to me that nothing can be more improving to a young naturalist, than a journey in distant countries.”  He might as well have written, “…nothing can be more improving to a young naturalist, than a journey to distant worlds.”
If we add to this prospect (to me a pleasant prospect) the possibility of a few extraterrestrial civilizations lurking among the stars of the Milky Way, at a pre-industrial level of development and therefore unable to engage with us until we stumble upon them directly , I cannot image a more fascinating and intriguing galaxy to explore.
 Quoted in Visions: How Science Will Revolutionize the Twenty-First Century (1999) by Michio Kaku, p. 295.
 I take these three kinds of scientific concepts – classification, comparison, and quantification – from Rudolf Carnap’s Philosophical Foundations of Physics, section 4; cf. my post The Future Science of Civilizations. We can classify, compare, and quantify energy usage, and it is this approach that gives us Kardashev civilization types; we can also classify, compare, and quantify information storage and retrieval, which gives us the metric proposed by Carl Sagan for giving a numerical value to civilization, but I take these to be reductive approaches to civilization, and therefore inadequate.
 The law of trichotomy for exocivilizations is simply a particular example of the law of trichotomy for real numbers, though applied to civilizations in time – time being a continuum that can be described by the real numbers.
 The idea of the punctiform present is that of the present moment as a durationless instant of time that is the unextended boundary between past and present. Note that the idea of the punctiform present is an idealization, like Clarke’s tertium non datur and the law of trichotomy of exocivilizations; as such it is a formal conception of time, and not an empirical claim about time. Like the distinction between pure geometry and physical geometry, we can distinguish between pure time, which is a formal idea parallel to pure geometry, and physical time.
 Cf. the Wikipedia entry on metallicity: http://en.wikipedia.org/wiki/Metallicity
 “Populations of Stars” http://www.astronomynotes.com/ismnotes/s9.htm
 I am employing the older distinction between primates and hominids. It has become commonplace in recent anthropological thought to introduce a new distinction between hominids and hominims, according to which hominids are all the great apes, including extinct species, and hominims are all human species, extinct and otherwise; This new distinction adds nothing to the older distinction. Moreover, from a purely poetic point of view, “hominim” is an unattractive word with an unattractive sound, with a series of insufficiently contrasting consonants (especially in contradistinction to “hominid”), so I prefer not to use it. I realize that this sounds eccentric, but I wanted my readers to be aware, both of the distinction and my reasons for rejecting it.
 I leave it as an exercise to the reader to reformulate my developmental account of the emergence of industrial-technological civilization on Earth into the more familiar terms of the developmental account implicit within the Drake equation.
 A recent study was widely publicized as predicting that 8.8 billion Earth-like planets are to be found in the habitable zones of sun-like stars in the Milky Way galaxy. “Prevalence of Earth-size planets orbiting Sun-like stars,” Erik A. Petigura, Andrew W. Howard, and Geoffrey W. Marcy doi: 10.1073/pnas.1319909110 (http://www.pnas.org/content/early/2013/10/31/1319909110)
 I wrote, “almost completely blind,” instead of, “completely blind,” as there obviously are predictions that can be made about the future of industrial-technological civilization, and some of these are potentially very fruitful for SETI and related efforts. More on this another time.
 The universe is neither random nor arbitrary; Earth is not random; life, intelligence and civilization are not random. Neither, however, are they planned; the order that that exhibit is not on the order of conscious construction anticipating future developments. It is one of the great weaknesses of our conceptual infrastructure that we have no (or very little) terminology and concepts to describe or explain empirical phenomena that are neither arbitrary nor teleological. We have, perhaps, the beginnings of such a conceptual infrastructure (starting with natural selection and moving on to contemporary conceptions of emergentism), but this has not yet pervasively shaped our thought, and it remains at present sufficiently counter-intuitive that we must struggle against our own cognitive biases in order to consistently and coherently think about the world without reference to teleology.
 According to Wikipedia, stromatolites are, “layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Stromatolites provide the most ancient records of life on Earth by fossil remains which date from more than 3.5 billion years ago.” I employ stromatolites merely as an example of early terrestrial life sufficiently robust to endure up to the present day; no weight should be attached to this particular example, as any number of other examples would serve equally as well. I could have said, perhaps with greater justification, that we may live in the universe of extremophiles.
 Charles Darwin, Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2d edition. London: John Murray, 1845, Chap. XXI (http://darwin-online.org.uk/content/frameset?itemID=F14&viewtype=text&pageseq=1)
 Our galaxy may host hundreds or thousands of civilizations at a stage of pre-electrification, prior to any possibility of technological communication or travel, and therefore beyond the possibility of observation until we can send a probe or visit ourselves. But keep in mind that a thousand civilizations unable to communicate by technological means, and distributed throughout the disk of the Milky Way, may as well be so many needles in a haystack.