Centauri Dreams
Imagining and Planning Interstellar Exploration
‘Hot Jupiters’: Drier Than Expected
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).
Tight Measurement of Exoplanet Radius
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.
A Needle in the Cosmic Haystack: Formal and Empirical Approaches to Life in the Universe
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.” [1] 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. [2] 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. [3] 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 [4]. 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 [5], 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 [6], 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. [7]
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. [8] If we take current estimates of Earth twins of 8.8 billion in the Milky Way galaxy alone [9], 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. [10]
Image: Stromatolites in Shark Bay, photograph taken by Paul Harrison. Is this what most habitable planets in the galaxy look like? Credit: Wikimedia Commons.
[http://en.wikipedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg]
A universe of stromatolites
Of course, it is misleading to speak of taking an Earth twin at random. The universe is not random. [11] 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 [12]. 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.” [13] 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 [14], I cannot image a more fascinating and intriguing galaxy to explore.
Notes
[1] Quoted in Visions: How Science Will Revolutionize the Twenty-First Century (1999) by Michio Kaku, p. 295.
[2] 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.
[3] 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.
[4] 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.
[5] Cf. the Wikipedia entry on metallicity: http://en.wikipedia.org/wiki/Metallicity
[6] “Populations of Stars” http://www.astronomynotes.com/ismnotes/s9.htm
[7] 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.
[8] 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.
[9] 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)
[10] 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.
[11] 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.
[12] 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.
[13] 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)
[14] 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.
SETI: The Pollution Factor
We tend to assume that our mistakes as a species flag us as immature, a young civilization blundering about with tools it is misusing on a course that may lead to extinction. But assume for a moment that an intelligent extraterrestrial civilization goes through phases more or less like our own. If we’re sifting through radio signals and looking for optical flashes to find them, shouldn’t we consider other ways such a civilization announces itself? What if we’re not the only polluters in the universe, for example, and other cultures are making the same mistakes?
In a 2010 paper, Jean Schneider (Observatoire de Paris-Meudon) and colleagues noted the possibility of using pollutants as a way of moving beyond biosignatures to find ETI. Let me quote from the paper:
…another type of far from equilibrium signals can be seen as techno-signatures, i.e., spectral features not explained by complex organic chemistry, like laser emissions. In the present state of our knowledge one cannot eliminate them a priori, although we have no guiding lines to search for them. For instance, in the present Earth atmosphere, CFC (Carbon Fluoro Compounds) gases are the result of technological chemical synthesis. Observed over interstellar distances, they would reveal to the observer the presence of a technology on our planet.
Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and James Kasting (Pennsylvania State) have been looking at the CFC idea for some time. As Kasting told New Scientist in 2009, “There’s a whole host of things we make industrially as solvents, cleaners and refrigerants – they certainly have absorption lines. If you had a big enough telescope, you could detect them.” CFCs themselves absorb infrared light at specific wavelengths and are detectable at very low concentrations, as low as a few parts in a trillion. Moreover, they do not form naturally, and though a detection would be tricky, Kaltenegger has suggested a future array of space telescopes working at infrared wavelengths should be able to do the job. For more on this, see To Spot an Alien, Follow the Pollution Trail, the original article in New Scientist.
Image credit: CfA.
But would it take such a futuristic flotilla of telescopes to spot pollution? New work out of the Harvard-Smithsonian Center for Astrophysics suggests even the James Webb Space Telescope may be up to the challenge. The paper argues that atmospheric levels of CFCs about ten times greater than we have produced here on Earth could be traced by JWST. The focus here is on tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), described as the easiest CFCs to detect. Henry Lin and team point out that the lifetimes of CFCs range from 10 to 105 years, meaning that a polluting civilization existing any time in the past 105 years would be theoretically detectable.
But the strategy only works in detecting pollutants on Earth-like planets circling a white dwarf star, a scenario that maximizes the atmospheric signal. Recent work has shown that white dwarfs can have long-lived habitable zones, and the similarity in size between the planet and star offers the best contrast between the planet’s atmospheric transmission spectrum and the star it is transiting. The paper considers white dwarfs that have cooled to surface temperatures around 6000 K, the same surface temperature as the Sun, so that the habitable zone is close in, at about 0.01 AU, which greatly increases the chance of a transit.
From the paper:
…a recent study by Worton et al. (2007) estimates the atmospheric concentration of CF4 at ~75 parts per trillion (ppt), whereas CF4 levels were at ~40 ppt around ~1950. Assuming a constant production rate…we expect as a very crude estimate that in roughly ~1000 years, the concentration of CF4 will reach 10 times its present levels. Coupled with the fact that the half-life of CF4 in the atmosphere is ~50,000 years, it is not inconceivable that an alien civilization which industrialized many millennia ago might have detectable levels of CF4. A more optimistic possibility is that the alien civilization is deliberately emitting molecules with high GWP [global warming potential] to terraform a planet on the outer edge of the habitable zone, or to keep their planet warm as the white dwarf slowly cools.
The JWST should be able to detect CF4 and CCl3F signatures in the atmospheres of transiting Earths around white dwarfs as long as their concentrations are on the order of ten times that of the Earth — CF4 detection demands 1.7 days of exposure time on the instrument and CCl3F 1.2 days. The exposure time is already built into biosignature study times, given that these will take on the order of an entire day to detect. Looking for pollutants, then, adds little in terms of additional observing costs.
Bear in mind that while some CFCs last for tens of thousands of years in the atmosphere, others persist no more than ten. Harvard’s Avi Loeb, one of the trio of researchers in this project, points out that finding a short-lived CFC on an exoplanet would signal an active civilization. The other scenario may be starker. If we detect molecules from long-lived CFCs but none of the short-lived variety, we could be witnessing a changing civilization. “In that case,” says Loeb, “we could speculate that the aliens wised up and cleaned up their act. Or in a darker scenario, it would serve as a warning sign of the dangers of not being good stewards of our own planet.”
The paper is Lin et al., “Detecting Industrial Pollution in the Atmospheres of Earth-like Exoplanets,” accepted by The Astrophysical Journal and available as a preprint. The Jean Schneider paper is “The Far Future of Exoplanet Direct Characterization,” Astrobiology 10 (2010), p. 121.
Transiting World at the Snow Line
It’s 9000 times easier to find a ‘hot Neptune’ than a Neptune out around the ‘snow line,’ that region marking the distance at which conditions are cold enough for ice grains to form in a solar system. Thus says David Kipping (Harvard-Smithsonian Center for Astrophysics), who is lead author on the paper announcing the discovery of Kepler-421b, an interesting world about which Kipping has been sending out provocative tweets this past week. Kepler-421b draws the eye because its year is 704 days, making it the longest orbital period transiting planet yet found. The intriguing new world is located about 1000 light years from Earth in the direction of the constellation Lyra.
The transit method works by detecting the characteristic drop in brightness as a planet moves across the face of the star as seen from Earth. What’s unusual here is that Kepler-421b moved across its star only twice in the four years that the Kepler space telescope monitored it. As Kipping explains on this CfA web page, the further a planet is from its host star, the lower the probability that it will pass in front of the star as seen from Earth. Kepler-421b should have had, by Kipping’s calculations, a tiny 0.3% chance of being observed in a transit. We can be happy for the discovery while also considering how tricky it will be to find worlds like it by transit methods.
Image: Transit light curve of Kepler-421b. Blue and red points denote the two different transit epochs observed, offset in time by 704 days. Credit: David Kipping et al.
Also known as the ‘frost line,’ the snow line in our own Solar System is the divider between the rocky inner planets all the way out to Mars, and the outer gas giants. The kind of planet you get depends in part on whether, during the early period of planet formation, the emerging planet is inside or outside the snow line. According to our current formation models, gas giants form beyond the snow line, where the temperatures are low enough that water condenses into ice grains. The planetary embryos that become the gas giants should have abundant ice grains sticking together to create worlds rich in ice and water compared to the inner system.
That has major implications, of course, because we have discovered a large number of ‘hot Jupiters’ and Neptune analogues that orbit far inside the snow line in their respective systems. That makes for migration scenarios where gas giants forming in the outer system move inward as the result of likely gravitational encounters with other worlds. Kepler-421b, however, orbits its K-class primary at a distance of about 177 million kilometers, a gas giant that may never have migrated, and the first example of such ever found using the transit method.
The snow line moves inward over time as the young planetary system evolves, and Kipping and team’s calculations show that when this system was about three million years old, early in the era of planet formation, its snow line should have been at about the same distance as Kepler-421b’s present location. The planet is roughly the size of Uranus, about four times the size of Earth, which may be an indication that it formed late in the planet formation era, at a time when not enough material was left in the system to allow it to become as large as Jupiter.
But is Kepler-421b truly an ice giant or could it actually be a large, rocky world? The evidence strongly favors the former. From the paper (internal citations deleted for brevity):
Although calculating detailed formation scenarios for Kepler-421b is outside the scope of this work, simple arguments suggest Kepler-421b is an icy planet which formed at or beyond the snow line. With a radius of roughly 4 R? and a mass density of at least 5 g cm-3, a rocky Kepler-421b has a mass of at least 60 M?. Growing such a massive planet requires a massive protostellar disk with most of the solid material at 1-2 AU. Among protoplanetary disks in nearby star-forming regions, such massive disks are rare. Thus, a rocky Kepler-421b seems unlikely.
And as to the place of formation:
For Kepler-421b, in situ formation is a reasonable alternative to formation and migration from larger semi-major axes. Scaling results from published calculations, the time scale to produce a 10-20 M? planet is comparable to or larger than the median lifetime of the protoplanetary disk. Thus formation from icy planetesimals is very likely. If significant migration through the gas and leftover planetesimals can be avoided, Kepler-421b remains close to the ‘feeding zone’ in which it formed.
To place the planet in context, consider that Mars orbits the Sun every 780 days, as compared to Kepler-421b’s 704 day orbit (around, as mentioned above, a K-class star that would be cooler and dimmer than the Sun). The researchers’ calculations indicate a temperature of about -135 Fahrenheit (180 K). At least one recent paper, cited by Kipping and colleagues, suggests that planets near the threshold of the snow line may be common, but finding them by transit methods will be difficult because of the low transit probability. As for radial velocity detection, the planet poses what the paper calls “a significant challenge to current observational facilities,” but determining the mass of worlds like this could help us understand the relationship between mass and radius as we move further from the parent star.
The paper is Kipping et al., “Discovery of a Transiting Planet Near the Snow-Line,” accepted by The Astrophysical Journal and available as a preprint online.
A Spacecraft in Your Pocket
Last week we looked at Mason Peck’s ideas on ‘Sprites,’ tiny spacecraft the size of computer chips that could be sent in swarms to targets near and far. I was particularly interested in Peck’s idea of using Jupiter as a massive particle accelerator, bringing huge numbers of Sprites up to speeds in the range of hundreds of kilometers per second. Growing out of Clifford Singer’s insights in the 1970s and given onboard intelligence by Gerald Nordley, the idea of ‘smart pellets’ thus moves beyond a propulsion method to become a fleet of networked space probes.
Perhaps one day we’ll be able to use the tools of nanotechnology to create highly intelligent vehicles of extremely small size, rendering the propulsion problem a bit more tractable. But until we’re at that level, it’s fascinating to see the groundwork being laid in work like Peck’s. Today I want to talk about another experiment with space vehicles that are smaller than a compact disc and as thin as a piece of paper. Pocket Spacecraft are being developed for launch in a CubeSat for a range of potential missions in an energetic attempt at space crowdsourcing.
We’ve already seen crowdsourcing at work in projects like KickSat, which is still in the works in the capable hands of Zachary Manchester at Cornell despite an initial setback last April. KickSat would have deployed a large number of Sprites for early testing, as will, presumably, the follow-up KickSat-2. Some members of the KickSat team have also gone on to work on Pocket Spacecraft, a project that has tapped the skills of volunteers from over twenty countries, and one that has played a major role in conferences like the Interplanetary CubeSat Workshop (MIT) and a Caltech event called Small Satellites: A Revolution in Space Science.
So what are Pocket Spacecraft? The image below gives you the gist of a spacecraft small enough to fit in your pocket, essentially a disk made out of the same material you find in flexible circuit boards. The polyimide disk is ringed by a nickel titanium hoop that contains memory and doubles as an antenna for the diminutive vehicle. Let me quote from the Pocket Spacecraft site to continue the description:
Solar cells, a thinned commercial off the shelf system-on-a-chip die (ground down with diamond sand paper) and support components, sensors and instruments are bonded or printed on the polyimide and protected with a conformal coating resulting in a spacecraft with an average thickness less than one twentieth of a millimetre (two thousandths of an inch), and a mass much less than a gram (a thirtieth of an ounce).
Image: Thin-Film Scout prototype consisting of a polyimide substrate, bonded solar cells and thinned die, printed passive components, antennas and images. Credit: Pocket Spacecraft.
The upshot is that Pocket Spacecraft can be packed tightly, thousands to a single CubeSat ‘mothership.’ We’ve seen that CubeSats are destined to be true workhorses for inexpensive space missions, a fact underlined by recent NASA studies on CubeSats in interplanetary space. In February of 2013, NASA selected 24 small satellites to fly as auxiliary payloads on rockets planned for launch in the next two years, the CubeSats being proposed coming not only from NASA centers (three from JPL) but educational institutions and non-profit organizations.
We’ve also seen that The Planetary Society has been engaged in a lengthy study of solar sails, with plans for its LightSail-1 to be deployed from a CubeSat. Potentially, the tools are all here to allow us to fly CubeSats with different instrument configurations to a wide range of targets in the Solar System using solar sails for propulsion. The Pocket Spacecraft team also speaks about another option, a system based on electrolysis, using solar power to break down liquid fuel. Its CubeSats are radiation hardened and contain the subsystems needed to communicate with Earth as well as to release and photograph the numerous CD-sized Pocket Spacecraft.
Image: An engineering model of a 3U CubeSat, the largest most commonly launched CubeSat format. Credit: Pocket Spacecraft.
Call them ‘Scouts,’ as the Pocket Spacecraft team does. The Scouts house integrated optical and radio transceivers and sensors including an accelerometer, gyroscope, temperature sensor, and single pixel optical sensor. Using online tools including smartphone apps, contributors to the project can take a role in personalizing an individual Scout and, using a Web browser, can participate in science experiments. The more technically inclined can use Arduino tools to run their own software on Scouts, backed by a Web-based integrated development environment.
So the crowdsourced side of this project is engaging because it’s not just a matter of contributing money, but of participating in an active way with a space mission. Some Scouts are to be released from Earth orbit to test their ability to re-enter the atmosphere, taken measurements of the Earth’s thermosphere along the way. Others will be flown on a low energy transfer orbit to the Moon for release and landing on the surface. The attempt is to show that a collaboration of private citizens operating on a shoestring budget can design and build spacecraft that can travel not just into orbit but to the Moon and theoretically further given the new generation of sail-enabled CubeSats now beginning to come online.
The issue I’d be most concerned with in this scenario is communications. Telemetry is to be transmitted either directly from the Scout spacecraft to Earth or through the CubeSat mothership, depending on the distance of the Scout. Pocket Spacecraft says it will take care of the communications infrastructure, which could involve amateur radio equipment at one end and, as Scouts get closer to the Moon, repurposed radio telescopes. Assuming this works, the telemetry is to be made available through a smartphone app as well as public servers.
Image: Lapping (thinning) a pocket spacecraft chip die using diamond ‘sandpaper’ (left) and testing the final part (right). Credit: Pocket Spacecraft.
The theme of miniaturization is the obvious driver for this entire project. From the site:
Even though our spacecraft are small, they are mighty. If you look beneath the superficial cosmetic customization of the surface of the spacecraft, you will find computing power comparable to that of the Voyager spacecraft and Apollo flight computers. Thanks to the spectacular advances in semiconductor technology and the widespread low cost availability of what once would have been considered high precision scientific instruments that are now commonly found in cell phones such as accelerometers and magnetometers, we are building tiny high performance inexpensive scientific spacecraft accessible to all.
I’m glad to see that the Pocket Spacecraft team has software development options for Scouts that allow schools or clubs to support up to 50 users for customizing onboard systems. A software Scout simulator and virtual Solar System are being established to test out customization ideas along with swarm communications possibilities. So the intent is to let users be ‘hands-on’ throughout the process from Scout building and lab testing to space operations.
We’ve talked about a future where long-haul ‘swarm’ spacecraft are a possibility, but one that demands huge advances in nanotechnology, artificial intelligence, networking and communications. We get to a future like that by making experiments one step at a time to discover the issues that need to be addressed. We move incrementally forward. Pocket Spacecraft, using off-the-shelf equipment and private money, should engage the interest of the public and, if successful, provide useful data applicable to future feats of miniaturization. Click here for further information.
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