Centauri Dreams
Imagining and Planning Interstellar Exploration
SETI: Musings on the Barrow Scale
John Barrow has been on my mind these past few days, for reasons that will become apparent in a moment. In my eulogy for Barrow (1952-2020), I quoted from his book The Left Hand of Creation (Oxford, 1983). I want to revisit that passage for its clarity, something that always inspired me about this brilliant physicist. For it seemed he could render the complex not only accessible but encouragingly pliable, as if scientific exploration always unlocked doors of possibility we could use to our advantage. His was a bright vision. The notion that animated him was that there was something in the sheer process of research that held its own value. Thus:
Could there be any shortcuts to the answers to the cosmological questions? There are some who foolishly desire contact with advanced extraterrestrials in order that we might painlessly discover the secrets of the universe secondhand and prematurely extend our understanding. Such a civilization would surely resemble a child who receives as a gift a collection of completed crossword puzzles. The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.
You can see that for Barrow, the question of values was not separated from scientific results, and in a sense transcended the data we actually gathered. He goes on:
About 50 years ago a group of eminent cosmologists were asked what single question they would ask of an infallible oracle who could answer them with only yes or no. When his opportunity came, Georges Lemaître made the wisest choice. He said, “I would ask the Oracle not to answer in order that a subsequent generation would not be deprived of the pleasure of searching for and finding the solution.”
Image: Cosmologist, mathematician and physicist John D. Barrow, whose books have been a personal inspiration for many years. Credit: Tom Powell.
Leave it to Lemaître (and Barrow to quote him) as we reach beyond the immediately practical to unlock what it is about human experience that compels us to push into new terrain, whether it be physical exploration or flights of the imagination as we pursue a new hypothesis about nature. Barrow comes to mind because we’ve just been talking about the scales by which a civilization can be measured. Some of these are well established, as for example the Kardashev scale, with its familiar Types I, II and III keyed to the scale of a civilization’s energy use. In Clarke’s The Fountains of Paradise we find an alien scale based on the use of tools. It’s possible to imagine other scales, and Barrow’s own contribution takes us into the nano-realm.
As best I can determine, Barrow first floated the scale in his 1998 book Impossibility: The limits of science and the science of limits (Oxford University Press). Inverting Kardashev, Barrow was interested in a civilization’s ability to control smaller and smaller things, relying on the observed fact that as we have explored such micro-realms, our technologies have proliferated. Nanotechnology and biotechnology are drawn out of our ability to manipulate matter at small scales, and in fact the development of nanotech is one marker for a Barrow scale IV culture.
Barrow I: The ability to manipulate objects at the same scale as the person or being involved. In other words, simple activities involving basic tools.
Barrow II: The control of genetic information.
Barrow III: The ability to control molecules.
Barrow IV: The ability to control individual atoms.
Barrow V: The manipulation of atomic nuclei..
Barrow VI: Control of elementary particles.
Barrow Omega (Ω): The ability to control fundamental elements of spacetime.
Table: Energetic and inward civilization development. Kardashev’s (1964) types refer to energy consumption; Barrow’s (1998, 133) types refer to a civilization’s ability to manipulate smaller and smaller entities. Credit: Clément Vidal.
I’ve drawn the above table from a paper by French philosopher and SETI scientist Clément Vidal, who is one of the few who have explored this realm (citation below). Here we get both Kardashev and Barrow at once, a convenience, and central to Vidal’s argument that black holes are going to draw advanced civilizations to extract their energies and explore what he calls “the computational density of matter.” On this score, it’s interesting to note that Freeman Dyson proposed in 1979 that a civilization exploiting time dilation effects near black holes could survive effectively forever (a later revision had to take into account the accelerating expansion of the universe).
What all this means for SETI is intriguing – almost punchy – and I’ll send you to Vidal’s superb The Beginning and the End: The Meaning of Life in a Cosmological Perspective (Springer, 2014) for a deep dive into the concepts involved. But consider this for a starter: Dysonian SETI assumes civilizations far more advanced than our own, the reasoning being that their works should be apparent even at astronomical scales. Thus searching our astronomical data as far back as we can could conceivably flag an anomaly that merits investigation as a possible civilizational marker.
What Clément Vidal has been investigating is where such markers would turn up, and for this he deploys the scales of both Kardashev and Barrow. I think the easiest assumption is that we would find an alien civilization at its home world, but of course this needn’t be the case. Vidal speaks of ‘attractors’ as those sources of energy that an advanced civilization would increasingly exploit. Take a culture a billion years older than our own and ponder energy needs that might require it to exploit things like the energies of close binary neutron stars or black holes themselves. Such a civilization would be far flung, with operations well beyond its local group of stars.
Now ponder Barrow Type Ω. This ‘omega’ culture is free of the constraints of spacetime, having achieved the ability to manipulate both. It’s anyone’s guess whether such a civilization would be noted by achievements on a truly celestial scale, or whether its works would actually be embedded in the nature of space and time themselves, so that to us they appear the simple functioning of nature. In this mode of thinking, the more advanced a civilization becomes as it moves up the Barrow scale, the more it begins to effectively disappear. Barrow thus channels Richard Feynman and anticipates Lee Smolin’s notions about cosmological evolution, a kind of self-selection for universes.
I’m going to swipe the chart below from Vidal’s 2010 paper on black hole attractors, showing the entertaining fact that as he puts it, “from the relative human point of view, there is more to explore in small scales than in large scales.”
Table: That humans are not in the center of the universe is also true in terms of scales. This implies that there is more to explore in small scales than in large scales. Richard Feynman (1960) popularized this insight when he said “there is plenty of room at the bottom”. Figure adapted from (Auffray and Nottale 2008, 86). Credit: Clément Vidal.
Futurist John Smart has dug into what he calls STEM Compression, with STEM in this case meaning Space/Time/Energy/Matter, and the compression being the idea that in terms of density and efficiency, we can as Vidal puts it “do more with less.” For going deeper into the Barrow scale, we see that as things get smaller, we are not hampered by the speed of light problem. In fact, our endgame barrier is at the Planck scale. A Kardashev II civilization extracting energy from a rotating black hole using technologies far up the Barrow scale may well be indistinguishable from an X-ray binary of the sort that has been cataloged in the astronomical literature.
Such speculations are on the far edge of SETI (and again, I refer you to Vidal’s book), but it’s also true that whether or not extraterrestrial civilizations exist, our own ability to chart futures for an expanding civilization may well come in handy if we can somehow punch through whatever ‘great filter’ may be out there and become a species that survives on the scale of deep time. There is no knowing whether this is even possible, and it may be that the galaxy is filled with the ruins of those who have gone before us.
It is also true, of course, that no one may have gone before us. Maybe N really does equal 1. But I return to Barrow: “The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.” And the human imagination is currency of the realm in matters like these.
The Vidal paper is “Black Holes: Attractors for Intelligence?” presented at the Kavli Royal Society International Centre, “Towards a scientific and societal agenda on extra-terrestrial life”, 4-5 Oct 2010 (abstract). The Dyson paper is “Time Without End: Physics and Biology in an Open Universe,” Review of Modern Physics 51: 447-460 (abstract). My eulogy for Barrow is On John Barrow. John Smart contributed a fascinating essay on cosmic evolution in these pages in The Goodness of the Universe.
Talking to Starglider
When we’ve discussed interstellar ‘interlopers’ like ‘Oumuamua and 2I/Borisov, the science fiction-minded among us have now and then noted Arthur Clarke’s Rendezvous with Rama (Gollancz, 1973). Although we’ve yet to figure out definitively what ‘Oumuamua is (2/I Borisov is definitely a comet), the Clarke reference is an imaginative nod to the possibility that one day an alien craft might enter our Solar System during a gravitational assist maneuver and be flung outward on whatever its mission was (in Rama’s case, out in the direction of the Large Magellanic Cloud).
Since we’ll never see ‘Oumuamua again, we wait with great anticipation the work of the Legacy Survey of Space and Time (LSST), which will be run via the Vera Rubin Telescope (first light in 2025). Estimates vary widely but the consensus seems to be that with a telescope capable of imaging the entire visible sky in the southern hemisphere every few nights, the LSST should produce more than a few interstellar objects, perhaps ten or more, every year. We probably won’t find a Rama, but who knows?
Meanwhile, I’m reminded of another Clarke novel that rarely gets the attention in this regard that Rendezvous with Rama does. This is 1979’s The Fountains of Paradise (BCA/Gollancz). Although known primarily for its exploration of space elevators (and its reality-distorting geography), the novel includes as a separate theme another entry into the Solar System, this time by a craft that, unlike Rama, is willing to take notice of us. Starglider is its name, and it represents a civilization that is cataloging planetary systems through probes scattered across a host of nearby stars.
Starglider has a 500 kilometer antenna to communicate with its home star (humans name this Starholme), and in the words of a report on its activities within the novel, it more or less ‘charges its batteries’ each time it makes a close stellar pass. Having explored the Alpha Centauri trio, its next destination after the Sun is Tau Ceti. The game plan is that each stellar encounter will gather data and open communications with any civilization found there as a precursor to long-term radio contact and, presumably, entry into some kind of interstellar information network.
This is rather fascinating. For Starglider is smart enough to have studied human languages and is able to converse, after a fashion. From the novel:
It was obvious from its first messages that Starglider understood the meaning of several thousand basic English and Chinese words, which it had deduced from an analysis of television, radio, and especially broadcast video-text services. But what it had picked up during its approach was a very unrepresentative sample from the whole spectrum of human culture; it contained little advanced science, still less advanced mathematics, and only a random selection of literature, music, and the visual arts.
Like any self-taught genius,therefore, Starglider had huge gaps in its education. On the principle that it was better to give too much than too little, as soon as contact was established, Starglider was presented with the Oxford English Dictionary, the Great Chinese Dictionary (Mandarin edition), and the Encyclopedia Terrae. Their digital transmission required little more than fifty minutes, and it was notable that immediately thereafter Starglider was silent for almost four hours — its longest period off the air. When it resumed contact, its vocabulary was immensely enlarged, and more than ninety-nine percent of the time it could pass the Turing test with ease — that is, there was no way of telling from the messages received that Starglider was a machine, and not a highly intelligent human.
Clarke slyly notes the cultural differences between species as opposed to the commonality of, say, mathematics, saying that Starglider had little comprehension of lines like this from Keats:
Charm’d casements, opening on the foam
Of perilous seas, in faery lands forlorn…
And it drew a blank on Shakespeare as well:
Shall I compare thee to a summer’s day?
Thou art more lovely and more temperate…
Well, these are aliens, after all. We have enough trouble with cross-cultural references here on Earth. Humans broadcast thousands of hours of music and video drama to Starglider to help it out, but here, of course, we run into the messaging problem. Just how much do we want to reveal of ourselves to a culture about which we have all too little information other than that it is markedly more advanced than our own? You’ll find that aspect of the METI debate explored as a core part of the Starglider subplot.
Some have panned Starglider’s appearance in the novel because it seems intrusive to the plot (although I suppose I could argue that autonomous probes cataloging stellar systems almost have to be intrusive to get their job done). But in the midst of the Starglider passages, we learn that the chatty aliens, now freely talking to humans via radio, catalog the civilizations they find on a scale based on their technological accomplishments. Is this Clarke channeling Nikolai Kardashev?
Whatever the case, Clarke as always takes the long view, and the long view by its very nature always pushes out into mystery. Consider the scale used by Starglider:
I. Stone Tools
II. Metals, fire
III. Writing, handicrafts, ships
IV. Steam power, basic science
V. Atomic energy, space travel
VI. “…the ability to convert matter completely into energy, and to transmute all elements on an industrial scale.”
On this scale of one through six we can place our species at level 5, as Starglider sees us. But are there further levels? Clarke is wise to imply their existence without exploring it any further, as this lets the reader’s imagination do the job. He’s expert at this:
“And is there a Category Seven?” Starglider was immediately asked. The reply was a brief “Affirmative.” When pressed for details, the probe explained: “I am not allowed to describe the technology of a higher-grade culture to a lower one.” There the matter remained, right up to the moment of the final message, despite all the leading questions designed by the most ingenious legal brains of Earth.
When the University of Chicago’s Department of Philosophy transmits the whole of Thomas Aquinas’ Summa Theologica to Starglider, all hell breaks loose. I turn you to the novel for more.
Image; Hubble took this image on Oct. 12, 2019, when comet 2I/Borisov was about 418 million kilometers from Earth. The image shows dust concentrated around the nucleus, but the nucleus itself was too small to be seen by Hubble. We are on the cusp of a windfall of ‘interstellar interloper’ data as the LSST comes online within a few years. Will we ever find a Rama, or a Starglider, amidst our observations? Credit: NASA, ESA and D. Jewitt (UCLA).
As I mentioned, some critics fault The Fountains of Paradise for Starglider’s very presence, noting that there are essentially two plots at work here. In fact there are in fact three plots taking place on different timescales here, one of them dating back several thousand years, and recall that the voyage of Starglider itself spans millennia, the mission having began some 60,000 years before the events of the main part of the novel – construction of the space elevator – take place. This kind of chronological juggling, allows Clarke to inspire deeper reflection on humanity’s place in the universe and I find it enormously effective.
Wonders fairly pop out of Clarke’s early novels and much of his later work. On that score, I likewise refuse to fault him severely because he cannot achieve complex characterization. A case can be made (James Gunn makes it strongly) that science fiction of Clarke’s ilk needs to put the wonder first. Rich, strange and complicated characters confronting rich, strange and wondrous events may lead to one richness too many. For we, the readers, to absorb the mystery, we need to see how a relatively straightforward character reacts. It’s that contrast that Clarke aims to mine.
That’s only one way of doing science fiction, but much science fiction of the 1950s, which I consider the genre’s true golden age (with a nod to the late 1930s, as one must) often operated with precisely this conceit. And that’s okay, because when writers of greater literary style began to emerge – writers like Alfred Bester, say, with his staggering The Stars My Destination (1956) we were able to see complex characters confronting the deeply strange in ways that simply added depth to the experience. Look at Robert Silverberg in the 1960s as an exemplar of an almost magical insight into what makes the individual human tick. Once you’ve begun on that journey, the field is altered forever, but that doesn’t negate its rich past.
In fact, none of this subsequent growth nullifies Clarke’s accomplishment in the realm of big ideas. Consider him a writer of a kind of SF that flourished and fed a mighty stream into what has now become a river of wildly untamed ideas and insights. And sometimes only Clarke will do. Thus when i read, for the umpteenth time, The City and the Stars, I’m again dazzled by the very title, and the first few pages take me back into a realm where there are suns not quite our own casting a numinous glow over landscapes we learn to navigate through characters who learn with us. Like Stapledon’s, like Asimov’s, Clarke’s is a voice we’ll celebrate deep into the future.
A Resonant Sub-Neptune Harvest at HD 110067
The ancient notion of the ‘music of the spheres’ sounds primitive until you learn something about planetary dynamics. Gravity is wondrous and can nudge planets in a given system into orbits that show an obvious mathematical ratio. Two planets in resonance can emerge, for instance, in a 2:1 ratio, where one goes around its star twice in the time it takes the second to orbit it once. Such linkages might seem almost coincidental to the casual observer until the coincidences begin to pile up.
In the exoplanet system at HD 110067, for example, resonance flourishes, so much so that we have six planets moving in a ‘resonance chain.’ No coincidence here, just gravity at work, although an actual coincidence is that just when I finished a post highlighting system dynamics in closely packed environments like TRAPPIST-1 as a ‘brake’ on inbound comets, an international team should reveal HD 110067’s resonance chain. It’s a beauty, for all six planets not only move in harmonic rhythm but also turn out to be transiting worlds. An orbital dance this complex is rare, but even more so is the ability to study such worlds thanks to the happenstance of our viewing angle.
Transits allow us to extract information, and plenty of it, including analysis of planetary atmospheres as light from the central star passes through them. Because complex resonances are in some sense ‘self-correcting,’ they tell us something about the history of the system, for planet migration during the period when the resonance is being established influences the final state of the system. In HD 110067 we have a mother lode of system harmonics around a star that, usefully enough, is fifty times brighter than TRAPPIST-1, where we have seven rocky planets in a resonant chain.
HD 110067 offers up all of this for that highly interesting category of planets called ‘sub-Neptunes,’ about which we’d like to know a lot more. 100 light years away in the constellation Coma Berenices, HD 110067’s resonance chain is obviously complex. The innermost planet makes three orbital revolutions as the second world makes two – a 3:2 resonance. But the chain continues: 3:2, 3:2, 3:2, 4:3, and 4:3, with the innermost planet making six orbits as the outermost planet completes one.
Image: A rare family of six exoplanets has been unlocked with the help of ESA’s Cheops mission. The planets in this family are all smaller than Neptune and revolve around their star HD110067 in a very precise waltz. When the closest planet to the star makes three full revolutions around it, the second one makes exactly two during the same time. This is called a 3:2 resonance. The six planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3, and 4:3, resulting in the closest planet completing six orbits while the outermost planet does one. CHEOPS confirmed the orbital period of the third planet in the system, which was the key to unlocking the rhythm of the entire system. This is the second planetary system in orbital resonance that CHEOPS has helped reveal. The first one is called TOI-178. Credit and copyright: ESA.
Untangling this particular chain was not easy. The astronomers used data from both ESA’s CHEOPS mission and the TESS space observatory to nail down the system architecture. Data from TESS determined the orbital periods of the innermost worlds to be 9 and 14 days. Observations from CHEOPS tagged planet d at 20.5 days and thus demonstrated that while the innermost planet revolves 9 times around the star, the second revolves six, and the third planet four times. The periods of the three outer planets could then be deduced as 31, 41 and 55 days respectively, with further analysis of the TESS data showing that no solution other than the 3:2, 3:2, 3:2, 4:3, 4:3 chain would work. Ground-based observations supplemented the TESS and CHEOPS data.
The analysis was led by Rafael Luque (University of Chicago) and published in Nature. Says Luque:
“This discovery is going to become a benchmark system to study how sub-Neptunes, the most common type of planets outside of the solar system, form, evolve, what are they made of, and if they possess the right conditions to support the existence of liquid water in their surfaces.”
TOI-178 offers a five-planet resonance chain that may include a sixth world in this system of transiting planets in the constellation Sculptor, some 200 light years out. The paper on HD 110067 takes note of the fact that resonant architectures like these imply a situation that has remained unchanged since the birth of the system, making them useful laboratories for planet formation and evolution. The planetary radii at HD 110067 range from 1.94 that of Earth to 2.85 times as large (1.94R⊕ to 2.85R⊕), and the low densities found in the three planets whose mass has been measured point to the likelihood of large atmospheres dominated by hydrogen.
Image: Tracing a link between two neighbor planets at regular time intervals along their orbits creates a pattern unique to each couple. The six planets of the HD110067 system create together a mesmerizing geometric pattern due to their resonance-chain. © CC BY-NC-SA 4.0, Thibaut Roger/NCCR PlanetS.
Ann Egger (a graduate student at the University of Bern and a co-author of the paper on this work) notes what is ahead in the study of this system:
“The sub-Neptune planets of the HD110067 system appear to have low masses, suggesting they may be gas- or water-rich. Future observations, for example with the James Webb Space Telescope (JWST), of these planetary atmospheres could determine whether the planets have rocky or water-rich interior structures.”
The sheer beauty of the HD 110067 system comes across in the animation below:
Image: To-scale animation of the orbits of the six resonant planets in the HD110067 system. The pitch of the notes played when each planet transits matches the resonant change in orbital frequencies between each subsequent planet. The relative sizes of the planets is accurate, although their true size compared to the star is much smaller. Also available at https://www.youtube.com/watch?v=2rrODAG7nmI.
The paper is Luque et al., “A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067,” Nature 623 (November 29, 2023), 932-937 (abstract).
Cometary Impacts: Looking for Life in the Right Places
If you had to choose, which planetary system would you gauge most likely to house a life-bearing planet: Proxima Centauri or TRAPPIST-1? The question is a bit loaded given that there are seven TRAPPIST-1 planets, hence a much higher chance for success there than in a system that (so far) has produced evidence for only two worlds. But there are other factors having to do with the delivery of prebiotic materials by comet, which is the subject of a new paper from Richard Anslow (Cambridge Institute of Astronomy). “It’s possible that the molecules that led to life on Earth came from comets,’’ Anslow reminds us, “so the same could be true for planets elsewhere in the galaxy.”
So let’s untangle this a bit. We don’t know whether comets are vital to the origin of life on Earth or any other world, and Anslow (working with Cambridge colleagues Amy Bonsor and Paul B. Rimmer) does not argue that they are. What their paper does is to examine the environments most likely to be affected by cometary delivery of organics, which in turn could be useful as we begin to study exo-atmospheres for biosignatures. If we can narrow the kind of systems where cometary delivery is likely, that could explain future findings of life signs as opposed to systems without such mechanisms, ultimately supporting the comet delivery model.
Image: Comets contain elements such as water, ammonia, methanol and carbon dioxide that could have supplied the raw materials that upon impact on early Earth would have yielded an abundant supply of energy to produce amino acids and jump start life. Credit: Lawrence Livermore National Laboratories.
The idea of delivering life-promoting materials by impacts is hardly new. We’ve learned from asteroid return samples like those from Ryugu and now Bennu that the inventory of prebiotic molecules is rich. We’ve also found intact amino acids in meteorite samples, showing the survival of these materials from their entry into the atmosphere. The authors point out that comets contain what they call ‘prebiotic feedstock molecules’ like hydrogen cyanide (HCN) along with basic amino acids, and some studies suggest that by way of comparison with asteroids, comets have delivered two orders of magnitude more organic materials than meteorites because of their high carbon content. But survival is the key, and that involves impact velocity upon arrival.
HCN is particularly useful to consider because its strong carbon-nitrogen bonds may make it more likely to survive the high temperatures of atmospheric entry. What Anslow and team call the ‘warm comet pond’ scenario demands a relatively soft landing. This is interesting, so let me quote the paper on it. A ‘soft landing’:
…excavates the impact point and forms a dirty pond from the cometary components. Climatic variations are thought to cause the episodic drying of these ponds, promoting the rapid polymerisation of constituent prebiotic molecules. It is thought this wet-dry cycling will effectively drive the required biogeochemical reactions crucial for RNA production on the early-Earth, and therefore play an important role in the initial emergence of life… Relatively high concentrations of prebiotic molecules are required for there to be sufficient polymerisation, and so this scenario still requires low-velocity impacts. Specific prebiotic molecules are more (or less) susceptible to thermal decomposition by virtue of their molecular structure, and so the inventory of molecules that can be effectively delivered to a planet is very sensitive to impact velocity.
The question that emerges, once we’ve examined the delivery of molecules like HCN, is what kind of stellar system is most likely to benefit from a cometary delivery mechanism? To address this, the authors construct an idealized planetary system with planets of equal mass that are equally spaced to study the minimum impact velocity that can emerge on the innermost habitable planet. They then use N-body simulations to model the necessary interactions between comets and planets in terms of their position and velocity through time. The snowline marks the boundary between rocky and volatile-rich materials in the disk, as below:
Image: This is Figure 1 from the paper. Caption: Schematic diagram of the idealised planetary system considered in this work with equally spaced planets (brown circles, semi-major axis ai ) scattering comets (small dark blue circles) from the snow-line. The blue region represents the volatile-rich region of the disc where comets occur, and the green region represents the habitable zone. Low velocity cometary impacts onto habitable planets will follow the lower arrows, which sketch the dynamically cold scattering between adjacent planets. The dynamically hot scattering as shown by the upper arrows, will result in high velocity impacts.
That equal spacing of planets is interesting. The authors call it a ‘peas in a pod’ system and note what other astronomers have observed, that “…individual exoplanet systems have much smaller dispersion in mass, radius, and orbital period in comparison to the system-to-system variation of the exoplanet population as a whole.” And indeed, tightly packed systems with equal and low-mass planets have been shown to be highly efficient at scattering comets into the inner system and hence into the habitable zone. Giant outer planets, it’s worth noting, may form in these tight systems, their effects helping to scatter comets inward, but they are not assumed in the author’s model.
The impactor’s size and velocity tell the tale. What we’d like to see is a minimum impact velocity below 15 kilometers per second to ensure the survival of the interesting prebiotic molecules like HCN. The simulations show that the impact velocity around stars like the Sun is reduced for lower mass planets, the effect being enhanced if there are planets in nearby orbits. Impact velocities drop even more for planets around low mass stars in tightly-packed systems (here again, think TRAPPIST-1), for here the comets tend to be delivered on low eccentricity orbits, a significant factor because impact speeds around low-mass stars are typically high. From the paper:
…the results of our N-body simulations demonstrate that the overall velocity distribution of impactors onto habitable planets is very sensitive to both the stellar-mass and planetary architecture, with the fraction of low-velocity impacts increasing significantly for planets around Solar-mass stars, and in tightly-packed systems. It will be these populations of exoplanets where cometary delivery of prebiotic molecules is most likely to be successful, with significant implications for the resulting prebiotic inventories due to the exponential decrease in survivability with impact velocity.
We learn from all this that we have to be attuned to the mass of the host star and the nature of planetary distribution there to be able to predict whether or not comets can effectively deliver prebiotic materials to worlds in the habitable zone. If this seems purely theoretical, consider that telescope time on future missions to study exoplanet atmospheres will be a precious commodity, and these factors may emerge as an important filter for observation. But we’ll also find out whether the correlations the authors have uncovered re lower mass, tightly packed planets are demonstrated in the presence of the biosignatures we are looking for. That may tell us whether comets are a significant factor for life’s emergence on distant worlds as well as our own.
The paper is Anslow, Bonsor & Rimmer. “Can comets deliver prebiotic molecules to rocky exoplanets?” Proceedings of the Royal Society A (2023). Full text. Thanks to my friend Antonio Tavani for the pointer to this work.
Tightening Proxima Centauri’s Orbit (and an Intriguing Speculation)
Although I think most astronomers have assumed Proxima Centauri was bound to the central binary at Alpha Centauri, the case wasn’t definitively made until fairly recently. Here we turn to Pierre Kervella (Observatoire de Paris), Frédéric Thévenin (Côte d’Azur Observatory) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève). We last saw Dr. Kervella with reference to a paper on aerographite as a sail material, but his work has appeared frequently in these pages, analyzing mission trajectories and studying the Alpha Centauri system. Here he and his colleagues use HARPS spectrographic data to demonstrate that we have at Centauri a single gravitationally bound triple system. This is important stuff; let me quote the paper on this work to explain why (italics mine):
Although statistical considerations are usually invoked to justify that Proxima is probably in a bound state, solid proof from dynamical arguments using astrometric and radial velocity (RV) measurements have never been obtained at a sufficient statistical significance level. As discussed by Worth & Sigurdsson (2016), if Proxima is indeed bound, its presence may have impacted planet formation around the main binary system.
This is a six-year old paper, but I want to return to it now because a new paper from the same team will tighten up its conclusions and slightly alter some of them. We’ve gone from resolving whether Proxima is bound to the A/B binary to pondering the issues involved in the dynamical history of this complex system. That in turn can inform the ongoing search for planets around Centauri A and B at least in terms of explaining what we might find there and how these two systems evolved. The original paper on this work lays out the challenges involved in tracing the orbit of the red dwarf. For HARPS is exquisitely sensitive to the Doppler shifts of starlight, and these data, obtained between 2004 and 2016, contain potential booby traps for analysis.
Image: Pierre Kervella, of the Observatoire de Paris/PSL.
Convective blueshift is one of these. We’re looking at the star’s spectral lines as we calculate its motion, and some of these are displaced toward the blue end of the spectrum because of the structure of its surface convection patterns. The lifting and sinking of hot internal gases has to be factored into the analysis and its effect nulled out. The spectral lines are displaced toward the blue, in effect a negative radial velocity shift, although the effect is stronger for hotter stars. In the case of Proxima, Kervella’s team finds a relatively small convective blueshift, though still one to be accounted for.
A similar though more significant issue is gravitational redshift, which occurs as photons climb out of the star’s gravity well. Here the effect is “an important source of uncertainty on the RV of Proxima” whose value can be established and corrected. How the astronomers went about making these corrections is laid out in a discussion of radial velocities that aspiring exoplanet hunters will want to read.
Image: Orbital plot of Proxima showing its position with respect to Alpha Centauri over the coming millenia (graduations in thousands of years). The large number of background stars is due to the fact that Proxima is located very close to the plane of the Milky Way. Credit: P. Kervella/ESO/Digitized Sky Survey 2/Davide De Martin/Mahdi Zamani.
Out of all this we learn that Proxima’s elliptical orbit around Centauri A and B’s barycenter extends from 800 billion kilometers when closest (periastron) to 1.9 trillion kilometers at apastron, its farthest distance, with an orbital period of approximately 550,000 years. The orbital phase is currently closest to apastron.
The Astronomy & Astrophysics site (this is the journal in which the paper above appeared) is currently down, so I’m quoting from the version of the paper on arXiv, which after noting that the escape velocity of Alpha Centauri at Proxima’s distance (545 +/- 11 m/s) is about twice as large as Proxima’s measured velocity, goes on to speculate in an intriguing way:
Proxima could have played a role in the formation and evolution of its planet (Anglada-Escudé et al. 2016). Conversely, it may also have influenced circumbinary planet formation around αCen (Worth & Sigurdsson 2016). A speculative scenario is that Proxima b formed as a distant circumbinary planet of the αCen pair, and was subsequently captured by Proxima. Proxima b could then be an ocean planet resulting from the meltdown of an icy body (Brugger et al. 2016). This would also mean that Proxima b may not have been located in the habitable zone (Ribas et al. 2016) for as long as the age of the αCen system (5 to 7 Ga; Miglio & Montalbán 2005; Eggenberger et al. 2004; Kervella et al. 2003; Thévenin et al. 2002).
The idea of Proxima b as a captured planet has not to my knowledge appeared anywhere else in the literature. I was fascinated, enough so that I dashed off a quick email to Dr. Kervella asking about this as well as the current status of the orbital calculations. And indeed, his response indicates new work in progress:
… we identified a mistake in our 2017 determination of the orbital parameters of Proxima. In the papier, they are expressed in the Galactic coordinate system, and the orbital inclination is thus not directly comparable to that of the Alpha Cen AB orbit. We are preparing a new publication with revised orbits and parameters for all three stars. The main difference is that now the orbital plane of Proxima is better aligned with that of AB. The gravitationally bound nature of Proxima with Alpha Cen AB is also strengthened, as we include new astrometry and radial velocities.
I’ll cover the new paper as soon as it appears. Dr. Kervella also observes that confirming a scenario of Proxima b as a captured planet would be difficult (Proxima b has ‘forgotten’ the history of its orbital evolution, as he puts it), meaning that working with astrometric data alone will not be sufficient. But the arrival of telescopes like the Extremely Large Telescope, now under construction in Chile’s Atacama Desert with first light planned for 2028, should signal a treasure trove of new information. A spectrum obtained by ELT could show us whether Proxima b is indeed an ocean planet.
The paper on Proxima Centauri’s orbit is Kervella, Thévenin & Lovis, “Proxima’s orbit around α Centauri,” Astronomy & Astrophysics Vol. 598 (February 2017), L7 (abstract/preprint).
The Odds on Alpha Centauri
How extraordinary that the nearest star to Earth is actually a triple system, the tight central binary visually merged as one bright object, the third star lost in the background field but still a relatively close 13000 or so AU from the others. Humans couldn’t have a better inducement to achieve interstellar flight on the grounds of these stars alone. We get three stellar types: The G-class Centauri A, the K-class Centauri B, both of which are capable of hosting planets, perhaps habitable, of their own.
And then we have Proxima Centauri, opening up M-class red dwarf stars to close investigation, and we already know of a planet in the habitable zone there, adding to the zest of the venture. If extraterrestrial beings in a system like this would have even more inducement to travel, with another star’s planets perhaps as close to them as our own system’s worlds are to us, we humans are also spurred to undertake a journey, because 4.2 light years is a mere stone’s throw in the overall galactic distribution.
Image: The central binary at Alpha Centauri, with the two stars only resolved in the x-ray image. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zdenek Bardon/ESO.
I like this image, used by Dirk Schulze-Makuch to illustrate a recent popular science article, because it includes the Chandra X-Ray imagery. That’s how we can separate the central stars, which are at times nearly as close as Saturn is to the Sun while they orbit their common barycenter. Centauri Dreams readers will recognize Schulze-Makuch (Technical University Berlin and an adjunct professor at Washington State) not only as a prolific writer but the author of a host of scientific papers including many we’ve looked at in these pages. He’s played a valuable role in presenting astrobiological matters to the general public, part of the flowering of interstellar investigation that continues as we keep finding interesting worlds to explore.
If you’re wondering about Proxima Centauri’s location, the image below flags it. Credit: ESO/B. Tafreshi (twanight.org)/Digitized Sky Survey 2; Acknowledgement: Davide De Martin/Mahdi Zamani).
I like to keep an eye on what appears in the popular press from respected scientists, because they’re bringing credibility to matters that often get distorted by mainstream media attention (not to mention what happens on social media sites). We should always give a nod to scientists willing to explain their work and the broader issues involved given that kind of competition for the public’s attention. It’s interesting in this case to get Schulze-Makuch’s take on habitability at Alpha Centauri. He’s pessimistic about Proxima but is surprisingly bullish on Centauri A and B:
The other two stars in the system are believed to have planets, although they have not been confirmed. (A possible Neptune-size planet was reported in 2021 orbiting Alpha Centauri A at roughly the same distance as Earth orbits the Sun, but this could turn out to be a dust cloud instead.) The apparent lack of any brown dwarfs or gas giants close to Alpha Centauri A and B make the likelihood of terrestrial planets greater than it would be otherwise, at least in theory. The chances of a rocky, potentially habitable planet in our neighboring solar system might therefore be as high as 75 percent.
The Proxima Centauri problem is, of course, the X-ray flux, although Schulze-Makuch also considers tidal lock a distinct negative. The Chandra data (citation below) revealed a relatively benign influx of X-rays for Centauri A and B, making them fine hosts for life if it can develop there. But Proxima is deeply problematic, receiving an average dose of X-rays some 500 times greater than Earth’s, and some 50,000 times as great during periods of flare activity, which M-dwarfs are particularly prone to in their younger days.
Here the word ‘younger’ is a bit deceptive. Recall that this kind of star can live for several trillion years. That’s a bit humbling, considering that the universe itself is thought to be 13.8 billion years old. In that sense all M-dwarfs are ‘young.’
Just as we can zoom in via X-ray to see the central stars, we can also take a look at Proxima Centauri’s movements via spectroscopic data, which we’ll examine next time, along with a fascinating speculation on the origin of Proxima b.
For more on the X-ray environment at Alpha Centauri, see Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (January, 2018), 17 (abstract). The possibility of a ‘warm Neptune’ at Alpha Centauri is discussed in Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12, Article number: 922 (2021). Full text. We’ll be talking about this one a bit more in coming days.
Follow with RSS or E-Mail
