Centauri Dreams
Imagining and Planning Interstellar Exploration
A New Class of Astronomical Transients
Some of the fastest outflows in nature are beginning to turn up in the phenomena known as Fast Blue Optical Transients (FBOTs). These are observed as bursts that quickly fade but leave quite an impression with their spectacular outpouring of energy. The transient AT2018cow was found in 2018, for example, in data from the ATLAS-HKO telescope in Hawaii, an explosion 10 to 100 times as bright as a typical supernova that appeared in the constellation Hercules. It was thought to be produced by the collapse of a star into a neutron star or black hole.
Now we have a new FBOT that is brighter at radio wavelengths than AT2018cow, the third of these events to be studied at radio wavelengths. The burst occurred in a small galaxy about 500 million light years from Earth and was first detected in 2016. Let’s call it CSS161010 (short for CRTS-CSS161010 J045834-081803), and note that it completely upstages its predecessors in terms of the speed of its outflow. The event launched gas and particles at more than 55 percent of the speed of light. Such FBOTs, astronomers believe, begin with the explosion of a massive star, with differences from supernovae and GRBs only showing up in the aftermath.
Deanne Coppejans (Northwestern University) led the study:
“This was unexpected. We know of energetic explosions that can eject material at almost the speed of light, specifically gamma-ray bursts, but they only launch a small amount of mass — about 1 millionth the mass of the sun. CSS161010 launched 1 to 10 percent the mass of the sun at more than half the speed of light — evidence that this is a new class of transient.”
Image: Keck’s view of where the CSS161010 explosion (red circle) occurred in a dwarf galaxy. Credit: Giacomo Terreran/Northwestern University.
Meanwhile, a second explosion, called ZTF18abvkwla (“The Koala”), has turned up in a galaxy considerably further out at 3.4 billion light years. Caltech’s Anna Ho led the study of this one, with both teams gathering data from the Very Large Array, the Giant Metrewave Radio Telescope in India and the Chandra X-ray Observatory. In both cases, it was clear that the type of explosion, bright at radio wavelengths, differed from both supernovae explosions and gamma-ray bursts. “When I reduced the data,” said Ho, “I thought I had made a mistake.”
FBOTs became recognized as a specific class of object in 2014, but the assumption is that our archives contain other examples of what Coppejans’ co-author Raffaella Margutti calls ‘weird supernovae,’ a concession to the fact that it is hard to gather information on these objects solely in the optical. The location of the CSS161010 explosion is a dwarf galaxy containing roughly 10 million stars in the southern constellation Eridanus.
Bright FBOTs like CSS161010 and AT2018cow have thus far turned up only in dwarf galaxies, which the authors note is reminiscent of some types of supernovae as well as gamma-ray bursts (GRBs). A transient like this flares up so quickly that it may prove impossible to pin down its origin,, but black holes and neutron stars are prominent in the astronomers’ thinking:
“The Cow and CSS161010 were very different in how fast they were able to speed up these outflows,” Margutti said. “But they do share one thing — this presence of a black hole or neutron star inside. That’s the key ingredient.”
Even so, the differences between the three FBOTs thus far studied at multiple wavelengths is notable. In the excerpt below, the authors of the Coppejans paper use the term ‘engine-driven’ to refer to the rotating accretion disk that produces jets in a neutron star or black hole produced by a supernova core-collapse, which can propel narrow jets of material outward in opposite directions. The authors believe that FBOTs produce this kind of engine, but in this case one surrounded by material shed by the star before it exploded. The surrounding shell as it is struck by the blast wave would be the source of the FBOT’s visible light burst and radio emission.
From the paper:
The three known FBOTs that are detected at radio wavelengths are among the most luminous and fastest-rising among FBOTs in the optical regime… Intriguingly, all the multi-wavelength FBOTs also have evidence for a compact object powering their emission… We consequently conclude… that at least some luminous FBOTs must be engine-driven and cannot be accounted for by existing FBOT models that do not invoke compact objects to power their emission across the electromagnetic spectrum. Furthermore, even within this sample of three luminous FBOTs with multiwavelength observations, we see a wide diversity of properties of their fastest ejecta. While CSS161010 and ZTF18abvkwla harbored mildly relativistic outflows, AT 2018cow is instead non-relativistic.
Which is another way of saying that we have a long way to go to understand FBOTs. We see characteristics of supernovae as well as GRBs but distinctive differences. Further observations in radio and X-ray wavelengths are critical for learning more about their physics.
Image: Artist’s conception of the new class of cosmic explosions called Fast Blue Optical Transients. Credit: Bill Saxton, NRAO/AUI/NSF.
The first paper is Coppejans, Margutti et al., “A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy,” Astrophysical Journal Letters Vol. 895, No. 1 (26 May 2020). Abstract.
On the FBOT ZTF18abvkwla, see Ho et al., “The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z = 0.27,” Astrophysical Journal Vol. 895, No. 1 (26 May 2020). Abstract.
Star Formation and Galactic Mergers
Our galaxy is 10,000 times more massive than Sagittarius, a dwarf galaxy discovered in the 1990s. But we’re learning that Sagittarius may have had a profound effect on the far larger galaxy it orbits, colliding with it on at least three occasions in the past six billion years. These interactions would have triggered periods of star formation that we can, for the first time, begin to map with data from the Gaia mission, a challenge tackled in a new study in Nature Astronomy.
The paper in question, produced by a team led by Tomás Ruiz-Lara (Instituto de Astrofísica de Canarias, Tenerife), argues that the influence of Sagittarius was substantial. The data show three periods of increased star formation, with peaks at 5.7 billion years ago, 1.9 billion years ago and 1 billion years ago, corresponding to the passage of Sagittarius through the Milky Way disk.
The work is built around Gaia Data Release 2, examining the photometry and parallax information combined with modeling of observed color-magnitude diagrams to build a star formation history within a bubble around the Sun with a radius of 2 kiloparsecs (about 6500 light years). The star formation ‘enhancements,’ as the paper calls them, are well-defined, though with decreasing strength, with a possible fourth burst spanning the last 70 million years
Ruiz-Lara sees the disruption caused by Sagittarius as substantial, a follow-on to an earlier merger:
“At the beginning you have a galaxy, the Milky Way, which is relatively quiet. After an initial violent epoch of star formation, partly triggered by an earlier merger as we described in a previous study, the Milky Way had reached a balanced state in which stars were forming steadily. Suddenly, you have Sagittarius fall in and disrupt the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples on the water.”
Image: The Sagittarius dwarf galaxy has been orbiting the Milky Way for billions for years. As its orbit around the 10,000 times more massive Milky Way gradually tightened, it started colliding with our galaxy’s disc. The three known collisions between Sagittarius and the Milky Way have, according to a new study, triggered major star formation episodes, one of which may have given rise to the Solar System. Credit: ESA.
The idea is that higher concentrations of gas and dust are produced in some areas as others empty, the newly dense material triggering star formation. According to the paper, the 2 kiloparsec local volume is:
…characterized by an episodic SFH [star formation history], with clear enhancements of star formation ~ 5.7, 1.9 and 1.0 Gyr ago. All evidence seems to suggest that recurrent interactions between the Milky Way and Sgr dwarf galaxy are behind such enhancements. These findings imply that low mass satellites not only affect the Milky Way disk dynamics, but also are able to trigger notable events of star formation throughout its disk. The precise dating of such star forming episodes provided in this work sets useful boundary conditions to properly model the orbit of Sgr and its interaction with the Milky Way. In addition, this work provides important constraints on the modelling of the interstellar medium and star formation within hydrodynamical simulations, manifesting the need of understanding physical processes at subresolution scales and of further analysis to unveil the physical mechanisms behind global and repeated star formation events induced by satellite interaction.
Could the passage of Sagittarius through the Milky Way be behind the Sun’s formation? That seems a stretch given the length of time between the first disruption and the Sun’s formation some 4.6 billion years ago, but co-author Carme Gallart (IAC) doesn’t rule it out:
“The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius. We don’t know if the particular cloud of gas and dust that turned into the Sun collapsed because of the effects of Sagittarius or not. But it is a possible scenario because the age of the Sun is consistent with a star formed as a result of the Sagittarius effect.”
What I learned here is that understanding the physical processes behind star formation and incorporating that understanding into workable models is a problematic issue for astronomers today, because ongoing work is challenging earlier views of what happens when galaxies merge. The paper points out that while we have a number of colliding galaxies to examine, there is little theoretical work on the impact of a single satellite galaxy on a spiral galaxy.
And a key point: “…although we can easily link the reported enhancements with possible perientric passages of Sgr, we cannot pinpoint what exact physical mechanisms are triggering such events.” Plenty of opportunity ahead for researchers looking into the Milky Way’s history.
The paper is Ruiz-Lara et al., “The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way,” published in Nature Astronomy 25 May 2020 (abstract).
On SETI, International Law, and Realpolitik
When Ken Wisian and John Traphagan (University of Texas at Austin) published “The Search for Extraterrestrial Intelligence: A Realpolitik Consideration” (Space Policy, May 2020), they tackled a problem I hadn’t considered. We’ve often discussed Messaging to Extraterrestrial Intelligence (METI) in these pages, pondering the pros and cons of broadcasting to the stars, but does SETI itself pose issues we are not considering? Moreover, could addressing these issues possibly point the way toward international protocols to address METI concerns?
Ken was kind enough to write a post summarizing the paper’s content, which appears below. A Major General in the USAF (now retired), Dr. Wisian is currently Associate Director of the Bureau of Economic Geology, Jackson School of Geosciences at UT. He is also affiliated with the Center for Space Research and the Center for Planetary Systems Habitability at the university. A geophysicist whose main research is in geothermal energy systems, modeling, and instrumentation & data analysis, he is developing a conference on First Contact Protocols to take place at UT-Austin this fall, a follow-on to his recent session at TVIW 2019 in Wichita.
by Ken Wisian
The debate over the wisdom of active Messaging to ExtraTerrestrial Intelligence (METI), has been vigorously engaged for some time. The progenitor of METI and the more accepted passive Search for ExtraTerrestrial Intelligence (SETI) has been largely assumed to be of little or no risk. The reasons for this assumption appear to be:
1. It does not alert ETI to our existence and therefore we should not face a threat of invasion or destruction from aliens (if it is even practical to do so over interstellar distances)
2. The minor Earthbound threat from extremists (of various possible persuasions) who might not like the possibility of ETI’s existence conflicting with their “world view” would be no more than an annoyance.
Implicit in the above is the underlying assumption that the only realistic threat that could arise from METI or SETI is that from a hostile ETI. In other words, the threat is external to humanity. What this too simple reasoning overlooks is human history, particularly international affairs, conflicts and war. [1]
SETI as used here is the passive searching for electromagnetic signals from ETI. This is currently primarily considered to be in the form of radio or laser signal, deliberately sent to somewhere. The search for non-signal evidence (e.g. inadvertent laser propulsion leakage, etc) is not considered here, though it could tie in to the discussion in a distant, indirect manner. Note: an ETI artifact (e.g. a spaceship) could have similar import as a SETI detection discussed here.
So what harm could SETI do? Looking at current and historical international affairs, particularly great-power competition, the answer is readily apparent – competition for dominance. In international affairs, nations compete, sometimes violently, for position in the world. This can be for economic or military advantage, more land or control over the seas, or merely survival. Witness the South China Sea today, stealing the secrets to nuclear weapons in the 1940’s and 1950’s, or the Byzantine Empire engaging in industrial espionage to steal the secret to making silk from China.
Now contemplate the potential technology advances that could come with a SETI detection. This could range from downloading the “Encyclopedia Galactica” to establishing two-way dialogue that includes sharing technology. With the potential for revolutionary science and technology leaps, whether directly destructive or not, to say the great & super powers would be “interested” is a monumental understatement.
Now think about the potential advantage (read as domination-enabling) that could accrue to one country if they were the only beneficiaries of said technology advances. “How?” you ask. “Anyone can point a radio telescope to the sky” Not so fast. Unless the signal comes from within our own galactic back yard, most likely within the Solar System, it will take a relatively large, complicated industrial complex (physical plant) with very specialized personnel to run, in order to send and receive interstellar communications. This is the key fact that could lead to SETI/METI being the next “Great Game” [2] of international affairs.
Large, specialized complexes & associated personnel are limited in number and therefore subject to physical control. For the sake of argument, let’s say there are a dozen such facilities in the world. This is far less than the number of critical infrastructure sites the US and coalition forces decided had to be taken out in Iraq in the Gulf Wars in order to reduce their military capability – a very manageable target set size. Now you begin to see the problem; superpowers, seeing a potentially world-dominating advantage in monopolizing the ETI communication channel, might also see as feasible preserving their access to ETI while at the same time denying the same to all other countries.
While “Great Games” like this can sometimes be kept in the purely political realm, that is relatively rare. Competition of this sort often includes violent espionage or proxy wars and occasionally can escalate to direct super-power competition. Thus, an actual SETI detection could lead rapidly to the first true information war – a superpower war (with all the existential risk that carries) fought purely for control of knowledge.
Monopolizing communication with ETI could be the trigger for the first information-driven world war.
Realization of the risk that even passive SETI presents should drive further actions:
1. The development of realistic and binding international treaties on the handling of first contact protocols – admittedly a long-shot. The existing post-detection protocol is a very small and completely non-binding first step in this direction.
2. Formation of deliberately international SETI facilities with uninterruptible data sharing to partner countries (and/or the UN). These would also have interleaved internal chains of command from member countries. While this would be somewhat inefficient, the offset to risk is well worth the effort. A phase 2 to this would be a similar arrangement for METI. This would implicitly force the adoption of international standards and provide a process for METI.
3. Further (renewed?) discussion and research into SETI risk. This should bring in many disciplines that are often not involved deeply in the SETI/METI fields, from government policy to history to psychology and many others. In staring so hard at the very obvious risk of METI, we missed the risk from SETI alone. We need to turn around and explore that road before proceeding further down the highway to METI.
Notes
[1] What I am getting at here is that unfortunately, this is a stereotypical “ivory tower” point of view, too idealistic and disconnected from messy, illogical human affairs. I say this reluctantly as a “card-carrying” (i.e. Ph.D.) member of the academic world.
[2] I am definitely abusing the term “Great Game” in multiple ways. The term refers to the 19th competition between the British and Russian empires for control of Central and South Asia. It was a deadly serious and deadly game in actuality, but the term captures well the feeling of being in a fierce competition.
PG: Let me insert here this excerpt from the paper highlighting the question of international law and the issues it raises:
The potentially enormous value to nation states of monopolizing communication with ETI, for the purpose of technological dominance in human affairs, is a significant factor in understanding possible scenarios after a confirmed contact event and bears further thinking by scholars and policy specialists. History shows that in circumstances that states perceive as vital they will likely act in their perceived best interest in accordance with principles of realpolitik thinking. In these circumstances, international law is frequently not a strong constraint on the behavior of governments and a protocol developed by scientists on how to handle first contact is unlikely to be of any concern at all. This risk needs to be acknowledged and understood by the larger international community to include scientists active in SETI in addition to political leaders and international relations scholars.
The paper is Wisian and Traphagan, “The Search for Extraterrestrial Intelligence: A Realpolitik Consideration,” Space Policy May, 2000 (abstract).
Astrobiological Science Fiction
I had never considered the possibilities for life on Uranus until I read Geoffrey Landis’ story “Into the Blue Abyss,” which first ran in Asimov’s in 1999, and later became a part of his collection Impact Parameter. Landis’ characters looked past the lifeless upper clouds of the 7th planet to go deep into warm, dark Uranian oceans, his protagonist a submersible pilot and physicist set to explore:
Below the clouds, way below, was an ocean of liquid water. Uranus was the true water-world of the solar system, a sphere of water surrounded by a thick atmosphere. Unlike the other planets, Uranus has a rocky core too small to measure, or perhaps no solid core at all, but only ocean, an ocean that has actually dissolved the silicate core of the planet away, a bottomless ocean of liquid water twenty thousand kilometers deep.
It would be churlish to give away what turns up in this ocean, so I’m going to direct you to the story itself, now available for free in a new anthology edited by Julie Novakova. Strangest of All is stuffed with good science fiction by the likes of David Nordley, Gregory Benford, Geoffrey Landis and Peter Watts. Each story is followed by an essay about the science involved and the implications for astrobiology.
Although I’ve been reading science fiction for decades, our discussions of it in these pages are generally sparse, related to specific scientific investigations. That’s because SF is a world in itself, and one I can cheerfully get lost in. I have to tread carefully to be able to stay on topic. But now and then something comes along that tracks precisely with the subject matter of Centauri Dreams. Strangest of All is such a title, downloadable as a PDF, .mobi or .epub file. I use both a Kindle Oasis and a Kobo Forma for varying reading tasks, and I’ve downloaded the .epub for use on the Forma, but .mobi works just fine for the Kindle.
What we have here is a collaborative volume, developed through the European Astrobiology Institute, containing work by authors we’ve talked about in these pages before because of their tight adherence to physics amidst literary skills beyond the norm. The quote introducing the volume still puts a bit of a chill down my spine:
“…this strangest of all things that ever came to earth from outer space must have fallen while I was sitting there, visible to me had I only looked up as it passed.”
That’s H. G. Wells from The War of the Worlds (1898), still a great read since the first time I tackled it as a teenager. What Novakova wants to do is use science fiction to make astrobiology more accessible, which is why the science commentaries following each story are useful. Strangest of All looks to be a classroom resource for those who teach, part of what the European Astrobiology Institute plans to be a continuing publishing program in outreach and education. We’ve talked before about science fiction’s role as a career starter for budding physicists and engineers.
Gerald Nordley’s “War, Ice, Egg, Universe” takes us to an ocean world with a frozen surface on top, a place like Europa, where the tale has implications for how we approach the exploration of Europa and Enceladus, and perhaps Ganymede as well. In fact, with oceans now defensibly proposed for objects ranging from Titan to Pluto, we are looking at potential venues for astrobiology that defy conventional descriptions of habitable zones as orbital arcs supporting liquid water on the surface. Referring to characters in the story, the EAI essay following Nordley’s tale comments:
Chyba (2000) and Chyba & Phillips (2001) tried to work even with these unknowns and calculate the amount of energy for putative Europan life, and to describe what ecosystems might potentially thrive there. According to these estimates, even a purely surface radiation-driven ecosystem might yield cell counts of over one cell per cubic centimeter; perhaps even a thousand cells per cubic centimeter in the uppermost ocean layers. Putative hydrothermal vents, of course, would create a different source of energy and chemicals for life (albeit one much more difficult to discover – in contrast, life near the icy shell might erupt into space in the geysers and be discovered by “simple” flybys). Any macrofauna, though, seems highly improbable given the energy estimates. Since Loudpincers was about eight times larger than the human Cyndi, by his own account, we’ll really have to look for his civilization elsewhere, perhaps on a larger moon of some warm Jupiter.
You see the method — the follow-up essay explores the ideas, but goes beyond that to provide references for continuing the investigation in the professional literature. This essay also speculates about Ganymede, where a liquid water ocean may be caught between two layers of ice. The ‘club sandwich’ model for Ganymede posits several layers of oceans and ice, which would make Ganymede perhaps the most bizarre ocean-bearing world in the Solar System, one with incredible pressures bearing down on high-pressure ice at the bottom (20 times the pressure of the bottom of the Mariana Trench on Earth).
Gregory Benford’s “Backscatter” is likewise ice-oriented, this time in the remote reaches of the Kuiper Belt and the Oort Cloud beyond. From the essay following the story:
Although it’s difficult to imagine a path from putative simple life in early water-soaked asteroids heated by the radioactive aluminum to vacflowers blooming on the surface of an iceteroid, life in the Kuiper Belt, the Oort Cloud and beyond cannot be ruled out – and we haven’t even touched the issue of rogue planets, which might have vastly varying surface conditions stemming from their size, mass, composition, history and any orbiting bodies.
The essay gives us an overview of the science that, as in Benford’s story, conceives of possible life sustained by sparse inner heat and the presence of ammonia and salts, perhaps with tidal heating thrown in for good measure. Cold brines would demand chemical and energy gradients to sustain life, a difficult thing to discover or measure unless cryovolcano activity coughs up evidence of the ocean below the ice. Some silicon compounds may support a form of life in ice as far out as the Oort, or perhaps in liquid nitrogen. Usefully, the essay on “Backscatter” runs through the scholarship.
The European Astrobology Institute has put together a project team around “Science Fiction as a Tool for Astrobiology Outreach and Education,” out of which has come this initial volume. The references in the science essays make Strangest of All valuable even for those of us who have encountered some of these stories before, for the fiction has lost none of its punch. Thomas Bucknell’s “A Jar of Goodwill” looks at new forms of plant metabolism on a world dominated by chlorine and a key question in addressing alien life: Will we know intelligence when we see it? Peter Watts’ “The Island” looks at Dyson spheres in an astrobiologically relevant form that Dyson himself never thought of (well, he probably did — I bet it’s somewhere in his notebooks).
All told, there are eight stories here along with the essays that explore their implications, an easy volume to recommend given the EAI’s willingness to make the volume available at no cost to readers. See what you think about the Fermi paradox as addressed in D. A. Xiaolin Spires “But Still I Smile.” Plenty of material here for discussion of the sort we routinely do here on Centauri Dreams!
The Odds on Intelligent Life in the Universe
If we could somehow rewind time to the earliest days of the Solar System and start over again, would life — and intelligence — reappear? It’s an experiment science fiction authors are able to try, but it defies real world science. Nonetheless, we can make approaches to the problem through the analysis of probabilities. In particular, we can use statistics, and the technique known as Bayesian inference, which weighs probabilities updated by new evidence.
This is a helpful exercise given that so often I hear people referring to the idea that intelligent life must be everywhere because the universe is so vast and there are so many opportunities for it to arise. But does life inevitably emerge on what we might consider habitable worlds?
What if this process of abiogenesis is rare? The question points to the fact that we have absolutely no idea what the likelihood is, and therefore assumptions about intelligent life based solely on numerical opportunity are nothing but speculations.
Enter Columbia University’s David Kipping, whose work has been featured often in these pages. Kipping tackles the question of the likelihood of life and the development of intelligence in a new paper in Proceedings of the National Academy of Sciences. He has a chronology to work with, one involving life’s earliest appearance as found in fossils, the emergence of humans, and the habitability constraints of our planet’s surface conditions, using it to draw inferences on how quickly life can arise, and how unusual intelligence may be.
The scenarios — derived as what in Bayesian terms are called ‘objective priors’ — are relatively straightforward, each of them worth examining in light of the fact that we have no observational evidence for life beyond the Earth. Our planet is our data point, and with all the disadvantages that produces, we can still draw inferences about life elsewhere from the constraints we can establish here. We know that abiogenesis is possible because we are here to write about it. But Kipping applies statistical methods based on Bayesian mathematics to consider the odds.
The first scenario (and the one I favor): Life is common, but rarely develops intelligence. I suspect we’re going to find evidence for simple life all over the Orion Arm as we extend our technologies outward, but little evidence for technologies. But other scenarios exist: Life is common and so is intelligence. And perhaps abiogenesis is rare. In that case, we may find life unusual but intelligence a common consequence when it does happen. Finally, life may be as rare as intelligence.
Image: Are we alone in the universe? A new study uses Bayesian statistics to weigh the likelihood of life and intelligence beyond our solar system. Credit: Shutterstock/Amanda Carden.
Which scenario to choose? Bayesian techniques involve testing a position against new evidence that can be applied to the question, which allows estimates to get better as they are refined. Bayesian mathematical formulae tackle how to model one scenario against another. And I found the result Kipping arrived at encouraging. Let me quote him on the matter:
“In Bayesian inference, prior probability distributions always need to be selected. But a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one.”
Drawing on our single data point — Earth — Kipping points out that we know life emerged quickly. We have to factor in the impact with the Mars-sized “Theia” some 4.51 billion years ago (leading to the formation of the Moon), but mineralogical evidence from zircons points to an atmosphere and liquid water present on Earth’s surface by roughly 4.4 billion years ago. The earliest evidence for life is found in 4.1 billion year old zircon deposits in the form of depleted carbon inclusions, a controversial datapoint, but undisputed evidence for life turns up in microfossils found in 3.465 billion year old rocks in western Australia.
We can come up, then, with the length of time for which Earth is expected to persist as habitable for intelligent beings, factoring in the growing luminosity of the Sun and the increased rate of weathering of silicate rocks on Earth and eventual depletion of carbon dioxide in the atmosphere. Kipping arrives at a habitable ‘window’ of 5.304 billion years. That’s from the beginning of life to its likely end, and it shows how significant is the question of how fast abiogenesis happens. If it takes too long, life would never emerge under the conditions most planets would face as their star continued to evolve. 900 million years from now, Earth will be a hostile place indeed.
But back to the key result — and I have to send the reader to the paper for the complex Bayesian mathematics involved — Kipping draws on a 2012 paper from Spiegel and Turner to refine the Bayesian formalism produced there for interpreting life’s early emergence on Earth. He considers it against the broader context of the habitable ‘window.’ From the Kipping paper:
The early emergence of life on Earth is naively interpreted as meaning that if we reran the tape, life would generally reappear quickly. But if the timescale for intelligence is long, then a quick start to life is simply a necessary byproduct of our existence—not evidence for a general rapid abiogenesis rate. Using our objective Bayesian framework, we show that the Bayes factor between a fast versus a slow abiogenesis scenario is at least a factor of 3—irrespective of the prior or the timescale for intelligence evolution. This factor is boosted to 9 when we replace the earliest microfossil evidence… with the more disputed 13C-depleted zircon deposits…
Thus the common life scenario gains odds, and markedly so. As for intelligence, Kipping’s analysis precludes the possibility that it emerges quickly (in less than billions of years), while the idea that intelligence is rare remains viable. Even so, he finds betting odds of only 3:2 that intelligence rarely emerges — this slight preference for rare intelligence is consistent with our lack of SETI results but leaves the question of searching for intelligence elsewhere wide open. Life is likely to emerge on other worlds, in other words, but our one data point — Earth — tells us that intelligence emerges only with time and difficulty. We have no outstanding way to choose here one way or another. Let me quote from the paper on this:
…our work supports an optimistic outlook for future searches for biosignatures…The slight preference for a rare intelligence scenario is consistent with a straightforward resolution to the Fermi paradox. However, our work says nothing about the lifetime of civilizations, and indeed the weight of evidence in favor of this scenario is sufficiently weak that searches for technosignatures should certainly be a component in observational campaigns seeking to resolve this grand mystery.
Life commonly found, the prevalence of intelligence still a mystery. Keep looking, but you now have some insight into where to place your chips in your next trip to the astrobiological casino.
If you’re interested in learning about Bayesian inference and the surprising successes of Bayesian analysis, I recommend Sharon McGrayne’s book The Theory That Would Not Die: How Bayes’ Rule Cracked the Enigma Code, Hunted Down Russian Submarines, and Emerged Triumphant from Two Centuries of Controversy (Yale University Press, 2011) as an excellent backgrounder.
The paper is Kipping, “An objective Bayesian analysis of life’s early start and our late arrival,” Proceedings of the National Academy of Sciences 18 May 2020 (full text). You can see Kipping’s lively video presentation on this work at https://www.youtube.com/watch?v=iLbbpRYRW5Y.
TRAPPIST-1: Orbital Alignment Among Rocky Worlds
You would think that the orbits of planets would align closely with the spin of their star, since they emerged from the same primordial disk. Many planets do just that, and in our own system, the orbits of the planets are aligned within 6 degrees of the Sun’s rotation. But the numerous cases of star-planet orbital misalignment around other stars cause us to question whether these systems formed out of alignment or were influenced by later perturbations. A massive companion in a wide orbit could do the trick, and other mechanisms to tilt the orbital or spin axes are discussed in the literature.
To examine the question, the Rossiter-McLaughlin effect comes into play. Discovered by studying binary stars, the effect is named after the two University of Michigan graduate students who figured it out back in the 1920s. They realized that as a star rotates, part of it seems to be coming toward the observer, creating a blueshift, while the other side seems to be moving away, producing a redshift. As a transiting planet blocks part of the background stellar disk, it creates an observable effect in the redshift that flags the direction of the planet’s rotation.
Thus we tease out yet further information about planets we cannot directly see. Now a team of astronomers using the Subaru Telescope has been able to deploy the Rossiter-McLaughlin effect to measure the obliquity, or spin-orbit angle, of three of the planets in the intriguing TRAPPIST-1 system. Here we have an M-dwarf orbited by seven small planets, evidently rocky, with three in or near the habitable zone as defined by the possibility of liquid water on the surface.
What we learn is that the three transiting planets the team observed on August 31, 2018 (two of them near the habitable zone) have an obliquity that is near zero. This marks the first time that stellar obliquity has been measured in a system around a very low-mass star like TRAPPIST-1. Led by Teruyuki Hirano (Tokyo Institute of Technology), the astronomers used the InfraRed Doppler (IRD) spectrograph, a new instrument on the Subaru Telescope, to make the measurement. Says Hirano:
“The data suggest alignment of the stellar spin with the planetary orbital axes, but the precision of the measurements was not good enough to completely rule out a small spin-orbit misalignment. Nonetheless, this is the first detection of the effect with Earth-like planets and more work will better characterize this remarkable exoplanet system.”
Image: Artist’s impression of the TRAPPIST-1 exoplanet system.(Credit: NAOJ).
Learning about spin-orbit misalignment (or the lack of same) is useful as we try to understand how planets around low mass stars evolve. The lack of larger worlds around this star, or the presence of a nearby star, means that the planetary orbits here are probably located close to where the planets first formed, so the orbits offer a window into the early days of the system. Rossiter-McLaughlin analyses have hitherto been restricted to planets of at least Neptune mass, so we’re pushing into new territory as we take advantage of IRD’s high spectral resolution.
Assuming the orbits of the TRAPPIST-1 system are coplanar, the authors offer their take on the system’s evolution:
Our result supports the idea that the known planets in the TRAPPIST-1 system achieved their compact configuration through convergent migration, and did not experience any substantial misaligning torques from processes such as planet-planet scatterings or long-term gravitational perturbations from a massive outer companion on an inclined orbit. It is unlikely that any primordial obliquity has been erased by tidal realignment between the star and these low-mass planets.
And on that assumption of a coplanar system, the authors add that they were unable to test it by measuring the mutual inclinations between the planets:
The mutual inclinations might be measurable in the future using repeated observations of Doppler transits to give a higher S/N. The mutual inclination between two planetary orbits might also be measured by observing the photometric effect of a planet-planet eclipse during a double transit event (Hirano et al. 2012). This would be another important clue to understand the architecture and dynamical history of the TRAPPIST-1 system.
No equivalent studies have been performed on a star this small, but the window is clearly opening on methods for studying the orbital architectures of such planetary systems.
The paper is Hirano et al., “Evidence for Spin-Orbit Alignment in the TRAPPIST-1 System,” Astrophysical Journal Letters Vol. 890, No. 2 (25 February 2020). Abstract.
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