Centauri Dreams
Imagining and Planning Interstellar Exploration
Does Artificial Intelligence Explain the Fermi Question?
Science fiction has been exploring advanced machine intelligence and its consequences for a long time now, and it’s now being bruited about in service of the Fermi paradox, which asks why we see no intelligent civilizations given the abundant opportunity seemingly offered by the cosmos. A new paper from Michael Garrett (Jodrell Bank Centre for Astrophysics/University of Manchester) explores the matter in terms of how advanced AI might provide the kind of ‘great filter’ (the term is Robin Hanson’s) that would limit the lifetime of any technological civilization.
The AI question is huge given its implications in all spheres of life, and its application to the Fermi question is inevitable. We can plug in any number of scenarios that limit a technological society’s ability to become communicative or spacefaring, and indeed there are dozens of potential answers to Fermi’s “Where are they?” But let’s explore this paper because its discussion of the nature of AI and where it leads is timely whether Fermi and SETI come into play or not.
A personal note: I use current AI chatbots every day in the form of ChatGPT and Google’s Gemini, and it may be useful to explain what I do with them. Keeping a window open to ChatGPT offers me the chance to do a quick investigation of specific terms that may be unclear to me in a scientific paper, or to put together a brief background on the history of a particular idea. What I do not do is to have AI write something for me, which is a notion that is anathema to any serious writer. Instead, I ask AI for information, then triple check it, once against another AI and then against conventional Internet research. And I find the ability to ask for a paragraph of explanation at various educational levels can help me when I’m trying to learn something utterly new from the ground up.
It’s surprising how often these sources prove to be accurate, but the odd mistake means that you have to take great caution in using them. For example, I asked Gemini a few months back how many planets had been confirmed around Proxima Centauri and was told there were none. In reality, we do have one, that being the intriguing Proxima b, which is Earth-class and in the habitable zone. And we have two candidates: Proxima c is a likely super-Earth on a five-year orbit and Proxima d is a small world (with mass a quarter that of Earth) orbiting every five days. Again, the latter two are candidates, not confirmed planets, as per the NASA Exoplanet Archive. I reported all this to Gemini and yesterday the same question produced an accurate result.
So we have to be careful about AI in even its current state. What happens as it evolves? As Garrett points out, it’s hard to come up with any area of human interest that will be untouched by the effects of AI, and commerce, healthcare, financial investigation and many other areas are already being impacted. Concerns about the workforce are in the air, as are issues of bias in algorithms, data privacy, ethical decision-making and environmental impact. So we have a lot to work with in terms of potential danger.
Image: Michael Garrett, Sir Bernard Lovell chair of Astrophysics at the University of Manchester and the Director of the Jodrell Bank Centre for Astrophysics (JBCA). Credit: University of Manchester.
Garrett’s focus is on AI’s potential as a deal-breaker for technological civilization. Now we’re entering the realm of artificial superintelligence (ASI), which was Stephen Hawking’s great concern when he argued that further developments in AI could spell the end of civilization itself. ASI refers to an independent AI that becomes capable of redesigning itself, meaning it moves into areas humans do not necessarily understand. An AI undergoing evolution and managing it at an ever increasing rate is a development that could be momentous and one that poses obvious societal risks.
The author’s assumption is that if we can produce AI and begin the process leading to ASI, then other civilizations in the galaxy could do the same. The picture that emerges is stark:
The scenario…suggests that almost all technical civilisations collapse on timescales set by their wide-spread adoption of AI. If AI-induced calamities need to occur before any civilisation achieves a multiplanetary capability, the longevity (L) of a communicating civilization as estimated by the Drake Equation suggests a value of L ∼ 100–200 years.
Which poses problems for SETI. We’re dealing with a short technological window before the inevitable disappearance of the culture we are trying to find. Assuming only a handful of technological civilizations exist in the galaxy at any particular time (and SETI always demands assumptions like this, which makes it unsettling and in some ways related more to philosophy than science), then the probability of detection is all but nil unless we move to all-sky surveys. Garrett notes that field of view is often overlooked amongst all the discussion of raw sensitivity and total bandwidth. A telling point.
But let’s pause right there. The 100-200 year ‘window’ may apply to biological civilizations, but what about the machines that may supersede them? As post-biological intelligence rockets forward in technological development, we see the possibility of system-wide and even interstellar exploration. The problem is that the activities of such a machine culture should also become apparent in our search for technosignatures, but thus far we remain frustrated. Garrett adds this:
We…note that a post-biological technical civilisation would be especially well-adapted to space exploration, with the potential to spread its presence throughout the Galaxy, even if the travel times are long and the interstellar environment harsh. Indeed, many predict that if we were to encounter extraterrestrial intelligence it would likely be in machine form. Contemporary initiatives like the Breakthrough Starshot programme are exploring technologies that would propel light-weight electronic systems toward the nearest star, Proxima Centauri. It’s conceivable that the first successful attempts to do this might be realised before the century’s close, and AI components could form an integral part of these miniature payloads. The absence of detectable signs of civilisations spanning stellar systems and entire galaxies (Kardashev Type II and Type III civilisations) further implies that such entities are either exceedingly rare or non-existent, reinforcing the notion of a “Great Filter” that halts the progress of a technical civilization within a few centuries of its emergence.
Biological civilizations, if they follow the example of our own, are likely to weaponize AI, perhaps leading to incidents that escalate to thermonuclear war. Indeed, the whole point of ASI is that in surpassing human intelligence, it will move well beyond oversight mechanisms and have consequences that are unlikely to merge with what its biological creators find acceptable. Thus the scenario of advanced machine intelligence finding the demands on energy and resources of humans more of a nuisance than an obligation. Various Terminator-like scenarios (or think Fred Saberhagen’s Berserker novels) suggest themselves as machines set about exterminating biological life.
There may come a time when, as they say in the old Westerns, it’s time to get out of Dodge. Indeed, developing a spacefaring civilization would allow humans to find alternate places to live in case the home world succumbed to the above scenarios. Redundancy is the goal, and as Garrett notes: “…the expansion into multiple widely separated locations provides a broader scope for experimenting with AI. It allows for isolated environments where the effects of advanced AI can be studied without the immediate risk of global annihilation. Different planets or outposts in space could serve as test beds for various stages of AI development, under controlled conditions.”
But we’re coming up against a hard stop here. While the advance of AI is phenomenal (and some think ASI is a matter of no more than a few decades away), the advance of space technologies moves at a comparative crawl. The imperative of becoming a technological species falls short because it runs out of time. In fact – and Garrett notes this – we may need ASI to help us figure out how to produce the system-wide infrastructure that we could use to develop this redundancy. In that case, technological civilizations may collapse on timescales related to their development of ASI.
Image: How will we use AI in furthering our interests in exploring the Solar System and beyond? Image credit: Generated by AI / Neil Sahota.
We talk about regulating AI, but how to do so is deeply problematic. Regulations won’t be easy. Consider one relatively minor current case. As reported in a CNN story, the chatbot AI ChatGPT can be tricked into bypassing blocks put into place by OpenAI (the company behind it) so that hackers can plan a variety of crimes with its help. These include money laundering and the evasion of trade sanctions. Such workarounds in the hands of dark interests are challenging at today’s level of AI, and we can see future counterparts evolving along with the advancing wave of AI experiments.
It could be said that SETI is a useful exercise partly because it forces us to examine our own values and actions, reflecting on how these might transform other worlds as beings other than ourselves face the their own dilemmas of personal and social growth. But can we assume that it’s even possible to understand, let alone model, what an alien being might consider ‘values’ or accepted modes of action? Better to think of simple survival. That’s a subject any civilization has to consider, and how it goes about doing it will determine how and whether it emerges from a transition to machine intelligence.
I think Garrett may be too pessimistic here:
We stand on the brink of exponential growth in AI’s evolution and its societal repercussions and implications. This pivotal shift is something that all biologically-based technical civilisations will encounter. Given that the pace of technological change is unparalleled in the history of science, it is probable that all technical civilisations will significantly miscalculate the profound effects that this shift will engender.
I pause at that word ‘probable,’ which is so soaked in our own outlook. As we try to establish a regulatory framework that can help AI progress in helpful ways and avoid deviations into lethality, we should consider the broader imperative. Call it insurance. I think Garrett is right in noting the lag in development in getting us off-planet, and can relate to his concern that advanced AI poses a distinct threat. All the more reason to advocate for a healthy space program as we face the AI challenge. And we should also consider that advanced AI may become the greatest boon humanity has ever seen in terms of making startling breakthroughs that can change our lives in short order.
Call me cautiously optimistic. Can AI crack interstellar propulsion? How about cancer? Such dizzying prospects should see us examining our own values and how we communicate them. For if AI might transform rather than annihilating us, we need to understand not only how to interact with it, but how to ensure that it understands what we are and where we are going.
The paper is Garrett, “Is artificial intelligence the great filter that makes advanced technical civilisations rare in the universe?” Acta Astronautica Vol. 219 (June 2024), pp. 731-735 (full text). Thanks to my old friend Antonio Tavani for the pointer.
Streams of Stars and What They Tell Us
A quick follow-up to yesterday’s post. The idea of a stream of debris or even large objects like comets or asteroids from another star continues to resonate with me. The odds on identifying such a stream in terms of origin seem stupendous, but the benefits of doing so would be obvious. I notice that another kind of stellar stream is in the news, one involving not debris but entire stars. The Icarus stream is a grouping of stars that seem to have been tidally disrupted by the Milky Way, probably from an earlier encounter between the parent galaxy and a dwarf galaxy.
Digging a bit, I learned that we can carry the idea of stellar streams back to the work of Donald Lynden-Bell, who in 1995 proposed the stream concept to explain the long structure or filament of stars evidently tidally stripped from the Sagittarius Dwarf Spheroidal Galaxy, the latter being a satellite galaxy of the Milky Way. The Sgr dSph, as it is known, actually contains four globular clusters within it. It travels a polar orbit around the Milky Way at a distance of some 50,000 light years from galactic center, and has evidently passed through the plane of the galaxy at least several times.
Image: The Sagittarius dwarf galaxy, a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our Galaxy’s gravitational tug, is visible as an elongated feature below the Galactic center and pointing in the downwards direction in the all-sky map of the density of stars observed by ESA’s Gaia mission between July 2014 to May 2016. Scientists analyzing data from Gaia’s second release have shown our Milky Way galaxy is still enduring the effects of a near collision that set millions of stars moving like ripples on a pond. The close encounter likely took place sometime in the past 300–900 million years, and the culprit could be the Sagittarius dwarf galaxy. Credit: ESA/Gaia/DPAC,CC BY-SA 3.0 IGO.
Stellar streams, like the debris streams we considered yesterday, are the result of tidal interactions that are now well studied, resulting in the identification of further structures like the Icarus stream (the Gaia observatory is frequently relied upon for the relevant data). Such stretched out streams of stars contain clues about the Milky Way’s gravitational potential and the distribution of mass, which accounts for the continuing interest in the formations. The Icarus stream apparently came from a dwarf galaxy with a stellar mass of about one billion solar masses in a low-inclination orbit.
We now have a new paper from a team of scientists led by Paola Re Fiorentin (Observatory of Turin), again using Gaia data along with results from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Galactic Archaeology with HERMES (GALAH). 81 members of the Icarus stream have been identified, upon which the group has managed to conduct chemical analysis and investigate the dynamics of the stream, learning that it is at least 12 billion years old. Everything about this work supports the notion of the disrupted galaxy remnant scenario.
But we’re early in the game when it comes to these probes of galactic interactions and what they tell us about the earliest days of the Milky Way. The paper adds this:
…even though we cannot exclude the contamination of unevolved low metallicity in situ stars, we argue that the Icarus stream is consistent with debris of a low-inclination prograde dwarf galaxy with a stellar mass ∼ 109M⊙. In any case, we remark that this scenario assumes the existence of a protogalactic disk formed in situ at ages > 12 Gyr (cf. Xiang & Rix 2022). A similar scenario was suggested by Carter et al. (2021) that proposed the accretion of low-α stars from a co-rotating dwarf galaxy onto a primordial high-α disk.
Image: The Milky Way and its halo of stars, many of which have been cannabalised via collisions with other galaxies (Image credit: ESA/Gaia/DPAC, T Donlon et al. 2024; Background Milky Way and Magellanic Clouds: Stefan Payne-Wardenaar).
In the quotation above, the term low-α stars refers to stars low in the elements formed through the fusion of helium nuclei in what are known as alpha-capture processes. These would include oxygen, magnesium, sulfur, silicon, etc. Low-α stars are considered to have formed in regions where star formation was relatively slow. Studying this variable can offer insights into star formation histories. What all this means is that the Icarus stream seems consistent with the scenario of galaxy disruption.
But the paper adds:
However, the origin of metal-poor stars with disk kinematics is currently a matter of lively debate in the astronomical community. We think that such controversial interpretations could derive from the different selection criteria that may generate kinematically or chemically biased samples.
Such studies probe the formation history of the galaxy, and are in their early stages. The Icarus stream does appear to have formed outside the Milky Way, which would be consistent with everything we’re learning about the formation of stellar streams. All of which points to the complex interactions that move material among the stars, but reinforces the difficulty of deducing the origin of material from any single star.
The Lynden-Bell paper on the Sagittarius stream is “Ghostly streams from the formation of the Galaxy’s halo,” Monthly Notices of the Royal Astronomical Society, Volume 275, Issue 2 (July 1995), pp. 429–442 (abstract). The Paola Re Fiorentin paper is “Icarus revisited: An Ancient, Metal-poor Accreted Stellar Stream in the Disk of the Milky Way,” accepted at The Astrophysical Journal and available as a preprint.
An Incoming ‘Stream’ from Alpha Centauri?
Here’s an interesting thought. We know that at least two objects from outside our Solar System have appeared in our skies, the comet 2I/Borisov and the still enigmatic object called ‘Oumuamua. Most attention on these visitors has focused on their composition and the prospects of one day visiting such an interloper, for it is assumed that with new technologies like the Vera Rubin Observatory and its Legacy Survey of Space and Time (LSST), we will be picking up more of the same.
But consider origins. Extrapolating backward to figure out where either object came from quickly exhausts the most patient researcher, for it only takes the slightest changes in trajectory to widen the search field so broadly as to be useless. That’s especially true since we don’t know the ages of the objects, which may span hundreds of millions of years.
Enter Cole Gregg (University of Western Ontario), who has embarked on a project to study the question from a different perspective. Gregg asks how likely it is that material from another star could be passing through Sol’s neighborhood. As presented at the American Astronomical Society’s Division for Planetary Sciences meeting in Boise, Idaho earlier this month, his calculations explore the gravitational interactions such objects would experience. And he studies whether ‘streams’ of such material could identify by their common characteristics the likely system of origin.
The question is intriguing given how difficult it is to get a probe to Alpha Centauri at present levels of technology. Material from that system, if identified as such, would have interesting implications, especially with regard to the dispersal of chemical elements and organic molecules. The idea of panspermia rides on the possibility that life itself might move between planets and even stars in such material, so getting our hands on something from another star would offer obvious benefits.
Gregg is working on this project with colleague Paul Wiegert, who is perhaps best known for pioneering work with Matt Holman back in the 1990s on planetary orbits possible in the Alpha Centauri system. That work established that rocky terrestrial worlds could remain on stable orbits around Centauri A or B within 3 AU of the host stars, and a bit further out if the planetary orbit was retrograde. As these orbits were stable for billions of years, this finding was an incentive for deeper investigation into Alpha Centauri as a possible home for life.
Gregg and Wiegert now turn their attention to what they call ‘interstellar meteor streams,’ analyzing the development of such streams as they form and “as the material evolves in time through a time-independent, asymmetric potential model for our galaxy.” Their method is to conduct simulations with existing galactic models that involve ejected material in the galactic bulge, the halo and the disk, with the direction of ejection oriented randomly and at speeds of up to 15 kilometers per second.
We’re early in the process here (remember that this work has just been presented), but quoting from a brief write-up made available for a 2023 conference:
As expected, since the physics of their motion is identical, we see the ejecta travel through the disk of the Galaxy in much the same way that stars do. However, through the progression of the simulation we see complicated and interesting effects on the ejecta swarm, hinting at the complicated evolution of interstellar meteoroid streams. This talk will also discuss the implications of the modeled motion on the identification of interstellar meteoroid streams.
For stars in the galactic disk (which is where we are), material ejected from their systems evolves in a way similar to meteoroid streams we see in the Solar System, undergoing ‘orbital shear’. These streams can be long-lived and may include a flux of material passing through our Solar System that, while small, would be of great interest. Gregg sees his ongoing work as refining the process of interstellar transfer and producing population estimates for interstellar visitors currently nearby.
Image: A hypothetical interstellar meteoroid stream after 185 Myr of evolution from a burst ejection originating in the Alpha Centauri system. Credit: Cole Gregg.
As to the Centauri system, Gregg told Sky & Telescope in an article published this month that his calculations using a simplified model of the Milky Way show that about 0.03 percent of material ejected from Alpha Centauri could reach the Solar System, and perhaps most important, be recognized as coming from that source. We’re talking about material that could be as small as dust grains and as large as a comet or asteroid, but moving within a stream that has similar orbital velocities within the galaxy and similar positions, the latter traits making their source identifiable.
Gregg’s work is intriguing and also preliminary, for he intends to produce a full follow-up study looking to refine the galactic model and include better estimates on the likely size of the material that would have made such a crossing. So at this point what we have is only a possibility that may be excluded if the further analysis rules it out. But it’s a worthwhile study given the implications of finding such a stream.
The Sky & Telescope article is from October 18, 2024, titled “Are Objects from Alpha Centauri Streaming by Earth?” (available here). Gregg’s writeup for a 2023 conference is “The Development of Interstellar Meteoroid Streams” (full text). A PowerPoint presentation can be found here. The Wiegert and Holman paper, a key paper in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).
SETI: Learning from TRAPPIST-1
Given our decades-long lack of success in finding hard evidence for an extraterrestrial civilization, it hardly comes as a surprise that a recent campaign studying the seven-planet TRAPPIST-1 system came up without a detection. 28 hours of scanning with the Allen Telescope Array by scientists at the SETI Institute and Penn State University produced about 11,000 candidate signals for further analysis, subsequently narrowed down to 2,264 of higher interest. None proved to be evidence for non-human intelligence, but the campaign is interesting in its own right. Let’s dig into it.
The unique configuration of the TRAPPIST-1 planets allowed the scientists involved to use planet-planet occultations (PPOs). A cool M-dwarf star, TRAPPIST-1 brings with it the features that make such stars optimal for detecting exoplanets. The relative mass and size of the planets and star mean that if we’re looking for rocky terrestrial-class worlds, we’re more likely to find and characterize them than around other kinds of star. True, they’re also orbiting a class of star that is dim, but another beauty of TRAPPIST-1 is that it’s only 40 light years out, and we see its seven planets virtually edge-on.
Planets e, f and g can be squeezed into the star’s habitable zone (liquid water on the surface) if we tweak our numbers for possible atmospheres. The edge-on vantage means that planets can pass in front of each other from our viewpoint, with the additional advantage that this well-studied system has planetary orbits that are sharply defined. This raises intriguing possibilities when you consider our own space activities. The Deep Space Network sends powerful signals to communicate with distant craft like the Voyagers, signals wide enough to propagate beyond them and into deep space. The right kind of receiver, if by chance aligned with them, might make a detection, producing evidence for a technology by the nature of its signal.
At TRAPPIST-1, there are seven planet-planet occultations, with two of them involving a potential transmission-source planet within the star’s habitable zone. But we have to consider that transmissions between planets might not be this limited, for radio traffic could move through relays placed for communications purposes on worlds that are uninhabitable. This would obviously be traffic never intended for interstellar reception, the kind of ongoing activity that marks a society communicating with itself, but perhaps leaving a technosignature that would reach the Earth through the width of its beam.
Image: A look from above at the communications line of sight between two worlds in the TRAPPIST-1 system, illustrating the PPO method used in this study. Credit: SETI Institute/Zayna Sheikh.
The possibilities of a detection using this PPO technique vary, of course, with the orbital parameters of the planets in any given system. We must also account for the drift rates produced by orbital motion. The paper explains the recent search’s technique this way:
…it is assumed that the TRAPPIST-1 planets are tidally locked due to their proximity to their host star and will have a negligible rotational contribution to the drift rate of a transmitter on their surfaces. Additionally, their orbital parameters are well constrained, making it possible to calculate the drift rate contributions from their orbital motion. Satellite transmitters in circular orbit around each planet could produce much higher drift rates, up to an additional ∼45 nHz on top of the contribution from the planet’s orbit around the star. However, we have chosen to limit our scope to analogues of our deep space communications, the strongest of which are surface transmitters to deep space probes.
The seven planet-planet occultations studied during the 28 hours of observation ranged from 8.6 minutes to 99.4 minutes. And it turns out that widening that window of observation through simulations produces numerous PPOs with a similarly large range of duration, making this strategy still more interesting. The animation below shows the TRAPPIST-1 system in motion and the possible communications opportunities. Credit: Tusay et al., citation below.
Animation: This is Figure 10 from the paper, the caption of which reads: Simulated potential PPO events during our observations. Online viewers will see a concatenated video of the orbital configuration of the system during each of the observations, including any potential PPO events that we found to occur during those windows. A still image of a PPO event during the observation on Oct 29, 2022 is included where the animation is not accessible. The top panel shows a bird’s-eye view of the system with planet radii scaled up for better viewing. The distances and beam sizes are to scale, assuming a beam created with a 3.4m dish at 3.3 GHz (the maximum frequency observed during this particular session) from the surface of planet g aimed at planet e. The bottom panel shows the edge-on view with planet sizes scaled with distance, showing how much of the beam spills over the planet toward the direction of Earth in the negative z-direction. The red dashed lines in the illustrated beam is the inner angle blocked by the occulting planet, e. The blue dashed lines show the outer angle of the beam that would spill over the planet. The window for this event lasted roughly 95 minutes. Credit: Tusay et al.
Is this method the longest of longshots? SETI itself might be described that way, depending on your views of life in the cosmos. But our steadily growing capabilities at signal detection can’t be ruled out when we consider the possibilities. From the paper:
The analysis of the observations presented here demonstrates that precise characterization of ideal systems, like TRAPPIST-1, enabling orbital dynamical modeling and prediction of PPO events offer practical application for leaked emission searches. This provides SETI a powerful new observational tool and search strategy. As signal detection and RFI mitigation pipelines improve, the inclusion of PPOs to provide narrow search windows may make it more feasible to increase time resolution and sensitivity at higher drift rates.
What beckons most strongly about technosignatures is that they assume no intent (which in any case would be unguessable) on the part of a hypothetical alien civilization. We would essentially be eavesdropping on their activities. Grad student Nick Tusay (Pennsylvania State), lead author of the paper on this work, adds this: “[W]ith better equipment, like the upcoming Square Kilometer Array (SKA), we might soon be able to detect signals from an alien civilization communicating with its spacecraft.” And that would be a SETI detection for the ages.
The paper is Tusay et al., “A Radio Technosignature Search of TRAPPIST-1 with the Allen Telescope Array,” currently available as a preprint.
A Gravitational Wave Surprise
I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers.
But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical gravitational wave background, or AGWB, as the inevitable acronym would have it. Astronomers also have to consider GW signals associated with events in the early universe, stochastic background ‘static’ that could have originated, for example, in cosmic inflation or the creation of cosmic strings.
The AGWB is the background noise of countless astrophysical events, a ‘hum’ from all sources emitting gravitational waves in the universe. Recent work has been showing that this collective signal, primarily from black hole and binary neutron star mergers, is detectable by the technologies we’ll be deploying in the 2030s in the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission. And it’s clear that for gravitational wave astronomy to proceed, we need to remove the AGWB to uncover underlying signals.
New work now makes the case that, surprisingly, we also have to reckon with the background noise of binary white dwarfs, although I see in the literature that scientists were delving into this as early as 2001 (citation below). In two recent papers, Dutch astronomers have developed models demonstrating that the background noise of white dwarfs would actually be stronger than that produced by black holes. Gijs Nelemans (Radboud University (Nijmegen, the Netherlands), who is working with the software and guidance mechanisms for the LISA mission, is a co-author on two papers on the subject. He sees white dwarf background noise as a way of studying stellar evolution on a galactic scale:
“With telescopes you can only study white dwarfs in our own Milky Way, but with LISA we can listen to white dwarfs from other galaxies. Moreover, in addition to the background noise of black holes and the noise of white dwarfs, perhaps other exotic processes from the early universe can be detected.”
Image: Dutch astronomer Gijs Nelemans. Credit: TechGelderland.
Nelemans has been developing the models described in the two recent papers with students Seppe Staelens and Sophie Hofman. Their work is significant given that until now, the LISA mission had not factored in a noisy white dwarf background problem. In a paper published in Astronomy & Astrophysics, the authors point out:
Given the amplitude of the WD component… it is expected that it can be very well measured by LISA. Furthermore, the relative amplitudes show that, if LISA detects an AGWB signal in the mHz regime, it is likely dominated by the WDs. This means that it is likely hard to make statements about the BH (and NS) population based on a measurement of the AGWB unless there is a way to disentangle the two, or to detect the high-frequency component of the AGWB above 40 mHz.
And in terms of the study of white dwarfs, the paper adds:
This offers an opportunity to study the WD binary population to much larger distances, while hampering the detection of the BH AGWB with missions such as LISA. The WD signal reaches a peak around 10 mHz and at higher frequencies the BH AGWB will become the dominant signal. The detectability of this transition by LISA and other mHz missions ought to be studied in detail.
Image: The LISA mission consists of a constellation of three identical spacecraft, flying in formation. They will orbit the Sun trailing the Earth, forming an equilateral triangle in space. Each side of the triangle will be 2.5 million km long (more than six times the Earth-Moon distance), and the spacecraft will exchange laser beams over this distance. This illustration shows two black holes merging and creating ripples in the fabric of spacetime. Some galaxies are visible in the background. In the foreground, the shape of a triangle is traced by shining red lines. It is meant to represent the position of the three LISA spacecraft and the laser beams that will travel between them. Credit: ESA.
This is indeed a unique kind of probe, because we’re talking about studying white dwarf evolution at high redshift in ways beyond the range of optical astronomy. Realize that only a small selection of gravitational wave sources can be detected with our current technologies. Millions of binaries in the Milky Way will simply merge into the stochastic foreground, a signal that is highly anisotropic (i.e., not uniform in all directions) while unresolved binary sources outside the galaxy produce a background signal that is profoundly isotropic, one that “encodes the combined information about the different source populations,” to quote the Hofman & Nelemans paper.
So we learn that filtering out white dwarf background mergers will be a major part of LISA’s investigations, but that the WD background is also a source of new information. LISA is to be the first dedicated space-based gravitational wave detector, involving three spacecraft in an equilateral triangle 2.5 million kilometers long in a heliocentric orbit. The European Space Agency hopes to launch LISA in 2035 on an Ariane 6.
The papers are Hofman & Nelemans, “On the uncertainty of the white dwarf astrophysical gravitational wave background,” accepted at Astronomy & Astrophysics (preprint); and Staelens & Nelemans, “Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band,” Astronomy & Astrophysics Vol. 683, A139 (March 2024). Full text. The 2001 paper is “Low-frequency gravitational waves from cosmological compact binaries,” Monthly Notices of the Royal Astronomical Society Vol. 324, Issue 4 (July 2001), pp. 797-810 (abstract).
Catches, Comets and Europa
If the public seems more interested in spaceflight as a vehicle for streaming TV dramas, the reality of both the Europa Clipper liftoff and the astounding ‘catch’ of SpaceX’s Starship booster may kindle a bit more interest in exploring nearby space. When I say ‘nearby,’ bear in mind that on this site the term refers to the entire Solar System, as we routinely discuss technologies that may one day make travel to far more distant targets possible. But to get there, we need public engagement, and who could fail to be thrilled by a returning space booster landing as if in a 1950’s SF movie?
Europa may itself offer another boost if Europa Clipper’s science return is anything like what it promises to be. Closing to 15 kilometers from the surface and making 49 passes over the icy ocean world, the spacecraft may give us further evidence that outer system moons can be venues for life. We also have the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), which will study Europa, Callisto and, in a spectacular move, end up orbiting Ganymede for extended close-up observations.
Image: Europa Clipper begins its journey. Credit: SpaceX.
JUICE gets to Jupiter in July of 2031, while Europa Clipper starts its flybys in the same year, though arriving in 2030. As a measure of how tricky it can be to get to these destinations, both craft make flybys of other worlds, returning in fact to the Earth for some of these. Europa Clipper’s journey will be marked by gravity assists from Mars in February of 2025 and Earth in December 2026. JUICE has already performed one Earth/Moon flyby and will make a flyby of Venus (August, 2025) followed by two Earth flybys (September 2026 and January 2029). A long and winding road indeed!
Speaking of flybys, it’s interesting to note that we have two cometary appearances this month. Comet C/2023 A3 (Tsuchinshan-ATLAS) and C/2024 S1 (ATLAS) are both likely to be visible in October, with the latter closest to Earth on October 24 as it swings toward Sol where it will likely disintegrate. The former should make an appearance in the western sky just after sunset before growing fainter in the latter part of the month. C/2023 A3 appears to be an Oort Cloud object, or long period comet, with an orbital period of some 80,000 years. Short-period comets (Halley’s Comet is one of these) have much shorter orbits, with Halley’s showing up every 76 years.
I find the Oort Cloud a fascinating subject, for it’s based on deduction and not observation. Astronomer James Wray (Georgia Tech), writing in The Conversation, makes the point that while we can’t directly image this vast collection of comets, likely numbering in the hundreds of billions, we can estimate that it extends possibly as far as halfway to the Alpha Centauri system. That’s an intriguing thought, for it means our cometary cloud may intermingle with an equivalent cloud (if one exists) from the Centauri stars. The space covered by our first interstellar probes is not vacant, though the distances between individual objects would still be vast. On the other hand, if the theory that the Oort Cloud formed because of interactions with the giant planets, it’s possible that in the absence of such planets (still not demonstrated), Centauri A and B may not have formed such a cloud.
Wray makes the case that long-period comets are conceivably our greatest planetary threat, outranking near Earth asteroids in degree of danger since an incoming Oort object would likely not be spotted until well inside the planetary system, giving us little time to react. ‘Oumuamua, after all (not an Oort object) was discovered after its closest approach to Earth.
Cometary flybys of our Sun will always be cherished for their visual appeal as ices evaporate and a tail forms, and a collision course with Earth is a highly unlikely scenario, but it’s always best to consider the prospects. Wray puts it this way:
One way to prepare for these objects is to better understand their basic properties, including their size and composition. Toward this end, my colleagues and I work to characterize new long-period comets. The largest known one, Bernardinelli–Bernstein, discovered just three years ago, is roughly 75 miles (120 kilometers) across. Most known comets are much smaller, from one to a few miles, and some smaller ones are too faint for us to see. But newer telescopes are helping. In particular, the Rubin Observatory’s decade-long Legacy Survey of Space and Time, starting up in 2025, may double the list of known Oort Cloud comets, which now stands at about 4,500.
The European Space Agency’s Comet Interceptor mission, scheduled for launch later in this decade, should offer an option for intercepting an Oort Cloud comet when one appears, making it possible to learn more about these objects in terms of their composition and possible role in the delivery of volatiles to the inner system. Oort comets are tricky because their wide orbits mean gravitational influences from other stars can nudge one into a solar close pass without any prior warning. An incoming long-period comet, writes Wray, might offer mere weeks or days to prepare any defense measures we have in place. Even so, the odds of an impact are extremely low.
Image: A stunning return. The Starship booster comes home. Credit: SpaceX.
All this is by way of hoping public interest in space will be quickened both by recent mission successes, ongoing exploration of possible sources of life, and the appearance of the occasional comet. The startling SpaceX success with Starship’s ‘catch’ underlines that technological advances, like comets, can seem to come out of nowhere when we’re not paying attention. I’m thinking back to the science fiction I read as a kid and realizing that watching Starship’s booster descend was right out of Astounding Stories. Heinlein would have loved it, and indeed foreshadowed what unfolded on Sunday.
As SpaceX communications manager Dan Huot put it: “What we just saw, that looked like magic.”
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