Centauri Dreams
Imagining and Planning Interstellar Exploration
Tightening our Understanding of Circumbinary Worlds
I’m collecting a number of documents on gravitational wave detection and unusual concepts regarding their use by advanced civilizations. It’s going to take a while for me to go through all these, but as I mentioned in the last post, I plan to zero in on the intriguing notion that civilizations with abilities far beyond our own might use gravitational waves rather than the electromagnetic spectrum to serve as the backbone of their communication system. It’s a science fictional concept for sure, though there may be ways it could be imagined for a sufficiently advanced culture.
For today, though, let’s look at a new survey that targets highly unusual planets. Binaries Escorted by Orbiting Planets has an acronym I can get into: BEBOP. It awakens the Charlie Parker in me; I can almost smell the smoky air of a mid-20th century jazz club and hear the clinking of glasses above Parker’s stunning alto work. I was thinking about the great sax player because I had just watched, for about the fifth time, Clint Eastwood’s superb 1988 film Bird, whose soundtrack is, of course, fabulous.
On the astronomy front, the BEBOP survey is a radial velocity sweep for circumbinary planets, those intriguing worlds, rare but definitely out there, that orbit around two stars in tight binary systems. Beginning in 2013, BEBOP targeted 47 eclipsing binaries, using data from the CORALIE spectrograph on the Swiss Euler Telescope at La Silla, Chile. This is intriguing because what we know about circumbinary planets has largely come from detections based on transit analysis. Radial velocity work has uncovered planets orbiting one star in a wide binary configuration but until now, not both.
Image: Artist’s visualization of a circumbinary planet. Credit: Ohio State University / Getty Images.
The new work adds data from the HARPS spectrograph at La Silla and the ESPRESSO spectrograph at Paranal to confirm one of two planets at TOI-1338/BEBOP-1. Thus we have radial velocity evidence for the gas giant BEBOP-1 c, massing in the range of 65 Earth masses, in an orbit around the binary of 215 days. A second world, referenced as TOI-1338 b because it shows up only in transit data from TESS, complements the RV find, making this only the second circumbinary system known to host multiple planets. TOI-1338 b is 21.8 times as massive as the Earth and as a transiting world could well be a candidate for atmospheric studies by the James Webb Space Telescope.
But BEBOP-1 c is the planet that stands out. I think I am safe in calling a co-author on this paper, David Martin (Ohio State University), a master of understatement when he describes the problems in extracting radial velocity data on a circumbinary world. After all, we’re relying on the tiniest gravitational effects flagged by minute changes in wavelength, and now we have to factor in multiple sets of stellar spectra. Here’s Martin:
“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space. So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different. Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries.”
Nice work indeed. Circumbinary planets are what the paper describes as ‘harsh environments’ for planet formation given the gravitational matrix in which such formation occurs, and thus we should be able to use the growing number of such systems (now 14 including this one) in the study of how planets form and also migrate. BEBOP should be a useful survey in providing accurate masses for planets in systems we’ve already discovered with the transit method.
Image: This is Figure 3 from the paper, offering an overview of the BEBOP-1 system. Caption: The BEBOP-1 system is shown along with the extent of the system’s habitable zone (HZ) calculated using the Multiple Star HZ website. The conservative habitable zone is shown by the dark green region, while the optimistic habitable zone is shown by the light green region. The binary stars are marked by the blue star symbols in the centre. The red shaded region denotes the instability region surrounding the binary stars as described by Holman and Wiegert. BEBOP-1 c’s orbit is shown by the red orbit models…shaded from the 50th to 99th percentiles. TOI-1338 b’s orbit is shown by the yellow models, and is also based on 500 random samples drawn from the posterior in its discovery paper. Credit: Standing et al.
Learning more about how planets in such perturbed environments emerge should advance the study of planet growth around single stars. It’s likely that the increased transit probabilities of circumbinary planets should play into our efforts to study planetary atmospheres as well. And while transits should provide the bulk of new discoveries in this space, radial velocity follow-ups should expand our knowledge of individual systems, being less dependent on orbital periods and inclination. BEBOP presages a productive use of these complementary observing methods.
The paper is Standing et al., “Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system,” Nature Astronomy 12 June 2023 (full text). See also Martin et al., “The BEBOP radial-velocity survey for circumbinary planets I. Eight years of CORALIE observations of 47 single-line eclipsing binaries and abundance constraints on the masses of circumbinary planets,” Astronomy & Astrophysics Vol. 624, A68 (April 2019), 45 pp. Abstract.
Building the Gravitational Machine
In fact, gravity is some 1038 times weaker than the strong force that holds atomic nuclei together, easily illustrated by pointing out to my friend that I was overcoming an entire planet’s worth of gravity by lifting the salt shaker on the table. I learned from Greg Matloff and Eugene Mallove’s The Starflight Handbook that despite Freeman Dyson’s early interest in using the gravitational force to capture energy from astronomical objects, it was Stanislaw Ulam who first pondered the idea in print.
Now Ulam is an interesting figure, a name that resonates on Centauri Dreams in the context of nuclear pulse propulsion, which he first analyzed as far back as 1947 in a report for Los Alamos Scientific Laboratory. This grew into the Project Orion concept, with nuclear bombs exploded behind a flat steel plate, the crew protected by the mother of all shock absorbers. Ulam’s work on gravitational machines, however, analyzed how much energy could be extracted in a three-body system, one of which was a rocket, and what kind of velocities such a rocket could attain.
Image: Stanislaw Ulam (1909-1984), well known for his work on Orion, but also an early analyst of the extraction of gravitational energy.
Freeman Dyson’s notion, explained in the paper “Gravitational Machines” that we looked at last week, was to extract energy from a binary star system, as shown in the figure from the paper below. The ever imaginative Dyson, remember, was captivated by the possibilities of engineering on the part of advanced civilizations, whose works we might observe in the form of technosignatures. Here, the idea involves two stars of mass equal to the Sun revolving around a common barycenter. A spacecraft can be injected into an orbit that maximizes the gravitational effect, as Dyson explains:
The exploiters of the device are living on a planet or vehicle P which circles around the double star at a distance much greater than R. They propel a small mass C into an orbit which falls toward the double star, starting from P with a small velocity. The orbit of C is computed in such a way that it makes a close approach to B at a time when B is moving in a direction opposite to the direction of arrival of C. The mass C then swings around B and escapes with greatly increased velocity. The effect is almost as if the light mass C had made an elastic collision with the moving heavy mass B. The mass C will arrive at a distant point Q with velocity somewhat greater than 2V.
Image: This is Figure 1 from the Dyson paper. Caption: The solid line indicates the orbit of A and B; the dashed line indicates the orbit of C. Credit: Freeman Dyson.
Two options open up as we reach point Q:
At Q the mass C may be intercepted and its kinetic energy converted into useful form. Alternatively the device may be used as a propulsion system, in which case C merely proceeds with velocity 2V to its destination. The destination might be a similar device situated very far away, which brings C to rest by the same mechanism working in reverse.
Why call this a ‘machine’? Dyson speculated that if the advanced civilization would create ‘a whole ring of starting points P and end points Q’ around the same binary system, masses dropped into the system would emerge as a continuous stream of payloads, or cargo, or whatever. The point is that the energy source for this system is simply the gravitational potential between the two stars.
Then we can go further and extrapolate what happens as the machine continues to function. Over large timespans, the two stars will be drawn closer together, with the effect that their orbital velocity will necessarily increase. Thus the machine continues to operate, extracting energy from the system until the stars close to such a tight distance that no passage between them is possible. Dyson thinks this would be a point where the distance between the centers of the two stars is 4 times the radius of each star.
Dyson calculated that the luminous energy radiated by Sun-like stars in a three-body system like this would be a more practical source than gravitational energy, but white dwarfs, far less luminous than the Sun, would ramp up the gravitational energy by a factor of a hundred. So there’s an interesting technosignature for you, a search for white dwarf binaries with the parameters defined by Dyson, marking a system that could accelerate objects to 2000 kilometers per second without any propellant.
The ever imaginative Dyson thought such a system of white dwarf binaries scattered around the galaxy could serve as a long-haul freight transportation network. More significantly, he went on to consider the more condensed form of star known as the neutron star, which as the time of writing was still a theoretical concept. “[T]he fact that none has yet been observed does not argue strongly against their existence.” And of course, it would not be long before Jocelyn Bell and Antony Hewish found the first pulsar in 1967.
If we were to choose a pair of white dwarf stars as our binary system, Greg Matloff notes in The Starflight Handbook, we might reach velocities of 0.009 c. This is roughly 2700 kilometers per second, not bad given our Voyager 1 travel speed of a mere 17.1 kilometers per second. Even so, it’s a long way to Proxima Centauri. If we could work with a pair of neutron stars, according to the calculations Dyson made, we might reach 0.27 c,or almost 81,000 kilometers per second. Now we’re moving out, reaching Proxima in a couple of decades. Then, of course, we’ve got to slow down.
Adds Dyson:
…it may be said that the dynamics of stellar systems, under conditions in which gravitational radiation is important, is a greatly neglected field of study. In any search for evidences of technologically advanced societies in the universe, an investigation of anomalously intense sources of gravitational radiation ought to be included.
What an extraordinary thinker Dyson was! I look forward to the recent essay collection “Well, Doc, You’re In”: Freeman Dyson’s Journey through the Universe (MIT Press, 2022), just arrived here and placed at the top of my stack of necessary reading. Meanwhile, it’s intriguing to take the subject further still. Although Dyson didn’t push into this direction, Greg Benford has examined how truly advanced civilizations might create a different kind of gravitational machine to enable communications systems that would make using the electromagnetic spectrum seem quaint. More on that soon.
Inadvertent Test Post
Those of you who follow Centauri Dreams through email probably received an inadvertent test post this morning. My apologies. The post was triggered by work on the site’s internals and was generated automatically by the email software module. Work on the site continues, but I think the email issue is fixed, so I anticipate no more of these. Thanks for your patience.
Freeman Dyson’s Gravitational Machines
But what a tangled history this paper presents. First of all, how does a 1962 paper get onto arXiv? A quick check reveals the uploader as David Derbes, a name that should resonate with Dyson purists. Derbes (University of Chicago Laboratory Schools, now retired) is the power behind getting Dyson’s lectures on quantum electrodynamics, first given at Cornell in 1951, into print in the volume Advanced Quantum Mechanics (World Scientific Publishing, 2007). He’s also an editor on Sidney Coleman’s Lectures on Relativity (Cambridge University Press, 2022) and has written a number of physics papers.
“Gravitational Machines” has been hard to find. Dyson wrote it, according to my polymath friend Adam Crowl, for the Gravitational Research Foundation in 1962; Centauri Dreams regular Al Jackson corroborates this in an email exchange, noting that the GRF was created by one Roger Babson, who offered a prize for such papers. Astrophysicist Alastair G. W. Cameron added it to his early SETI tome Interstellar Communications: A Collection of Reprints and Original Contributions (W. A. Benjamin, 1963). The paper, a tight six pages, does not appear in the 1996 volume Selected Papers of Freeman Dyson with Commentary (American Mathematical Society, 1996).
So we can be thankful that David Derbes saw fit to post it on arXiv. Al Jackson noted in his email that Greg Benford and Larry Niven have used Dyson’s gravitational concepts in their work, so I suspect “Gravitational Machines” was a paper known to them at this early stage of their career. A recent phone call with Jim Benford also reminded me of the Dyson paper’s re-emergence. Listen to Dyson’s familiar voice in 1962:
The difficulty in building machines to harness the energy of the gravitational field is entirely one of scale. Gravitational forces between objects of a size that we can manipulate are so absurdly weak that they can scarcely be measured, let alone exploited. To yield a useful output of energy, any gravitational machine must be built on a scale that is literally astronomical. It is nevertheless worthwhile to think about gravitational machines, for two reasons. First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.
There’s the Dysonian reach into the far future, sensing where exponential technology growth might lead a civilization, and speculating at the most massive scale on the manipulation of matter as a form of engineering. But here too is the Dyson of ‘Dyson Sphere’ fame, tackling the question of whether or not such a project would be observable if undertaken elsewhere in the cosmos, just as he would go on to bring numerous other ideas on ‘technosignatures’ to our consciousness. Hence the term ‘Dysonian SETI,’ which I’ve often used here on Centauri Dreams.
Dyson goes on to speculate on the nature of eclipsing white dwarf binaries and their output of gravitational radiation, working the math to demonstrate the strength of such systems in terms of gravitational wave output, and finding that the output might be detectable. However, what catches his eye next is the idea of neutron star binaries, although he notes that at the time of writing, these objects were entirely hypothetical. But if they did exist (they do), their gravitational output should be “interesting indeed.”
…the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity.
It’s interesting that at the time Dyson wrote, Joseph Weber was mounting what must be the first attempt to detect gravitational waves, although he seems to have found nothing but instrumental noise. The LIGO (Laser Interferometer Gravitational-Wave Observatory) team would go on to cite Weber’s work following their successful detection of GW170817 in 2017, produced just as Dyson predicted by a neutron star binary. Calling such waves “a neglected field of study,” the 1962 paper adds this:
…the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.
At the end of the paper posted on arXiv, David Derbes adds a useful note, pointing out Dyson’s prescience in this field, and adding that he had secured Dyson’s permission to publish the article before the latter’s death. But as typical of Dyson, he also stressed that he wanted Weber’s contribution to be noted, which Derbes delivered on by inserting a footnote to that effect in the text. We can all thank David Derbes for bringing this neglected work of a masterful scientist back into wider view.
In the next post, I want to talk about how these gravitational wave energies might be exploited by the ‘machines’ Dyson refers to in the title of the paper. The paper is Dyson, “Gravitational Machines,” now available on arXiv.
The Prevalence of ‘Jupiters’ around Larger Stars
Work on the Centauri Dreams internals continues, with the unwelcome result that the site has been popped offline twice because of a possible security problem. Needless to say, this has to be resolved before I can move forward on other aspects of the rebuild. While I deal with that issue, let me respond to a few backchannel questions about yesterday’s post on gas giants in red dwarf planetary systems. What I’m being asked about is my comment that gas giants like Jupiter, at similar distances and installation, around other classes of stars are common compared to what we see at red dwarfs.
This has been a problematic issue, and the matter is a long way from achieving a consensus among researchers. A moment’s reflection yields the reason: Finding gas giants in outer system orbits around a star like the Sun is no easy matter. Radial velocity is most sensitive when dealing with large planets in tight orbits, which is why the first detections in main sequence stellar systems, beginning back in 1995 with 51 Pegasi b, were of the ‘hot Jupiter’ variety. That in itself offered new insights into planetary formation and dynamics. As physicist Isidor Isaac Rabi cogently asked when the muon was first detected, “Who ordered that?”
We’re making all kinds of advances in radial velocity as we use ever more sophisticated instruments to measure the motion induced by orbiting bodies around distant stars, but if we back out to, say, 5 AU, Jupiter’s distance from the Sun, we’re still dealing with extremely tiny effects. Transits are problematic because a planet on a five-year orbit obviously transits its host on long timeframes. Gravitational microlensing is an interesting prospect, because here we can detect planets at the needed distances, but even so the catalog isn’t large and there is much we don’t know.
Fortunately, resources like the California Legacy Survey (719 stars over three decades) are available and have produced data on what we can call ‘cold giants.’ I made my comment because of a paper in the Astrophysical Journal Supplement Series that I learned about through the Pass et al. paper we looked at in the previous post. This is from Caltech’s Lee Rosenthal and colleagues, and it examines the combination of small rocky planets with outer gas giants using the CLS for the bulk of its data. The result is a look at the occurrence of close-in planets with outer giant companions.
The Rosenthal paper addresses radial velocity work on F-, G-, K- and M-class stars and targets both categories of planets, finding that roughly 41 percent of systems with a close-in small planet also host an outer giant. By close-in small planet, the authors mean planets orbiting from 0.023–1 AU with a mass twice to 30 times that of Earth. And the giant planets examined are from 0.23–10 AU and 30 to 6000 Earth masses.
The implication is that stars hosting small inner planets are more likely to have an outer gas giant, for the number is roughly 17 percent for stars irrespective of small planet presence. There is much to be done with data from the California Legacy Survey (the baseline of RV observations goes back to 1988, and is invaluable), but studies like these lead to the conclusion that planets in Jupiter-like orbits are not uncommon among F-, G- and K-class stars. As to the M-dwarfs, the Pass paper indicates the scarcity of gas giants around them, with all that may imply about inner planet habitability. Note that the CLS is made up mostly of FGK stars, with 98% of stars in the sample having stellar masses above 0.3 solar masses..
I haven’t had time to dig into a previous paper using the California Legacy Survey data, this one from Benjamin Fulton (Caltech) with Rosenthal as a co-author, but do note that the authors find that the occurrence of planets less massive than Jupiter (from 30 Earth masses up to 300 as per RV data) is enhanced near 1–10 AU “in concordance with their more massive counterparts.” The complete citation is below.
We still have much to learn about exoplanet system architectures, but we’re making progress as the inflowing current of high-quality data grows ever more powerful.
The paper is Rosenthal et al., “The California Legacy Survey. III. On the Shoulders of (Some) Giants: The Relationship between Inner Small Planets and Outer Massive Planets,” Astrophysical Journal Supplement Series, Vol. 262, No. 1 (17 August 2022), 262 1 (abstract). The Fulton paper is “California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line,” Astrophysical Journal Supplement Series Vol. 255, No. 1 (9 July 2021), 255, 13 (abstract).
A Scarcity of ‘Jupiters’ in Red Dwarf Systems
Gas giant worlds like Jupiter may be uncommon around red dwarf stars, as a number of recent studies have found. It would be useful to tighten up the data, however, because many of the papers on this matter have used stellar samples at the high end of the mass range of M-dwarfs. At the Center for Astrophysics | Harvard & Smithsonian (CfA), Emily Pass and colleagues have gone to work on the question by looking at lower-mass M-dwarfs and working with a lot of them, some 200 in their sample, all within 15 parsecs.
The question is not purely academic, for some scientists suggest that the presence of a Jupiter-class planet – not uncommon around G-class stars like the Sun – is a factor in the development of life. Migrating inward from a formation in the first few hundred million years of the Solar System’s existence, Jupiter would have stirred up plenty of icy cometary bodies through gravitational interactions. Impacts from this infall into the inner system likely delivered a great deal of water and organic molecules to the young Earth, thus becoming a factor in the development of life.
Thus a system like TRAPPIST-1, with its seven rocky planets orbiting a nearby red dwarf, raises the question of whether such a system would have gone through this kind of mixing. No one knows whether life would have begun on Earth without these effects, but the suggestion that systems without a gas giant are barren is plausible. So just how common are red dwarf systems with gas giants equivalent to Jupiter based on what we know so far? It’s telling that only two of the known gas giants orbiting a red dwarf occur around stars of less than 30 percent of the Sun’s mass: LHS 252 b and GJ 83.1 b.
Image: A gas giant around an M-class dwarf, as visualized by artist Melissa Weiss, CfA.
What Pass and team deliver is a statistical analysis, using spectroscopic surveys and radial velocity data on nearby M-dwarfs in the mass range of 0.10–0.30 stellar masses. The data are presented in a paper now in process at The Astronomical Journal. The results confirm the belief that red dwarfs are seldom the hosts for Jupiter-class worlds. In fact, in the entire sample, not a single Jupiter-equivalent planet occurred, allowing the authors to conclude that Jupiter analogues must be found in fewer than 2 percent of low-mass red dwarf systems:
Planets that are Jupiter-like in mass and instellation are rare around low-mass M dwarfs, consistent with expectations from core accretion theory. Compared with previous radial-velocity and microlensing studies that consider broader distributions of M-dwarfs with higher mean stellar masses, our results are consistent with a decrease in giant planet occurrence with decreasing M-dwarf mass…
The authors note the complications of comparing occurrence rate between the various surveys that have so far attempted it, but add:
…the picture of giant planet occurrence from microlensing is still unclear. If Poleski et al. (2021) are correct in their assertion that every microlensing star has a wide-orbit giant planet, our results imply that the distribution of giant planets around low-mass M dwarfs must differ dramatically from more massive stars, whose giant planets are more prevalent near the water snow line than on wide orbits.
These are interesting findings especially in terms of habitability. Rather than assuming that red dwarf planets are unlikely to have life, they could just as easily point to the differences between these systems and our own as offering other avenues for life to develop. CfA’s David Charbonneau makes the point explicitly: “We don’t think that the absence of Jupiters necessarily means rocky planets around red dwarfs are uninhabitable.”
What we do have are planetary systems different enough from ours to encourage speculation on what factors might produce life in different ways than our own system. Consider that the lack of gas giants also indicates more raw material for planetary formation on the scale of smaller rocky worlds. Given the proximity of red dwarf stars with rocky planets, they’ll be at the forefront of astrobiological investigation as we develop the ability to study their atmospheres. The possibilities remain open, and perhaps exotic, as we continue the hunt for life elsewhere. Adds Pass:
“We have shown that the least massive stars don’t have Jupiters, meaning Jupiter-mass planets that receive similar amounts of starlight as Jupiter receives from our Sun. While this discovery suggests truly Earth-like planets might be in short supply around red dwarfs, there still is so much we don’t yet know about these systems, so we must keep our minds open.”
The paper is Pass et al., “Mid-to-Late M Dwarfs Lack Jupiter Analogs,” in process at The Astronomical Journal (preprint).
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