Centauri Dreams
Imagining and Planning Interstellar Exploration
Star in Tight Orbit around Black Hole
Beyond their obvious value in advancing our knowledge, astronomical discoveries can be thought of as exercises for the imagination, making us think about what we would see if we were actually near the phenomenon being observed. The view from a planet deep in a globular cluster can only be spectacular, and has been the subject of my own musings for many years. But this morning’s topic, a white dwarf star in a fantastically tight orbit around a black hole, leaves my imagination reeling. Just what would a scenario like the one playing out in the globular cluster 47 Tucanae, some 14,800 light years from Earth, look like up close?
Here we find, thanks to the space-based Chandra X-Ray Observatory and NuSTAR (Nuclear Spectroscopic Telescope Array), as well as the Australia Telescope Compact Array, a binary that consists of an apparent white dwarf orbiting a black hole every 28 minutes. The presence of large amounts of oxygen in the system gives weight to the idea that the companion is indeed a white dwarf, as an abundance of oxygen is a characteristic feature of white dwarfs.
The orbit here is so tight that the star is separated from the apparent black hole by a distance about 2.5 times the separation between the Earth and the Moon.
“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” says lead author Arash Bahramian (University of Alberta and Michigan State University). “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”
Image: An artist’s impression of a white dwarf star (left) in orbit around a black hole and so close that much of its material is being pulled away. There is a hotspot where the gas from the white dwarf hits the disc of matter swirling around the black hole. The black hole itself is surrounded by a cloud of ionised gas, which contains large amounts of oxygen. Credit: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss.
The orbital determination is made from the changes of brightness observed on a regular 28 minute cycle by Chandra. And the likely outcome of this tight pairing for the white dwarf is that it will eventually become something even more unusual. Says James Miller-Jones (Curtin University, Western Australia and the International Centre for Radio Astronomy Research):
“We think the star may have been losing gas to the black hole for tens of millions of years and by now has now lost the majority of its mass. Over time, we think that the star’s orbit will get wider and wider as even more mass is lost, eventually turning into an exotic object similar to the famous diamond planet discovered a few years ago.”
Small digression: More commonly known as a ‘carbon planet,’ an exotic world like the one Miller-Jones mentions draws on work from Marc Kuchner (NASA GSFC) and Sara Seager (MIT), who investigated the possibility that carbon-rich, oxygen-poor protoplanetary disks might form such planets. Here the visualization exercise gets another workout: Layers of diamond could exist within the mantle, possibly erupting to the surface during volcanic episodes. In addition to mountains of diamond, the surface could contain liquid or frozen tar and methane.
Although 55 Cancri e is frequently cited as a possible carbon planet, pulsar planets have also been suggested as candidates, and in connection with today’s story, it’s worth pointing out that carbon planets are also speculated to be found in globular clusters, where stars have a higher carbon-to-oxygen ratio than the Sun — hence Miller-Jones’ allusion to diamond planets in the quotation above. As to the binary in the globular cluster 47 Tucanae, called X9, the origin scenarios being discussed are intriguing. From the paper:
Low mass X-ray binaries (LMXBs) are systems in which a compact object [neutron star (NS) or black hole (BH)] accretes matter from a low mass companion (typically a main sequence star) through Roche-lobe overflow or wind-fed accretion (from a red giant). It has been long noticed that the population of LMXBs per unit mass in globular clusters (GCs) is orders of magnitude higher than that of the Galactic field… This overabundance has been associated with the high stellar density in GCs. While most LMXBs in the Galactic field are formed through binary evolution of primordial binaries, it has been shown that in GCs, the dominant channel to form LMXBs is through encounters. These scenarios include tidal capture by an NS or BH, exchange of a compact object into a primordial binary, or collision of a compact object with a red giant…
Here too the visualization game stretches the imagination. A black hole encounters a red giant star, with gas from the star being ejected and the remnants of the red giant’s atmosphere becoming a companion to the black hole in the form of a white dwarf. Whether or not we are looking at the result of such an encounter in X9, or whether this is actually a white dwarf paired with a neutron star that is stripping material from its companion, is not entirely clear, though X9 lacks many features of this kind of transitional millisecond pulsar.
“We’re going to watch this binary closely in the future, since we know little about how such an extreme system should behave,” says co-author Vlad Tudor (Curtin University/ICRAR). “We’re also going to keep studying globular clusters in our galaxy to see if more evidence for very tight black hole binaries can be found.”
Before now, the closest star around a likely black hole was the system called MAXI J1659-152, where the orbital period is 2.4 hours, much longer than the 28 minutes we see in X9. There is also evidence for a similar system in the galaxy M49 (about 56 million light years away in the Virgo Cluster). That one is believed to have an orbit even tighter than X9’s, but Miller-Jones points out that there is no direct measurement of its period, making this work on X9 the first time that an orbital period this short in a system containing a likely black hole has ever been measured. An interesting exercise for the imagination indeed.
Although the gravitational waves from this system are too weak to detect with present ground-based observatories, future space-based platforms may be able to detect them.
The paper is Bahramian et al., “The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9,” Monthly Notices of the Royal Astronomical Society 14 March 2017 (abstract / preprint).
Giant Planet Clues to a Debris Disk Anomaly
A massive young planet on the borderline between gas giant and brown dwarf is telling us a bit more about planet formation in general, and circumstellar disk dynamics in particular. Known as HD 106906b, the world is 11 times the mass of Jupiter and no more than 13 million years old. Its position 650 AU from its star creates an orbit that takes 1500 years to complete.
The host HD 106906, about 300 light years from Earth, is an F5-class star in the constellation Crux, the southern constellation dominated by the asterism we call the Southern Cross.
What we find here is a debris disk that is non-circular, its shape evidently explained by the presence, well outside the disk, of HD 106906b, whose orbit is elliptical. Observations through the Gemini Planet Imager, the Hubble Space Telescope and ESO’s SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch instrument) show that we are viewing the disk nearly edge-on. The inner region appears cleared of small dust grains.
Working with these observations, a team led by Erika Nesvold, a postdoctoral fellow at the Carnegie Institution for Science, used a software tool she created called Superparticle-Method Algorithm for Collisions in Kuiper belts and debris disks (SMACK) to model the planet’s orbital path and assess its effects on the circumstellar disk.
Image: Erika Nesvold, creator of the software that made possible the new analysis of HD 106906b.
The disk, an analog of our own Kuiper Belt, is much closer to the star on one side than on the other, producing a stronger infrared signature on the warmer side. The model accurately reproduces the shape of the disk as a result of the perturbations caused by this distant planet. The team’s analysis shows that the planet must have formed outside the disk; otherwise, the disk would have been perturbed in an entirely different manner. From the paper:
Constraining the orbit of HD 106906b could have implications for its formation scenario. Prior to the publication of resolved images of the disk, it was suggested (using N-body simulations) that the companion formed interior to the disk and was scattered onto a highly eccentric orbit (Jílková & Portegies Zwart 2015). This study concluded that the disk can survive perturbations by a companion with an apocenter distance of 650 au and a pericenter distance interior to the disk if the companion’s inclination is ? 10°. However, this resulted in a significantly vertically perturbed disk by 10 Myr, regardless of the companion’s inclination. Our simulations indicate that a companion with an orbit completely exterior to the disk can reproduce the observed asymmetries without vertically extending the disk, supporting the scenario in which the companion formed in situ.
Image: Two images of the HD 106906 stellar system created by Erika Nesvold and her team’s simulation. The left panel shows a zoomed-in image of the ring of leftover rocky and icy planet-forming material that is rotating around the star. (The star is masked by the black circle.) The different hues represent gradients of brightness in the disk material. (Yellow is the brightest and blue the dimmest.). The right panel shows a farther-out view of the simulated system. The star is represented by the yellow circle with an arrow pointing to the exoplanet, HD 106906b. Nesvold’s team demonstrated that the exoplanet is shaping the structure of the debris disk, which is shown by the white and blue dots encircling the star. Credit: Erika Nesvold/Carnegie Institution for Science.
SMACK is able to re-create the shape of the disk without the addition of any planets within the disk itself, although the question of possible planets there remains open. The paper acknowledges that while HD 106906b can account for the disk’s shape, there are alternative mechanisms, including a planet within the debris disk, that could also force its eccentricity. Tightening up the parameters of HD 106906b’s orbit should help to resolve the question.
Image: This is an actual observation of HD 106906 taken by the European Southern Observatory’s planet-finding tool SPHERE. The star is blacked out by a circle (which masks its glare from blinding the instrument) and the debris disk can be seen in the lower left. In the upper right is the exoplanet, 106906b. The simulation created by Erika Nesvold and her team accurately recreated the observed characteristics of the disk: the disk is brighter on its eastern (left) side, and oriented about 20 degrees clockwise from the planet’s position on the sky. Credit: ESO and A.M. Lagrange of Université Grenoble Alpes.
Modeling tools like SMACK are going to be turned to other young system disks as we work to understand planetary evolution. After all, we’re seeing the process here in its infancy. “In our solar system, we’ve had billions of years of evolution,” said Michael Fitzgerald, UCLA associate professor of physics and astronomy, and a co-author on the study. “We’re seeing this young system revealed to us before it has had a chance to dynamically mature.”
The paper points out that previously undetected planets may be investigated by modeling the asymmetries in such disks. We do know that systems like this, with a large, exterior perturber of some kind, are not uncommon. Some 25 percent of known debris disks exist, for example, in binary or triple star systems. Understanding disk perturbations, whether they are caused by planets within the disk or outside of it, may point us toward the worlds that cause them.
The paper is Nesvold, Naoz & Fitzgerald, “HD 106906: A Case Study for External Perturbations of a Debris Disk,” Astrophysical Journal Letters Vol. 837, No. 1 (1 March 2017). Abstract / preprint.
Titan: Nitrogen Bubbles and ‘Magic Islands’
With Cassini now in the final stages of its mission, we can look forward to just one more close flyby of Titan, the 127th targeted encounter, on April 22. ‘Targeted’ means that Cassini has to use its thrusters to position itself optimally for the flyby. The first of the images below, by contrast, comes from a ‘non-targeted’ flyby, one of several anticipated for 2017.
The close pass will give researchers a chance to probe the moon’s northern seas one last time, which may prove useful in the investigation of the transient features some have dubbed ‘magic islands.’ Even as these studies proceed, Cassini will also be using the Titan flyby to alter its course enroute to the series of plunges through the gap between Saturn and its innermost rings now being called the Cassini Grand Finale. The spacecraft will plunge into Saturn’s atmosphere on September 15.
Image: As it sped away from a relatively distant encounter with Titan on Feb. 17, 2017, NASA’s Cassini spacecraft captured this mosaic view of the moon’s northern lakes and seas. Cassini’s viewing angle over Kraken Mare and Ligeia Mare was better during this flyby than previous encounters, providing increased contrast for viewing these seas. Because the spacecraft is peering through less of Titan’s haze toward Kraken and Ligeia, more details on their shorelines are visible, compared to earlier maps. Credit: NASA/JPL-Caltech/Space Science Institute.
But back to those ‘magic islands.’ Several previous Cassini flybys of Titan have revealed the radar signature of small areas on the seas that have appeared and then disappeared. In one instance, the feature re-emerged in subsequent imagery. Have a look at Ligeia Mare in the image below. With a total area of about 130,000 kilometers, this huge Titan lake is 50 percent larger than Lake Superior. The Cassini flyby on April 22 will re-observe this area. Similar transient features have been found on Kraken Mare, the largest of Titan’s seas. In both cases, we’re learning that Titan’s seas are more active places than originally thought.
The April 22 observations may help us to distinguish between floating or suspended solids, waves or bubbles as the culprit for the ‘magic islands,’ assuming that Cassini finds the phenomenon again. And if bubbles seem an unlikely cause, consider a new paper in the journal Icarus, which gives us an interesting look into how nitrogen behaves as it interacts with methane and ethane in conditions like those on Titan’s surface.
Image: These images from the radar instrument aboard NASA’s Cassini spacecraft show the evolution of a transient feature in the large hydrocarbon sea named Ligeia Mare on Saturn’s moon Titan. Analysis by Cassini scientists indicates that the bright features, informally known as the “magic island,” are a phenomenon that changes over time. They conclude that the brightening is due to either waves, solids at or beneath the surface or bubbles, with waves thought to be the most likely explanation. They think tides, sea level and seafloor changes are unlikely to be responsible for the brightening. The images in the column at left show the same region of Ligeia Mare as seen by Cassini’s radar during flybys in (from top to bottom) 2007, 2013, 2014 and 2015. The bottom image was acquired by Cassini on Jan. 11, 2015, and adds another snapshot in time as Cassini continues to monitor the ephemeral feature. The feature is apparent in the images from 2013 and 2014, but it is not present in other images of the region. Credit: NASA/JPL-Caltech/ASI/Cornell.
Through simulations of Titan’s surface conditions, JPL researchers have learned that a great deal of nitrogen can be dissolved in the cold liquid methane that rains out of Titan’s skies and flows through its rivers and lakes to the seas. This JPL news release likens the nitrogen release to the fizz that happens when you open a bottle of carbonated soda. The nitrogen bubbles would vary with the composition of the moon’s lakes and seas, depending on the concentration of ethane vs. methane, as JPL’s Michael Malaska, who led the study, explains:
“Our experiments showed that when methane-rich liquids mix with ethane-rich ones — for example from a heavy rain, or when runoff from a methane river mixes into an ethane-rich lake — the nitrogen is less able to stay in solution.”
This release of nitrogen could be a widespread phenomenon on Titan, and one that could also mark changes in season as methane seas warm and cool during the year. So the possibility exists that the ‘magic islands’ may be explained by fields of bubbles emerging from below. Nitrogen, like carbon dioxide being absorbed in Earth’s oceans, moves in both directions.
“In effect, it’s as though the lakes of Titan breathe nitrogen,” Malaska said. “As they cool, they can absorb more of the gas, ‘inhaling.’ And as they warm, the liquid’s capacity is reduced, so they ‘exhale.'”
The team’s simulations also show that when ethane ice forms — it appears on the bottom of a simulated Titan lake, rather than, like water on Earth, floating on the top — nitrogen can be released. So we have plentiful ways of coaxing bubbles out of liquid on Titan.
Whether or not these nitrogen bubbles are the cause of the ‘magic islands’ on Titan, we need to learn as much as we can about them. Proposals to send robotic vessels to float through Titan’s seas may have to be modified to account for them, for heat from the probe could cause bubbles to form around its various surfaces, possibly inducing problems in stability. The JPL work shows that even the slightest changes in temperature, air pressure or composition can cause the absorbed nitrogen to separate rapidly, a process we’ll have to anticipate.
The paper is Malaska et al., “Laboratory measurements of nitrogen dissolution in Titan lake fluids,” published online at Icarus 2 February 2017 (abstract).
JAXA Sail to Jupiter’s Trojan Asteroids
I like the way Jun Matsumoto approaches his work. A researcher with JAXA (Japan Aerospace Exploration Agency), Matsumoto is deeply involved in the design of the space sail that will pick up where Japan’s IKAROS left off. Launched in 2010, the latter was a square sail 14 meters to the side that demonstrated the feasibility of maneuvering a sail on interplanetary trajectories. JAXA has talked ever since about going to Jupiter, but the challenges are formidable, not the least of which is the question of generating enough power to operate over 5 AU from the Sun.
Image: A computer rendering shows what JAXA’s solar sail may look like as it approaches an asteroid. The probe is at the sail’s center. Credit: Japan Aerospace Exploration Agency.
But back for a moment to Matsumoto, who has the kind of long-term approach to his work that this site has long championed. I ran into him in an article in the Japan Times that ran last summer (thanks to James Jason Wentworth for the pointer). Matsumoto knows he is tied up in a project that will take decades, and he relishes the notion. Let me quote from the newspaper:
“I am currently the youngest JAXA staffer on this project. But by the time it is completed, I might no longer be part of the team due to retirement,” said Matsumoto, 27, who dreamed of becoming an astronaut as a child. “Wherever I go, people always talk about how to train young people to become the researchers of the next generation.
“Personally, I want to show children that there are adults who aim to explore places no human has ever gone.”
Image: JAXA workers and others attach electric wiring to a thin film that contains solar panels on July 13 in Sagamihara, Kanagawa Prefecture. Credit: Satoko Kawasaki / Japan Times.
It’s been too long since I’ve talked about this mission. The time frames here come from the necessary travel and exploration time, and the lengthy process of designing and building the spacecraft in the first place. The new JAXA sail, which has been in the planning pipeline since before IKAROS flew, will span 50 meters to the side, 2500 square meters that will contain the 30,000 solar panels — thin film solar cells attached to the entire surface of the sail membrane — necessary to operate at the 5.2 AU distance of Jupiter’s trojan asteroids.
Like IKAROS, the sail will use liquid crystal reflectivity control devices as a means of attitude control. But the new sail will also carry a high specific impulse ion engine for maneuvering among the trojan asteroid population. Here we’re at a key issue in the mission, for operating this far from the Sun, generating electrical power becomes increasingly difficult, and the craft will also need to perform numerous trajectory changes. Just as significant as the sail itself, then, will be the operational success of the new sail’s solar panels and ion engine.
The sail is to be made up of 10-micrometer-thick polyimide, with the payload attached to the center of the sail. Current plans are for launch in the early 2020s. The Jupiter trojans are a group of asteroids that share orbits with the giant planet, clustering in its L4 and L5 Lagrangian points. There should be no shortage of candidates, for the total number of Jupiter trojans greater than 1 kilometer in size is estimated at 106. The JAXA sail will perform both flyby and rendezvous operations, with a landing on the surface of a 20-30 km asteroid, operations there and, if all goes well, a sample return to the Earth in the 2050s.
Image: A slide from a JAXA presentation by Osamu Mori et al. on the new sail, called Direct Exploration of Jupiter Trojan Asteroid using Solar Power Sail, available online.
Last summer at a gymnasium in Sagamihara, Kanagawa Prefecture, a team of JAXA workers, academics and students unfolded a full-scale model of one of the sail’s four trapezoidal components before an audience of 250. As the article in the Japan Times points out, a mission lasting decades has to look not only to current researchers but the young scientists to whom its fate will be handed off, making Matsumoto’s long-term outlook germane not just to sail missions to Jupiter space but any long-haul ventures with decadal timeframes.
TRAPPIST-1h: Drawing on K2 Data
The data recently made available from Campaign 12 of K2 (the Kepler spacecraft’s two-reaction wheel mission) is already paying off in the form of information about the outermost planet in the TRAPPIST-1 system. Campaign 12 (described in Kepler Data on TRAPPIST-1 Coming Online) began on December 15 of 2016 and ran until March 4 of this year, though the spacecraft was in safe mode for a time, producing a 5-day data loss.
An international team including lead author Rodrigo Luger (University of Washington) and TRAPPIST-1 planet discoverer Michaël Gillon (Université de Liège) used the K2 data to constrain the period of TRAPPIST-1h, the outermost planet in this seven-planet system, which had only been observed to transit once before now. The team was also looking for additional planets (none were found) and, of course, examining resonances with the inner worlds.
The result: The orbital period of TRAPPIST-1h is found to be 18.764 days, a figure that fits into the pattern of resonance that the team’s theoretical work had predicted. TRAPPIST-1h is thus “…the seventh member of a complex chain, with three-body resonances linking every member.” The paper goes on to tell us that the planet has a radius of 0.715 R?, and an equilibrium temperature of 169 K, meaning it orbits at the snow line.
Image: Figure 3 from Luger et al. Caption: The short cadence data folded on the four transits of planet h after correcting for TTVs and subtracting a simultaneous transit of b and a near-simultaneous flare. Other transits of b ? g have not been removed and are visible in parts of the data. The data downbinned by a factor of 30 is shown as the orange line, and a transit model based solely on the Spitzer parameters is shown in red. The residuals (data minus this model) are shown at the bottom. Credit: Luger et al.
The paper notes that the stellar flux TRAPPIST-1h receives from its star is below what would be required to sustain liquid water under an atmosphere dominated by nitrogen, carbon dioxide and water (a hydrogen dominated atmosphere could theoretically make it possible). Nor is it on an orbit eccentric enough for tidal heating to warm the surface sufficiently. Nonetheless, tidal interactions play an important role in the evolution of the TRAPPIST-1 planets’ orbits. On the matter of formation and evolution, this was interesting:
The resonant structure of the system suggests that orbital migration may have played a role in its formation. Embedded in gaseous planet-forming disks, planets growing above ? 1 MMars create density perturbations that torque the planets’ orbits and trigger radial migration. One model for the origin of low-mass planets found very close to their stars proposes that Mars- to Earth-sized planetary embryos form far from their stars and migrate inward. The inner edge of the disk provides a migration barrier such that planets pile up into chains of mean motion resonances.
We can even extrapolate something about the speed of formation which, in turn, would have affected the compactness of the resulting system:
The TRAPPIST-1 system may thus represent a pristine surviving chain of mean motion resonances. Given that TRAPPIST-1’s planet-forming disk was likely low in mass and the planets themselves are low-mass, their migration was likely relatively slow. This may explain why TRAPPIST-1’s resonant chain is modestly less compact than chains in systems with more massive planets, which may have protected it from instability.
Image: An artist’s illustration of the seven TRAPPIST-1 planets. Sizes and relative positions are to scale. Credit: NASA/JPL-Caltech.
On the star TRAPPIST-1 itself, the researchers discovered it to be prone to star spots, making it possible to determine a rotational period of about 3.3 days, roughly comparable to nearby late M-dwarfs. The paper suggests an age in the range of 3 to 8 billion years. And note this re flare activity:
The presence of star spots and infrequent weak optical flares (0.38 d?1) for peak fluxes above 1% of the continuum, 30 times less frequent than active M6-M9 dwarfs are consistent with a low-activity M8 star, also arguing in favor of a relatively old system.
I notice that an energetic flare occurred at the end of the K2 campaign, and the authors promise that a full model of flare activity will be offered in an upcoming paper. This is useful information as we investigate questions of habitability among the inner worlds, for flares can damage atmospheres and have other adverse consequences for life.
It’s fascinating to consider how work like this develops from a damaged spacecraft. The paper points out that because of its two failed reaction wheels, the Kepler spacecraft’s rolling motion (created by imbalances in torque) induces strong instrumental effects, which lead to an increase of between 3 and 5 times in photometric noise compared to the original mission. The paper explains how removing these instrumental effects is done, but I continue to marvel at the fact that Kepler is still producing good science despite its serious internal problems.
The paper is Luger et al., “A terrestrial-sized exoplanet at the snow line of TRAPPIST-1,” submitted to Nature Astronomy (preprint).
Fast Radio Bursts: Signature of Distant Technology?
We have a lot to learn about Fast Radio Bursts (FRBs), a reminder that the first of these, the so-called Lorimer Burst (FRB 010724) was detected only a decade ago. Since then we’ve found 16 others, all thought to be at cosmological distances. The 2015 detection of FRB 150418, at first thought to have left an afterglow, has now been traced to an active galactic nucleus powered by a supermassive black hole. FRB 121102 appears to be a rare case of a repeating FRB (about which more a bit later). The distances involved and the brightness of the FRBs have led to source hypotheses ranging from gamma ray bursts to massive neutron stars.
But as Avi Loeb (Harvard University) speculates in a new paper slated to appear in Astrophysical Journal Letters, we could conceivably be dealing with an engineering phenomenon rather than a natural one. What Loeb and Manasvi Lingam, a Harvard postdoctoral fellow at Harvard’s School of Engineering, discuss is whether FRBs could be interpreted as artificial beams set up as an infrastructure for pushing lightsails.
Analyzing the distance of the beam sources through their dispersion measures — the delay of radio frequencies as they sweep through charged particles between the Earth and the source — Loeb and Lingam go on to look at the spectra of FRBs and find them consistent with an artificial origin. Indeed, FRBs can be analyzed as the result of energetic natural processes, but we can equally well work out their parameters in terms of engineering requirements.
Image: Harvard’s Avi Loeb. Credit: Kris Snibbe (Harvard Staff Photographer).
If an extraterrestrial civilization were using the energy of a star to power these beams, and if water were being used as a coolant, such beamers, though enormous, would not violate known physics. The paper extracts a minimum aperture diameter to maintain the operation, finding it to be roughly twice the diameter of the Earth, meaning that the beamer would be an object on the order of a planet, a large ‘super-Earth’ or even a ‘mini-Neptune.’ And in keeping with the speculative nature of the inquiry, the authors point out that we can also consider beam emitters along the lines of the Stapledon-Dyson spheres that have been postulated to use stellar system materials to build artifacts surrounding and enveloping the host star.
(Let me digress to note approvingly the authors’ use of ‘Stapledon-Dyson spheres’ in place of the more common ‘Dyson spheres.’ I heartily approve of this tribute to the great philosopher and science fiction writer Olaf Stapledon, who developed the idea in works like Star Maker (1937), and think I will start using the term on Centauri Dreams).
The power involved in such a beamer could push a payload of a million tons, and the paper speculates that if a beam were indeed being used to power a spaceship, it would have to be a large one, perhaps the kind of ‘worldship’ imagined in science fiction, though not one as large as some of the worldships that have been discussed in the interstellar literature. As to what we would see of this operation on Earth, the motion of the beamer relative to the receding sail would be changing relative to the observer, which means we would detect no more than a brief flash, although powering a lightsail would require the transmitter to focus its beam continuously.
Image: An artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. The leakage from such beams as they sweep across the sky would appear as Fast Radio Bursts (FRBs), similar to the new population of sources that was discovered recently at cosmological distances. M. Weiss/CfA.
These are fascinating speculations, but what can we do to delve further into the origin of FRBs? The paper offers multiple ways into the problem:
It should be possible to distinguish between FRBs of natural and artificial (light sail) origin based on the expected shape of the pulse, as the beam sweeps by to power the light sail (Guillochon & Loeb 2015). More specifically, the sail would cast a moving shadow on the observed beam, thereby leading to a diffraction pattern and multiple peaks in the light curve based on the sail geometry (Manchester & Loeb 2016). A series of short symmetric bursts would be observed as the beam’s path intersects with the observer’s line of sight (Guillochon & Loeb 2015). Hence, looking for similar signatures in the signal could help determine whether FRBs are powered by extragalactic civilizations (although the use of a broad range of frequencies might smear these signals).
Here, then, is a target for Breakthrough Listen or other SETI initiatives, which Loeb and Lingam advocate should direct their attention toward the repeating FRB 121102, the only source known to repeat. Continuing study of this source is reasonable because we would expect astrophysical explosions of the kind advanced by some theorists to be single events, while artificial sources like lightsail beamers or beacons can repeat in the course of their normal operations.
Another interesting thought: Other models of FRBs that assume natural phenomena like gamma-ray bursts would have an FRB occurrence rate determined by the formation rate of the massive stars that produce the bursts. If, on the other hand, FRBs really are artificial and have a planetary origin, then their rate would be set by the number of planets with advanced civilizations (this draws on an interesting discussion in the paper on the upper bound of extraterrestrial civilizations — fewer than 104 FRB-producing civilizations in a galaxy like our own). Complicating this, however, is the issue of whether all FRBs have the same origin — perhaps we should focus only on repeating FRBs as possibly artificial.
For that matter, as Loeb and Lingam point out, a given civilization could set up more than one beamer, especially if it had reached a state as advanced as Kardashev II or III. As we continue investigating FRBs, it’s worthwhile remembering that 104 FRBs are now thought to occur in our sky each day. Working out detection rates for galaxies within 100 Gpc3, the authors calculate that each galaxy has a probability of 10-5 FRBs per day. We might expect one from within our own galaxy every 300 years or so, a spectacular event if it occurred anywhere within 20 parsecs, and one that could “…reveal everything that can be known about the true origin of FRBs, and thereby settle this FRB origin debate once and for all.”
It’s a fascinating discussion, and one that makes me wonder whether other signals in our astronomical data at different wavelengths might be similarly interpreted. As to the nature of such investigations, Avi Loeb makes this statement in a CfA news release:
“Science isn’t a matter of belief, it’s a matter of evidence. Deciding what’s likely ahead of time limits the possibilities. It’s worth putting ideas out there and letting the data be the judge.”
A sound reminder indeed of how to proceed in such speculative realms. The paper is Loeb and Lingam, “Fast Radio Bursts from Extragalactic Light Sails,” accepted for publication at The Astrophysical Journal Letters (preprint).
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