Centauri Dreams
Imagining and Planning Interstellar Exploration
Toward Gravitational Wave Astronomy
The second detection of gravitational waves by the LIGO (Laser Interferometer Gravitational-Wave Observatory) instruments reminds us how much we gain when we move beyond the visible light observations that for so many millennia determined what people thought of the universe. In the electromagnetic spectrum, it took data at long radio wavelengths to show us the leftover radiation from the Big Bang, and we’ve used radio ever since to study everything from quasars and supernovae to interesting molecules in interstellar space. Infrared helps us penetrate dust clouds and see not only into star-forming areas but the galactic center.
So much is learned by taking advantage of the enormous width of the electromagnetic spectrum, wide enough that, as Gregory Benford points out, visible light is a mere one octave on a keyboard fifteen meters wide. Ultraviolet tells us about the gaseous halo around the Milky Way and shows us active galaxies and quasars while helping us analyze interstellar gas and dust. X-rays and gamma rays deepen our understanding of black holes and matter moving at extremely high velocities, tuning up our knowledge of supernovae.
And now gravitational waves are taking us off the keyboard entirely. We’re at the dawn of gravitational wave astronomy, using distortions of spacetime itself to learn about the merging of black holes. The new detection from the LIGO team came on 26 December of last year, making the case that if we have found two black hole mergers in three months, such events must be relatively common in the universe. The work was announced at the recent meeting of the American Astronomical Society in San Diego and published in Physical Review Letters.
Image: Simulation of the motion of two black holes just before merging, and the gravitational waves they produce. Credit: Max Planck Institute for Gravitational Physics.
At a confidence level of 99.99999%, the GW151226 detection shows us a pair of black holes merging as they lose energy in the form of gravitational waves. The first detection, on 14 September 2015, involved black holes of 29 and 36 solar masses respectively. The second event involves black holes between 8 and 14 times as massive as the Sun, revealed to us in a signal that lasted about one second. Researchers believe the event took place about 1.4 billion light years from the Earth. A third possible detection came in October of 2015 but was at a much lower degree of certainty. It too is discussed in the Physical Review Letters paper.
As to GW151226 itself, we learn that the data are consistent with previous theory (public data are available here). From the Abbott et al. discovery paper:
Binary black hole formation has been predicted through a range of different channels involving either isolated binaries or dynamical processes in dense stellar systems. At present all types of formation channels predict binary black hole merger rates and black hole masses consistent with the observational constraints from GW150914. Both classical isolated binary evolution through the common envelope phase and dynamical formation are also consistent with GW151226, whose formation time and time delay to merger cannot be determined from the merger observation. Given our current understanding of massive-star evolution, the measured black hole masses are also consistent with any metallicity for the stellar progenitors and a broad range of progenitor masses.
Unlike electromagnetic waves, gravitational waves have the intriguing property that they propagate unperturbed once they have been created, which places the remote corners of the universe into our field of ‘view,’ as Asimina Arvanitaki (Stanford University) and Andrew Geraci (University of Nevada, Reno) pointed out in a 2013 paper that looked at ways to enhance gravitational wave detection. As our sensitivity to such signals increases, we should be able to move from black holes to neutron stars and supernovae, and perhaps the merger of binary stars, as events that can be examined by these techniques.
Two gravitational wave detectors are online in the United States (in Louisiana and Washington state) and one in Italy (the European Gravitational Observatory near Pisa), with Japan’s Kamioka Gravitational Wave Detector expected to become available in 2018 (I don’t know what the gravitational wave equivalent of ‘first light’ is, but maybe we should dream one up). India is working on a GW detector of its own. Five detectors will make locating the source of gravitational waves more accurate. Meanwhile, ESA’s LISA Pathfinder spacecraft (Laser Interferometer Space Antenna) is testing technologies that the upcoming eLISA (Evolved Laser Interferometer Space Antenna) will be using upon its planned launch in the 2030s.
The papers are Abbott et al., “GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence,” Physical Review Letters 116, 241103 (2016) (abstract); Barausse et al., “Theory-Agnostic Constraints on Black-Hole Dipole Radiation with Multiband Gravitational-Wave Astrophysics,” Physical Review Letters 116, 241104 (2016) (preprint). The Arvanitaki and Geraci paper is, “Detecting high-frequency gravitational waves with optically-levitated sensors,” Physical Review Letters 110, 071105 (2013) (abstract).
FU Orionis: Implications of Sudden Brightening for Planet Formation
I would like to thank the many Centauri Dreams readers who contributed to the successful Kickstarter campaign to fund a year’s worth of study of KIC 8462852. As I write, there is less than an hour to go, but we have already gone well over the needed $100,000 mark. Congratulations to Tabitha Boyajian, and thanks for all the work she and her colleagues have put into this effort. Now we have a year of observations ahead using the Las Cumbres Observatory Global Telescope Network. The long-term observations will be crucial because we don’t know what to expect in terms of sudden dimming in this star’s light curve.
What a pleasure it is to write for this audience. Readers here have played a large role in pushing this project over the top, and we’ll follow the work on KIC 8462852 closely in coming days. Meanwhile, have a look at Penn State’s Jason Wright discussing ‘Tabby’s Star.’
Speaking of Unusual Stars…
If KIC 8462852 is a star that some believe is becoming less bright over time (the controversy between Bradley Schaefer and Michael Hippke is well documented here), FU Orionis is the opposite, a star whose notable brightening in 1936 is the most extreme such event we have seen around a star of Sun-like size. This is intriguing stuff, because we seem to be looking at a solar system in its infancy, undergoing a process that our own Sun may have experienced on the way toward the formation of the planets in our system. If so, this stellar activity would have changed the chemistry in the surrounding disk, with implications for how planets form.
The 1936 brightening event took FU Orionis, some 1500 light years away in the constellation Orion, from an apparent visual magnitude of 16.5 to 9.6, and the star is now around magnitude 9, with visible light observations showing a slow fade in progress. Stars like this — V1057 Signi is another — are young pre-main sequence stars in a class named after FU Orionis and nicknamed FUors. All exhibit extreme changes in magnitude and spectral type, evidently caused by the star consuming inner disk materials in a sudden feeding spree.
Joel Green (Space Telescope Science Institute) and colleagues wondered when FU Orionis might return to pre-1936 brightness levels, and what effect such brightening events could have on early system formation. The team used infrared data from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and checked it against Spitzer Space Telescope data from 2004. Green presented the results, showing the star’s behavior over the twelve-year interval, at the just concluded meeting of the American Astronomical Society in San Diego.
Image: This artist’s concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 (left) to 2016 (right). Credit: NASA/JPL-Caltech.
The brightening of FU Orionis observed in 1936 was the result of the infant star devouring gas and dust from its surrounding disk, a three-month process that caused it to become 100 times brighter in short order, heating the disk to temperatures up to 7000 K. It’s clear from the most recent study that the star continues to ingest disk material. According to this JPL news release, it has consumed the equivalent of 18 Jupiters in the eighty years since the first outburst.
Even so, the SOFIA measurements confirm that the total amount of visible and infrared light from FU Orionis has decreased by about 13 percent during the twelve years since the Spitzer data were taken. The dimming is occurring at short infrared wavelengths only, an indication that about 13 percent of the hottest disk material has disappeared even as cooler materials have remained intact. Says Green:
“A decrease in the hottest gas means that the star is eating the innermost part of the disk, but the rest of the disk has essentially not changed in the last 12 years. This result is consistent with computer models, but for the first time we are able to confirm the theory with observations… The material falling into the star is like water from a hose that’s slowly being pinched off. Eventually the water will stop.”
Based on this work, we could expect FU Orionis to return to 1936 brightness levels within the next several hundred years. Still unanswered is the question of what set off the rampant activity in the first place. The fact that a protoplanetary disk can experience such sudden activity is striking. Green’s team believes that a similar event in the early Sun’s history could explain why materials close to the Sun have a different composition than those farther out, accounting for the different abundances of certain elements on Mars than on the Earth. The brightening would have had a much larger effect on the inner disk than on more distant materials.
The presentation is Green, “The Evolution of FU Orionis Disks,” American Astronomical Society, AAS Meeting #228, id.#308.05 (abstract).
Stéphane Dumas (1970 – 2016)
The interstellar community is a small one, and reporting the loss of one of our number is not easy. SETI researcher Stéphane Dumas, who had been working with Claudio Maccone on the application of the Karhunen-Loève transform (KLT) for SETI observations, has died unexpectedly at his home in Quebec. I remember a wonderful conversation with Stéphane at one of the 100 Year Starship meetings in Houston, where we got into a spirited exchange about interstellar propulsion. It was, alas, the only time I spent with the man, but he was also active on the advisory board for Jon Lomberg’s One Earth Message project, and so we interacted electronically. Below is a video of Stéphane and Claudio Maccone presenting the latest work in mathematical SETI. You’ll find Stéphane’s talk at about 34:56 on the counter.
Structure and Composition of a White Dwarf Planet
Given everything we’re learning about planets around other suns, it’s frustrating that we have so little information about the chemical composition of the rocky planets we’ve found thus far. Now we have a new study, announced at the San Diego meeting of the American Astronomical Society, that offers data on a ‘planet-like body’ whose surface layers are being consumed by the white dwarf SDSSJ1043+0855. Although it’s been known for some time that the star has been devouring rocky material orbiting around it, the new work offers a striking view of the accretion process and the composition of what was once a differentiated body.
At least, that’s the best interpretation of the data taken from the Keck Observatory’s HIRES spectrometer (installed on the 10-meter Keck I instrument) and the Hubble Space Telescope. White dwarf stars are the remains of stars like the Sun — this one was once a few times the Sun’s mass — that have gone through their red giant phase and expelled all their outer material. The ‘planet-like body’ the researchers refer to is likely the remnant of a surviving planet.
To study the chemical composition of such a world, Carl Melis (UC-San Diego) and Patrick Dufour (Université de Montreal) used the spectra of the rocky accretion material as filtered through the atmosphere of the star. The researchers believe that using these methods, they are able to analyze specific layers of the body undergoing accretion. We learn that the object shows large amounts of carbon, combined with smaller amounts of calcium and oxygen.
We may be looking, Melis and Dufour suggest, at calcium carbonate (CaCO3), a mineral widely found in shelled marine organisms here on Earth. As this news release from Keck Observatory points out, incorporating carbon in the surfaces of rocky objects is difficult, which is why the terrestrial planets in our own Solar System are sometimes described as being in a ‘carbon desert.’ The surface being accreted by SDSSJ1043+0855 shows perhaps as much as several hundred times the amount of carbon found on the surface of the Earth.
Is it possible that life played a role in this object’s history? Melis comments:
“…the presence of such high levels of carbon is unique and really needs to be explained. Our choice of calcium-carbonate as a potential carrier of the carbon provides a natural way for it to be locked up in the planet and eventually delivered to the white dwarf star, is entirely consistent with the observations in hand, and of course is suggestive. That’s really the hidden subtext. When people think about finding extra-terrestrial life, they think about Hollywood dramatizations. But the first evidence of life outside of our Solar system will probably come in a much subtler form. More likely than not, it’s going to come as a nuanced signature that may not be immediately recognizable.”
Image (click to enlarge): Artist’s impression of the surface of the massive, planet-like body being devoured by the white dwarf SDSSJ1043+0855. The Keck Observatory and Hubble Space Telescope data (shown in inset) show calcium and carbon, the presence of which can be explained with a model suggesting the surface of the planet may have been encrusted in limestone (calcium carbonate). This material was removed from the surface of the massive rocky body, probably through large-scale collisions, subsequently shredded into a disk of material, and accreted by the white dwarf star (ringed object seen in the planet’s sky). Credit: A. Hara/C. Melis/W. M. Keck Observatory.
But calcium carbonate isn’t always the result of biology, and the current work examines only the accretion materials that have been absorbed by the parent white dwarf. Melis and Dufour would like to look next at surrounding dust before it falls into the star, using the James Webb Space Telescope if possible to confirm whether calcium carbonate is present. This would allow a better estimate of whether the amount of calcium carbonate is consistent with natural processes.
Centauri Dreams‘ take: Calcium carbonate or not, it’s striking that using accreted material in the region of a white dwarf and in its atmosphere can help us understand the structure of an exoplanet. We move beyond bulk chemical composition to differentiate between the layers of the body being accreted. That’s a highly useful tool for studying planetary structure.
The presentation is Melis and Dufour, “The Surface of a Limestone-Rich World?” American Astronomical Society 20 June 2016, AAS Meeting #228, id.#201.03 (abstract).
A Long-Period Circumbinary World
Before getting into today’s subject, the discovery of an interesting long-period circumbinary planet, I want to make another pitch for Centauri Dreams readers to support the Kickstarter campaign for Tabby’s Star. I’ve written often about this mysterious star whose light curves are anomalous and demand further study. Trying to find out what’s happening around KIC 8462852 means acquiring more data, and the Kickstarter campaign would provide an entire year of observations using the Las Cumbres Observatory Global Telescope Network.
We’re now down to 48 hours and of the $100,000 needed, about three-fourths has been raised. Coming down the homestretch, the remaining $24,000 should be achievable, but it looks to be a dramatic finish. If you haven’t been following the KIC 8462852 story, you can check the archives here, or for a quick overview, see my article A Kickstarter Campaign for KIC 8462852. Whatever you can do to help would be hugely appreciated as we try to learn as much as possible about what Penn State’s Jason Wright has called ‘the most mysterious star in our galaxy.’
Kepler-1647b: Insights into Planet Formation
On to the ongoing meeting of the American Astronomical Society in San Diego, where the discovery of the largest planet yet found around a double-star system was announced. That’s ‘around’ a double-star system rather than ‘in’ one, the planet in question orbiting both stars. Such circumbinary planets are welded to the imagination of Star Wars viewers because of the world Tatooine, the cinematic home of Luke Skywalker, and as in the film, they cast a hypnotic spell on the imagination as we think about what such worlds might look like.
No standing on a planetary surface to watch interesting sunsets here, though. Kepler-1647b is a gas giant, about 3700 light years away in the constellation Cygnus, and at 4.4 billion years old, it’s roughly the same age as the Earth. The planet orbits its eclipsing binary host — two solar mass stars — every 1100 days, making it the longest-period transiting circumbinary planet yet discovered. Despite the lengthy orbital period, three times longer than the Earth’s, Kepler-1647b appears to be in the circumbinary habitable zone for the entire duration of its orbit. Terrestrial moons are theoretically possible, but no evidence for them turns up in the data.
Image: Artist’s impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647 b. Such a double eclipse event is known as a syzygy. Credit: Lynette Cook.
The discovery of this Jupiter-like world is the work of a team led by Veselin Kostov (NASA Goddard). A transit was originally detected back in 2011 but additional data were needed before the circumbinary planet could be confirmed, part of the problem being the length of the planet’s orbital period. As co-author William Welsh (San Diego State University) notes: “…finding circumbinary planets is much harder than finding planets around single stars. The transits are not regularly spaced in time and they can vary in duration and even depth.”
At three years, Kepler-1647b’s orbital period is the longest of any confirmed transiting planet. The gas giant is also the largest circumbinary planet yet found since the first such world, Kepler-16b, was detected through its transits. We now know of 10 transiting circumbinary planets in eight eclipsing binary systems, and what’s intriguing here is that all of these except Kepler-1647b are near the critical orbital distance for dynamical stability. Get any closer to the binary host, in other words, and their orbits would become unstable.
But Kepler-1647b is on a much wider orbit and its large size contrasts with all previously discovered circumbinary planets, which have been Saturn-sized or smaller. Theoretical models had predicted that Jupiter-mass circumbinary worlds should be less common and should orbit at large distances from the central binary. The discovery of a gas giant on a wide orbit is thus consistent with these models. No long-period circumbinary worlds have been detected before this, a fact whose implications for planetary evolution are discussed in the paper:
As important as a new discovery of a CBP [circumbinary planet] is to indulge our basic human curiosity about distant worlds, its main significance is to expand our understanding of the inner workings of planetary systems in the dynamically rich environments of close binary stars. The orbital parameters of CBPs, for example, provide important new insight into the properties of protoplanetary disks and shed light on planetary formation and migration in the dynamically-challenging environments of binary stars. In particular, the observed orbit of Kepler-1647b lends strong support to the models suggesting that CBPs form at large distances from their host binaries and subsequently migrate either as a result of planet-disk interaction, or planet-planet scattering…
Image: The orbit of Kepler 1647b (white dot) around its two suns (red and yellow circles). Kepler-1647 b was observed transiting each of its two suns during a single orbit (days 0 and 4.3). Credit: Kostov et al.
The paper is Kostov et al., “Kepler-1647b: the largest and longest-period Kepler transiting circumbinary planet,” accepted at the Astrophysical Journal (preprint). A news release from the University of Hawaii at Manoa is also available.
Asteroids as Spacecraft
Rama is a name that resonates with science fiction fans who remember Arthur C. Clarke’s wonderful Rendezvous with Rama (1973). The novel depicts a 50-kilometer starship that enters the Solar System and is intercepted by a human crew, finding remarkable and enigmatic things that I will leave undescribed for the pleasure of those who haven’t yet read the book. Suffice it to say that among Clarke’s many fine novels, Rendezvous with Rama is, along with The City and the Stars, a personal favorite.
What a company called Made in Space Inc. has in mind is something different than Clarke’s vision, though it too evokes names from the past, as we’ll shortly see. Based in Mountain View, CA the company is embarking on an attempt to turn asteroids into small spacecraft that can move themselves to new trajectories. RAMA in this case stands for Reconstituting Asteroids into Mechanical Automata, and it proceeds by putting ‘Seed Craft’ on asteroids that will use materials found on the surface. This is the kind of in situ resource utilization (ISRU) that Ian Crawford discussed in his essay in these pages last Friday.
A suitably modified asteroid could take itself to the nearest extraction point for human mining, while the seed craft could be sent on to another asteroid. Build the system right, Made in Space believes, and you can do away with at least some of the human control needed for space operations. 3-D printing plays a big role here, no surprise given the company’s background in providing the first such printer (to the ISS in 2014) that can function in zero-g. Our friend Jon Lomberg worked with Made in Space to create a ‘Golden Plate’ commemorating the first space manufacturing operation, now attached internally to the functioning space printer.
That name from the past I mentioned above is Dandridge Cole, an aerospace engineer, former paratrooper and futurist whose death in 1965 at the young age of 44 cost the space community one of its true visionaries. Cole had plenty of ideas of his own on moving asteroids, but in his case, the idea involved more than robotic transfer into a new orbit. Much more. Why not, thought Cole, actually hollow out an asteroid to create an internal habitat? Here’s how Alex Michael Bonnici described Cole’s idea in a tribute written in 2007:
In 1963, Cole wrote Exploring the Secrets of Space: Astronautics for the Layman with I. M. Levitt. In this book they suggested hollowing out an ellipsoidal asteroid about 30 km long, and rotating it about its major axis to simulate gravity. By reflecting sunlight inside with mirrors, and creating, on its inner surface, a pastoral setting an asteroid could be transformed into a permanent space colony. Cole and Cox also envisioned that asteroids would provide the raw materials to form the basis of a spacefaring civilization. And, that asteroidal materials would also serve terrestrial needs. In their view these materials could be transported using mass drivers or linear motors. Cole’s work largely presages that of Gerard K. O’Neill by more than a decade.
Image: Aerospace engineer and futurist Dandridge Cole, who coined the term ‘macro-life’ to refer to human colonies in space and their evolution. Credit: Wikimedia Commons.
Hollow asteroids are an idea familiar to science fiction fans, who will have encountered the trope in various short stories and perhaps in George Zebrowski’s 1979 novel Macrolife. The name is carefully chosen because Cole used ‘macro-life’ to describe future human evolution within space habitats like these, a development he thought would involve a life-form incorporating technology and intimately synchronized with its environment. Putting large colonies of hollow asteroids into play would ensure our species’ survival while allowing us to progress, he believed, beyond dangers like nuclear proliferation and population pressure.
Here’s Cole in 1961, from an essay called “The Ultimate Human Society”:
This concept of a new life form which I call Macro Life and Isaac Asimov calls ‘multiorganismic life’ serves as a convenient shorthand whereby the whole collection of social, political, and biological problems facing the future space colonist may be represented with two-word symbols. It also communicates quickly an appreciation for the similar problems which are rapidly descending on the whole human race. Macro Life can be defined as ‘life squared per cell.’ Taking man as representative of multicelled life we can say that man is the mean proportional between Macro Life and the cell, or Macro Life is to man as man is to the cell. Macro Life is a new life form of gigantic size which has for its cells individual human beings, plants, animals and machines.”
Arthur Clarke liked the notion enough to call Zebrowski’s novel ‘a worthy successor to Olaf Stapledon’s Star Maker,’ which had been a major influence on Clarke and most of his contemporaries. As to the notion of moving asteroids about, an early treatment was Robert Heinlein’s ‘Misfit,’ in which an asteroid is moved out of the main belt to an orbit between Mars and the Earth. This one made its appearance in the November 1939 issue of Astounding Science Fiction, and would hardly be the last asteroid-themed tale. A more modern take shows up in Larry Niven’s Known Space stories and the memorable ‘Belters.’
Image: An engineered asteroid from without and within. Illustrator Roy Scarfo worked with Cole on the 1965 book Beyond Tomorrow. Credit: Roy Scarfo.
We’ve come a long way from Made in Space and their plans to move asteroids through ‘seed craft’ and in situ resource utilization, but what I find exciting here is the synergy between some of these ideas from the past and the conceptual studies Made in Space is performing, with help from NASA’s Innovative Advanced Concepts Program. Asteroid mining gives us a route forward as we contemplate infrastructures within the Solar System, building, we can hope, toward a society comfortable working in deep space and continuing to explore.
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