by Paul Gilster | Oct 31, 2014 | Exoplanetary Science |
Because building an economically sustainable infrastructure in the Solar System is crucial for the development of interstellar flight, I was interested to learn about a game called High Frontier, which looks to combine O’Neill habitats with a steady expansion of our species outward. Have a look at the Kickstarter campaign page if the idea of modeling space colonies as an extension of human civilization appeals to you. High Frontier seems to be a chance to get involved in game creation from the ground up to create models of how a starfaring culture might grow.
I’ve never gotten involved in gaming, but I can see the potential for education in games that accurately model complex economies or cultural interactions. In the case of deep space scenarios, it’s possible to model an interstellar mission that does not rely on an established infrastructure. Indeed, we just looked at one in Dana Andrews’ recent paper, which asks how a mission without such resources could be mounted. But building a system-wide economy that can sustain an interstellar effort and tunes up the basic technologies seems like the most likely outcome, and I’m all for exploring the various scenarios within which that could occur.
Back to the Exoplanet Chase
As we build infrastructures, whether in simulations or in reality, we keep looking outward at potential targets for exploration, and at stellar systems that tell us more about how planets form. The news about a multiple-star system called GG Tauri-A, some 450 light years from Earth in the constellation Taurus, is intriguing. This is a system with a large, circumbinary outer disk as well as a second, inner disk around one of the two binary components. The inner disk is losing material to its star at a rate that should have made it disappear a long time ago.
Image: Artist’s impression of the double-star system GG Tauri-A. Credit: ESO.
A research team led by Anne Dutrey (Laboratory of Astrophysics of Bordeaux and CNRS) used the Atacama Large Millimeter/submillimeter Array (ALMA) in new observations of the dust and gas dispersed in the GG Tau-A system. What has turned up are clumps of gas flowing between the outer and inner disk, replenishing and sustaining the latter. Here’s Dutrey’s comment on the finding:
“Material flowing through the cavity was predicted by computer simulations but has not been imaged before. Detecting these clumps indicates that material is moving between the discs, allowing one to feed off the other. These observations demonstrate that material from the outer disc can sustain the inner disc for a long time. This has major consequences for potential planet formation.”
Indeed, it’s an interesting situation. In close binaries, we might expect to find a circumstellar disk around each star, and an outer circumbinary disk around both. The paper makes the case that inner disks should be depleted on timescales of no more than a few thousand years as their material is accreted onto the parent star. Here we’re seeing a replenishment process that heightens the possibility of planet formation by continually feeding this region with new materials.
This isn’t the first time that a gas flow between two disk systems — or between gaps within a single disk — has been found. Around the young star HD 142527, Simon Casassus (Universidad de Chile, Chile) and colleagues found streams of gas flowing across such a gap and described it in a paper published in 2013. At HD 142527, the inner disk extends to about 10 AU, with the outer disk about 14 times further out. As with GG Tau-A, these findings were made with ALMA.
We’re looking at a mechanism that could play a significant role in planet formation around both single and binary stars. The paper summarizes the finding, noting that the outer ring shows a distinctive ‘puffed-up’ rim which the researchers think could be caused by stellar heating:
Our observations demonstrate that active replenishment from the outer disk can sustain the circumprimary disc surrounding GG Tau-Aa beyond accretion lifetime, increasing its potential for planet formation. The presence of the condensation at the inner edge of the outer ring is puzzling and needs further investigations to determine its links with accretion processes and possible planet formation. Since almost half of Sun-like stars were born in multiple systems, our observations provide a step towards understanding the true complexity of protoplanetary discs in multiple stellar systems and unveiling planet formation mechanisms for a significant fraction of stellar systems in our Galaxy.
Notice the reference to GG Tau-Aa above. GG Tau-A is part of a still more complex star system known as GG Tauri. This ESO news release points out that recent observations of the multiple star system show that one of its stars — GG Tau Ab, the one that is not surrounded by a disk — is itself a close binary, consisting of GG Tau-Ab1 and GG Tau-Ab2. Beneath the cumbersome nomenclature is the fact that we have identified five objects altogether in the GG Tauri system.
The paper is Dutrey et al., “Planet formation in the young, low-mass multiple stellar system GG Tau-A,” Nature 514 (30 October 2014), pp. 600-602 (abstract).
by Paul Gilster | Oct 30, 2014 | Astrobiology and SETI |
Last year the New Frontiers in Astronomy & Cosmology program, set up by the John Templeton Foundation as a grant-awarding organization, dispensed three grants with a bearing on what Clément Vidal calls ‘Zen SETI.’ The idea of looking into our astronomical data and making new observations to track possible signs of an extraterrestrial civilization at work is not new, and yesterday we looked at Freeman Dyson’s early contribution. Carl Sagan and Josif Shklovskii are also among those in a lineage we can extend back at least to the early 20th Century.
The recent grants show a gathering momentum for extending SETI in new directions. The team of Jason Wright (Pennsylvania State) and colleagues Steinn Sigurðsson and Matthew Povich is embarking on a hunt for Dyson spheres, which if observed in a distant galaxy colonized by a Kardashev Type III civilization, should throw an unmistakable signature in the infrared. Could we find such an object in our data from WISE, the Wide-field Infrared Survey Explorer satellite?
Or how about Kepler? Lucianne Walkowicz at Princeton was a winner of one of the 2013 grants, looking for hints of technology — of artificiality — around distant stars. The third recipient was exoplanet hunter Geoff Marcy (UC-Berkeley), working with Andrew Howard (University of Hawaii) and John Johnson (Caltech) on data from Kepler. When Clément Vidal writes about SETI as an observing rather than a communications program (in The Beginning and the End (Springer, 2014), he gives a powerful boost to the principles behind such searches.
Vidal’s book is rich and densely textured, which is why what I’ve tried to do in the last few days is to extract a few core ideas in the area most related to what we do here on Centauri Dreams. The early chapters are primarily concerned with building a worldview that is consistent with the latest thinking in cosmology, and Vidal speculates as well not only on multiverse theories but on a role for life in the cosmos that includes cosmogenesis, the creation of new universes. Olaf Stapledon immediately comes to mind because Vidal’s ambitious lunge into new intellectual terrain reminds me so much of the British writer. He would be at home with Vidal’s ideas of a cosmological artificial selection, one that draws on and extends ideas originally put forward by another deeply creative thinker, Lee Smolin.
Black Holes and their Uses
What can we, for example, say about black holes in a SETI context? For one thing, they form what would surely be the most powerful gravitational lensing opportunity available. Claudio Maccone has written about the potential of the central black hole in galaxies like the Milky Way becoming surrounded by a swarm of observing stations aligned with targets throughout the universe. For that matter, the lensing of electromagnetic radiation around a black hole is so intense that a communications channel could be set up for intergalactic distances (waiting out the answer is a different problem).
But black holes offer more than this. As the densest known objects in the universe, they can meet the needs of a Type III civilization faced with a continuing demand to support its energy consumption. Vidal runs through the literature on the matter, starting with Roger Penrose, who imagined extraction of black hole rotational energy by injection of matter, and through other scientists (the bibliography is extensive and quite good) who worked out the specifics of drawing energy from rotating black holes. Another possibility: Collecting energy from gravitational waves generated when black holes collide, or actually manipulating the merger of smaller black holes. In recent days, Louis Crane has studied small black holes as a power source — these objects convert matter into energy (Hawking radiation) at high levels of efficiency.
There are computational uses for black holes that push us out to the boundaries of computer science in the form of theorized ‘hypercomputers’ that draw on relativistic effects to dilate time in the proximity of black holes. Vidal’s philosophical ideas of cosmological artificial selection draw on the prospect that a Type III civilization may learn how to use black holes to create entirely new universes. However we view such prospects, the idea here is that for a wide variety of reasons, black holes should be attractors for intelligence. Vidal wants to know what the observable manifestations of any of these uses might be. Would such things be detectable?
Energy Sources for Advanced Civilizations
But we don’t have to confine our search to black holes themselves. If extracting energy from the thin accretion disk around a rotating black hole may be one of the most efficient power sources we can imagine, we can also look for similar configurations around neutron stars or white dwarfs. A key question, then is this: Could a civilization harness its star’s energy with efficiencies that approach black hole densities? The interesting family of binary systems called X-ray binaries (because of their emissions in the X-ray electromagnetic spectrum) should, Vidal believes, intrigue us as one possible sign of an artificial astrophysical system.
Image: An artist’s impression of GRS 1915, which is thought to be an X-ray binary. The black hole sucks material off the companion star, which is heated by friction, emitting X-rays. Credit: Rob Hynes, from http://www.phys.lsu.edu/~rih/.
There are others, including a whole range of contact binaries where stars exchange matter and energy in complicated ways. Vidal’s assumption is that these binaries are natural objects, but he doesn’t want to rule out the possibility that in at least some, we may be seeing something else at work.
Let me quote the author on this:
Accretion is a ubiquitous astrophysical process in galaxy and planet formation, so we may object that all binaries may simply always be natural. But let me introduce an analogy. Fission can be found in natural processes, as well as fusion, which is one of the core energetic processes in stellar evolution. Yet humans seek to copy them, and would certainly benefit greatly from — always — controlling them. So it is not because a process is known to occur naturally that its use in a given case is not under intelligent control. In fact, the situation may even be more subtle. The formation of XRBs might be natural, but they may later be controlled or taken over by ETIs, just as a river flowing down a mountain is a natural gravitational energy source that humans can harness with hydroelectric power stations.
In other words, there is a wide variety of binary stars in which we find accretion disks forming that could provide useful sources of energy to an advanced civilization. Vidal creates the term starivore to describe a civilization that could ‘feed’ on stars. More specifically:
It is an extraterrestrial civilization using stellar energy (Type KII) in the configuration of a slow non-conservative transient accreting binary…, with the dense primary… being either a planet, a white dwarf, a neutron star, or a black hole.
And indeed, Vidal quotes Stapledon’s novel Star Maker by way of showing that the idea of energy extraction from binary systems is not new. A speculative scenario that grows out of this is where our own civilization may one day go. As our technologies function on smaller and denser scales and we continue to move up the Kardashev ladder, thus using more and more energy, we run out of energy even if we cover the Earth with solar panels. So we bring Earth closer to the Sun (a Stapledon notion) to get more energy, with our descendants now living as postbiological beings. Still we need more energy, so stellar engineers create active accretion from, the Sun, transforming what had once been human life into a starivore civilization.
The density of the evolved Earth now approaches that of a white dwarf, and the new binary resembles what we see in our data as a cataclysmic variable, a binary system with white dwarf component. Vidal:
If such binary systems are starivores, then we should find that the primitive versions of them extract energy from a star paired with a planet that is not dense compared to WDs, NS, or BHs. This would happen at a low accretion rate, so planetary accretion is one of the concrete predictions from the starivore hypothesis (and indeed planet-star interactions have recently been discovered…)
Image: An artist’s concept of the accretion disk around the binary star system WZ Sge. P. Marenfeld and NOAO/AURA/NSF.
Vidal’s hypothesis of starivores lets us see high energy astrophysics from an astrobiological point of view. Speculative? Of course, but Vidal is a philosopher for whom the play of ideas is as entrancing as the flow of notes in a Bach fugue. Rather than claiming the existence of starivore civilizations, he offers data on the wide variety of binary systems and the possibilities for energy extraction, with predictions about what we might see if such civilizations exist. A high energy astrobiology agenda is presented containing proposals for specific research. I do not have time this morning to go through the wealth of supporting argument but the book is well worth extended study.
Ultimately, the starivore idea is Vidal’s way of describing SETI’s new direction, a concrete example of how we can study objects in our data that may show the signature of extraterrestrial engineering. Building a robust scientific structure for such inquiries is at the heart of The Beginning and the End, whose principles are being played out and refined in the ongoing SETI searches mentioned at the beginning of this post. As with the original SETI work back to the days of Project Ozma, we can’t know what we’re going to find until we mount the actual search. Finding a Type II or III culture — or its remnants — would show us what intelligent life is capable of, while raising the familiar question of how long any technological species can hope to survive.
by Paul Gilster | Oct 28, 2014 | Astrobiology and SETI |
If we’re trying to extend the boundaries of the search for extraterrestrial intelligence, how do we proceed? A speculative mind is essential, and one of the delights of science fiction is the ability to move through an unrestricted imaginative space, working out the ramifications of various scenarios. But we have to prioritize what we’re doing, which is why Freeman Dyson settled on the idea of looking for conspicuous examples of intelligence using technology. It’s no surprise that the term ‘Dysonian SETI’ has arisen to describe how such a search might proceed.
The Dyson sphere is a case in point. We can imagine a civilization vastly more ancient and technologically adept than our own deciding to maximize the amount of power it can draw from a star. Although Dyson spheres are sometimes pictured as shells completely surrounding a star, Dyson’s ideas are more readily thought of in terms of a ‘swarm’ of objects soaking up as much power as possible. Other configurations are in the mix, including the ‘ringworld’ envisioned by Larry Niven in the novel of the same name. Such engineering would throw a unique astronomical signature. Even a completely enclosed star could be detected by its radiation in the infrared, which is where previous searches for Dyson spheres have been conducted.
Image: Physicist Freeman Dyson, whose work has inspired not only SETI proponents but aerospace engineers, science fiction authors and philosophers of science. Credit: Wikimedia Commons.
Would a powerful civilization build such things? It’s a key question, and as Clément Vidal points out in The Beginning and the End (Springer, 2014), Dyson’s 1966 paper on the matter made the assumption that an alien intelligence would use a technology we can understand. The idea has been rightfully criticized as anthropocentric, even by Dyson himself, who called the notion ‘utterly unrealistic.’ But we have to start somewhere, acknowledging the very real prospect that a truly advanced civilization might operate in ways that mimic natural processes. Developing criteria based on what we do understand at least gives us an opening into studying things we see in our astronomical data that might flag the presence of astroengineering. We’re limited by but must employ our own level of scientific knowledge.
As I mentioned yesterday, Dysonian SETI (or Vidal’s ‘Zen SETI’) does not conflict with older radio and optical SETI methods. By looking for manifestations of technology at work in our astronomical data, Zen SETI largely abandons the idea of SETI as an attempt at receiving intentional communications, and looks instead to identify large-scale anomalies that show us another civilization at work. This form of SETI also pushes not only deep into our own galaxy but into any observable astronomical objects we can see with our telescopes. As I said yesterday, this is a bit like archaeology, with conceivable discoveries that are billions of years old.
Where Life Can Emerge
So while traditional SETI pushes on with its entirely valid search, newer forms of SETI widen the search space and cause us to question the philosophical bases of our assumptions. Should we, for example, assume we are looking for forms of life reliant on carbon and water? Vidal notes a 1980 definition of life by Gerald Feinberg and Robert Shapiro (in Life Beyond Earth, Morrow) that describes life as highly ordered systems of matter and energy ‘characterized by complex cycles that maintain or gradually increase the order of the system through the exchange of energy with the environment.’ Vidal comments:
It is important to notice the high generality of such a definition. There is no mention of carbon, water or DNA. What remains are energetic exchanges leading to an increase of order. Free from the limiting assumptions of [carbon and water], the two authors conceive possible beings living in lava flows, in Earth’s magma, or on the surface of neutron stars. The idea of life on neutron stars was explored not only in science fiction… but also by scientist [Frank] Drake.
The reference to science fiction takes in Robert Forward’s Dragon’s Egg (Ballantine, 1980), a punchy tale driven by the usual Forward gusto. Drake lesser known article is “Life on a Neutron Star,” which ran in Astronomy (Vol. 1, No. 5) in 1973, and which I still have buried in the stacks of old magazines that fill a cabinet here in my office. I remember the Drake with pleasure as one of those eye-opening things that make you look at the world a bit differently when you see how much you are a creature of your own environment. And then you start thinking about how many environments are out there…
The field for speculation is wide — Robert Freitas has even written about the possibility of metabolisms of living systems based on the four fundamental physical forces: the strong nuclear force, electromagnetism, the weak force, and gravitation. We should also consider the possibility (likelihood?) that an advanced civilization will be comprised largely of postbiological beings. Vidal reminds us of the many generations computers have gone through in our own lifetimes, with three-dimensional molecular computing as a possible follow-on to today’s integrated circuits. And he asks what a computer scientist from the 1940s would make of today’s digital world. Would he be able to find large parts of our technology, much less understand it? Vidal adds:
The moral of the story is that in SETI, matter doesn’t matter (much). What is important is the ability to manipulate matter-energy and information, not the material substrate itself. The case for postbiology is strong… Abandoning the hypothesis of ET using a biological substrate such as carbon, water, DNA molecules, or proteins makes us focus on the functional systems theory, which aims to be independent of a particular material substrate. This makes system theory the interdisciplinary research field par excellence and also an indispensable tool in astrobiology and SETI.
Image: Philosopher Clément Vidal approaches SETI with a multidisciplinary background that he uses to question underlying assumptions that affect the search. Credit: Sébastien Herrmann.
Maybe these extracts give some sense of how provocative this tightly written study is, and how often it questions the assumptions we bring to astrobiology. In fact, Vidal thinks a tight analysis of SETI can help us rid ourselves of those assumptions that apply only to terrestrial life so that we can try to uncover what the essential characteristics of all life must be. He’s looking for concepts of living systems and especially intelligence that can be generalized to extraterrestrial venues as we proceed to tighten our criteria for studying anomalous astronomical data.
What kind of things might we hope to find if there is such a thing as astroengineering on an interstellar scale? Beyond the aforementioned Dyson spheres, could we detect extensive mining in asteroid belts in exoplanetary systems? How about anomalous stars, far too young to be in the region we find them, or stars that display unusual spectra that may indicate a civilization trying to lengthen the hydrogen fusion burning cycle of its home sun? As I mentioned yesterday, you can see a summary of recent ideas on the matter in my essay Distant Ruins, which ran in Aeon.
Tomorrow I’ll wrap up this discussion of The Beginning and the End with Vidal’s own thinking on what may be a candidate for what we can call ‘high energy astrobiology,’ an astronomical phenomenon that is curious enough to provoke Dysonian SETI theorists. But I’ll argue in advance that the value of this book isn’t in a specific SETI candidate but in the far broader context Vidal brings to the human quest for other civilizations, a context that challenges readers to examine their own views of the place of intelligent beings in the universe.
The original Dyson paper covering a broadened search for ETI is “The Search for Extraterrestrial Technology,” in Marshak, R.E. (ed.) Perspectives in Modern Physics (Wiley, 1966), pp. 641-655.
by Paul Gilster | Oct 27, 2014 | Astrobiology and SETI |
The SETI challenge has often been likened to archaeology, and for good reason. In both cases, we are trying to recover information about cultures from the past. When Heinrich Schliemann dug into the numerous layers of Troy — and in the process inadvertently damaged precious remnants of later eras — he and his team were exploring the heroic age of Homer. Any SETI detection will likewise deal with a signal from the past. Just how old it is will depend upon how far away the source world is, for this information travels at the speed of light.
The archaeology analogy is hardly perfect, because on Earth we are dealing with artifacts of our own species and are often working with linguistic remains we can decipher to aid our understanding. Figuring out Egyptian hieroglyphs wasn’t easy, but the stele known as the Rosetta Stone gave us a text in three scripts that helped us make sense of them. Even Linear B, the script of the Mycenaean Greeks before the emergence of the Greek alphabet, can be placed into context as the oldest Greek dialect, apparently borrowed as a script from the Minoan Linear A. But a SETI reception will be pure message, and absent the numerous cultural and linguistic cues we rely on to make sense of an undeciphered language, how will we approach it?
While we have significant problems with some ancient languages — the resolution of Mayan awaited seeing the glyphs in an entirely new context, looking at them phonemically and morphologically, and Etruscan is a challenge to this day — the problems with a genuinely alien message from another star dwarf these issues. Which brings me around to Clément Vidal, who has written a book that digs with gusto into the SETI question by way of asking what kind of detection we may expect to make. What we might call ‘traditional’ SETI by and large supposes that a distant civilization will be trying to get a message to us, for we’re highly unlikely to pick up radio signals not beamed in our direction.
The Beginning and the End (Springer, 2014) is subtitled ‘The Meaning of Life in a Cosmological Perspective,’ and SETI is only one aspect of its tripartite discussion. But its analysis of SETI and its application of a cosmological worldview help us see current SETI efforts as part of a larger picture. The communication assumption is sensible given SETI’s roots and the deliberate decision to look for signals in the most probable part of the spectrum, which early advocates saw as the region between the spectral lines of hydrogen and the hydroxyl radical — between 1420 and 1665 MHz. The ‘water hole’ for communication was quiet and presumably would attract cultures looking for other intelligent beings. But there are other ways to search for life that add valuable tools to our quest.
Vidal is a philosopher and, as his book attests, a polymath who delves into astrobiology, complexity science, cosmology and much else in the course of his discussion. Analyzing the weaknesses of our underlying assumptions is a key part of his argument. He believes we do not need to assume communication to conduct a SETI search, nor do we need to confine our efforts to our own galaxy. Radio methods offer us the hope of one day engaging in a two-way conversation with another species, putting the premium on nearby stars, but what I often call ‘interstellar archaeology’ — the science of looking for ETI in the data, and that data may be spread over numerous galaxies — yields on communications while stressing detection of civilizations that may be far more powerful than our own.
‘Zen SETI’ is the entertaining term Vidal coins for this approach, one that has been championed in recent years by Milan ?irkovi?, though analyzed by many scientists over the years, from Freeman Dyson to Nikolai Kardashev, James Annis, Richard Carrigan and current working groups like Penn State’s Jason Wright, Matthew Povich and Steinn Sigurðsson. I won’t go through a complete round-up here, but you can find current work discussed in my essay Distant Ruins, which ran in Aeon. The point is to think creatively about information that may already be in our astronomical data, and about new searches that put a premium on the signature that a Kardashev Type II or III civilization would leave.
Needless to say, Zen SETI makes no claim at being the only approach to the discipline, and indeed, these methods should be seen as complementary to ongoing radio and optical searches. When Richard Carrigan went to work searching infrared data from the IRAS satellite for the signature of possible Dyson spheres (see Toward an Interstellar Archaeology), he was broadening the effort to study SETI targets in places as distant as M51, the Whirlpool galaxy, pondering how a Kardashev Type III culture might begin turning stars en masse into a wavefront of such spheres as it maximized its energy resources.
Image: M51, the Whirlpool Galaxy. If a Kardashev Type III culture were active here building Dyson spheres, would we be able to see its signature as a growing void in visible light? Credit: NASA/ESA.
Such a search doesn’t preclude communications from a much closer civilization, but it does ask a thoughtful question. We now peg the age of our universe at roughly 13.7-13.8 billion years. We also think that the oldest Sun-like stars formed as early as about 12.5 billion years ago, with rocky planets beginning to emerge at that time. Give life five billion years to emerge, as it did on Earth, and you have the possibility of the earliest intelligence appearing as early as six billion years after the Big Bang. Because the Milky Way is thought to have formed between 10 and 11 billion years ago, intelligence may have appeared in our galaxy as much as five billion years before we humans began turning radio telescope dishes toward the nearby stars.
Charles Lineweaver (Australian National University) is the go-to guy on these matters, with work showing that on average, Earth-like planets around other stars are 1.8 billion years older than our planet, give or take 0.9 billion years either way. Given Lineweaver’s findings, isn’t it likely that any civilization we do discover is going to be significantly advanced over our own? Milan ?irkovi? made the case in a 2006 paper:
Applying the Copernican assumption naively, we would expect that correspondingly complex life forms on those others to be on the average 1.8 Gyr older. Intelligent societies, therefore, should also be older than ours by the same amount. In fact, the situation is even worse, since this is just the average value, and it is reasonable to assume that there will be, somewhere in the Galaxy, an inhabitable planet (say) 3 Gyr older than Earth. Since the set of intelligent societies is likely to be dominated by a small number of oldest and most advanced members…we are likely to encounter a civilization actually more ancient than 1.8 Gyr (and probably significantly more).
All of which compels Vidal to argue that the terms of our SETI search must be flexible:
We need not be overcautious in our astrobiological speculations. Quite the contrary, we must push them to their extreme limits if we want to glimpse what such advanced civilizations could look like. Naturally, such an ambitious search should be balanced with considered conclusions. Furthermore, given our total ignorance of such civilizations, it remains wise to encourage and maintain a wide variety of search strategies. A commitment to observation, to the scientific method, and to the most general scientific theories remains our best touchstone.
Paul Davies makes much the same point and is quoted by Vidal as saying that “the universe is a rich and complex arena in which signs of alien intelligence might be buried amid a welter of data from natural processes, and unearthed only after some ingenious sifting.” Tomorrow I want to go further into our SETI assumptions and where they might be challenged, using Clément Vidal’s fine discussion of Zen SETI and its consequences for how we proceed.
The ?irkovi? paper is “Macroengineering in the Galactic Context” (full text). Charles Lineweaver’s study is “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect,” Icarus Vol. 151, No. 2 (2001), pp. 307-313 (full text).
by Paul Gilster | Oct 24, 2014 | Exoplanetary Science |
Imaging planets around other stars is challenging enough because their light is overwhelmed by the proximity of the parent star. But what about comets? We may not be able to see them directly, but minute variations in light can mark their passage across the stellar disk. Nearly 500 comets have been detected around the star Beta Pictoris using these methods. New work led by Flavien Kiefer (IAP/CNRS/UPMC) analyzes this cometary hoard to give us a look at what is happening in a young planetary system.
Using the HARPS instrument at the European Southern Observatory’s site at La Silla in Chile, Kiefer and team have compiled what the ESO is calling ‘the most complete census of comets around another star ever created.’
Beta Pictoris is becoming an old friend, a young star some 63 light years from the Sun that is no older than 20 million years. The star is surrounded by a disk of material that has been the subject of intense study as we watch the interaction between gas, dust and the asteroids and comets that continue to produce them. Cometary ices evaporate as the comets approach the star, producing the familiar cometary tails we associate with the objects. Usefully, light passing through the released gas and dust can be analyzed to tell us about cometary composition.
Image: An artist’s impression of exocomets around the star Beta Pictoris. Credit: ESO.
Working with data from over 1000 observations obtained between 2003 and 2011, Kiefer’s team produced measurements of the size and speed of the gas clouds and was able to deduce orbital properties for a subset of the comets studied. What emerges is the presence of two distinct families of exocomets, one of them old and showing orbits highly influenced by the massive planet Beta Pictoris b, which was thought to orbit at a distance of one billion kilometers from the star. That number may now be reduced, for the eccentricity and orientation of the comets indicate they are in orbital resonance with an object about 700 million kilometers from the star.
The other comet family is newer and more active, with comets that are all on similar orbits, an indication of a common origin. The likelihood, the researchers say, is that this newer family of exocomets results from the breakdown of a larger object whose remains are now in an orbit that grazes the star. Says Kiefer: “For the first time a statistical study has determined the physics and orbits for a large number of exocomets. This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
The paper is Kiefer et al., “Two families of exocomets in the ? Pictoris system,” Nature 514 (23 October 2014). Abstract available.
An Awakening Comet of Our Own
Meanwhile, a good deal closer to home, the comet 67P/Churyumov-Gerasimenko, under close scrutiny by the Rosetta spacecraft and its OSIRIS imaging system, is showing increasing signs of life. Still more than 450 million kilometers from the Sun, the comet is producing jets of dust along much of its surface, whereas in past months the dust was confined to the ‘neck’ region that connects the two lobes. OSIRIS principal investigator Holger Sierks (MPS) notes that the jets are now appearing on the ‘body’ and ‘head’ of the comet.
Image: Two views of the same region on the “neck” of comet 67P/Churyumov-Gerasimenko. The right image was taken with an exposure time of less than a second and shows details on the comet’s surface. The left image was overexposed (exposure time of 18.45 seconds) so that surface structures are obscured. At the same time, however, jets arising from the comet’s surface become visible. The images were obtained by the wide-angle camera of OSIRIS, Rosetta’s scientific imaging system, on 20 October, 2014 from a distance of 7.2 kilometers from the surface. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
We should see much more activity as the comet reaches 300 million kilometers from the Sun and closer. Interestingly, the soon to be renamed ‘Site J’ at the ‘head’ of the comet, designated as the landing site, remains comparatively quiet. Rosetta’s Philae lander will make a landing attempt in November, and we’ll have the opportunity to see an awakening comet close up. Both lander and orbiter are to remain in operation until the comet’s closest approach to the Sun in August of 2015. Keep an eye on the European Space Agency’s Rosetta page for updates.
