Centauri Dreams
Imagining and Planning Interstellar Exploration
Red Dwarfs: Planets in Abundance
Whether or not they’re suitable for life, habitable zone ‘super-Earths’ are seeing increased scrutiny around M-class dwarf stars because the mass ratio of planet to star makes detection easier than around more massive stars. We need radial velocity surveys to help us here because planets on orbits longer than 200-300 days will definitely be out of Kepler’s reach. Moreover, while Kepler targets many K, G and F-class stars, M-dwarfs aren’t bright enough to show up in large numbers in its field of view, making occurrence rates around such stars problematic.
A 2013 paper by Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) found that the Kepler sample contains 3897 stars with estimated effective temperatures below 4000K. Out of these, 64 are planet candidate host stars, with 95 candidate planets orbiting them. The researchers deduced from their analysis that about 15 percent of all red dwarfs have an Earth-sized planet in the habitable zone. Ravi Kopparapu (Penn State) recast these results with a revised set of habitable zone parameters. The result: Four out of ten of the nearest small stars are likely to have planets in the habitable zone.
These results are fascinating because they suggest that the nearest habitable planet could be as close as seven light years away. Bear in mind that we know of eight stars within 10 light years of the Sun that fit this definition, so we might find three Earth-sized planets in habitable zones in relatively nearby space. We looked at Kopparapu’s work in Habitable Zone Planets: Upping the Numbers about a year ago, noting that his work on an improved climate model (developed with Penn State’s James Kasting) allows the habitable zone to be moved out further from the host star than it had been before, another finding with promising astrobiological implications.
Now we have word of a new study from Mikko Tuomi (University of Hertfordshire) and colleagues, who have combined data from the HARPS (High Accuracy Radial Velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph) instruments operated by the European Southern Observatory in Chile. Using Bayesian signal detection criteria and noise models that take into account correlations in the data, the team found three habitable zone super-Earths among eight new planets it discovered orbiting nearby red dwarfs. The stars — GJ 27.1, GJ 160.2, GJ 180, GJ 229, GJ 422, and GJ 682 — are between 15 and 80 light years away, with planetary orbital periods ranging from two weeks to nine years. The researchers were also able to con?rm the existence of a companion around GJ 433.
Image: Recent work confirms the existence of a long-period planet around the M-class dwarf GJ 433, about 30 light years from the Sun. Credit: Wikimedia Commons.
But the paper has implications well beyond these new worlds, for Tuomi’s group went on to calculate, using the estimated detection probability function, the occurrence rate of low-mass planets around nearby M-dwarfs. Habitable zone super-Earths, their paper deduces, should orbit at least a quarter of the red dwarfs in the Sun’s neighborhood. The paper on this work compares these results briefly with the Kepler work of Dressing and Charbonneau, but notes a key difference:
…such a comparison is not necessarily reliable because the properties of Kepler’s transiting planet candidates can only be discussed in terms of planetary radii and the radial velocity method can only be used to obtain minimum masses. Because of this, it is not surprising that there are remarkable differences that are unlikely to arise by chance alone.
So we emerge with somewhat different occurrence rates, with Tuomi’s team finding habitable zone super-Earths occurring in a range between Dressing and Charbonneau’s 15 percent and Kopparapu’s 40 percent. Bear in mind that the radius of some of the planet candidates in both the Dressing and Charbonneau paper as well as Kopparapu’s may change later with more accurate observations of the host star, a possibility that would change the occurrence rates from both these studies.
In any case, it makes sense that these estimates might vary. Dressing and Charbonneau, for example, worked with planetary radii between 0.5 and 1.4 times that of Earth, while Tuomi and colleagues made their calculations based on masses between 3 and 10 times that of Earth, and Tuomi points out that his group couldn’t assess the occurrence rates of planets with masses below 3 Earths because they failed to detect any in their sample. The detection methods, transit and radial velocity, differed, and in any case, the relationship between mass and radius is not well established for super-Earths.
The occurrence rate of low-mass planets in general, however, is high:
We find that low-mass planets are very common around M dwarfs in the Solar neighborhood and that the occurrence rate of planets with masses between 3 and 10 M? is 1.08 [+2.83/-0.72] per star. This estimate is likely consistent with that suggested based on the Kepler results for a sample of stars with Teff < 4000 K…, although the comparisons are not easily performed because we could not assess the occurrence rates of companions with periods up to the span of the radial velocity data of a few thousand days. On the other hand, we confirm the lack of planets with masses above 3 M? on orbits with periods between 1-10 days.
Bear in mind that M-class dwarfs are the most common type of star in the galaxy, perhaps comprising up to 80 percent of the total. The new work gives additional weight to the idea that these stars have low-mass planets around them in abundance, and a high probability of at least a super-Earth class world in their habitable zones. Given our ability to detect low-mass planets around cool stars with both transit and radial velocity methods, their stock can only rise as targets for future searches for Earth-sized planets and studies of planetary atmospheres.
The paper is Tuomi et al, “Bayesian search for low-mass planets around nearby M dwarfs. Estimates for occurrence rate based on global detectability statistics,” MNRAS, in press (full text). Ravi Kopparapu’s 2013 paper is “A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs,” Astrophysical Journal Letters Vol. 767, No. 1, L8 (abstract). The Dressing and Charbonneau paper is “The occurrence rate of small planets around small stars,” The Astrophysical Journal Vol. 767, No. 1, 95 (abstract).
An Interstellar Mission Statement
Yesterday I wrote about what Michael Michaud calls ‘the new cosmic humanism,’ looking back at an essay the writer and diplomat wrote for Interdisciplinary Science Reviews in 1979. Intelligence, Michaud believes, creates the opportunity to reverse entropy at least on the local scale, and to impose choice on a universe whose purpose we do not otherwise understand. Continuing growth into space, expansion and discovery are the kind of long-term goals humans can share, highlighting the extension of knowledge and the rediversification of our species.
What Michaud is talking about is nothing less than a mission statement for extraterrestrial man, one that trades off a key uncertainty: In the face of an indifferent universe, intelligence itself may prove to be an evolutionary quirk that is of little consequence. Whether or not this is the case could depend on the decisions and purposeful choices of intelligent beings, assuming they choose to expand into the cosmos. Let me quote Michaud on this:
Intelligent beings evolved from planets can do that [i.e., influence the universe] only by adopting extraplanetary models of their futures, by committing themselves to purposes for the species that reach beyond individual lifetimes, and perhaps eventually by adopting goals that reach beyond the species itself. If evolution suggests any ultimate moral task, it is survival; intelligent beings with technology and social purpose are equipped with unique means to assure it.
Image: The galaxy cluster Abell 1689, as seen from NASA’s Hubble Space Telescope. Faced with such immensity, what role do we assign intelligence in the ultimate fate of the universe? Credit: AFP/NASA.
The Price of Purpose
What Michaud calls cosmic humanism is not without its detractors, whose case emerges on practical grounds, and the author is wise to present it in the context of his larger argument. Anyone involved with the space community is challenged to explain why we should be looking beyond the Earth when we have so many problems here. Zero-sum arguments suggest that money spent on space is money taken away from social programs, leaving space advocates to argue the technological benefits that have accrued from the space program and future savings in areas like power generation.
These are arguments the space community has to be prepared to engage in with tangible benefits and a sensible eye toward budget realities. The moral argument can be trickier, and it also becomes ideological. Is space expansion just another way of imposing human chauvinism or preserving a Western model of industrial growth, a kind of futuristic ‘manifest destiny’? Is it in fact a thinly cloaked imperialism that turns the conflict of nation states into a model for future human conflict on other worlds, perhaps imposing our values on alien cultures?
We also run into the issue of limits. Michaud cites Philip Morrison, who has criticized the idea that there is any imperative for human expansion, arguing that as finite beings, we should be more attuned to our own limits. Added to this is the understandable argument that we have evolved in a biosphere for which we are now uniquely suited, and that expansion to other worlds would dilute and transform the species. Whether this is something to be deplored or celebrated is controversial, but certainly Michaud falls into line with Freeman Dyson as seeing diversification in society and ultimately biology as inevitable outcomes of an interstellar culture.
Behind all these objections lies the undeniable fact that a wide range of futures is within the realm of human choice, a philosophical stance varying from bounded to unbounded, a closed model vs. one open to expansion. Michaud points out that the window of opportunity for making a choice for the extraterrestrial paradigm is not necessarily infinite. We may run into serious issues of resource depletion within the next century that stymie our hopes for expansion.
Whether we take these outward steps depends on our motivations, on the values of our most powerful societies, and on political events within and between them. One might argue that, without an external goal, the world’s nations may have no purpose other than the periodic redistribution of wealth, status and power, often by force. By pointing out the promise of an extraterrestrial future, one might strengthen the case for solving current problems so that such a future can be realized.
Evolution of an Extraterrestrial Ethos
We cannot know what our species will ultimately choose, or whether natural or self-inflicted disaster will punctuate or even end future attempts at expansion into space. But what if our own extraterrestrial paradigm runs into the paradigm of another species? We may encounter, in the distant future, intelligent species that have chosen for their own reasons not to expand, perhaps through a lack of nearby destinations and resources or because of cultural values incompatible with colonizing other worlds. Or we may find expansionist beings whose interest is in shaping the universe to their own design to form a noosphere of interstellar dimension.
Here we can see Michaud the diplomat at work, asking how we might handle inevitable questions of contact and cooperation, subjects he would later address in Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials (Copernicus, 2006). Would the need for survival of a single species lead to conflict between it and the newly discovered others, a Darwinian struggle that could produce permanent limits to growth? We will need to be thinking about such issues down the road if interstellar travel ever becomes a reality, for ultimately we will be called upon to make ethical decisions — the hard currency of intelligence — involving both survival and altruism.
For intelligence to survive, shared purpose will need to come to the fore in our dealings with other civilizations, with an emerging macro-intelligence being made up of the individual contributions of cultures in control of their local sphere of space. Such a ‘conscious universe’ gives itself purpose through its own survival and the effort to impose structure and meaning upon the cosmos. If intelligence is to have a long-term purpose, something like this may be it.
Lighting the Cosmos
And if we are alone in the universe? Just as I was thinking about these issues yesterday I ran across Vinton Cerf’s What If It’s Us?, a short essay on the Communications of the ACM website. Cerf will be known to Centauri Dreams readers and almost all digitally aware people as the father of the TCP/IP protocols that are at the heart of the Internet. He’s also fascinated with the human future in space, and when I read his new piece, I found a useful synergy with what Michael Michaud is saying in “The Extraterrestrial Paradigm.”
Cerf was at a conference held by the Internet Society to discuss the emerging protocols for interplanetary communication when guest speaker David Brin asked what we would do if there were no other civilizations out there. What if we are the ones who are supposed to light the galaxy? If our species is destined to bring intelligence into at least the local cosmos and, in Michaud’s terms, impose choice on inevitability, just think of the responsibility that places upon us. Do we have the capacity as a species to survive long enough to make it happen? Cerf’s thought echoes Michaud:
I cannot speak for anyone else, but I think I would think somewhat differently about a lot of things. I would be thinking more long-term and be worried about the sustainability of our planet and the species that inhabit it. I would wonder what we should be developing to fulfill this mission. What technologies do we need to expand beyond our planet and our solar system? How should we prepare ourselves for such an ultimate goal?
Either way, you see, we face the need for sustained vision and developing purpose, our choices governing the outcome either here on among the stars. We must be prepared, as Michaud explains, to impose that purpose on a cosmos in which intelligence is rare just as we are prepared for possible contact with other species whose ideas on these matters may differ. Culturally, developing an outward-looking paradigm is not the work of years or decades but of centuries, requiring sustained effort and continuing debate. Those who choose to ponder these things on the public stage are playing a valuable role in the evolution of human purpose. They are helping us create the interstellar mission statement that will guide our way forward.
Toward an Extraterrestrial Paradigm
Growing up in the Sputnik era, I followed the fortunes of space exploration with huge enthusiasm. In those days, the model was primarily planetary in nature, the progression from the Moon to the nearest planets and then beyond seemingly inevitable. At the same time, a second model was developing around the idea of space stations and self-contained worlds built by man, one that would reach high visibility in the works of Gerard O’Neill, but one that ultimately reached back as far as the 1920’s (Oberth and Noordung) and further back to the science fiction of Jules Verne. In fact, E. E. Hale’s “The Brick Moon” explored a space station as early as 1869.
But even as our Mariners and Veneras explored other planets, an interstellar thread was also emerging. Robert Goddard wrote about interstellar journeys in 1918, science fiction was full of such travel as the field matured in the 1940s and ’50s, and serious scientific study of interstellar flight became established by mid-century. Writing in 1979, Michael Michaud could point to Project Daedalus as the first serious starship design, and could note the continuing work of Robert Forward in presenting what he thought of as a roadmap for interstellar expansion.
Reasons for the Journey
It’s useful to frame these issues by seeing how they have been examined in the past, for if we are beginning to adapt to what Michaud calls an ‘extraterrestrial paradigm,’ it is because we have, in reaction to technological breakthroughs and exploration, been forced to adjust our ideas about our place in the cosmos. Michaud’s “The Extraterrestrial Paradigm: Improving the Prospects for Life in the Universe” pulls together his own prior work in the Journal of the British Interplanetary Society and the thinking of the scientists and scholars of the day to make the case for a human culture that will expand to the stars out of choices made in the service of a newly emerging sense of purpose.
Image: Aswarm with galaxies, this part of the Hubble Ultra Deep Field survey cuts across billions of light-years. Such glimpses of immensity make us ponder the purpose behind human exploration of the cosmos. Credit: NASA, ESA, and R. Thompson (Univ. Arizona).
Solving the riddle of purpose must underlie all our explorations. Why, after all, do we choose to go into space in the first place? Robert Jastrow could find meaning in the linkage between cosmology and the workings of evolution, believing that persistent struggle and striving gave purpose to our existence. Michaud looks toward the loss of energy and structure we call entropy and asks whether life and the intelligence that can grow out of it are the only chance to reverse entropy on a local scale, and in his memorable phrase, “to impose choice on inevitability.”
Michaud’s essay deserves a wider audience, published originally in the academic journal Interdisciplinary Science Reviews and unavailable online. For the choices we make going forward are going to be continuous, given the decades- and probably centuries-long commitment that reaching the stars will require. But the extraterrestrial paradigm begins right here in our own system as we adapt our philosophy and perhaps one day our biology to moving off-planet. And while we have made the case for exploration and scientific research in spaceflight all along, Michaud here explores social and economic issues and the long term survival of the species as key drivers for the development of this new framework.
Expanding the economy beyond Earth’s limits allows us to surmount huge issues of resource depletion on the home world while developing new industrial processes in space. Earth’s energy demands can be met by abundant solar energy at the same time that we expand the human ecosphere to create new habitats beyond the Earth, perhaps modifying existing habitats through terraforming or other means. Moreover, the creation of a multitude of separate biospheres ensures us against collective disaster, whether natural or man-made. In terms of human liberty, freedom is enhanced and diversity encouraged as we experiment with new societies separate from those on Earth. All of these work together in shaping the extraterrestrial paradigm.
Centauri Dreams readers will be familiar with Michaud’s work in these pages, and also with his long-term commitment to developing a strategy for human expansion to the stars. His JBIS work in the late 1970s — three papers collectively known as “Spaceflight, Colonization and Independence: A Synthesis,” drawn on heavily in this essay — extends the colonization and humanization of the Solar System to an outward push to other star systems, one that by its nature changes who we are as we make the choice of who to become.
A New Cosmic Humanism
For humans, Michaud believes, must create their own goals in the absence of a pre-determined purpose imposed on them. The future, on this planet or elsewhere, depends on an evolutionary paradigm that relies on technology as a way to extend human influence into larger environments. It’s a view that suggests an inevitability about the rise of life and intelligence, but this determinism is modulated by chance as well as individual and social choice. That choice offers us the opportunity to bring meaning into the cosmos. As Michaud puts it:
The extraterrestrial paradigm suggests such goals: endless growth, expansion and discovery, the enrichment and rediversification of the species, and the increasing of human knowledge and power in the universe. Extraterrestrial growth could be a grand shared enterprise for humanity. It would define Homo sapiens by contrast with the external environment into which we ventured, and by contrast with nonexpanding forms of life and possibly intelligence on Earth. It suggests a way to free humans of Darwinian competition among themselves.
Above all, Michaud believes that the development of the extraterrestrial outlook helps us create a purposeful place for ourselves in the greater cosmos:
It suggests that, by our own activities, we can ensure the survival of our species, our intelligence, our consciousness, our culture, and that we can make intelligence have an impact on an inanimate, unfeeling universe, giving at least part of it an intelligent purpose. Thus the extraterrestrial paradigm may be an essential part of building a new cosmic context for mankind. It could be the basis for a sort of cosmic humanism that might be a factor in the philosophical history of the future.
Survival of the species demands short-term practical thinking but also the development of shared, long-term purpose, the latter more distant in time and requiring consensus and commitment. It may be that our movement into space expresses that purpose. But there are numerous challenges to the notion, all of which Michaud addresses in his essay. Tomorrow I’ll run through objections to this cosmic context, and go on to discuss how SETI comes into play. What happens to our own sense of purpose if and when we run into another intelligence?
The paper is Michaud, “The Extraterrestrial Paradigm: Improving the Prospects for Life in the Universe,” Interdisciplinary Science Reviews Vol. 4, No. 3 (1979), pp. 177-192. Michaud’s three-part study “Spaceflight, Colonization and Independence: A Synthesis” appeared in JBIS 30, 83-95 (Part I, March 1977); 203-212 (Part II, June 1977); 323-331 (Part III, September 1977).
‘Super-Earths’ Problematic for Life
The Kepler announcements yesterday were greatly cheering to those of us fascinated with the sheer process of doing exoplanetology. The ‘verification by multiplicity’ technique propelled the statistical analysis that resulted in 715 newly verified worlds, and we have yet to turn it loose on two more years of Kepler data (check Hugh Osborn’s excellent Lost in Transits site for more on the method). For those who focus primarily on habitable worlds, the results seemed a bit more sparse, with just four planets found in the habitable zone. And even where we find such, there are reasons to wonder whether a ‘super-Earth’ could actually sustain life.
Apropos of this question, a team of researchers led by Helmut Lammer (Austrian Academy of Sciences) has just published the results of its modeling of planetary cores, looking at the rate of hydrogen capture and removal for cores between 0.1 and 5 times the mass of the Earth found in the habitable zone of a G-class star. Cores like these inevitably attract hydrogen from the surrounding protoplanetary disk, although some of it will be stripped away by ultraviolet light from the hot young star they orbit. The question is, can enough of this primordial hydrogen envelope be blown off to allow the formation of a more benign secondary atmosphere?
The results are not promising for even relatively small super-Earths. Planetary cores with a mass similar to the Earth capture a hydrogen envelope and can also lose it — the paper suggests that terrestrial planets like Mercury, Venus, Earth and Mars lost their proto-atmospheres because of ultraviolet light. But high-mass cores similar to many of the super-Earth discoveries like Kepler-62e and -62f, wind up with atmospheres much thicker than ours. The habitable zone isn’t enough to guarantee a habitable world, as Lammer notes:
“Our results suggest that worlds like these two super-Earths may have captured the equivalent of between 100 and 1000 times the hydrogen in the Earth’s oceans, but may only lose a few percent of it over their lifetime. With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist.”
Image: The mass of the initial rocky core determines whether the final planet is potentially habitable. On the top row of the diagram, the core has a mass of more than 1.5 times that of the Earth. The result is that it holds on to a thick atmosphere of hydrogen (H), deuterium (H2) and helium (He). The lower row shows the evolution of a smaller mass core, between 0.5 and 1.5 times the mass of the Earth. It holds on to far less of the lighter gases, making it much more likely to develop an atmosphere suitable for life. Credit: NASA / H. Lammer.
The constraints here get to be pretty tight. Let me quote the paper on this:
Therefore we suggest that ‘rocky’ habitable terrestrial planets, which can lose their nebula-captured hydrogen envelopes and can keep their outgassed or impact delivered secondary atmospheres in habitable zones of G-type stars, have most likely core masses with 1±0.5M? and corresponding radii between 0.8–1.15R?. Depending on nebula conditions, the formation scenarios, and the nebula life time, there may be some planets with masses that are larger than 1.5M? and lost their protoatmospheres, but these objects may represent a minority compared to planets in the Earth-mass domain.
The super-Earths we’re likely to find in habitable zones, according to this work, are going to be uninhabitable places with hydrogen dominated atmospheres. The paper goes on to note that, based on its results, there may well be Earth-size and mass planets that have not been able to shed their captured, nebula-based hydrogen envelope. The upshot: Extreme caution is advised when speculating about how habitable planets evolve, particular when mass and density are still problematic. The variety of outcomes is wide even for small worlds inside the habitable zone.
The paper is Lammer et al., “Origin and Loss of nebula-captured hydrogen envelopes from “sub”- to “super-Earths” in the habitable zone of Sun-like stars,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
Kepler: Opening the Planet Verification Bottleneck
A planet like Kepler-296f is bound to get a lot of publicity. Orbiting a star half the Sun’s size and only five percent as bright, this world, twice the size of the Earth, appears to orbit in the habitable zone, where liquid water could exist on its surface. We focus so much on the potential of life that the four planets announced yesterday (out of 715 newly verified worlds) inevitably get special treatment. And we learn that Kepler-296f exists in a system with four other planets, orbiting the star every thirty days. What we don’t know is whether we’re dealing with a small Neptune-class world surrounded by a thick hydrogen/helium atmosphere or a water world with a deep ocean.
An interesting world, to be sure, but the real story in yesterday’s announcements from the Kepler team has to do with the ‘verification by multiplicity’ technique used to validate the existence of so many planets in 305 star systems. One of the findings papers titled “Almost All of Kepler’s Multiple Planet Candidates are Planets” (to be published March 10 in The Astrophysical Journal) makes the case that the vast majority of Kepler’s multiple planet candidates are true multi-planet systems, the number of false positives lower for multiples than for single detections. The paper continues (internal citations removed for brevity):
Kepler has found far more multiple candidate systems than would be the case if candidates were randomly distributed among target stars. False positives are expected to be nearly randomly distributed among Kepler targets, whereas true transiting planets could be clustered if planets whose sizes and periods are adequate for transits to be detected often come in multiples, as is the case for planets detected by radial velocity variations, and/or if planetary systems tend to be flat, so geometry leads to higher transit probabilities for other planets if one planet is seen to transit.
The paper gives a blow by blow description of the statistical analysis used here, but for our purposes, Kepler looked at over 150,000 stars and found only a few thousand that showed the characteristic lightcurve of a planetary transit. If this transit-like pattern were random, only a few stars would have shown more than one pattern, but we find hundreds of stars that have multiple transit-like patterns. As Jack Lissauer (NASA Ames) pointed out in yesterday’s teleconference, multiplicity is not random. Instead, it is a technique for opening up what Lissauer calls the ‘planetary verification bottleneck.’ Joining Lissauer in the teleconference were Jason Rowe (SETI Institute), Douglas Hudgins (NASA Astrophysics Division), and Sara Seager (MIT).
Image: The histogram shows the number of planets by size for all known exoplanets. The blue bars on the histogram represents all the exoplanets known, by size, before the Kepler Planet Bonanza announcement on Feb. 26, 2014. The gold bars on the histogram represent Kepler’s newly-verified planets. Image Credit: NASA Ames/W Stenzel.
Lissauer talks about the method as a way ‘to verify multiple planet candidates in bulk to deliver planets wholesale,’ and it yields 715 worlds of which 94 percent are smaller than Neptune, in the process establishing that multiple planet systems around a single star are common. The number of confirmed exoplanets now nears 1700. Besides Kepler-296f, three other verified worlds are less than 2.5 times the size of Earth and orbiting in their star’s habitable zone.
Overall, these results offer the largest number of validated planets ever to be announced at one time, a number sufficient for the transit technique to overtake radial velocity as the most prolific technique for exoplanet detection. And as Jason Rowe pointed out, these results show us small planets in multiple planet systems that move in circular, flat orbits, a configuration not unlike what we find in our own inner Solar System. “The more we explore,” Rowe added, “the more we find familiar traces of ourselves amongst the stars that remind us of home.”
The paper cited above is Lissauer et al. “Almost All of Kepler’s Multiple Planet Candidates Are Planets,” in press at The Astrophysical Journal (preprint). See also Rowe et al. “Validation of Kepler’s Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems,” also in press at The Astrophysical Journal (preprint). The archived Kepler media teleconference can be accessed here.
Science Fiction in Extreme Environments
I’ve had pulsars on the mind the last couple of days after our discussion of PSR 1257+12 and its contribution to exoplanetology. A bit more about pulsars today and the way we look at extreme objects through science fiction. PSR 1257+12 was discovered in 1990 by Aleksander Wolszczan using data from the Arecibo dish, and it was in 1992 that Wolszczan and Dale Frail published a paper outlining their discovery of the first planets ever found outside our Solar System. The two planets were joined by a third in 1994, but evidence for a fourth was later shown to be mistaken. In any case, the three planets confounded many astronomers, who hardly expected the first extrasolar planets to be found orbiting a radiation-spewing neutron star.
Centauri Dreams regular Al Jackson was co-author of a 1992 study of PSR1257+12 that examined orbital resonance in the planets around the pulsar. In a note last night, Al mused “Just think of a K2 civilization setting up a research station on one of those to study a pulsar – maybe someone has written the SF story?” I can’t think of a story specifically targeting a pulsar planet, though if memory serves, Alastair Reynolds deals with a pulsar in one of the early Revelation Space novels. But the notion reminds me of other extreme environment classics like Hal Clement’s Mission of Gravity (1954), which covers the fantastically spinning world Mesklin, where surface gravity gets up close to 700 g at the poles (the complete Mesklin material is available in the collection Heavy Planet: The Classic Mesklin Stories, an Orb title from 2002).
High-energy environments are made to order for hard science fiction. Consider what our best authors might do with this: The pulsar IGR J11014-6103, now the subject of observations by the Chandra X-Ray Observatory, is producing a jet of high-energy particles that this Chandra news release claims is the longest of any object in the Milky Way, a whopping 37 light years. The pulsar is moving away from a supernova remnant in the constellation of Carina at a speed somewhere between 1100 and 2200 kilometers per second, making it one of the fastest moving pulsars yet observed.
Image: An extraordinary jet trailing behind a runaway pulsar is seen in this composite image that contains data from NASA’s Chandra X-ray Observatory (purple), radio data from the Australia Telescope Compact Array (green), and optical data from the 2MASS survey (red, green, and blue). The pulsar – a spinning neutron star – and its tail are found in the lower right of this image. The tail stretches for 37 light years, making it the longest jet ever seen from an object in the Milky Way galaxy. Credit: X-ray: NASA/CXC/ISDC/L.Pavan et al, Radio: CSIRO/ATNF/ATCA Optical: 2MASS/UMass/IPAC-Caltech/NASA/NSF.
A 37 light-year jet is remarkable enough (nine times the distance to Proxima Centauri!), but we also find that there is a corkscrew pattern in the jet suggesting that the pulsar is wobbling as it spins. In addition, it’s producing a comet-like tail behind it due to the effects of a pulsar wind nebula (PWN) — a sheath of high-energy particles that enshrouds the pulsar. And while a pulsar’s direction of motion is usually aligned with its jet and the pulsar wind nebula around it, IGR J11014-6103’s own PWN is almost perpendicular to the direction of the jet, possibly indicating high rotation speeds in the iron core of the supernova that produced the pulsar.
It’s fascinating to explore such extreme environments, and one of the things I love about science fiction is that a combination of scientific rigor and imagination can let us see things like this up close. Clement’s Mesklin was put together using a carefully wrought model that the author went on to describe in an article called ‘Whirligig World,’ which ran in Astounding Science Fiction in June of 1953. He based it on an object then believed to exist in the 61 Cygni system, and although the latter turned out to be a chimera, his concern for getting the science right built a memorable world we can still enjoy reading about today. Now who will take up Al Jackson’s challenge to describe a research station on a pulsar planet?
Image: The April, 1953 issue of Astounding contained the first installment of Hal Clement’s Mission of Gravity, published in book form the following year.
The paper on the light jet is Pavan et al. “The long helical jet of the Lighthouse nebula, IGR J11014-6103,” Astronomy & Astrophysics 562 (2014): A122 (preprint). Al Jackson’s paper on PSR 1257+12 is Malhotra et al. “Resonant orbital evolution in the putative planetary system of PSR1257 + 12,” Nature 356 (16 April 1992), pp. 583-585 (abstract).
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