Centauri Dreams
Imagining and Planning Interstellar Exploration
A Thoroughly Disrupted Solar System
A quick follow-up on our most recent discussion of KIC 8462852 (and thanks to all for the continuing high level of discussion in the comments) because today’s topic touches on a bit of the same ground. Centauri Dreams regular Harry Ray was first to notice a paper from Eva Bodman and Alice Quillen (University of Rochester) titled “KIC 8462852: Transit of a Large Comet Family.” From the paper:
…if the comet family model is correct, there is likely a planetary companion forming sungrazers. Since the comets are still tightly clustered within each dip, a disruption event likely occurred recently within orbit, like tidal disruption by the star. This comet family model does not explain the large dip observed around day 800 and treats it as unrelated to the ones starting at day 1500. The flux changes too smoothly and too slowly to be easily explained with a simple comet family model.
I’ve only had the chance to glance at this work so far, but it’s heartening to see another paper analyzing the KIC 8462852 light curves. Massimo Marengo (Iowa State), whose own paper we just looked at, notes in the comments to that article that star-grazing comets regularly fragment, and to produce a ‘family of comets’ at KIC 8462852 simply requires a large, icy body (think, for example, of a Kuiper Belt object) breaking up after a close pass by the star. The follow-up work that Marengo, Hulsebus and Willis recommend in their paper give us a useful way to proceed.
Addendum: Dr. Marengo just alerted me to another new paper, one in which he had a hand. It’s Lisse et al., “IRTF/SPEX Observations of the Unusual Kepler Lightcurve System KIC8462852,” now available here on the arXiv site. From the abstract, this interesting bit:
Our results are inconsistent with large amounts of static close-in obscuring material or the unusual behavior of a YSO system, but are consistent with the favored episodic models of a Gyr old stellar system favored by Boyajian et al. (2015). We speculate that KIC8462852, like the ~1.4 Gyr old F2V system ? Corvi (Wyatt et al. 2005, Chen et al. 2006, Lisse et al. 2012), is undergoing a Late Heavy Bombardment, but is only in its very early stages.
Exoplanets at System’s Edge
The thought of comets on interesting courses provides a helpful segue into today’s topic, a study by Paul Kalas (UC-Berkeley) and colleagues that looks at the star HD 106906, where images from the Gemini Planet Imager and new analysis of data from the Hubble instrument show us a cometary belt that is lopsided. The disturbed nature of this solar system is obvious, telling us of likely planetary interactions that caused the comet disruptions and may well have caused an exoplanet seen in the GPI image to be driven to the remote outskirts of its system.
Image: Astrophysicist Paul Kalas. Credit: UC Berkeley.
We may be looking at disruptions caused by a passing star, although the researchers do not rule out a second massive planet as the culprit in the HD 106906 system. The planet HD 106906 b was discovered by Vanessa Bailey (University of Arizona) in 2014, a gas giant of approximately eleven times Jupiter’s mass. 300 light years from us in the constellation Crux, the young star (about 13 million years old) is separated from its companion by an eyebrow-raising 650 AU. This is sixteen times farther than Pluto is from the Sun, a region far beyond where planets are normally thought to form.
Image (click to enlarge): Two direct images of the cometary dust and exoplanet surrounding the young star HD 106906. The wider field in blue shows Hubble Space Telescope data where the star’s blinding light is artificially eclipsed (gray circular mask). The point to the upper right is an 11 Jupiter mass planet located over 650 times the Earth-Sun distance. A new discovery in these Hubble observations is an extremely asymmetric nebulosity indicating a dynamically disturbed system of comets. Surprisingly, the planet is located 21 degrees above the plane of the nebulosity. The circular orange inset shows a region much closer to the star that can only be detected using advanced adaptive optics from the ground-based Gemini Observatory. Using the Gemini Planet Imager (GPI), researchers found a narrow loop of nebulosity suggesting that a planetary system formed close to the star, but somehow the architecture of the outer regions is severely disrupted. Credit: Paul Kalas / UC Berkeley.
Several explanations for the location of this planet can be advanced, one of them being that the planet formed where it is today from its own cloud of gas and dust, but the Kalas paper looks at the options involving system disruption. For archival images from Hubble’s Advanced Camera for Surveys show us that HD 106906 is surrounded by a ring of material reminiscent of our own Kuiper Belt, with a largely empty central region, about 50 AU in radius, that implies a planetary system has formed there.
The outer ring, however, extends further than expected and is lopsided, reaching almost out to the known planet on one side, while being thick and truncated on the side opposite the planet. Moreover, the planet’s orbit appears to be tilted 21 degrees away from the plane of the inner system. “These discoveries,” said Kalas, “suggest that the entire planetary system has been recently jostled by an unknown perturbation to its current asymmetric state.”
In a UC Berkeley news release, Kalas adds this:
“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud. We conducted three tests and found tentative evidence for a dust cloud, but the jury is still out.”
The paper analyzes three dynamical scenarios, two of which link the planet with the large-scale disk asymmetry, while the third assumes a stellar flyby that perturbed the disk, causing the asymmetry but not implicating the planet in its formation. If the planet is surrounded by a dust shroud or ring, this along with the misshapen debris belt would point to a planet that formed in the inner system and experienced gravitational interactions that forced it into the present outer orbit, as opposed to a planet that formed in that remote region in the first place.
Focusing in on dust near the planet, Kalas and colleagues conducted several experiments in their search for evidence of a circumplanetary disk. From the paper:
Based on the combination of evidence from the IR color, HST optical radial profile, and the optical flux level, we conclude that there may be a disk of material that was either captured in an encounter with the primary star’s disk, or retained from the time of formation of the planetary mass companion. Additional observations are required to clarify these tentative conclusions about the environment surrounding HD 106906b.
So we have only tentative evidence for a dust cloud of captured material surrounding the planet, perhaps as a large dust ring, but the paper outlines follow-up observations that can clarify the situation. In any case, we are learning how violent a place a young solar system can be, with gravitational disturbances that can profoundly affect dust and debris disks and potentially eject entire planets from the inner system. A passing star could explain the initial disruption here. We now wait to learn how a possibly ejected planet further affects the system’s configuration.
The paper is Kalas et al., “Direct imaging of an asymmetric debris disk in the HD 106906 planetary system,” The Astrophysical Journal Vol. 814, No. 1 (2015). Abstract / preprint.
Habitable Planets in the Same System
Learning that our own Solar System has a configuration that is only one of many possible in the universe leads to a certain intellectual exhilaration. We can, for example, begin to ponder the problems of space travel and even interstellar missions within a new context. Are there planetary configurations that would produce a more serious incentive for interplanetary travel than others? What would happen if there were not one but two habitable planets in the same system, or perhaps orbiting different stars of a close binary pair like Centauri A and B?
My guess is that having a clearly habitable world — one whose continents could be made out through ground-based telescopes, and whose vegetation patterns would be obvious — as a near neighbor would produce a culture anxious to master spaceflight. Imagine the funding for manned interplanetary missions if we had a second green and blue world that was as reachable as Mars, one that obviously possessed life and perhaps even a civilization.
Solar systems with multiple habitable planets are the subject of an interesting new paper from Jason Steffen (UNLV) and Gongjie Li (Harvard-Smithsonian Center for Astrophysics), who point to the fact that the Kepler mission has found planet pairs on similar orbits, with orbital distances that in some cases differ by little more than 10 percent. Mars is at best 200 times as far away as the Moon, but as Steffen notes, multi-habitable systems will produce much closer destinations:
“It’s pretty intriguing to imagine a system where you have two Earth-like planets orbiting right next to each other,” says Steffen. “If some of these systems we’ve seen with Kepler were scaled up to the size of the Earth’s orbit, then the two planets would only be one-tenth of one astronomical unit apart at their closest approach. That’s only 40 times the distance to the Moon.”
We can’t know at this point whether any of the Kepler candidates have life or not, but consider this: Kepler-36, about 1530 light years away in the constellation Cygnus, is known to be orbited by a ‘super-Earth’ and a ‘mini-Neptune’ in orbits that differ by 0.013 AU. The outer planet orbits close enough to the inner that we can pick up obvious transit timing variations (TTV) for both. Locked in a 7:6 orbital resonance, these worlds are representative of this kind of planetary configuration. Imagine the changing celestial display from the surface of the super-Earth as the larger planet sweeps out its orbit. Now consider the same view if both planets were clearly habitable!
Image: A gas giant planet spanning three times more sky than the Moon as seen from the Earth looms over the molten landscape of Kepler-36b. Credit: Harvard-Smithsonian Center for Astrophysics.
Steffen and Li focus in on two key issues. Specifically, what sort of variations in obliquity might be caused by such close planetary orbits? Obliquity measures the angle between the planet’s spin and its orbital axis — Earth’s 23.5° tilt relative to its orbit explains seasonal change and has obvious effects on climate. So we’d like to know whether close orbits disrupt planetary climates enough to wreak climatic havoc. The answer in most cases is no. From the paper:
We found that obliquity variations are generally not affected by the close proximity of the planets in a multihabitable system. Also, obliquity variations of close pairs embedded in the solar system or of potentially habitable planets in a system with a close pair were not sufficient to significantly reduce the probability of having a stable climate. Only in cases where the inclination modal frequencies coincide with the planetary precession frequency did large obliquity variation arise.
With planets this close, however, an even larger question is whether life on one planet can influence the other. We have abundant evidence of rocks from Mars that have fallen to Earth, leading to the possibility that other planets could have delivered life-bearing materials here in the process called lithopanspermia. Much closer planets should be far more susceptible to this process. Clearly, the possibility exists for life branching out from the same roots, taking different evolutionary paths just as occurs on remote islands here on Earth.
The simulations used by Steffen and Li demonstrate that two Earth-like planets in orbits like those found around Kepler-36 would have a much greater opportunity for exchanging materials than planets do in our Solar System. The relative velocities of the planets — and thus the velocities of the ejected particles — would be much less than in the case of a transfer of materials from Mars to the Earth. Biological materials transferred by collision ejecta have been considered on individual planets (with material falling back onto other parts of the same world), between binary planets or habitable moons, and even between different star systems.
Steffen and Li’s simulations show that the nearer the planets are to each other, the higher the success rate of ejecta transfer. Moving biological materials between two worlds becomes almost as feasible as moving them from one place on a single planet to another region of that planet, and the energy of the impact needed to make the transfer is much less than in our Solar System, leading to higher survivability. We can add in the fact that the time needed for biological materials to survive an interplanetary journey is that much shorter.
And there is this final factor:
…we found that the smaller the velocity of the ejected material the more uniformly they can be sourced across the originating planet. With high velocity ejecta, the range of initial longitudes is constrained relative to the direction of motion.
The result: Debris from a single impact is far more likely to strike the destination planet at multiple locations in rapid succession than when planets are spaced farther apart. That too increases the chance of life catching hold. Processes like these, which could also occur around planets with large habitable moons, allow life to spread readily within the same system. The paper adds:
Not only will panspermia be more common in a multihabitable system than in the solar system, but the close proximity of the planets to each other within the habitable zone of the host star allows for a real possibility of the planets having regions of similar climate—perhaps allowing the microbiological family tree to extend across the system. There are many things to consider in multihabitable systems, especially in the cases where intelligent life emerges.
In this UNLV news release, Steffen follows up that last thought:
“You can imagine that if civilizations did arise on both planets, they could communicate with each other for hundreds of years before they ever met face-to-face. It’s certainly food for thought.”
The scenario is striking, and I’m hoping readers can come up with some science fiction titles in which multiple habitable planets in the same system are the background for the tale.
The paper is Steffen and Li, “Dynamical considerations for life in multihabitable planetary systems,” accepted at The Astrophysical Journal (preprint).
No Catastrophic Collision at KIC 8462852
Last week I mentioned that I wanted to get into Massimo Marengo’s new paper on KIC 8462852, the interesting star that, when studied by the Kepler instrument, revealed an intriguing light curve. I’ve written this object up numerous times now, so if you’re coming into the discussion for the first time, plug KIC 8462852 into the archive search engine to get up to speed. Marengo (Iowa State) is himself well represented in the archives. In fact, I began writing about him back in 2005, when he was working on planetary companions to Epsilon Eridani.
In the new paper, Marengo moves the ball forward in our quest to understand why the star I’ll abbreviate as KIC 8462 poses such problems. The F3-class star doesn’t give us the infrared signature we’d expect from a debris disk, yet the light curves we see suggest objects of various sizes (and shapes) transiting across its surface. What we lacked from Tabetha Boyajian’s earlier paper (and it was Boyajian, working with the Planet Hunters group, that brought KIC 8462 to our attention) was data about infrared wavelengths after the WISE mission finished its work.
That was a significant omission, because the WISE data on the star were taken in 2010, while the first events Kepler flagged at KIC 8462 occurred in March of 2011, with a long series of events beginning in February of 2013 and lasting sixty days. That gave us a small window in which something could have happened — the idea of a planetary catastrophe comes to mind, perhaps even a collision between two planets, or a planet and large asteroid. What Marengo brings to the table are observations from the Spitzer Space Telescope dating from early 2015.
Image: Astrophysicist Massimo Marengo. Credit: Iowa State University.
We learn that the Spitzer photometry from its Infrared Array Camera (IRAC) finds no strong infrared excess — no significant amount of circumstellar dust can be detected two years after the 2013 dimming event at KIC 8462. Here is what Marengo concludes from this:
The absence of strong infrared excess at the time of the IRAC observations (after the dimming events) implied by our 4.5 µm 3? limit suggests that the phenomenon observed by Kepler produced a very small amount of dust. Alternatively, if significant quantity of dust is present, it must be located at large distance from the star.
This seems to preclude catastrophic scenarios, while leaving a cometary solution intact. The paper continues:
As noted by B15 [this is the Boyajian paper], this makes the scenarios very unlikely in which the dimming events are caused by a catastrophic collision in KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system, or a population of dust-enshrouded planetesimals. All these scenarios would produce very large amount of dust dispersed along the orbits of the debris, resulting in more mid-IR emission than what can be inferred from the optical depth of the dust seen passing along our line of sight to the star. Our limit (two times lower than the limit based on WISE data) further reduces the odds for these scenarios.
Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.
Tabetha Boyajian’s paper analyzed the natural phenomena that could account for KIC 8462’s light curve and concluded that a family of exocomets was the most promising explanation. Here the idea is that we have a family of comets in a highly elliptical orbit that has moved between us and the star, an idea that would be consistent with the lack of a strong infrared signature. Marengo has reached the same conclusion now that we are able to discount the idea of a large collision within the system. Both Boyajian and Marengo favor the comet hypothesis because it does not require a circular orbit and allows associated dust to quickly move away from the star.
In Marengo’s analysis, this fits the data, as the two-year gap between the Kepler light curves and the observations from Spitzer provide enough time for cometary debris to move several AU from the zone of tidal destruction from the star. The paper adds:
At such a distance, the thermal emission from the dust would be peaked at longer wavelengths and undetectable by IRAC. A robust detection at longer wavelengths (where the fractional brightness of the debris with respect to the star would be more favorable) will allow the determination of the distance of the cometary fragments and constrain the geometry of this scenario.
So we have a way to proceed here. Marengo notes that the measurements his paper presents cannot reveal the temperature or the luminosity of the dust that would be associated with such a family of comets, but long-term infrared monitoring would allow us to constrain both. The other day I also mentioned the small red dwarf (about 850 AU out) that could be the cause of instabilities in any Oort Cloud-like collection of comets around KIC 8462. Boyajian’s paper makes the case for measuring the motion or possible orbit (if bound) of this star as a way to tighten predictions on the timescale and repeatability of any associated comet showers.
Marengo dismisses SETI study of KIC 8462, with specific reference to Jason Wright’s recent paper on the matter, as “wild speculations,” an unfortunate phrase because Wright’s shrewd and analytical discussion of these matters has been anything but ‘wild.’
The Marengo paper is Marengo, Hulsebus and Willis, “KIC 8462852 – The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract / preprint). The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Jason Wright paper that examines KIC 8462 in the context of SETI signatures is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).
Will We Stop at Mars?
In the heady days of Apollo, Mars by 2000 looked entirely feasible. Now we’re talking about the 2030s for manned exploration, and even that target seems to keep receding. In the review that follows, Michael Michaud looks at Louis Friedman’s new book on human spaceflight, which advocates Mars landings but cedes more distant targets to robotics. So how do we reconcile ambitions for human expansion beyond Mars with political and economic constraints? A career diplomat whose service included postings as Counselor for Science, Technology and Environment at U.S. embassies in Paris and Tokyo, and Director of the State Department’s Office of Advanced Technology, Michael is also the author of Contact with Alien Civilizations (Copernicus, 2007). Here he places the debate over manned missions vs. robotics in context, and suggests a remedy for pessimism about an expansive future for Humankind.
by Michael A.G. Michaud
Many people in the space and astronomy communities will know of Louis Friedman, a tireless campaigner for planetary exploration and solar sailing. He was one of the co-founders of the Planetary Society in 1980, with Carl Sagan and Bruce Murray.
In his new book, entitled Human Spaceflight: From Mars to the Stars, Friedman states his argument up front: Humans will become a multi-planet species by going to Mars, but will never travel beyond that planet. Future humans will explore the rest of the universe vicariously through machines and virtual reality.
Friedman acknowledges that public interest in space exploration is still dominated by “human interest.” No one, he writes, is going to discontinue human spaceflight. Yet there is a conundrum. While giving up on manned missions to Mars is politically unacceptable, getting such a program approved and funded is not an achievable political step at this time. If another decade goes by without humans going farther in space, Friedman writes, public interest will likely decline and robotic and virtual exploration technologies will pass us by.
Friedman claims that going beyond Mars with humans is impossible not just physically for the foreseeable future but culturally forever. The long-range future of humankind, he declares, is to extend its presence in the universe virtually with robotic emissaries and artificial intelligence. This argument puts a permanent cap on human expansion, as if travel beyond Mars never will be possible.
Friedman sees having another world as a prudent step to prevent humankind being wiped out by a catastrophe. He argues that the danger of not sending humans to Mars is that we will become complacent. If that complacency overcomes making humankind a multi-planet species, we are doomed.
Friedman dismisses big ideas about exploiting planetary resources throughout the solar system and living everywhere to build civilizations and colonies on other worlds. He can’t see why or how we would do this, nor can he see waiting to do so. This illustrates an old split in the space interest community between those advocating space exploration and those supporting space utilization and eventual human expansion.
In his chapter entitled Stepping Stones to Mars, Friedman lists potential human spaceflight achievements with dates. An appendix presents a plan for a manned Mars mission in the 2040s. That first landing is to be followed later by missions establishing an infrastructure for human habitation, an effort that will take many decades.
Interstellar flight
This book’s subtitle is From Mars to the Stars. Yet Friedman dismisses interstellar travel by human beings as a subject of science fiction. People are too impatient, he writes, to wait for the necessary life-support developments. This contrasts with Carl Sagan’s 1966 comment that efficient interstellar spaceflight to the farthest reaches of our galaxy is a feasible objective for humanity.
Friedman argues that we have only one technology that might someday take our machines to the stars – light sailing. It may be another century before we have large enough laser power sources to drive small unmanned spacecraft over interstellar distances. The barrier of bigness will be overcome by the enablement of smallness.
Friedman suggests three interstellar precursor missions: the first launched in 2018 to the Kuiper Belt and onward to the heliopause; the second launched in 2025 to the solar gravity lens focus and on to 1,000 astronomical units; the third launched in 2040 to the Oort Cloud.
Virtual Reality
Friedman oversells virtual reality just as some others have oversold manned spaceflight. He acknowledges that we have yet to reach full cultural acceptance and satisfaction with the virtual world. Yet he seems to assume that such acceptance by the general population is inevitable.
Calling virtual reality human exploration may confuse many readers. Will we be content to watch all future exploration through robotic eyes?
There may be an unstated reason for preferring virtual reality over human presence. If future space exploration were entirely robotic, scientists would be in charge.
Cautions about Mars
Mars is far from ideal as a future home for humankind. The thin atmosphere is mostly carbon dioxide. Temperatures are low. The surface is more exposed to radiation and meteorites than Earth. Yet Mars remains the best candidate for a second planetary home within our own solar system.
Like other schedules proposed by some space advocates, Friedman’s plan for missions to Mars may be too optimistic. Yet such optimism keeps goals alive and encourages others to get involved.
What seems wildly optimistic now may be possible over the longer term. In the 1950s, some scientists thought that sending humans to the Moon was impossible.
The failure of grand visions
Friedman is correct in stating the biggest problem of space policy: the merging of grand visions with political constraints. In 1988, President Reagan’s statement on space policy included the idea of expanding human activity beyond Earth and into the solar system, an endorsement long sought by some elements of the space interest community. President George H.W. Bush fleshed out this idea in 1989 with his Space Exploration Initiative, urging that the U.S. develop a permanent presence on the Moon and the landing of a human crew on Mars by 2019. These visions failed to win the financing that would make them feasible.
Frustration and Patience
It is understandable that long-time campaigners for further exploration and use of space get frustrated, in some cases foreseeing the end of such endeavors. We all want to see major hopeful events occur in our own lifetimes. Yet we share some responsibility to look beyond.
Writing off human expansion beyond Mars for all the humans who follow us is, despite Friedman’s claim, pessimistic. The remedy is a younger generation of advocates.
A Little History
Friedman states that the settlement of Mars is the rationale for human spaceflight. The leaders of the Planetary Society did not initially support that goal. In the organization’s early years, its chief spokespersons criticized NASA’s emphasis on human missions (particularly the Space Station), which they saw as robbing funds that should have gone into further robotic exploration.
Sagan and others later realized that the planetary exploration budget rose and fell with the rise and fall of manned spaceflight programs. When NASA funding was rising, space science prospered; when NASA funding declined, space science funding declined with it. After the cancellation of further Apollo missions, planetary science was hit hardest by budget cuts . This revived a debate as old as the space program, between advocates of manned spaceflight and those who believe that priority should be given to exploration by unmanned spacecraft.
Friedman wrote in a 1984 article in Aerospace America about extending human civilization to space, suggesting a lunar base, a manned expedition to Mars, or a prospecting journey to some asteroids undertaken by an international team.
By the mid-1980s, the Planetary Society was advocating a joint U.S. Soviet manned mission to Mars. Senator Spark Matsunaga of Hawaii introduced legislation to support this idea and published a book in 1986 entitled The Mars Project: Journeys beyond the Cold War. Soviet leader Mikhail Gorbachev made overtures to the U.S. in 1987 and 1988 for a cooperative program eventually leading to a Mars landing.
Bruce Murray, reacting favorably to the 1989 Space Exploration Initiative, published an article in 1990 entitled Destination Mars—A Manifesto. Observing that the space frontier for the U.S. and the USSR had stagnated a few hundred miles up, Murray commented that neither the United States nor the Soviet Union is likely, by itself, to sustain the decades of effort necessary to reach Mars. Murray urged a joint U.S.-Soviet manned spaceflight program leading eventually to Mars.
This reviewer argued at the 1987 Case for Mars conference that relying on the Soviet Union during the Cold War made such a mission subject to political volatility. This turned out to be true. As Friedman reports, a brief flurry of interest by President Reagan and Gorbachev in a cooperative human mission to Mars disappeared quickly in the face of large global events such as the dissolution of the Soviet Union.
More recently, when the U.S. sought to punish Russia for invading Ukraine, Russian officials made public statements threatening the continuation of Russian transport of Americans to the International Space Station, even though the U.S. was paying for those flights.
References
Louis Friedman, Human Spaceflight: From Mars to the Stars, University of Arizona Press, 2015.
Louis D. Friedman, “New Era of Global Security: Reach for the Stars,” Aerospace America, August 1984, 4.
Michael A.G. Michaud, “Choosing partners for a manned mission to Mars,” Space Policy, February 1988, 12-18.
Chapter entitled “Scientists, Citizens, and Space” in Michael A.G. Michaud, Reaching for the High Frontier: The American Pro-Space Movement, 1972-1984, Praeger, 1986, 187-213.
Bruce Murray, “Destination Mars: A Manifesto,” Nature 345 (17 May 1990), 199-200.
Iosif Shklovskii and Carl Sagan, Intelligent Life in the Universe, Dell, 1966, 449.
A Cometary Solution for KIC 8462852?
KIC 8462852 is back in the news. And despite a new paper dealing with the unusual star, I suspect it will be in the news for some time to come, for we’re a long way from finding out what is causing the unusual light curves the Planet Hunters group found in Kepler data. KIC 8462, you’ll recall, clearly showed something moving between us and the star, with options explored by Tabetha Boyajian, a Yale University postdoc, in a paper we examined here in October (see KIC 8462852: Cometary Origin of an Unusual Light Curve? and a series of follow-up articles).
To recap, we’re seeing a light curve around this F3-class star that doesn’t look anything like a planetary transit, but is much more suggestive of debris. Finding a debris disk around a star is not in itself unusual, since we’ve found many such around young stars, but KIC 8462 doesn’t appear to be a young star when looked at kinematically. In other words, it’s not moving the way we would expect from a star that has recently formed. Moreover, the star shows us none of the emissions at mid-infrared wavelengths we would expect from a young, dusty disk.
Jason Wright and the team at the Glimpsing Heat from Alien Technologies project at Penn State have taken a hard look at KIC 8462 and discussed it briefly in a recent paper (citations for both the Boyajian and Wright papers are at the end of this entry). It seems entirely reasonable to do what Wright did in referencing the fact that the light curve we see around the star is what we would expect to see if an advanced civilization were building something. That ‘something’ might be a project along the lines of a ‘Dyson swarm,’ in which huge collectors gather solar energy, or it could be a kind of structure beyond our current thinking.
We all know that the media reaction was swift, and we saw some outlets acting as if Wright had declared KIC 8462 an alien outpost. He had done no such thing, nor has he or the Penn State team ever suggested anything more than continuing investigation of this strange star. What seems to bother others, who have scoffed at the idea of extraterrestrial engineering, is that Wright and company have not explicitly ruled it out as a matter of course. The assumption there is that no other civilizations exist, and therefore we could not possibly be seeing one.
I come down on the side of keeping our options open and studying the data in front of us. We have a lot of work ahead to figure out what is causing a light curve so unusual that at least one of the objects briefly occulting this star caused a 22 percent dip in its flux. That implies a huge object, evidently transiting in company with many smaller ones. There seems to be no evidence that the objects are spherically symmetric. What’s going on around KIC 8462?
A new paper from Massimo Marengo (Iowa State) and colleagues looks at what Tabetha Boyajian identified as the most likely natural cause of the KIC 8462 light curves. All I have at this point is the JPL news release and a release from Iowa State — the paper has not yet appeared online — describing evidence for a swarm of comets as the culprit. The study, which has been accepted at Astrophysical Journal Letters relies on Spitzer data dating from 2015, five years later than the WISE data that found no signs of an infrared excess.
Image: This illustration shows a star behind a shattered comet. Is this the explanation for the unusual light curves found at KIC 8462852? Image credit: NASA/JPL-Caltech.
If there had been a collision between planets or asteroids in this system, it was possible that the WISE (Wide-field Infrared Survey Explorer) data, taken in 2010, reflected conditions just before the collision occurred. Now, however, we can rule that out, because Spitzer, like WISE, finds no excess of infrared light from warm dust around KIC 8462. So the idea of planet or asteroid collisions seems even less likely. Marengo, according to the JPL document, falls back on the idea of a family of comets on an eccentric orbit. He’s also aware of just how odd KIC 8462 is:
“This is a very strange star,” [Marengo] said. “It reminds me of when we first discovered pulsars. They were emitting odd signals nobody had ever seen before, and the first one discovered was named LGM-1 after ‘Little Green Men… We may not know yet what’s going on around this star, but that’s what makes it so interesting.”
It would take a very large comet indeed to account for the drop in flux we’ve already seen, but a swarm of comets and fragments can’t be ruled out because we just don’t have enough data to make the call. I assume Marengo also gets into the fact that a nearby M-dwarf (less than 900 AU from KIC 8462, is a possible influence in disrupting the system. The comet explanation would be striking if confirmed because we have no other instances of transiting events like these, and we would have found these comets by just happening to see them at the right time in their presumably long and eccentric orbit around the star.
Image: Left: a deep, isolated, asymmetric event in the Kepler data for KIC 8462. The deepest portion of the event is a couple of days long, but the long “tails” extend for over 10 days. Right: a complex series of events. The deepest event extends below 0.8, off the bottom of the figure. After Figure 1 of Boyajian et al. (2015). Credit: Wright et al.
So, despite PR headlines like Strange Star Likely Swarmed by Comets, I think we have to take a more cautious view. We’re dealing with a curious star whose changes in flux we don’t yet understand, and we have candidate theories to explain them. We’re no more ready to declare comets the cause of KIC 8462’s anomalies than we are to confirm alien megastructures. At this point we should leave both natural and artificial causes in the mix and recognize how long it’s going to take to work out a viable solution through careful, unbiased analysis.
The Marengo paper is Marengo, Hulsebus and Willis, ”KIC 8462852: The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract). I write about it this morning only because it is getting so much media attention — more later when I can go through the actual paper. The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Wright paper is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).
Huge Flares from a Tiny Star
Just a few days ago we looked at evidence that Kepler-438b, thought in some circles to be a possibly habitable world, is likely kept out of that category by flare activity and coronal mass ejections from the parent star. These may well have stripped the planet’s atmosphere entirely (see A Kepler-438b Caveat – and a Digression). Now we have another important study, this one out of the Harvard-Smithsonian Center for Astrophysics, taking a deep look at the red dwarf TVLM 513–46546 and finding flare activity far stronger than anything our Sun produces.
Led by the CfA’s Peter Williams, the team behind this work used data from the Atacama Large Millimeter/submillimeter Array (ALMA), examining the star at a frequency of 95 GHz. Flares have never before been detected from a red dwarf at frequencies as high as this. Moreover, although TVLM 513 is just one-tenth as massive as Sol, the detected emissions are fully 10,000 times brighter than what our star produces. The four-hour observation window was short, which may be an indication that we’re looking at a star that is frequently active.
Now considered an M9 dwarf, TVLM 513 is about 35 light years away in the constellation Boötes. It is believed to be on the borderline between red and brown dwarfs, with a radius 0.11 that of the Sun, a temperature of 2500 K, and a rotation rate of a scant two hours (the Sun takes almost a month for a complete rotation). For a habitable planet to exist here — one with temperatures allowing liquid water on the surface — it would need to orbit at about 0.02 AU. That’s obviously a problem, as Williams explains in this CfA news release:
“It’s like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms. A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun.”
Image: Artist’s impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field (shown by the blue lines), potentially associated with a flurry of solar-flare-like eruptions. Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks.
Another unusual aspect of TVLM 513 is its magnetic field. Data from the Very Large Array in New Mexico had previously shown a magnetic field several hundred times stronger than the Sun’s. The paper argues that the emissions observed in the ALMA data are the result of synchrotron emission — radiation generated by the acceleration of high-velocity charged particles through magnetic fields — associated with the small star’s magnetic activity.
We have a lot to learn about small stars, their magnetic fields and their flare processes, and even in this study, the paper offers a caveat:
… confident inferences based on the broadband radio spectrum of TVLM 513 are precluded because the ALMA observations were not obtained contemporaneously with observations at longer wavelengths, and TVLM 513’s radio luminosity, and possibly its radio spectral shape, are variable. Additional support from the Joint ALMA Observatory to allow simultaneous observations with other observatories would be highly valuable.
The authors add that while it has long been known that both stars and gas giant planets have magnetic fields, the mechanisms at work are different and it is unclear what kind of magnetic activity we should expect from objects of intermediate size. Learning more about magnetic processes in small stars should help us understand more about exoplanets and their magnetic activity. This first result at millimeter wavelengths thus points to the work ahead:
Modern radio telescopes are capable of achieving ?µJy sensitivities at high frequencies (?20 GHz), raising the possibility of probing the means by which particles are accelerated to MeV energies by objects with effective temperatures of ?2500 K.
So we’re going to learn a lot more about small red dwarfs as we study whether or not such stars can host habitable planets. The argument against red dwarfs and astrobiology used to focus on tidal lock and the problems of atmospheric circulation, but we’re now wondering whether, particularly in young red dwarfs, flare activity may not be the key factor. If TVLM 513 is representative of a category of flare-spitting stars, the smallest red dwarfs may be hostile to life.
The paper is Williams et al., “The First Millimeter Detection of a Non-Accreting Ultracool Dwarf,” in press at The Astrophysical Journal (preprint).
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