by Paul Gilster | Dec 2, 2015 | Exoplanetary Science |
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.
by Paul Gilster | Oct 27, 2015 | Astrobiology and SETI |
How do you feel about a universe that shows no signs of intelligent life? Let’s suppose that we pursue various forms of SETI for the next century or two and at the end of that time, find no evidence whatsoever for extraterrestrial civilizations. Would scientists of that era be disappointed or simply perplexed? Would they, for that matter, keep on looking?
I suspect the latter is the case, not because extraterrestrial civilizations would demonstrate that we’re not alone, but because in matters of great scientific interest, it’s the truth we’re after, not just the results we want to see. In my view, learning that there was no other civilization within our galaxy — at least, not one we can detect — would be a profoundly interesting result. It might imply that life itself is rare, or even more to the point, that any civilizations that do arise are short-lived. This is that tricky term in the Drake equation that refers to the lifespan of a technological civilization, and if that lifetime is short, then our own position is tenuous.
The anomalous light curve in the Kepler data from KIC 8462852 focuses this issue because on the one hand I’m hearing from critics that SETI researchers simply want to see extraterrestrials in their data, and thus misinterpret natural phenomena. An equally vocal group asks why people like me are so keen on looking for natural explanations when the laws of physics do not rule out other civilizations. All I can say is that we need to be dispassionate in the SETI search, looking for interesting signals (or objects) while learning how to distinguish their probable causes.
In other words, I don’t have a horse in this race. The universe is what it is, and the great quest is to learn as much as we can about it. I am not going to lose sleep if we discover a natural cause for the KIC 8462852 light curves because whatever is going on there is astrophysically interesting, and will help us as we deepen our transit studies of other stars. The recent paper from Wright et al. discusses how transiting megastructures could be distinguished from exoplanets, and goes on to describe the natural sources that could produce such signatures. The ongoing discussion is fascinating in its own right and sharpens our observational skills.
Image: The Kepler field of view, containing portions of the constellations Cygnus, Lyra, and Draco. Credit: NASA.
Yesterday’s post looked at ‘gravity darkening’ as a possible explanation for what we see at KIC 8462852, with reference to conversations we’ve been having in the comments section here. Gravity darkening appears in the Wright paper, though not with reference to KIC 8462852, and is also under study in other systems, particularly the one called PTFO 8-8695. But its prospects seem to be dimming when it comes to KIC 8462852, as Wright explained in a tweet.
He went on to elaborate in yesterday’s comments section:
Gravity darkening might be a small part of the puzzle, but it does not explain the features of this star. Tabby’s star does not rotate fast enough to experience significant gravity darkening. That post also suggests that planets could be responsible, but planets are not large enough to produce the observed events, and there are too many events to explain with planets or stars.
The Wright paper lists nine natural causes of anomalous light curves in addition to gravity darkening, including planet/planet interactions, ring systems and debris fields, and starspots. Exomoons, the subject of continuing work by David Kipping and colleagues at the Hunt for Exomoons with Kepler project, also can play a role, with a sufficiently large moon producing its own transit events and leaving a signature in transit timing and duration variations.
We have examples of objects whose anomalies have been investigated and found to be natural, including the interesting CoRoT-29b, in which gravity darkening is likewise rejected. From the paper:
CoRoT-29b shows an unexplained, persistent, asymmetric transit — the amount of oblateness and gravity darkening required to explain the asymmetry appears to be inconsistent with the measured rotational velocity of the star (Cabrera et al. 2015). Cabrera et al. explore each of the natural confounders in Table 2.3 for such an anomaly, and find that none of them is satisfactory. Except for the radial velocity measurements of this system, which are consistent with CoRoT-29b having planetary mass, CoRoT-29b would be a fascinating candidate for an alien megastructure.
We can also assign a natural explanation to KIC 1255b, an interesting find because its transit depths vary widely even between consecutive transits, and its transit light curves show an asymmetry between ingress and egress. What we are apparently looking at here is a small planet that is disintegrating, creating a thick, comet-like coma and tail that is producing the asymmetries in the transit light curves. This is an intriguing situation, as the Wright paper notes, with the planet likely pared of 70 percent of its mass and reduced to an iron-nickel core.
We may well find a natural explanation that takes care of KIC 8462852 as well, and the large scope of the challenge will ensure that the object remains under intense scrutiny. Both CoRoT-29b and KIC 1255b are useful case studies because they show us how unusual transit signatures can be identified and explained. We also have to keep in mind that such signatures may not be immediately found because Kepler data assessment techniques are not tuned for them, as the paper notes:
…in some cases of highly non-standard transit signatures, it may be that only a model-free approach — such as a human-based, star-by-star light curve examination — would turn them up. Indeed, KIC 8462852 was discovered in exactly this manner. KIC 8462852 shows transit signatures consistent with a swarm of artificial objects, and we strongly encourage intense SETI efforts on it, in addition to conventional astronomical efforts to find more such objects (since, if it is natural, it is both very interesting in its own right and unlikely to be unique).
Thus we leave the KIC 8462852 story for now, although I would encourage anyone interested in Dysonian SETI to read through the Wright paper to get a sense of the range of transiting signatures that draw SETI interest. The 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).
by Paul Gilster | Jan 21, 2019 | Astrobiology and SETI |
One of the more important things about the interstellar object called ‘Oumuamua is the nature of the debate it has engendered. Harvard astronomer Avi Loeb’s paper examining it as a possible technology has provoked comment throughout the scientific community, as witness Jason Wright’s essay below. Dr. Wright (Penn State) heads the Glimpsing Heat from Alien Techologies (G-HAT) project, which he described in these pages, and is a key player in the rapidly developing field of Dysonian SETI, the study of possible artifacts as opposed to deliberate communications from extraterrestrial civilizations. Here he looks at the debate Loeb’s work has engendered and its implications not only for how we do science but how we teach its values to those just coming into the field. Jason’s essay was originally posted several days ago on his Astrowright blog, which should be a regular stop for Centauri Dreams readers.
by Jason T. Wright
Avi Loeb is the chair of the astronomy department at Harvard, a distinguished and well cited astronomer (he has an h-index of 87), and the chair of the Breakthrough Starshot initiative. He’s a strong proponent of making sure that science doesn’t succumb to groupthink and champion of outré ideas.
He also has been making headlines recently for articles he has co-authored, interviews he has given, and popular media columns he has written about the possibility that fast radio bursts, and now ‘Oumuamua, are artificial in origin. This has created a great deal of buzz in popular culture and a lot of hand-wringing and criticism on social media by scientists who find his actions irresponsible. Many have asked my opinion, so I’m collecting my many thoughts on the topic in this post.
I am happy to defend Avi on these grounds:
- He is driving us to have an important conversation about what “acceptable” SETI research looks like, and in this conversation I’m mostly on his side. He’s essentially moving the scientific equivalent of the “Overton Window” towards SETI, and that’s a good thing. These are exciting and interesting questions and we should not let the face-on-Mars/Ancient-Aliens/UFOlogy types prevent us from discussing them.
- He is using tenure and his stature the way we all imagine it’s supposed to be used: as a shield so that he can explore potentially unpopular research avenues without fear of retribution or ostracism. We all imagine that’s what we would do in his position (I hope!) but too often it ends up just being a club to get junior scientists to conform to one’s vision for what “proper” science looks like and what “good” problems are.
- The papers he and his postdocs are writing are important first steps in making Solar System and other forms of SETI a serious academic discipline.
- He is being a role model for how scientists can explore outré ideas and spend an appropriate amount of their time on potential breakthroughs.
- He is putting SETI in the public eye and doing a lot of outreach.
Image: Harvard’s Avi Loeb, at the center of the discussion of ‘Oumuamua. Credit: Harvard University.
Avi wouldn’t be pushing the envelope hard enough if he weren’t getting some pushback, and indeed there is plenty of fair and good-faith criticism that can be made about his approach (not all of which I agree with):
- The degree of certainty he expresses in ‘Oumuamua being artificial does seem unwarranted to me (though to be fair I’ve always been an ‘Oumuamua-might-be-artificial skeptic.)
- Given the way we know the press (especially the yellow press) will handle any story about “aliens”, one can argue that the “extraordinary claims require extraordinary evidence” maxim is especially applicable to SETI (I’ve made this argument strongly when discussing my own research in the press.) Avi could hew more closely to this maxim.
- The tone of his papers and his public comments are quite divergent. The body of the paper on ‘Oumuamua-as-lightsail, for instance, has a brief mention about the potential of the artifice of ‘Oumuamua at the end, but most of it is about the perfectly general problem of thin objects in interstellar space. Snopes highlights this divergence well pointing out that the paper is quite sober and restrained compared to some of the media coverage. (It’s true that the title and abstract of the paper are about ‘Oumuamua specifically, and that it serves as the case study for the whole analysis.) Avi’s public statements are much less conservative and equivocal.
- He is not just quietly following the evidence; he is using his platform to have a very public and high-visibility discussion about his research. I will concede that Avi is an exception to my earlier (somewhat petulant) protest that SETI scientists are not in it for the attention. That said, I will object to anyone who would claim Avi is only in it for the attention, or that such attention is inherently a bad thing.
- Many of his papers are de novo explorations of topics like the fate of comets in interstellar space, with little connection to the substantial amounts of work that has already been done on the topic, and his papers would be better and less naive if they had a closer connection to this prior work rather than starting from scratch.
More broadly, let’s look at two threads on Twitter criticizing Avi. I’ll start with this one by Bryan Gaensler:
Bryan makes the rather Popperian argument that if your model is too flexible then it can’t be falsified, so you’re not doing science. The implication is that since we don’t have a good model for aliens, we can always play the “aliens of the gaps” game and so SETI isn’t good science unless it’s looking for unambiguously artificial signals like narrow-band radio waves.
This argument isn’t as tight as it seems. Most interesting new theories start without concrete predictions—General Relativity was so hard to use that even Einstein wasn’t sure what it predicted (he got the deflection of starlight wrong the first time he calculated it; he wrote a paper saying gravitational waves don’t exist). Theories don’t spring fully-formed from theorists’ heads; many important breakthroughs start with something less than quantitative or precise (“maybe we need to modify gravity”; “maybe there is a new subatomic particle involved”) and let the data guide the theories’ details.
This is the normal progression of science. SETI is no different, and so no less scientific.
Then there is this one, by Eric Mamajek, which I mostly agree with:
It’s mostly fine through tweet #9, but then he conflates things in the last tweet using an unwarranted leap of logic.
Up until then he had been criticizing the Holmesian logic of how ‘Oumuamua must be alien because we had ruled out natural explanations. I quite agree with him.
But in the last tweet he jumps to criticizing even bringing up the hypothesis of ETI’s in general, implying that scientists who do are pulling a Giorgio Tsoukalos. (There’s also the assertion at the end such anomalies will “inevitably” turn out to be not just natural, but mundane, which is obviously not strictly true.)
But Tabby and I weren’t pulling a Tsoukalos when we submitted our proposal with Andrew Siemion to NRAO to study Tabby’s Star. We really weren’t. I have clarified the actual events with Eric, so I’m pretty sure that’s not what he meant to imply here, but that is how this tweet reads.
Bryan makes a similar (but softer) implication in his final tweets:
We all would! Indeed, it was Avi Loeb who made the suggestion that Breakthrough Listen point Green Bank at ‘Oumuamua [1] because he understands very well that the proof of alien technology is something like the bullets on Bryan’s list.
But the implications of these tweets aren’t just wrong, they’re harmful to the field of SETI. A very plausible path to SETI success will be that we will see something strange (not “Eureka!” but “That’s funny…” as the old fortune quip goes) and eventually, after lots of follow up, we might find the smoking gun, or perhaps it will just end up being a proof by exclusion. As I wrote in 2014:
Artifact SETI can thus proceed by seeking phenomena that appear outside the range that one would expect natural mechanisms to produce. Such phenomena are inherently scientifically interesting, and worthy of further study by virtue of their extreme nature. The path from the detection of a strange object to the certain discovery of alien life is then one of exclusion of all possible naturalistic origins. While such a path might be quite long, and potentially never-ending, it may be the best we can do.
Communication SETI, on the other hand, shortcuts this path to discovery by seeking signals of such obviously engineered and intelligent origin that no naturalistic explanation could be valid. Together, artifact and communication SETI thus provide us with complementary tools: the most suspicious targets revealed by artifact SETI provide the likeliest targets for communication SETI programs that otherwise must cast an impossibly wide net, and communication SETI might provide conclusive evidence that an extreme but still potentially naturalistic source is in fact the product of extraterrestrial intelligence (Bradbury et al. 2011).
Bryan’s thread and Eric’s final tweet could easily be read to foreclose this sort of research, essentially saying “it’s not worth thinking about the aliens hypothesis until it’s so unavoidable that you’ll get no flak for it” (radio signals à la Contact, the proverbial saucer on the White House lawn, etc.). They certainly make it clear that they won’t hesitate to chastise you on Twitter for going down this road.
But if we want to get to the end of that road, we’ve got to start walking down it at some point, and when the media very reasonably asks what we’re doing so they can report on it to a very understandably curious public, we should be allowed to answer their questions without having our motives (or scientific credibility) questioned by our peers.
In short: your mileage may vary on Avi’s particular style of public communication and conclusions on ‘Oumuamua, but when making your critique please be mindful that you are not slamming the whole endeavor. SETI as a serious science will make hypotheses, explore anomalies, and discuss the possibility of alien technology as the cause, and we need to be able to do so without obloquy from our peers, and without them policing which kinds of SETI we’re “allowed” to work on or talk about in public.
If I seem touchy about this, it’s actually not because I’m smarting from these Twitter threads or anything like that (which I don’t actually disagree with much—in particular I’m friends with Eric and I know I have his respect). As I wrote at the top, I’m glad we’re having this conversation and I hope it continues!
But another purpose of this post is that Avi and I (and other SETI researchers) have advisees that work on SETI and these sorts of messages are not lost on them: these tweets imply that senior people in your field will disapprove of you because of the topic of your research, and they will police what you’re allowed to say to the press, regardless of how good a scientist you are. Keep in mind, “Avi’s” paper on ‘Oumuamua that is being criticized has a postdoc as first author.
So in closing: I pledge to keep the SETI real and well grounded in science, to be responsible in my interactions with the media about it, and to train my students to do the same.
And, I hope my peers will pledge to create a welcoming environment for my advisees as SETI (hopefully!) comes back into the astronomy fold (even when—especially when—they are complaining about Avi).
[Updates: Bryan responds in this thread (click to expand):
also:
[1] = privately, Bryan clarified to me his tweet was referring to his team’s MWA search for signals, not the search by Breakthrough Listen at Avi’s suggestion, as I suggested in my post. My point that Avi appreciates the importance of dispositive evidence stands, but I should have read Bryan’s tweet more carefully and followed link before critiquing his tweet.]
Also, I’ve changed the language about who suggested that GBT observe ‘Oumuamua; Joe Lazio informs me that the observations were made with WVU time following discussions with Breakthrough Listen that preceded Avi’s recommendation. In spite of both errors on my part in the original post, my point that Avi appreciates the importance of dispositive evidence stands.
Also, Avi touches on his motives in this interview:
But the search for intelligent life remains outside the mainstream. I am trying to change that in two ways. First, by speaking out in the way that I did on ‘Oumuamua.
