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.
by Paul Gilster | Jan 18, 2019 | Exoplanetary Science |
One of the joys of science fiction is imagining landscapes. What would it be like to stand on Titan, for example, a question that was inescapably influenced in my youth by Chesley Bonestell’s wonderful depictions, as well as novels like Larry Niven’s World of Ptavvs (1966) or Michael Swanwick’s novelette “Slow Life” (Analog, December 2002). And then, of course, there were those multi-star skies, as in Asimov’s “Nightfall” (Astounding Science Fiction September, 1941.
The Science Fiction Writers of America, incidentally, voted “Nightfall” the best science fiction story written prior to 1965, when the Nebula Awards began. I would bet almost all Centauri Dreams readers are familiar with it, but if not, it’s widely anthologized.
And now we have another visual phenomenon to contend with, a landscape and its sky that had never occurred to me. A team led by Grant Kennedy (University of Warwick, UK) has discovered the first confirmed case of a multiple star system whose surrounding disk of gas and dust circles the central stars at right angles. The work grows out of data gathered with the Atacama Large Millimeter/sub-millimeter Array (ALMA). Says Kennedy:
“Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form.”
So let’s imagine a planet forming in the dust ring, just as we know planets form in the disks we’ve found around single stars. From the surface of such a world, our new science fictional setting shows us the disk as a band rising out of the horizon, with the twin stars moving in and out of the disk plane, so that we get two shadows much of the time. Our circumbinary planet in its all but perpendicular orbit of the primaries might see a scene like the one below.
Image: View from an orbiting planet. Copyright: University of Warwick/Mark Garlick. Used with permission.
The young system in question is found at HD 98800, also known as TV Crateris, in the constellation Crater, somewhere around 150 light years away from the Sun. This is actually a quadruple star system found in the TW Hydrae association. HD 98800 A is a K-class dwarf probably orbited by a red dwarf, while HD 98800 B is likewise a K-class, red dwarf pairing. A planet in this system — and bear in mind that this is a very young system, so the planet-forming process would be early — would have four nearby stars to color its landscape.
The authors do not believe such systems are rare. From the paper:
If planet formation can proceed equally efficiently in both coplanar and polar configurations, circumbinary planets on polar orbits are predicted to be nearly as common as their coplanar brethren (although these fractions may be modified by later dynamical evolution). The most eccentric binaries are the most likely to have polar disk configurations, so it is not surprising that the known transiting circumbinary planets, which are near to coplanar, are all in systems with e?0.52, with 8 out of 9 having e<0.22… Polar disks, and perhaps planets, may be a common outcome of circumbinary disk formation, and provide motivation for systematic searches for both.
Image: View of the double star system and surrounding disc. Copyright: University of Warwick/Mark Garlick. Used with permission.
Is there, then, a large population of such unusually aligned circumbinary planets awaiting discovery? If so, we’ll have a variety of further interesting landscapes to consider, even as we ponder the kind of seasonal variations that can occur on circumbinary worlds circling a wide range of stellar classes and their own possible companions. Plenty of material here for writers, or has some far-sighted SF wordsmith already depicted such a planet? If so, please let me know in the comments.
The paper is Kennedy et al., “A Circumbinary Protoplanetary Disc in a Polar Configuration,” Nature Astronomy 14 January 2019 (abstract).
by Paul Gilster | Jan 17, 2019 | Exoplanetary Science |
Barnard’s Star b, the planet announced last November around the second nearest star system to the Earth, has been the subject of intensive study by an international team led by Ignasi Ribas at the Institute of Space Studies of Catalonia (IEEC), and Institute of Space Sciences (ICE, CSIC). As announced at the recent meeting of the American Astronomical Society in Seattle, the work helps to refine the age of Barnard’s Star and examines its potential for supporting life on its known planet.
We don’t know whether there are other planets around Barnard’s Star, but the fact of Barnard Star b’s existence is significant, according to Scott Engle (Villanova University), who along with colleague Edward Guinan presented the results in Seattle. Says Engle:
“The most significant aspect of the discovery of Barnard’s Star b is that the two nearest star systems to the Sun are now known to host planets. This supports previous studies based on Kepler Mission data, inferring that planets can be very common throughout the galaxy, even numbering in the tens of billions.”
Indeed, the idea of at least one planet around every star gains currency, and in terms of our own position in the cosmos, it bears noting that many stars in our stellar neighborhood are far older than our own. On that point, the new work benefits from the running analysis performed by a Villanova program called Living with a Red Dwarf, which homes in on the radiative environments that planets around such stars would be subject to as their host evolves. The goal is to make a determination of the likelihood that complex molecules can form, and whether life can evolve.
Image: Model of the Barnard’s Star planet system (from Ribas et al. 2018) compared to the inner Solar System. Barnard b orbits at 0..404 AU from its M3.5V host star and has an equilibrium temperature of T=-168C° in its 233-day orbit. Credit: Edward Guinan, Scott Engle / Villanova University.
The gathering of photometric data on Barnard’s Star under this project goes back to 2003, determining a rotation period of 142±8 days, a value that agrees well with other recent studies. The team then used the rotation period to extract a likely age of 8.6 billion years. Estimating stellar age for low-mass stars through rotation is a field known as gyrochronology, one that has accumulated a significant history of published analysis in the past decade. The age determined here also fits other age indicators to establish a result with 1.2 billion years play on either side.
As to that interesting planet, Barnard’s Star b is a super-Earth orbiting far enough from the primary to be cold (-168 C°), with only about 2 percent of light relative to the Earth. What the researchers go on to point out in their presentation is that as a super-Earth with a minimum mass of 3.25 Earth masses, Barnard’s Star b could have a hot iron/nickel core with resulting geothermal activity. The potential, if water is present, is for liquid water under an icy surface.
Geothermal heating could support “life zones” under its surface, akin to subsurface lakes found in Antarctica,” Guinan said. “We note that the surface temperature on Jupiter’s icy moon Europa is similar to Barnard b but, because of tidal heating, Europa probably has liquid oceans under its icy surface.”
We can only speculate about such matters, and the range of outcomes depending on the mass of the planet is wide. Note the range of possibilities in the authors’ presentation, called “X-Ray, UV, Optical Irradiances and Age of Barnard’s Star’s New Super Earth Planet – ‘Can Life Find a Way’ on such a Cold Planet?”:
Although little is definitely known about geomagnetism of superearths like Barnard b, a large liquid iron core, that could strong generate geomagnetic fields, could offer protection from strong winds and coronal mass ejections when the star was young & magnetically active. However, if the mass of the Barnard b is much higher than about 7–10 M?, its higher gravity could result in it retaining a thick H2 -He atmosphere and thus be a dwarf gas giant (mini-Neptune). In this case all hope for life is probably lost unless by chance Barnard b hosts an icy moon (with a subsurface ocean) that could be tidally heated like Europa.
Image: (L) Possible model of Barnard b based on geothermal heating. If water is present, geothermal heating could create a subsurface ocean where primitive life could exist. The model would be a scaled-up Europa. (R) In another scenario if the mass of the exoplanet is > 7 M?, then the stronger gravity could cause the retention of its primordial H2/He atmosphere. These planets are known as Mini-Neptunes / Dwarf Gas Giants. Credit: Edward Guinan, Scott Engle / Villanova University.
To learn more, we need to image the planet, an observation that would tell us about its atmosphere, surface and potential for life. On this score, the news is promising. Barnard’s Star b has an angular separation from its host that is much larger than Proxima b from Proxima Centauri, and may well be imaged by the next generation of extremely large telescopes (ELTs). It may also prove a target for the James Webb Space Telescope or the WFIRST mission.
For more, see Toledo-Padrón, “Stellar activity analysis of Barnard’s Star: Very slow rotation and evidence for long-term activity cycle” (preprint), which includes the high-precision photometry data of Barnard’s Star used in this analysis.
by Paul Gilster | Jan 16, 2019 | Exoplanetary Science |
We can identify a number of circumstellar disks, but most are too far away to provide internal detail, much less the kind of activity that seems to be showing up around the red dwarf AU Microscopii. For at 32 light years out in the southern constellation Microscopium, AU Microscopii is presenting us with an unusual kind of activity that may have repercussions for the question of life around red dwarf stars in general. As presented at the recent meeting of the American Astronomical Society, fast-moving blobs of material are eroding the disk.
The consequence: Icy materials and organics that might have developed in asteroids and comets may instead be pushed out of the disk, long before they could provide the infall of materials thought to have benefited planets like ours. “The Earth, we know, formed ‘dry,’ with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system,” said co-investigator Glenn Schneider (Steward Observatory, Tucson, Arizona).
Image: These two NASA Hubble Space Telescope images, taken six years apart, show fast-moving blobs of material sweeping outwardly through a debris disk around the young, nearby red dwarf star AU Microscopii (AU Mic). The top image was taken in 2011; the bottom in 2017. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the images in visible light. This comparison of the two images shows the six-year movement of one of the known blobs (marked by an arrow). Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).
Researchers estimate that the blob of material in the image above is moving at about 24,000 kilometers per hour. It would have moved more than 1.3 billion kilometers between 2011 and 2017, roughly the distance between the Earth and Saturn when the two are at their closest approach to one another. Continually pushing small particles containing water and other volatiles out of the system, such circumstellar materials could cause the AU Microscopii disk to dissipate in 1.5 million years. Each blob — and thus far the team has found six of them — is thought to mass four ten-millionths the mass of Earth.
The ejection speeds among the six identified blobs range between 14,500 kilometers per hour and 43,500 kilometers per hour, well beyond escape velocity for the star. Their current distance ranges from 1.5 billion kilometers from the star to more than 8.8 billion kilometers. AU Microscopii’s relative proximity makes it possible for Hubble to resolve substructure in at least one of the blobs, which may eventually make it possible to discover their origins.
Image: The box in the image at left highlights one blob of material extending above and below the disk. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the picture in 2018, in visible light. The glare of the star, located at the center of the disk, has been blocked out by the STIS coronagraph so that astronomers can see more structure in the disk. The STIS close-up image at right reveals, for the first time, details in the blobby material, including a loop-like structure and a mushroom-shaped cap. Astronomers expect the train of blobs to clear out the disk within only 1.5 million years. The consequences are that any rocky planets could be left bone-dry and lifeless, because comets and asteroids will no longer be available to glaze the planets with water or organic compounds. Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).
We wind up with planets lacking the nearby volatiles to enrich them, giving us the prospect of dry, dusty worlds without life. We can add this to the other factors that challenge the emergence of life around red dwarf stars, such as possible tidal lock and the resulting climate issues, not to mention heavy ultraviolet flux from young stars that could strip away the atmosphere of planets in the habitable zone. AU Microscopii is itself 23 million years old, an infant in stellar terms. Bear in mind that red dwarfs are the most common type of stars in the galaxy.
“The fast dissipation of the disk is not something I would have expected,” says Carol Grady (Eureka Scientific, Oakland, California), a co-investigator on the Hubble observations. “Based on the observations of disks around more luminous stars, we had expected disks around fainter red dwarf stars to have a longer time span. In this system, the disk will be gone before the star is 25 million years old.”
The AU Microscopii data were gathered by the European Southern Observatory’s Very Large Telescope in Chile as well as the Hubble Space Telescope Imaging Spectrograph (STIS) by a team led by John Wisniewski (University of Oklahoma). The STIS visible light images, taken in 2010-2011, were followed up by near-infrared work at the the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) mounted on the VLT. The work also draws on disk observations of AU Microscopii by the Hubble Advanced Camera for Surveys in 2004.
by Paul Gilster | Jan 15, 2019 | Astrobiology and SETI |
‘Oumuamua continues to inspire questions and provoke media attention, not only because of its unusual characteristics, but because of the discussion that has emerged on whether it may be a derelict (or active) technology. Harvard’s Avi Loeb examined the interstellar object in these terms in a paper with Shmuel Bialy, one we talked about at length in these pages (see ‘Oumuamua, Thin Films and Lightsails). The paper would quickly go viral.
Those who have been following his work on ‘Oumuamua will want to know about two articles in the popular press in which Loeb answers questions. From the Israeli newspaper Ha’aretz comes an interview conducted by Oded Carmeli, while at Der Spiegel Johann Grolle asks the questions. From the latter, a snippet, in which Grolle asks Loeb what the moment would be like if and when humanity discovers an extraterrestrial intelligence. Loeb’s answer raises intriguing questions:
I can’t tell you what this moment will look like. But it will be shocking. Because we are biased by our own experiences. We imagine other beings to be similar to us. But maybe they are radically different. For example, it is quite possible that we won’t encounter the life forms themselves, but rather only their artifacts. In any case, we ourselves are not designed for interstellar journeys. The only reason astronauts survive in space is that they are under the protection of the Earth’s magnetic field. Even when traveling to Mars, cosmic rays will become a major problem.
Image: Avi Loeb (center) at the Daniel K. Inoue Solar Telescope (DKIST) in June of 2017. Credit: Avi Loeb.
Intriguing, given our conversations here about artificial intelligence and the emergence of non-biological civilizations. After all, we are in the nearby galactic company of numerous stars far older than our own. Would robotic beings supplant their biological cousins, or would the scenario be more like biological beings using artilects as their way of achieving interstellar travel? Either way, Loeb’s guess is that our first evidence will be an encounter with technological debris. The interview goes on to cover the ‘Oumuamua story’s outline thus far.
Meanwhile, two new papers from Loeb have appeared, the first written with John C. Forbes. “Turning Up the Heat on ‘Oumuamua” looks at the interstellar object, whatever it is, from another angle. If we were to discover more objects like this, how could we best analyze them? In earlier work with Manasvi Lingam, Loeb examined the population of interstellar objects that could be trapped within the Solar System, slung by Jupiter into parabolic orbits around the Sun.
The number could be as high as 6,000, a figure based on the deduced abundance of interstellar objects given the fact that we observed ‘Oumuamua as early as we did with instrumentation of the sensitivity of the Pan-STARRS telescopes. The paper references work on the overall abundance of these objects performed in 2017 by Greg Laughlin (UC-Santa Cruz) and Konstantin Batygin (Caltech), as well as a 2018 paper from Aaron Do (University of Hawai’i).
Learning more could involve a flyby mission, says Loeb, but there may be a better way:
In our new paper with John Forbes we proposed instead studying the vapor produced when such objects pass close to the Sun and get evaporated by the intense solar heat. We calculated the likelihood of that happening, keeping in mind that `Oumuamua did not show any signs of a cometary tail or carbon-based gas since it did not pass close enough to the Sun.
We used the known orbit of `Oumumua and assume a population of similar interstellar objects on random orbits in the vicinity of the Sun. This provided us with a likelihood of passages close to the Sun.
These objects would be expected to show a high orbital inclination, and assuming a population of this size, they should be readily detectable by future telescopes, such as the forthcoming Daniel K. Inoue Solar Telescope (DKIST). Another marker of interstellar origin, according to the paper, would be anomalous oxygen isotope ratios. If we can find interstellar objects that pass close to the Sun, we should be able to learn something about their composition. Loeb and Forbes use Monte Carlo methods to determine that such objects collide with the Sun once every 30 years, while about two should pass within the orbit of Mercury each year.
Usefully, spectroscopic study of cometary tails is a well-practiced science. As the paper notes:
Generally these studies are able to classify comets into different groups depending on the inferred production rates of H2O, C2, CN, and NH2 as well as dynamical properties, which likely reflect formation in different parts of the protoplanetary disk (Levison 1996)… The promise of using close encounters with the sun to learn about extrasolar small bodies is that the sun has the ability to disrupt even large cometary nuclei via its intense radiation, sublimating not just surface volatiles but even silicates and iron. In principle this exposes the interiors of these objects to remote spectroscopy, which could place strong constraints on the composition of these objects.
And indeed, two comets — 96P/Machholz 1 and Yanaka (1998r) — have been found to have depleted levels of CN and C2 relative to water. Sun-grazing comets of interstellar origin, assuming we can identify them early through instrumentation like the LSST (Large Synoptic Survey Telescope) should be available for such examination, a way to probe their composition without the need for sending fast flyby missions, although the latter would obviously be useful.
In a second paper, just accepted at Research Notes of the American Astronomical Society. Loeb and Harvard colleague Amir Siraj note that ‘Oumuamua’s shape may be more extreme than we have thought. Noting that the axis ratio for the object has been pegged at between 6:1 and 10:1, the paper delves into the lightcurve, with a startling result, as Loeb explained in an email this morning:
The lightcurve of the interstellar object Oumuamua showed a net brightening by one magnitude between October and November 2017, after corrections for the changing distances to the Sun and Earth and solar phase angle, assuming isotropic uniform albedo and the canonical phase function slope value for cometary and D-class objects of -0.04 magnitude per degree. We used the change in the orientation of `Oumuamua between October and November 2017 to show that this brightening implies a more extreme shape for the object. We inferred a ratio between its brightest and dimmest phases of at least 50:1 for a cigar shape and 20:1 for a pancake-like geometry. The revised values can be avoided if the phase function slope is 3 times larger than the canonical value, implying in turn another unusual property of `Oumuamua.
Variations in albedo could be in play, although here we would be looking at sharp variations for a minor change in viewing angle of ~ 11°, which Loeb and Forbes consider a possibility, though one without precedent in previous studies of asteroids and comets.
The papers are Forbes and Loeb, “Turning Up the Heat on ‘Oumuamua,” submitted to The Astrophysical Journal Letters (preprint); and Siraj and Loeb, “‘Oumuamua’s Geometry Could be More Extreme than Previously Inferred,” accepted at Research Notes of the American Astronomical Society (full text).
