Centauri Dreams
Imagining and Planning Interstellar Exploration
Bradley Schaefer: A Response to Michael Hippke
The question of a gradual dimming of KIC 8462852 continues to make waves, the most recent response being Michael Hippke’s preprint on the arXiv site, discussed in the post immediately below. Bradley Schaefer (Lousiana State University), who published his work on the dimming he found in now digitized photographs in the archives of Harvard College Observatory, disagrees strongly with Hippke’s findings and is concerned that the paper impugns the solid work being performed by DASCH (Digital Access to a Sky Century@Harvard). Below is Dr. Schaefer’s response with details on the astrophotographic photometry at the heart of the issue.
by Bradley E. Schaefer
A few hours ago, Michael Hippke posted a manuscript to arXiv (http://arxiv.org/abs/1601.07314), and submitted the same manuscript to the Astrophysical Journal Letters (ApJLett). This manuscript claims to have found that the DASCH data produces light curves with secular trends (both systematic dimmings and brightenings) over the century-long records. This same DASCH data (from the collection of archival sky photographs now at Harvard Observatory) was used to recognize a dimming of KIC 8462852 (a.k.a. ‘Tabby’s Star’ or the ‘WTF star’) at an average rate of 0.165±0.013 magnitudes per century from 1890 to 1989.
This dimming from the DASCH data is just a long-time scale version of the dimming also seen with the Kepler spacecraft, and these dimmings are still a high mystery and a perplexing problem. Hippke is taking his claimed result (that the majority of DASCH light curves have major and widespread calibration errors resulting in apparent secular trends) as then implying that KIC 8462852 does not have any secular trend. This claim is easily proved wrong.
Hippke made two major errors, both of which are beginner’s mistakes, and both of which will erroneously produce apparent dimmings and brightenings when none exist. First, Hippke explicitly includes red-sensitive and yellow-sensitive photographs together with the blue-sensitive photographs. The different colors will produce systematically different brightnesses (magnitudes). The trouble is further that the red and yellow photographs are predominantly at late times in the century-long light curve (in the 1970s and 1980s), so the inclusion of many magnitudes that are systematically high or low only at the end of the century will artificially make the star appear to brighten or dim over the century.
Second, Hippke explicitly includes magnitudes from photographs with known and recognized defects. The Harvard photographs are not perfect, with some having long-trailed images, some being double exposures with stars overlapping, and some having various plate defects where the emulsion is nicked or such. The DASCH scanning and software has a robust means of identifying problem photographs, and these are objective measures independent of the magnitude. These known-poor-quality magnitudes should not be used for any sensitive purposes. Colloquially put, these are ‘garbage’. Hippke keeps all the many good DASCH magnitudes and he also adds in the garbage magnitudes, so his final light curves have many points that are systematically skewed.
The frequency of the poor-quality magnitudes varies over time, usually with more early-on during the century. And the erroneous magnitudes are variously systematically brighter or dimmer, also varying over the century. The result of Hippke’s good+garbage light curves is that the garbage points tilt the light curve by a bit. This is seen when I take all of Hippke’s same stars and data and go from his sloped light curves (including his garbage points) to flat light curves (with only the good points). The bottom line is that Hippke’s second big mistake was to include the poor-quality photographs. Garbage-in, garbage-out.
So we understand why Hippke’s secular trends are wrong. But we already knew this very well anyway. The reason is that the DASCH people have already measured many (likely up around the millions) of light curves for single main sequence stars (i.e., stars that really should be perfectly constant) and found that their light curves are actually very flat. This is in stunning contradiction to the claims of Hippke that the majority show big secular trends.
Hippke’s paper has a title of “KIC 8462852 Did Likely Not Fade During the Last 100 Years”, yet his paper never discusses or analyses any data from KIC 8462852. One reason is perhaps that he cannot get around the flatness of the five check star light curves. That is, these five stars always appear within 3 millimeters of Tabby’s Star on these 10″x8″ phootgraphs, these stars are all of similar brightness as Tabby’s Star, and they all have similar color as Tabby’s Star.
If there were any systematic problems for the DASCH data with Tabby’s star, then we should see the exact same dimming trend in the check stars as is seen for Tabby’s Star. But we do not. These ‘check stars’ serve as the classic control study in science. They serve as proof that neither the check stars nor Tabby’s Star have any substantial systematic problem. They serve as proof that Hippke’s title is wrong.
Hippke submitted his draft manuscript to ApJLett, to arXiv and to reporters even before he had any checks with experts on archival sky photographs. For example, I first read his email just about the time that he was submitting his manuscript. He did not contact any of the DASCH people, despite them being the target of his attack. Indeed, he has not talked with anyone who has any experience with or knowledge of any archival photographs. This topic has a lot of detail and many quirks, but Hippke apparently did not have the realization or the will to check out his claims. And, in an email from Hippke from early this morning, he explicitly labelled himself as “a novice” for this technical topic. So he is a novice working without bothering to check with anyone knowledgeable. As such, it is not surprising that he made beginner’s blunders.
A broader problem is now that DASCH has the publicly-stated claim that it has major, widespread, persistent calibration and measurement errors. In knowledeable circles, Hippke’s paper won’t come to anything. But these circles are not large, because few people really understand the working of DASCH or plate photometry.
So most people will simply look at the paper’s conclusions, not recognize the horrible beginner’s blunders that create the false conclusion, and only come away thinking that the DASCH light curves are “wrong” or at least “questionable”. Public perceptions do matter for many aspects. Most important for DASCH is their future success rate in funding proposals, the reception of all future papers relating to DASCH, and the future useage of the DASCH data.
Perhaps from a journalistic point of view, any ‘stirring of the pot’ is good copy. But from the point of view of science and knowledge, putting up unchecked and false claims is bad all the way around. Science has a great strength of being error-correcting, with the normal procedure now for the DASCH people to put out a full formal refutation of Hippke’s claims, and such will appear in many months. But with the one-day turn-around of arXiv and with fast journalist response, there will be many months where the reputation of DASCH is maligned. So Hippke’s choice of running to reporters before the paper appeared publicly, and disdaining any experienced advice despite being a self-proclaimed “novice”, is not good science.
——-
For more on this controversy, see KIC 8462852: Where Are We After Eight Months?, Michael Hippke’s follow-up.
KIC 8462852: No Dimming After All?
As if the Kepler star KIC 8462852 weren’t interesting enough, Bradley Schaefer (Louisiana State) added to the controversy when he discovered what appeared to be a steady dimming of the star over the past century. Schaefer called the result “completely unprecedented for any F-type main sequence star,” and given the discussion about KIC 8462852 as a SETI target, this raised the stakes. Something just as odd as the object’s strange lightcurves was going on here, and it seemed natural to think that the dimming and the lightcurves were related.
But Michael Hippke now begs to disagree. An old friend of Centauri Dreams (see, for example, his Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak), Hippke takes a close look at Schaefer’s work and reaches a different conclusion. As he sees it, the ‘dimming’ of up 0.165 ± 0.013 magnitudes per century in this F3 star may actually be the result of imperfect calibration on the Harvard plates. In other words, while the lightcurve anomalies remain, the dimming may well be a data artifact rather than an astrophysical enigma.
Image: KIC 8462852 as photographed from Aguadilla, Puerto Rico by Efraín Morales, of the Astronomical Society of the Caribbean (SAC).
First, though, a word about Bradley Schaefer’s work, about which Hippke says “Schaefer had the excellent idea to look into the old plate archives. To solve this mystery, we need all the information we can get, and Schaefer did very careful and high-quality work.”
This parallels comments I’ve heard from other professionals, who praise the quality of Schaefer’s analysis. Submitted to Astrophysical Journal Letters, the Hippke paper looks to contrast the ‘dimming’ of KIC 8462852 with an analysis of other F-type main sequence stars from the same dataset. Along the way, Hippke double-checks Schaefer and finds sound work:
Although the process of data cleansing and binning involves arbitrary choices, we have reproduced this part of the analysis for all variants with virtually identical results. It is therefore important to note that the method and results in Schaefer (2016) appear to be adequately careful and accurate. In the following, we will thus concentrate solely on the interpretation of his result – whether the dimming is “unprecedented”.
Take away its odd lightcurves and KIC 8462852 appears to be a relatively normal star. Thus Hippke’s criteria for study are F-stars from the Kepler field of view, from which photometry is studied for the 3 most quiet F-dwarfs and 25 bright F-dwarfs in the Harvard DASCH (Digital Access to a Sky Century @Harvard) archive. Trends in the data may not, Hippke believes, be slow drifts but ‘structural breaks’ — in other words, changes caused by abrupt changes in technology or calibration techniques. Evidence for this occurs not only for KIC 8462852 but also for KIC 7180968, indicating we are dealing with a phenomenon not isolated to KIC 8462852.
From the paper:
The significant trends (and/or structural breaks) found in 18 of 28 comparison stars support the interpretation that the dimming of KIC 8462852 is not extraordinary. A careful analysis of each dataset is time-consuming, which is why we have not performed this analysis for hundreds of stars. In case of further doubt on the significance of such trends, the analysis presented could simply be expanded to more stars.
This would make an astrophysical interpretation of the ‘dimming’ unlikely because it would require that a number of main-sequence F-dwarfs fluctuate by 10% or more over the course of a century. “It seems more likely,” writes Hippke, “that the change of emulsions, errors in calibration etc. cause these trends.” In an email just received, Hippke notes of Schaefer’s work that “It might just be that his check stars were unusually stable, which obfuscated existing trends in the data.”
Thus the paper favors the notion that changes in technology and imperfect calibration — quality issues in the dataset itself — explain what otherwise appears to be long-term dimming of KIC 8462852. This leaves us, as the author notes, with the short-term dimmings found in KIC 8462852’s lightcurves, a problem that the question of century-long dimming does not address.
What can be done to investigate the dimming issue further? Hippke’s email suggests that other data, particularly plates from the Sonneberg Observatory in Germany, will be useful for comparison. “Unfortunately, these plates are not available online and have only partially been scanned, so checking these data might take several months.”
The paper is Hippke, “KIC 8462852 did likely not fade during the last 100 years,” submitted to the Astrophysical Journal Letters (preprint). Bradley Schaefer strongly disputes Hippke’s work, so we haven’t heard the end of this.
In Search of the First Rocket Man
If you’re interested enough in space to be reading this site, you’ve probably run into the name of Wan Hu. He’s the subject of a tale that may well be spurious, but it’s certainly lively. It seems that some time around the year 1500 AD, Wan Hu took his fascination with rocketry to the logical limit by building a chair equipped with 47 gunpowder rockets. Lit by 47 attendants, the combined rockets took Wan Hu somewhere, but just where is unknown, as he is said to have disappeared with a loud bang, leaving only a pall of smoke hanging over the scene.
The first rocket man? Maybe. But experts on science in China find it more likely that the tale was invented somewhere in Europe, during a period (17th-19th century) when Chinese motifs were much in vogue. Frank Winter (National Air and Space Museum, Washington DC) did his own investigation and could find no mention of Wan Hu in Ming Dynasty biographical guides or histories. And apparently there are variants involving not Wan Hu but ‘Wang Tu,’ who is said to have lived 2000 years ago. This story popped up in a 1909 issue of Scientific American and involves the same rocket chair, but Wang Tu manages to survive.
Image: The fabulous Wan Hu. Who knows where he wound up? Credit: National Air & Space Museum, Smithsonian Institution, Washington DC.
I have all this from a wonderful 1992 paper that Winter wrote for the Journal of the British Interplanetary Society, sent to me by my friend Al Jackson. Al was wondering about a fellow named Frederick Rodman Law, who like Wan Fu was obsessed with rockets. I had never heard of Law either, but in Winter’s view, in Law we have the first proven occurrence of pre-Space Age manned rocket flight, an event that took place in 1913. Known as ‘the human bullet’ and other professional names, Law was a showman and Hollywood stunt man whose sister was flyer Ruth Law.
Law’s plan (according to the New York Times: To blast off from Jersey City, NJ and land in Elizabeth, about nineteen kilometers away (Winter thinks the rocket would actually have landed either back in Jersey City or in Newark if it had worked). Law’s rocket was 14 meters tall, ending in a conical steel cap with an opening for Law to crawl into. 408 kg of gunpowder provided the punch, evidently supplied by the International Fireworks Co. Law intended to ride the rocket up to about 1000 meters and then jump out so as to parachute to Earth.
Winter draws on the New York Times account of what happened:
A crowd of about 150 people gathered that morning to see Law off. His rocket was set “on a mound of earth surrounded by marsh and rushes.” Continued the Times: “A motion picture operator became active when Law himself appeared, clad in khaki [if it still exists, this film has never been found]… When all the preliminary scenes had been duly recorded Law mounted a scaffold which had been built besides the rocket to enable him to reach his seat, which was about 18 feet (5.4m) above the ground. “You may light the fuse when ready, Sam (Samuel L. Serpico, Manager of the International Fireworks Co.), announced Law after he had taken the seat.
“The motion picture man rapidly placed a high-power lens on his machine to follow the shaft to Elizabeth… The spark climbed up several feet of fuse, sputtering menacingly… After a few seconds there was a terrific explosion with a shock that threw most of the crowd to the ground [as] the big projectile burst into a thousand pieces…
Image: Frederick Rodman Law atop his vehicle. Credit: National Air & Space Museum, Smithsonian Institution, Washington DC.
The amazing thing is that, unlike Wan Fu, Law did not join his ancestors at this point. He was thrown about nine meters from the rocket, and a physician in the crowd discovered that the only thing wrong with him was scorching to his hands and face, although his clothes were a mess. Law would go on to perform such feats as climbing the US Capitol dome (the police stopped the stunt), and dropping from an exploding balloon, exploding craft evidently being his trademark. When he did eventually die in 1919, it was quietly, of tuberculosis, in South Carolina.
Winter’s paper is worth checking out if you’re near a library with JBIS available. He goes into various tales worldwide, such as that of Lagari Hasan Çelebi, who was said to have flown a gunpowder rocket to altitude in the 17th Century and then coasted down after deploying ‘wings’ on the device to land. Like Wan Hu, the tale has never been corroborated. We also have stories like that of Claude-Fortuné Ruggieri, a doughty pyrotechnist who somewhere around 1830 constructed a rocket that was to launch a man from the Champ de Mars in Paris. This attempt was fairly well along, but when the volunteer pilot turned out to be a small boy, the police stepped in and banned the flight. Ruggieri’s tale, too, is hard to corroborate.
Perhaps the strangest of the many tales in Winter’s paper is his account of a ‘Signor Camarara,’ presented as an attraction at the 1901 Pan-American Exposition in Buffalo, NY. Said to be a ‘human skyrocket,’ Camarara was said in a newspaper account to have previously flown a rocket near the turn of the century in Callao, Peru. The vehicle was said to have had four exhausts, all of which used a strange, secret fuel Camarara called ‘dynoascenemite.’ Blasting off, Camarara was lofted to almost 5000 meters before landing safely by parachute.
Winter, as you would imagine, will have none of this one, pointing out that even Robert H. Goddard had trouble reaching altitudes like this with much more efficient liquid-fueled rockets decades later. Like other rocket man tales Winter has collected, Camarara’s story was surely a hoax, speaking to the power rockets held over daredevils and cranks early in their development, as well as to the gullibility of those willing to believe such astounding stories.
But we have to give Frederick Rodman Law credit. He had plenty of witnesses, was well advertised, and had the guts to climb into a gunpowder-fueled rocket with the cameras rolling. Good for Frank Winter for telling his tale. Just ahead were the Opel-RAK rocket vehicles, including the Opel RAK.1, a rocket glider flown in 1929 in Germany, and the various rocket-powered aircraft developed by Allied and Axis powers during World War II, the most famous being the Me-163 and the suicidal Japanese Yokosuka MXY-7 Ohka. The story of the rocket was quickly going from tall tale and stunt to scientific fact.
The paper is Winter, “Who First Flew in a Rocket?” Journal of the British Interplanetary Society Vol. 45 (1992), pp. 275-280.
Planet in Widest Orbit Yet Discovered
Free floating planets — planets without any star — are exotic things, presumably thrown out of their original solar system by gravitational interactions with other worlds. But the line between such wanderers and bound planets isn’t always clear. A case in point is the object 2MASS J2126, found in an infrared sky survey and at one point considered to be part of a group of young stars known as the Tucana Horologium Association. If linked to this group, its age could be inferred and it was young and low enough in mass to be considered an independent planet.
Now we learn otherwise, as a research team from the United Kingdom, Australia and the United States has determined that 2MASS J2126 is in an extraordinarily wide orbit around the star TYC 9486-927-1. Lead author Niall Deacon (University of Hertfordshire) has been focusing for several years on young stars with planetary companions in wide orbits. But this system has to come as a surprise. The young planet is about 1 trillion kilometers from the star, a whopping 7000 AU. By way of comparison, 2,000–20,000 AU is thought to be the extent of the inner Oort Cloud (also called the Hills cloud) in our system. Proxima Centauri is roughly 15000 AU from Centauri A and B, little more than twice the distance of 2MASS J2126 from its star.
In this work we present the identification of two previously known young objects in the solar neighbourhood, TYC 9486-927-1 and 2MASS J21265040-8140283, as a co-moving wide pair with a probable planetary mass secondary. During an examination of the literature we found that these two objects are separated by 217″ and have similar proper motions. Hence we attempted to better determine their properties to see if they were a likely young, bound system. Using revised astrometry and detailed kinematic analyses of nearby young stars and brown dwarfs, we have determined that this previously known young brown dwarf/free-floating planetary mass object and young low mass star are a likely widely separated bound pair.
Image: False colour infrared image of TYC 9486-927-1 and 2MASS J2126. The arrows show the projected movement of the star and planet on the sky over 1000 years. The scale indicates a distance of 4000 Astronomical Units (AU), where 1 AU is the average distance between the Earth and the Sun. Credit: 2MASS/S. Murphy/ANU.
Planets in very wide orbits are scarce (the paper calls them ‘a currently small but previously unexpected population of massive gas giant companions to stars’), but we do have four others: WD 0806-661B (2500 AU); GU Psc b (2000 AU); SR12 C (1100 AU); and Ross 458 C (1160 AU). All four of these objects were discovered through direct imaging. Just over 100 light years from Earth, 2MASS J2126 can now lay claim to the widest orbit of any planet found around another star, with each revolution taking some 900,000 years to complete.
“This is the widest planet system found so far and both the members of it have been known for eight years,” said Dr Deacon, “but nobody had made the link between the objects before. The planet is not quite as lonely as we first thought, but it’s certainly in a very long distance relationship.”
As mentioned above, the earlier presumed connection with the Tucana Horologium Association offered a way of establishing the age of the star. But without being able to confirm that connection, Deacon and team studied the parent star’s spectrum, finding that TYC 9486-927-1 shows a stronger signature for lithium that would be expected in 45 million year old stars. Lithium is destroyed early in a star’s life, so the lithium signature can be a valuable marker. The star’s lithium signature is also weaker than ten million year old stars, making the probable age somewhere between the two figures. The paper compares the star to different stellar moving groups (remnants of a stellar association still moving roughly together):
It is clear that the TYC 9486-927-1 has stronger lithium absorption than stars of similar spectral type in TucHor [Tucana Horologium], weaker absorption than TWA [TW Hydrae] members but in reasonable agreement with ? Pic [Beta Pictoris] members. Based on this comparison we conclude that TYC 9486-927-1 is older than TWA and likely of similar age or younger than TucHor. Thus, our Li analysis suggests an age comparable to the ? Pic moving group, but we note that Li depletion in low-mass stars can be affected by initial conditions (rotation, episodic accretion) and we therefore adopt a conservative age range of 10–45 Myr.
With these age estimates in mind, the team went on to estimate the mass of the planet, which turns out to be on the boundary between planets and brown dwarfs — 11.6 to 15 times the mass of Jupiter. The closest analogue we have to 2MASS J2126, then, is the planet ? Pictoris b, although the former is more than 700 times further from its host star. How a planet in such a wide orbit forms in such a system should be interesting fodder for future work.
The paper is Deacon et al., “A nearby young M dwarf with a wide, possibly planetary-mass companion,” in press at Monthly Notices of the Royal Astronomical Society (abstract). An RAS news release is also available.
Proxima Centauri & the Imagination
My essay Intensifying the Proxima Centauri Planet Hunt is now available on the European Southern Observatory’s Pale Red Dot site. My intent was to give background on earlier searches for planets around the nearest star, leading up to today’s efforts, which include the Pale Red Dot work using HARPS, the High Accuracy Radial velocity Planet Searcher spectrograph at La Silla, as well as David Kipping’s ongoing transit searches with data from the Canadian MOST satellite (Microvariability & Oscillations of STars), and gravitational microlensing studies by Kailash Sahu (Space Telescope Science Institute).
As it turned out, the choice of earlier Proxima planet hunts as a topic fit in where Alan Boss had left off. Boss (Carnegie Institution for Science) had led off the Pale Red Dot campaign’s outreach effort with a piece on the overall background of exoplanetology (Pale Blue Dot, Pale Red Dot, Pale Green Dot). Whatever the color of the distant world, our tools are developing rapidly, and there is a heady sense of optimism that we have what we need to find Proxima planets, assuming they are there. As Boss puts it:
We now know that nearly every star we can see in the night sky has at least one planet, and that a goodly fraction of those are likely to be rocky worlds orbiting close enough to their suns to be warm and perhaps inhabitable. The search for a habitable world around Proxima Centauri is the natural outgrowth of the explosion in knowledge about exoplanets that human beings have achieved in just the last two decades of the million-odd years of our existence as a unique species on Earth. If Pale Red Dots are in orbit around Proxima, we are confident we will find them, whether they are habitable or not.
Image: Proxima Centauri (Alpha Centauri C). Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews).
Saturday was cloudy, but Sunday offered up a fourth HARPS observation window, activity you can follow on Twitter @PaleRedDot. Let me also remind you that David Kipping wrote up the MOST work just prior to its inception in 2014 (see Proxima Centauri Transit Search to Begin). The latest I have from Dr. Kipping is that 13 days of data in that year were followed by another 30 in 2015. We should have results by the summer of this year. As to gravitational microlensing, we have a second occultation of a background star by Proxima coming up in February. Hope grows that we are getting close to a detection by one method or another.
and after losing a data-point on Saturday(clouds :S), we got a fourth HARPS obs today…yey! building up baseline https://t.co/BsC9M2fKNL
— Pale Red Dot (@Pale_red_dot) January 25, 2016
Proxima Centauri’s Emergence in Fiction
I’ve always been surprised that there wasn’t a greater flurry of interest in Proxima Centauri in early science fiction, given that the closest star to the Sun had just been discovered in 1915. Still, a few odd tales emerged, among the more interesting being Henri Duvernois’ L’homme qui s’est retrouvé (1936), which the indispensable Brian Stableford translated in 2010 as The Man Who Found Himself. Here we have a scientific romance in the grand style, with the journey to Proxima Centauri meshing with time travel and an encounter with the protagonist’s doppelgänger. Stableford has done wonders in reawakening interest in French scientific romance; his labors as translator and critic receive all too little credit.
And then there’s Murray Leinster (Will F. Jenkins), whose story “Proxima Centauri” ran in Astounding Stories in March of 1935. Leinster wanted to treat a journey to another star within a different context. Rather than presenting a dream-like moral tale, he showed a starship (the Adastra) that was capable of getting his characters to Proxima well within a human lifetime. The description is heavy on theatrics, minimal on detail, but it’s fascinating to see writers beginning to consider the sociological problems of long voyages. The Adastra will take ten years to reach its destination, and the crew will deal with mutinies, angst and utter boredom as the price of their ticket.
It’s interesting to see science fiction grappling with how to imagine starflight in this era. Coming out of the age of Gernsback, Leinster wanted solutions more satisfying to the science experimenter of the day than simply ignoring what physics was telling us about time and space. But how to do it? The results demanded giving up one kind of magic (faster than light methods) in favor of another, an authorial sleight of hand that tries to slip one over on the reader. Thus we read about the starship Adastra‘s “tenuous purple flames,” which were actually “disintegration blasts from the rockets” which had lifted the craft into space, and so on.
Anyone who digs up Leinster’s “Proxima Centauri” today will find that despite its reputation in its own time, it hasn’t aged well, and the interest of the effort will be purely an antiquarian one. But we do see the emergence of a greater appreciation of interstellar distances and the problems of staying within known physics in Leinster’s story (available in Asimov’s Before the Golden Age (Doubleday, 1974). Such efforts stand in stark contrast to anemic tales like Frederick Hester’s “Gipsies of Thos,” (Amazing Stories, May 1935) or Leslie Stone’s “Across the Void” (Amazing Stories April-June 1931), both tales that take us to the Centauri stars as if by magic carpet.
Today’s science fiction gives us a Proxima Centauri of considerably more detail, along with a more realistic assessment of the propulsion conundrum that accompanies the voyage. With that detail we’re also given further mysteries about which to speculate. Here’s Stephen Baxter describing the Proxima planet Per Ardua in his novel Proxima (Roc, 2014):
The weather was overcast, muggy, humid. For such a static world the weather had turned out to be surprisingly changeable, with systems of low or high pressure bubbling up endlessly from the south. It was warm in this unending season, always like a humid summer’s day in North Britain, from what Yuri remembered of the weather. But the ColU, ever curious in its methodical robot way, said it had seen traces of cold: frost-shattered rock, gravel beds, even glaciated valleys in the flanks of features like the Cowpat. Evidence that glaciers had come this way in the past, if not whole ice ages. Somehow this world could deliver up a winter.
It’s a winter caused not by axial tilt but by massive eruptions of starspots on Proxima’s face, driving weather patterns on the kind of tidally locked habitable world we may one day find through our ongoing planet hunts. If and when we do find the ‘pale red dot’ around Proxima, writers will continue imagining it, always tightening the detail as new facts emerge, until one day, we can hope, we have robotic or even human emissaries reporting back from the scene.
A New Filter for Life’s Survival
How do we make out the odds on our survival as a species? Philosopher Nick Bostrom (University of Oxford) ponders questions of human extinction in terms of a so-called Great Filter. It’s one that gives us a certain insight into the workings of the universe, in Bostrom’s view, because it seems to keep the galaxy from being positively filled with civilizations. Somewhere along the road between inert matter and transcendent intelligence would be a filter that screens out the vast majority of life-forms, keeping the population of the galaxy low, and offering us a way to gauge our own chances for survival.
Think of it this way. Perhaps the Great Filter has to do with the formation of life itself. If that is the case, then we have already made it through the filter and can go about exploring the universe. But if the Great Filter is in our future, then we can’t know exactly what it will be, and neither can we know whether we will survive it. Here the final term in Frank Drake’s equation comes to mind, the one specifying the lifespan of technological civilizations. Perhaps the Great Filter has to do with technology destroying itself. In that case, the filter could spell our doom. For more on Bostrom’s ideas, see Bostrom: From Extinction to Transcendence.
Through the Gaian Bottleneck
An ominous view, some would say, but along come Aditya Chopra and Charles Lineweaver (Australian National University), with a new paper suggesting a different kind of filter. The authors call it the Gaian Bottleneck, and it’s a filter that life on Earth has already passed through. The scenario is that life is fragile enough that it rarely develops into intelligence.
The reason: Young planetary environments are unstable. The life that does emerge needs to find ways to regulate greenhouse gases like water and carbon to keep surface temperatures in the habitable range. Normally we think of the decrease in the incoming bombardment of Solar System debris going back 4.5 billion years and extending to about 3.8 Gya as being a key to making the Earth more suitable for life, but the Gaian Bottleneck sees early life as being under strong selection pressure to modify and regulate its own environment. From the paper:
… bombardment rates inevitably decrease in the circumstellar habitable zones (CHZs) of stars, but the timescales for the evolution of Gaian regulation are probably unpredictable and would not inevitably evolve rapidly (or at all). Thus, if there is anything special about what happened on Earth to allow life to persist here, it might have less to do with the decreasing bombardment rate in the Hadean, or special chemical ingredients, or sources of free energy, or even a rare recipe for the emergence of life. The existence of life on Earth today might have more to do with the unusually rapid biological evolution of effective niche construction and Gaian regulation in the first billion years. Habitability and habitable zones would then not only be a passive abiotic property of stellar and planetary physics and chemistry (such as stellar luminosity, initial water content, and decreasing bombardment rate) but would also be a result of early life’s ability to influence initially abiotic geochemical cycles and turn them into the life-mediated biogeochemical cycles that we are familiar with on the current Earth…
In this view, we have gotten through the filter already, finding ourselves in a position not shared by planets around us. Conceivably, both Mars and Venus were once habitable, but a billion years after formation, Venus turns into the hell it is today and Mars goes into a deep freeze. Chopra and Lineweaver argue that if there was early microbial life on either world, it was unable to stabilize its environment, whereas on Earth, life played an active role in doing just that.
Image: Early Abiotic Feedbacks. During the first billion years after the formation of Earth (or of Earth-like planets), abiotic positive feedbacks (left) can lead to runaway surface temperatures outside the habitable range (both too hot and too cold). These positive feedbacks lead to the loss of liquid water [either from hydrogen escape to space or condensation into ice. Abiotic negative feedbacks (right) have been invoked to stabilize surface temperatures, but they may not be significant in the first billion years, hence the dashed lines and the question marks. As life evolves, it can strengthen or weaken these initially abiotic geochemical feedback loops and turn them into biogeochemical cycles and feedback loops. Evolving life can insert itself into these feedbacks at the points labeled A, B, C, and D. Credit: Aditya Chopra and Charles Lineweaver/ANU.
We can thus look without surprise on a galaxy where rocky terrestrial-class worlds are common even while life itself is not. Early extinction becomes a near universal phenomenon:
We argue that the habitable surface environments of rocky planets usually become uninhabitable due to abiotic runaway positive feedback mechanisms involving surface temperature, albedo, and the loss of atmospheric volatiles. Because of the strength, rapidity, and universality of abiotic positive feedbacks in the atmospheres of rocky planets in traditional CHZs, biotic negative feedback or Gaian regulation may be necessary to maintain habitability.
What, then, do we find on the planets of nearby stars? Not ancient ruins of advanced civilizations that have long ago destroyed themselves, but the fossilized remnants of extinct microbial life. No shattered remains of cities but long-gone remnants of simple life that failed in its mission to make its own planet a place where more complicated forms could develop. We would be living in a universe where almost all life is young, microbial, or extinct.
Chopra and Lineweaver run through a variety of ways their argument could be challenged. If Gaian regulation, for example, is a key to making a young planet into a place where life can survive, why should it necessarily be rare? Moreover, Gaian regulation is usually considered as starting in the Proterozoic, around 2.5 Gya. Invoking a pre-Proterozoic Gaia is even more controversial, to say the least. And the paper notes that abiotic negative feedback on the carbonate-silicate cycle could have stabilized surface temperatures on Earth without any recourse to Gaian regulation. Other potential objections are also weighed.
But knowing how many imponderables they are dealing with, the authors nonetheless suggest this sequence of events, which they consider ‘potentially universal,’ on young, wet planets:
First *0.5 Gyr: Hot, high bombardment, uninhabitable.
*0.5 to *1.0 Gyr: Cooler, reduced bombardment, continuous volatile loss.
*0.5 to *1.0 Gyr: Emergence of life in an environment with a tendency to evolve away from habitability.
*1.0 to *1.5 Gyr: Inability to maintain habitability, followed by extinction. As a rare alternative, this period would experience the rapid evolution of Gaian regulation and the maintenance of habitability, followed by the persistence of life for several billion more years.
The paper is Chopra and Lineweaver, “The Case for a Gaian Bottleneck: The Biology of Habitability,” Astrobiology 16(1) (January 2016), 7-22 (abstract).
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