The thought that water and organic molecules might have arrived on the early Earth from the impacts of comets and asteroids has long been provocative, and our missions to nearby comets are now paying off with insights into the possibility. It was back in 2004 that the Stardust mission flew past Comet Wild 2, collecting dust samples that showed traces of the amino acid glycine. Possible contamination of the samples during their analysis left the question open, however.
Now we have news that the European Space Agency’s Rosetta mission has also found glycine — a significant organic compound that appears in proteins — at Comet 67P/Churyumov-Gerasimenko. The spacecraft’s ROSINA instrument (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) detected glycine in October of 2014, with later measurements taken during the August 2015 perihelion event, where cometary outgassing was at its peak.
Kathrin Altwegg (University of Bern), who led the study, calls this “…the first unambiguous detection of glycine in the thin atmosphere of a comet.” From the paper:
ROSINA’s double focusing mass spectrometer (DFMS) ionizes the incoming volatiles by electron impact ionization and detects the corresponding positively charged fragments. Unlike for meteorites or Stardust grains, there is no chemical sample preparation involved. Furthermore, the absence of a terrestrial source of glycine from the spacecraft is verified from observations before arrival at the comet. Therefore, glycine detected by DFMS has to be in this form already in the coma of the comet and is clearly not the result of contamination.
Although little glycine is released from the cometary surface, the detection of precursors to glycine formation, the organic molecules methylamine and ethylamine, is also significant. Glycine is the only amino acid known to be able to form without liquid water. Says Altwegg:
“We see a strong correlation of glycine to dust, suggesting that it is probably released from the grains’ icy mantles once they have warmed up in the coma, perhaps together with other volatiles. The simultaneous presence of methylamine and ethylamine, and the correlation between dust and glycine, also hints at how the glycine was formed.”
Image (click to enlarge): An ESA infographic detailing the recent work on organic compounds found at Comet 67P/Churyumov-Gerasimenko.
We also get word, in the paper published by Science Advances, that Rosetta found phosphorus at Comet 67P/C-G. The researchers argue that the number of organic molecules ROSINA has detected at the comet, now including glycine and phosphorus, make a strong case for comets as a delivery mechanism for prebiotic chemistry. This marks the first time phosphorus, an ingredient in the framework of DNA and RNA, has been found at a comet.
The presence of glycine, phosphorus, and a multitude of organic molecules, including hydrogen sulfide (H2S) and hydrogen cyanide (HCN), seen in the coma of 67P/Churyumov-Gerasimenko supports the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and, in particular, to the early Earth, drastically increasing the concentration of life-related chemicals by impact on a closed water body. The simultaneous presence of methylamine and the correlation between dust and glycine also suggest that the pathways for glycine formation on dust grain ices, as described for the ISM or the protosolar nebula, could also account for the cometary glycine.
The paper is Altwegg et al., “Prebiotic chemicals – amino acid and phosphorus – in the coma of comet 67P/Churyumov-Gerasimenko,” Science Advances, 27 May 2016 (abstract / full text).
So often planets described as ‘potentially habitable’ turn out to be over-rated — we look deeper into their composition and characteristics only to find that the likelihood of liquid water on the surface is slim. How to make more accurate calls on the matter of habitability? One way may be to combine orbital and atmospheric models, adjusting each with the known parameters of the planet in question. A new study does just that for the interesting world Kepler-62f.
About 1200 light years from Earth in the direction of the constellation Lyra, Kepler-62f has a radius 40 percent larger than Earth’s, which puts it well below the 1.6 RE demarcation line that is increasingly thought to define the difference between Earth-like worlds and planets that are more like Neptune. We’re probably looking at a rocky planet here. It’s also a planet that orbits its K-class primary at a distance that could place it in the outer regions of the habitable zone (as defined, again, by the presence of liquid water on the surface).
Image: Kepler-62f, shown here in an artist’s rendering, is far enough from its star that its atmosphere would need a high concentration of carbon dioxide to maintain liquid water on the planet’s surface. Credit: NASA Ames/JPL-Caltech/T. Pyle.
The new work on this world was conducted by Aomawa Shields (UCLA), working with Rory Barnes, Eric Agol, Benjamin Charnay, Cecilia Bitz and Victoria Meadows (all at the University of Washington, where Shields received her PhD). It examines the planet’s atmosphere to consider scenarios that could produce habitability. The team modeled climate possibilities using two different methods — the Community Climate System Model and the Laboratoire de Météorologie Dynamique Generic Model. Informing the climate modeling was the use of another computer model, HNBody, which was used to analyze and adjust the planet’s orbital parameters.
This UCLA news release calls the work the first time astronomers have combined the two different kinds of models to study exoplanet habitability. Says Shields:
“We found there are multiple atmospheric compositions that allow it to be warm enough to have surface liquid water. This makes it a strong candidate for a habitable planet.”
The necessary composition involves a great deal more carbon dioxide that we find in Earth’s atmosphere, necessary to keep the surface from remaining in a deep freeze. Remember, the planet circles a K-class star cooler than the Sun, and at a distance further away from the primary than the Earth from our star. By adjusting carbon dioxide levels in the atmospheric model, the team learned that given various orbital configurations, there were circumstances where habitability was possible. Some configurations work better than others, but the team found that even with an Earth-like level of carbon dioxide, there were rare but possible orbital configurations that allowed at least part of the year to be warm enough for liquid water.
From the paper:
At 41% of the modern solar constant, this planet will likely require an active carbonate silicate cycle (or some other means by which to produce high greenhouse gas concentrations) to maintain clement conditions for surface liquid water. With 3 bars of CO2 in its atmosphere and an Earth-like rotation rate, 3-D climate simulations of Kepler-62f yielded open water across ?20% of the planetary surface at the upper limit of the stable eccentricity range possible for the planet, provided that it has an extreme obliquity (90°). With 5 bars of CO2 in its atmosphere, a global mean surface temperature similar to modern-day Earth is possible for the full range of stable eccentricities and at the present obliquity of the Earth. This higher CO2 level is therefore optimal, as it is below the maximum CO2 greenhouse limit, and generates habitable surface conditions for a wide range of orbital configurations throughout the entire orbital period of the planet. If Kepler-62f is synchronously rotating, CO2 concentrations above 3 bars would be required to distribute sufficient heat to the night side of the planet to avoid atmospheric freeze-out.
There is a faint chance that with CO2 levels like the Earth’s, Kepler-62f could experience surface melting of ice sheets on an annual cycle. This would depend on a high degree of axial tilt, or obliquity — the angle between the planet’s rotational axis and its orbit. The Earth’s obliquity is 23 percent. An axial tilt of 60° or higher, coupled with summer solstice at a given hemisphere occurring at the planet’s closest approach to its star, could produce this annual effect, allowing surface conditions that were at least periodically habitable.
The paper is Shields et al., “The Effect of Orbital Configuration on the Possible Climates and Habitability of Kepler-62f,” published online by Astrobiology 13 May 2016 (abstract / preprint).
I hadn’t planned the conjunction of the Breakthrough Starshot forum’s opening here on Centauri Dreams and the interesting news out of the NASA budget for 2017, but some things just fall into your lap. In any case, what happened in Washington makes a nice follow-up to yesterday’s post, considering that it calls up visions of fast probes to Alpha Centauri, and in a document coming out of the U.S. House of Representatives, of all things. As more than a few readers have noted, it’s not often that we hear interstellar issues discussed in the halls of Congress.
Call for a New Interstellar Study
The specifics are that space-minded John Culberson (R-TX), who has championed space exploration with abandon, has made sure that NASA will look at the possibilities of interstellar travel. Culberson chairs the House of Representatives sub-panel in charge of NASA appropriations, and the call for interstellar study comes in a report that accompanies the bill establishing the agency’s budget for the coming year. You can see the report as submitted by Culberson online. Let me quote the relevant paragraph. First, the statement of the problem:
Interstellar propulsion research.—Current NASA propulsion investments include advancements in chemical, solar electric, and nuclear thermal propulsion. However, even in their ultimate theoretically achievable implementations, none of these could approach cruise velocities of one-tenth the speed of light (0.1c), nor could any other fission-based approach (including nuclear electric or pulsed fission).
Now the call for study:
The Committee encourages NASA to study and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 0.1c. These efforts shall be centered on enabling such a mission to Alpha Centauri, which can be launched by the one-hundredth anniversary, 2069, of the Apollo 11 moon landing. Propulsion concepts may include, but are not limited to fusion-based implementations (including antimatter-catalyzed fusion and the Bussard interstellar ramjet); matter-antimatter annihilation reactions; multiple forms of beamed energy approaches; and immense ‘sails’ that intercept solar photons or the solar wind. At the present time, none of these are beyond technology readiness level (TRL) 1 or 2.
And finally, the course to follow:
The NASA Innovative Advanced Concepts (NIAC) program is currently funding concept studies of directed energy propulsion for wafer-sized spacecraft that in principle could achieve velocities exceeding 0.1c and an electric sail that intercepts solar wind protons. Over the past few years NIAC has also funded mission-level concept studies of two fusion-based propulsion concepts. Therefore, within one year of enactment of this Act, NASA shall submit an interstellar propulsion technology assessment report with a draft conceptual roadmap, which may include an overview of potential advance propulsion concepts for such an interstellar mission, including technical challenges, technology readiness level assessments, risks, and potential near-term milestones and funding requirements.
Notice a couple of things here. First, keying a major effort to a significant anniversary is interesting as a motivational driver. Hence the call for reaching the goal of a launch by the one-hundredth anniversary of the Apollo 11 landing. As noted in these pages at the time, Yuri Milner’s Breakthrough Starshot was announced in conjunction with another such milestone, the anniversary of Yuri Gagarin’s orbital flight. Humans love to link big ideas with major events in the past as a way of goal-setting and, I suppose, putting a new idea into a broader context.
Note as well some of the parameters. Culberson talks about a spacecraft reaching ten percent of lightspeed, while Breakthrough Starshot goes for twenty percent. And while Starshot focuses tightly on small sails driven by phased laser array, Culberson’s call to NASA incorporates propulsion concepts ranging from sails to antimatter and even Bussard ramjets. Moreover, the report makes a nod to fusion-based propulsion as studied by the NASA Innovative Advanced Concepts program (NIAC). NASA is called upon to submit a technology assessment within a year of the enactment of the budget bill.
Dr. Forward Goes to Washington
If any of this sounds familiar, it may be because the only other time interstellar flight was considered in the U.S. Congress at any level of detail was when Robert Forward made an appearance before the Subcommittee on Space Science and Applications of the House Committee on Science and Technology. That was in 1975, when interstellar flight was rarely discussed outside of science fiction magazines, and the work of scientists like Forward appeared largely at conferences where interstellar issues were only a small part of the proceedings. There was, of course, no Internet, and media attention was scant.
Forward’s “A National Space Program for Interstellar Exploration” was as ambitious as it gets, calling for the launch of automated probes to nearby stellar systems by the turn of the 21st Century, with manned exploration to commence a scant 25 years later. With a budget that today seems modest, Forward called for ‘a few million dollars a year’ during the initial study phase, climbing into the multi-billion dollar range as the program began launching its first automated probes. As always, Forward thought big, as witness this:
Development of man-rated propulsion systems would continue for 20 years while awaiting the return of the automated probe data. Assuming positive returns from the probes, a manned exploration starship would be launched in 2025 AD, arriving at Alpha Centauri 10 to 20 years later.
Image: Interstellar theorist Robert Forward, whose work on beamed propulsion began in the early 1960s.
All that is by way of placing Rep. Culberson’s call to NASA in historical context. What Forward didn’t have when he made his recommendations was an ongoing privately funded effort like Breakthrough Starshot to offer a parallel track of development. It would be fascinating to know how he would have played that card. Breakthrough Starshot aims to spend its $100 million endowment on shaking out the basic concepts involved in a sail mission to Alpha Centauri. Forward would have been all over its phased laser array concept and we’re the poorer for not having his insights, especially on the issue of Earth- vs. space-based deployment.
The parallels between Culberson’s report and the Starshot studies are interesting if only in that both make reference to Philip Lubin’s work at NIAC on beamed propulsion. Lubin (UC-Santa Barbara) is a major figure in the Starshot effort, but his work goes back several years at NIAC, and thus offers ideas to both projects. Moreover, the Starchip concept — a ‘nano-spacecraft’ — being studied by Breakthrough Starshot is heavily dependent on Mason Peck’s work at Cornell, which also has had NIAC support. So NASA can draw on NIAC.
In the broader sense, though, both NASA and Starshot owe their inspiration on the propulsion side to Forward and the many colleagues who developed the core ideas of beamed propulsion over the past fifty years. Beaming to a sail has long been under investigation, though never so publicly. In the near future, I’ll be publishing an extended take on the history of these studies and how we have arrived at today’s laser sail concepts.
Image: A beamed lightsail as envisioned by the space artist Adrian Mann.
Will NASA and Breakthrough Starshot duplicate each other’s efforts? It’s not likely given current funding constraints, and whether it’s based at MSFC Huntsville or the Jet Propulsion Laboratory, the NASA effort outlined by Culberson calls only for an assessment of an interstellar mission. But having interstellar flight’s new media prominence so prominently reinforced by its introduction into a Congressional report on the NASA budget can’t hurt as we launch a serious look at what it takes to reach Alpha Centauri. We’ll soon learn what kind of synergies may exist.
Forward’s presentation to Congress can be found in “A National Space Program for Interstellar Exploration,” Future Space Programs 1975, vol. VI, Subcommittee on Space Science and Applications, Committee on Science and Technology, U.S. House of Representatives, Serial M, 94th Congress (September, 1975).
Ever since coming back from the Breakthrough Discuss meeting in Palo Alto, I have been pondering the enormous issues the Breakthrough Starshot project will encounter. Getting a tiny spacecraft up to twenty percent of lightspeed is only the beginning of an effort that has to deal with power generation, a phased laser array of enormous strength and complexity, the miniaturization of critical components, lightsail integrity under thrust and much more.
These topics were freely discussed in Palo Alto, and especially at the Yuri’s Night party that Yuri Milner threw for the assembled conference goers. When I talked to Milner at the party, he suggested an idea that we have been working on ever since. In order to keep the discussion on the critical issues involving Breakthrough Starshot in front of the interstellar community, why not set up a linkage between the discussion areas of the Breakthrough site and Centauri Dreams? This site would maintain its usual structure and separate comments, but it would then include a section specifically devoted to pursuing the Breakthrough Starshot concept.
I am the world’s poorest programmer, but a crack team from Breakthrough Initiatives, led by Stepan Torchyan and including Peter Petrov and Konstantin Efimov knew exactly what to do. As of this morning, the new section goes live. You’ll find the ‘Challenges’ forum accesible through the ‘Breakthrough Starshot’ tab at the top of the Centauri Dreams logo, Click on ‘Breakthrough Starshot’ and you’ll see it is set up as a series of forums, each of them dealing with a single major issue for the project. A ‘Challenges’ link also appears on the Breakthrough Initiatives site.
What’s happening here is that any comments Centauri Dreams readers place in one of these forums will be mirrored on the Breakthrough Initiatives site, while comments posted there will appear on the ‘Challenges’ forum pages on this site. As long-time readers know, Centauri Dreams doesn’t require a login in order to post, but ‘Challenges’ does. You can set up a password on the ‘Challenges’ page. Click the ‘Breakthrough Starshot’ tab, then click ‘Sign In’ and, on the ensuing page, ‘Sign Up.’
When you go to the new forum, you’ll see that the list of topics is lengthy, and I hope that we can continue the many good discussions we’ve already had on Centauri Dreams in this new venue, even as we follow the project with articles and comments on Centauri Dreams itself. Many of the current topics came under discussion in Palo Alto, but as Breakthrough Starshot begins its work this summer, each will come into sharper focus.
Simply put, there could be a deal-breaker almost anywhere on the list of Challenges. If we build the phased laser array to boost a Starchip up to a substantial percentage of c, would the sail be stable under the beam? Would the phased array work as we hope through the atmosphere? What about objects in the beam’s path — a bird, an airplane? What of the collision potential for a spacecraft moving at these velocities in cruise, and how do we manage to adjust the spacecraft’s course on the way? And a huge one: How do we manage data return considering the minute size of the Starchip we hope to send to Alpha Centauri?
Have a look at the Challenges forum as we begin to see how it will work. Stepan Torchyan’s team will check into any bug reports if problems arise, and after an initial shakedown, we should be able to add to the Breakthrough Starshot discussion daily. I’m happy to be able to bring these forums to the attention of the Centauri Dreams audience considering how much I’ve learned from readers here since I began the site in 2004. I have no doubt we’ll be able to make a contribution to this ongoing study to achieve what I’ve always dreamed about, a human technology arriving intact and returning data from the Alpha Centauri system.
Is the anomalous star KIC 8462852 undergoing a long-term dimming or not? We’ve looked at Bradley Schaefer’s work on the star and the follow-ups disputing the idea from Michael Hippke and Daniel Angerhausen (NASA GSFC), with collaboration from Keivan Stassun and Michael Lund (both at Vanderbilt University) and LeHigh University’s Joshua Pepper. Dr. Schaefer (Louisiana State University) believes the evidence for dimming is still strong, and in the post below explains why. He has also provided a link to a more detailed analysis with supporting graphs and figures for those who want to go still deeper (further information below). As we embark on the Kickstarter campaign to put ‘Tabby’s Star’ in the sights of the Las Cumbres Observatory Global Telescope Network — an important project to which I have contributed and hope you will as well — we continue to monitor this evolving story. No matter how it turns out, the Kepler data are iron-clad, so the success of the Kickstarter campaign is vital to provide us with the further data we need to make sense of what we are seeing at this unusual star.
By Bradley Schaefer
The dips shown by KIC 8462852 (Tabby’s Star) are still a profound mystery. Further, I have found that Tabby’s star has faded by ~20% from 1890 to 1989 as measured from the Harvard plates. (This is now Schaefer 2016, ApJLett, 822, L34.) A straight line fit to the light curve gives a slope of +0.164 ± 0.013 magnitudes-per-century. The quoted error bar here is from the measurement error (as taken by a chi-square fit), whereas there is some larger systematic error associated with all the usual small problems in photographic photometry and the sampling of the plates.
To measure the systematic errors, I used 12 uncrowded check stars of the same magnitude and color as Tabby’s star, and all within ~22 arc-minutes. The average of the linear slopes is -0.007 mag/century, with an RMS scatter of 0.044 mag/cen. With the systematic error dominating, the century-long decline of Tabby’s Star is significant at the 4.0-sigma level (i.e., a probability of 0.000064 of such a high slope occurring by chance, even with systematic errors).
Two papers (Hippke et al. arXiv:1601.07314v4 & Lund et al. arXiv:1605.02760v1) have recently appeared with the basic claim that the historic light curves from Harvard (as part of the DASCH [Digital Access to a Sky Century@Harvard] database) have a much larger systematic error, more like ±0.15 mag/cen, with the agreed slope for Tabby’s Star then not being anything special. If the DASCH RMS scatter in the fitted linear slopes is really this large, then the existence of the century-long fading in Tabby’s Star would not be significant.
The two papers of Hippke and Lund have been widely publicized, because both authors have run to the press first. Indeed, Hippke contacted at least one reporter *before* he had submitted the first version of his paper. (At that time, Hippke had known about the existence of the Harvard plates for only two weeks, he had talked with zero people who had ever seen any archival plate, and Hippke still has never laid eyes on any archival plate.) In the usual way of ‘social media’, Hippke’s and Lund’s claims have been highlighted as a refutation of the century-long dimming, and this has been extended to everything about KIC 8462852. For example, on the first day of the launch of the Kickstarter program, the Reddit talk had the statement that the person ‘thought this has all been refuted’.
Well, despite ‘social media’, it is actually Hippke and Lund that are definitely wrong. As I’ll show, any experienced worker can quickly find exactly what mistakes they made, so that their claimed large scatter of slopes arises simply from two distinct mistakes on their part. But I have no ordinary venue to put out any effective counters or proofs. For example, any further submission to ApJ or ApJLett would not have any new data to show, and it would appear only many months from now. Research on Tabby’s Star is moving fast, so Hippke’s and Lund’s claims need to be challenged soon. The best way that I can think of to get the challenge and proofs out is to place them into a detailed document plus an email (*this* email), and to send this out to people who have queried me for an analysis of Hippke’s and Lund’s manuscripts on Tabby’s Star. A link to the detailed document appears below.
I present three reasons to show that Hippke and Lund have incorrect claims:
Reason #1: Hippke & Lund Both Made Two Killer Mistakes
Mistake #1 is that they selected many check stars that have some random nearby star at just the right distance so as to produce overlapping star images on the Harvard plates with large plate scales. The DASCH photometry uses SExtractor, and the algorithm returns something like the combined magnitude when the two star images overlap. This overlap produces an erroneously-bright magnitude for some plates. This occurs for most of the plates after the 1953-1969 Menzel gap (the Damon plates), resulting in an apparent jump across the Menzel gap. When the whole light curve is fit to a straight line, it will also result in an apparently brightening light curve.
Some crowding stars will cause this effect to be mainly visible on the RB & RH series or the AM & AC series, which result in the opposite sign for the jumps and slopes. In the linked PDF file below, I give many detailed examples, tables, and illustrations. That is, jumps in brightness across the Menzel gap and non-zero slopes are produced as pure artifacts of choosing check stars with nearby crowding stars. Now, critically, Tabby’s Star does not have any crowding stars. So it is not correct to choose any crowded-check-stars. No experienced researcher would make such a choice. It turns out that a large fraction of both Hippke’s and Lund’s stars with high claimed slopes are badly crowded. That is, many of their stars have high slopes simply due to this bad mistake.
Mistake #2 is that they have used the KIC magnitudes for calibration, rather than the APASS magnitudes as strongly recommended by DASCH in many places. The KIC calibration is based on the ‘g’ magnitudes as used by the Kepler satellite, whereas the APASS magnitudes directly give ‘B’ magnitudes. The native system of the Harvard plates is ‘B’. So the use of the KIC-calibration will always be problematic for some purposes because there must always be color terms needing correction. It is only a historical relic that the DASCH database allows the use of the KIC calibration. Yet most of Hippke’s and Lund’s results were made with the KIC calibration.
This actually matters. The reason is that the KIC-calibrated light curve for some presumably-constant star often shows an apparent slope (and possibly a jump in brightness across the Menzel gap), whereas the APASS-calibrated light curve for the same star shows a perfectly flat light curve. I show several examples of this effect in the attached PDF file. With this, we see that the use of the KIC-calibration by Hippke & Lund is causing the jumps and slopes as pure artifacts. Their Mistake #2 would not be made by anyone experienced with the Harvard plates (or anyone who reads the DASCH website or papers).
The attached PDF file gives a detailed account of the commission of the errors. Between the two killer mistakes, all of Hippke’s and Lund’s claims are shown to be artifacts of their bad analysis.
Reason #2: Two Measures by Experienced Workers give ±0.044 and ±0.048
Measure #1 is by myself, as given in fine detail in my ApJLett paper. I derive the century-long slopes for 12 uncrowded check stars that have essentially identical magnitude, color, and position as Tabby’s Star. Whatever systematic and measurement errors happen for Tabby’s Star on the DASCH photometry, the identical effects must be present on these 12 stars. No one can do any better than this for a direct measure of the real total errors. With this, the average slope is very close to zero, while the RMS of the slopes is ±0.044 mag/cen. The largest deviation from a flat slope is one at -0.070 mag/cen. I should mention that I have a vast experience with the Harvard plates, with nearly continuous work since 1979, something like 50 papers in refereed journals, plus five papers on the theory of photographic photometry.
Measure #2 is by Josh Grindlay. He is a professor at Harvard; he has been a long time user of the Harvard plates (going back before 1979), and he is the founder and leader of the DASCH program. He had long been using DASCH light curves, so he knew perfectly well that DASCH produces flat light curves for constant stars. With the spectacle of Hippke’s paper, he started a formal measure of many Landolt stars with the DASCH data. (Landolt stars have long served the community as standard stars, and they are most likely closely constant in brightness.) For 31 Landolt stars, Grindlay finds that the average fitted-linear-slope is -0.015±0.048 mag/cen.
So we have the two most experienced workers in the world, and we are getting an RMS in the fitted-linear-slope of 0.044-0.048 mag/cen. For Tabby’s Star, this results in the century-long dimming being near 4.0-sigma in significance. I think that these two solid measures by the most experienced people in the world are to be strongly preferred to a claim coming from people who have yet to lay eyes on any archival photographic plate.
Reason #3: The Dimming of Tabby’s Star Has Been Confirmed
Ben Montet is in the process of investigating the long-term photometric behavior of KIC 8462852 in a high quality independent data set, and his preliminary results support the finding that Tabby’s star is indeed fading.
Tabby’s Star is bright, and the Kepler data is legendary for its photometric accuracy and stability. If Tabby’s Star is fading at the rate of 0.164 mag/cen (which it might or might not still be doing), then it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus Field. This should be discoverable by a careful analysis.
Apparently Montet has made such an analysis, and finds Tabby’s Star to be fading at some unspecified fade-rate. So we have an apparent confirmation of the fading of Tabby’s Star over 4.5 years, although certainly we must await a definitive paper coming from Montet. [A group at Pulkova Observatory has claimed to provide a weak confirmation of a fading of Tabby’s Star. This is based on just ten plates from 1922 to 2001.There is indeed a formally fading slope, but the real uncertainties are greatly larger than any claimed slope. This result is not a confirmation.]
For those interested in following this matter further, the document I discuss above, my “ANALYSIS OF HIPPKE et al. (2016) and LUND et al. (2016) is available.” Often the refutations of claims are not short, so I have presented the full details in this document. In sum: We have three strong reasons to know that Hippke’s and Lund’s claims are certainly wrong.