100YSS: SETI, Sprites and Cutting Costs

by Paul Gilster on September 23, 2014

Gatherings like the 100 Year Starship Symposium have tough organizational choices to make, and the solutions aren’t always obvious. A good part of any aerospace conference is involved in presenting papers, but do you set up a multi-track system or take a single-track approach? In Houston, the 100 Year Starship organization chose multiple tracks: We had, for example, a track on Life Sciences, one on Data Communications, another on Propulsion & Energy, and there were several others including a useful track on Interstellar Education.

The problem is that with all these tracks running at once, it was a matter of picking and choosing, and that often meant getting up after a presentation, switching rooms, and entering another track. I missed papers in Kathleen Toerpe’s Education track that I wanted to hear because I needed to hear many of the Propulsion & Energy papers, and while I caught a paper in the Becoming an Interstellar Civilization track, it was at the expense of some promising offerings in another track called Uncharted Space & Destinations.

This is how many conferences operate and it allows for a greater number of papers, but I was glad that the symposium also opened up for plenary sessions, one of which, called State of the Universe, was a discussion between Jill Tarter (SETI Institute) and Mason Peck (Cornell University). Tarter’s lengthy involvement in SETI brings particular weight to her insights, and Centauri Dreams readers already know about my admiration for Peck’s work on the miniaturized ‘satellite on a chip’ designs he and his team call ‘Sprites.’ I’m convinced that shrinking payloads through such technologies is a promising path toward interstellar missions.

Miniaturization and Cost

KickSat was a plan to take a large number of Sprites into orbit aboard a CubeSat, although the initial attempt failed when, after a launch in April of this year as part of an ISS re-supply mission, the CubeSat orbited and re-entered without deploying the nanosatellites. But the technology is fascinating. Peck refers to Sprites as ‘Sputnik on a microchip,’ and we can think of them as robotic precursor missions on a very small scale, an economic approach to exploration. A swarm of cheap Sprites could be deployed, for example, from a CubeSat solar sail mission to an outer planet, replacing telescope imagery with direct sensor readings.

But Tarter pressed Peck on astrobiology, where for our immediate purposes, Sprites aren’t going to offer the flexibility of today’s large rovers on places like Mars. How do we cope with the need to keep the costs of large missions down? The commercial sector, as well as crowdsourced funding, came up again and again. In Peck’s words:

“For Mars, the long range plan is a sample return mission. To keep the prices as low as possible, what if we buy the science? Rather than having an agency declare the boundaries of a science mission, why not offer a prize per gram of Martian soil? Let companies bid and bring economics into play. If no one rises to the challenge, no money is spent.”


Tarter was interested in what she called a ‘mega-Kickstarter from the world’ as a funding source for another kind of technology, a starshade. In reports for the NASA Institute for Advanced Concepts a few years back (still available at the NIAC site), Webster Cash (University of Colorado at Boulder) outlined the possibilities. Even the largest planets are invariably drowned out by the glare of the parent star, but a starshade approximately 20,000 kilometers away from a large space instrument like the James Webb Space Telescope uses a highly-refined ‘flower-petal’ architecture to filter out the starlight, leaving the telescope with photons from the exoplanet itself. That makes spectroscopy possible, allowing us to study the constituents of a planetary atmosphere.

Starshades are big and have to be deployed, which led Tarter to her next question: Given the kind of miniaturized technology Peck was already working with, couldn’t we make a starshade out of a swarm of smaller objects? The problems are daunting, and include keeping the surface of these structures precise enough to prevent problems with diffraction that can cloud the image. Even so, Peck noted the radical change that computation has brought to optics in the last few years, offering hope of dealing with the optics of widely distributed systems.

And I love this idea: We have places in the Solar System like Enceladus, Europa and Triton, where cryovolcanoes are known to exist. What Tarter calls a ‘shortcut to a sample return’ is to fly a swarm of Sprites through material these cryovolcanoes are throwing into space. Peck’s response:

“I don’t see why not. Small sensors could be distributed throughout this environment. If you fly hundreds — or millions — of Sprites through a geyser, you hedge your bets for survival. Even better, you get spatially and temporally distributed sets of measurements that make a different kind of science possible. It’s a ‘village’ of satellites rather than a single big spacecraft that has to work or you lose the mission. A swarm offers a stochastic or random look at the target.”

The Assumptions of SETI

When the conversation turned to SETI, Tarter talked about current work in the radio and optical part of the spectrum and the need to move to larger apertures and greater computing power, noting that SETI would be part of the Square Kilometer Array that will go online in the 2023-2025 timeframe. In optical wavelengths, the coming generation of instruments like the European Extremely Large Telescope and the Thirty Meter Telescope might have places in the focal plane where SETI detectors could be deployed.

Peck wanted to know whether the assumption was that other civilizations are trying to contact us, or would it be possible to pick up accidental transmissions. Tarter’s response:

“We assume most of the gain is in the transmitter, so receivers don’t need to be as powerful. But in the case of other kinds of signals, those that are not deliberate — think of large astro-engineering programs, for example — any leakage is going to be extremely weak. For these we would have to build the gain into the receivers. But putting huge amounts of money into a single area doesn’t seem the right way to go. Better to do more small but different things. For now, the philosophy is to go for deliberate signals and try to be affordable.”

Driving the push into optical wavelengths has been our own experience. Peck likens the way we currently communicate and return data from spacecraft to ‘exploring the universe with a dial-up modem.’ We have low communications bandwidth and leave about 85 percent of the images we collect on Mars. Improving communications by a factor of ten through laser methods would allow far greater science return. And if we find lasers valuable, wouldn’t other civilizations come to the same conclusion? A large aperture doing individual photon counting can detect a very distant signal.

I liked the note the session ended on, the idea that working on a distant goal like a starship can have wide impact on Earth in the near future. Tarter noted that we’re going to have a large number of megacities before long, teeming with populations of 20 million or more each. Everything we need to learn about maintaining stable life support systems for a starship flows as well into how to keep such cities alive and healthy. Cities as starships. We don’t always plan the solutions that work, but experience has shown how often they emerge from pushing into the unknown.



The Morning the Earth Stood Still

by Paul Gilster on September 22, 2014

A long time ago in what now seems like a different lifetime, a colleague told me that the best parts of any conference were the accidental encounters in the hallways where you ran into old friends or people whose work you knew about but hadn’t yet met. That was back when I was going to conferences about medieval literature rather than starships, but the lesson holds. There were almost too many such encounters at the 100 Year Starship 2014 Symposium in Houston to count, and it seemed that around every corner was a chance to exchange ideas and opinions.


There were also enough tracks and ongoing events that it was impossible to get everything in. Claudio Maccone and I always get together, and when I saw him crossing the lobby of the Hilton Americas hotel, I intercepted him to see if he wanted to join a group of us for dinner. But Claudio was headed for a screening of the 1951 version of The Day the Earth Stood Still, a film he had never seen, and I could hardly ask him to turn down the opportunity.

Thus the gravitational lens gave way to Gort and Klaatu and Earth’s chance to live in peace among interstellar civilizations or be burned to a cinder for our transgressions. ‘The decision rests with you,’ as Michael Rennie would say. Unlike the later version, it really was a terrific film. And Claudio and I did have the chance to catch up at a breakfast encounter filled with interstellar talk that included the lens at 550 AU and beyond. I’ll have some thoughts on using it for communications on Friday.

Image: My favorite scene in The Day the Earth Stood Still. Interstellar visitor Klaatu (Michael Rennie) adds an equation to Professor Barnhardt’s blackboard, knowing the professor will soon see it.

Which brings me to the reason for the title of today’s post. I’m sure we’ve all had the dream where something is after you and you seem frozen into immobility, knowing you have to do something fast but are unable to act. I found myself in that position this morning. Still worn out from travel and pushed by non-aerospace obligations this afternoon, I fired up the computer intent on a first post about the symposium and an introduction to a week’s worth of musings, technical session notes and other observations about Houston. And then…

Software glitches. Operating system updates (why did I choose this morning of all mornings not to work as usual in Linux but in Windows 7?). The Mac to PC transfer of my session notes left them completely jumbled, which took time to fix. Then Internet connectivity became unpredictable, for reasons unknown. As soon as it came back, I turned to Dropbox to pull my photos from the symposium and discovered that, because I had upgraded my phone to IOS8, DropBox was now unable to download the Houston images. Multiple downloads of Dropbox updates, to no avail (DropBox: Please fix this!). Finally a Googled workaround to get the photos on the PC.

So it was a morning where time stood still. As it did in Dallas on the way to Houston. The clouds in the photo below were the remainder of a system that, the day before, delayed my Dallas-based flight for an interminable four hours. Now I seem to be running perpetually behind schedule, and am pushing up against an outside deadline. So tomorrow I’ll start digging into Houston issues, starting with a conversation between Jill Tarter and Mason Peck that evoked SETI, miniaturized spacecraft, and astrobiological signatures that might be detected by space-based telescopes.

2014-09-20 16.39.47

Surely the Earth will start moving normally again and I’ll have fixed the remaining software snags by then. My son Miles said he was walking down the hall when Eric Davis called him over to join a group of colleagues, saying, “We’ve just been talking about whether we’re all living in a simulation.” That was right after a lunch with Al Jackson at a nearby Starbucks where Al explained how Roy Kerr came up with his metric for rotating black holes — Al was there in 1963 when Kerr presented the paper! What’s not to like about a place where you get invited into conversations like this? Houston gave me much to think about and I’ll start digging into it tomorrow.



Habitable Moons: Background and Prospects

by Paul Gilster on September 19, 2014

While I’m in Houston attending the 100 Year Starship Symposium (about which more next week), Andrew LePage has the floor. A physicist and freelance writer specializing in astronomy and the history of spaceflight, LePage will be joining us on a regular basis to provide the benefits of his considerable insight. Over the last 25 years, he has had over 100 articles published in magazines including Scientific American, Sky & Telescope and Ad Astra as well as numerous online sites. He also has a web site, www.DrewExMachina.com, where he regularly publishes a blog on various space-related topics. When not writing, LePage works as a Senior Project Scientist at Visidyne, Inc. located outside Boston, Massachusetts, where he specializes in the processing and analysis of remote sensing data.

by Andrew LePage


Like many space exploration enthusiasts and professional scientists, I was inspired as a child by science fiction in films, television and print. Even as a young adult, science fiction occasionally forced me to think outside of the confines of my mainstream training in science to consider other possibilities. One example of this was the 1983 film Star Wars: Return of the Jedi which is largely set on the forest moon of Endor. While this was hardly the first time a science fiction story was set on a habitable moon, as a college physics major increasingly interested in the science behind planetary habitability, it did get me thinking about what it would take for a moon of an extrasolar planet (or exomoon) to be habitable. And not “habitable” like Jupiter’s moon Europa potentially is with a tidally-heated ocean that could provide an abode for life buried beneath kilometers of ice, but “habitable” like the Earth with conditions that allow for the presence of liquid water on the surface for billions of years with the possibility of life and maybe a technological civilization evolving.

A dozen years later, the first extrasolar planet orbiting a normal star was discovered and a few months afterwards on January 17, 1996, famed extrasolar planet hunters Geoff Marcy and Paul Butler announced the discovery of a pair of new extrasolar giant planets (EGPs) opening the floodgate of discoveries that continues to this day. One of these new EGPs, 47 UMa b, immediately caught my attention since it orbited right at the outer edge of its sun’s habitable zone based on the newest models by James Kasting (Penn State) and his colleagues published just three years earlier. While 47 UMa b was a gas giant with a minimum mass of about 2.5 times that of Jupiter and was therefore unlikely to be habitable, what about any moons it might have? If the size of exomoons scaled with the mass of their primary, one could expect 47 UMa b to sport a family of moons with minimum masses up to a quarter of Earth’s.

I was hardly the first to consider this possibility since it was frequently mentioned at this time by astronomers whenever new EGPs were found anywhere near the habitable zone. But this realization did get me seriously researching the scientific issues surrounding the potential habitability of exomoons and I started preparing an article on the subject for the short-lived SETI and bioastronomy magazine SETIQuest, whose editorial staff I had recently joined. While working on this article, I started corresponding with then-grad student Darren Williams (Penn State) who, it would turn out, was already preparing a paper on habitable moons with Dr. Kasting and Richard Wade (Penn State). Published in Nature on January 16, 1997, their paper titled “Habitable Moons Around Extrasolar Giant Planets” was the first peer-reviewed scientific paper on the topic. They showed that a moon with a mass greater than 0.12 times that of Earth would be large enough to hold onto an atmosphere and shield it from the erosive effect of an EGP’s radiation environment. In addition, tidal heating could potentially provide an important additional source of internal heat to drive the geologic activity needed for the carbonate-silicate cycle (which acts as a planetary thermostat) for much longer periods than would otherwise be possible for such a small body in isolation.

I published my fully-referenced article on habitable moons in the spring of 1997. In addition to incorporating the results from Williams et al. and related work by other researchers, I went so far as to make the first tentative estimate of the number of habitable moons orbiting EGPs and brown dwarfs in our galaxy based on the earliest results of extrasolar planet searches: 47 million compared to the best estimate of the time of about ten billion habitable planets in the galaxy (estimates that are in desperate need of revision after almost two decades of progress). Since my research showed it was likely that habitable moons would tend to come in groups of two or more, I further speculated about the possibilities of life originating on one of these moons being transplanted to a neighbor via lithospermia. And since I did not have to contend with scientific peer-review for this article, I even speculated about the effects multiple habitable moons would have on a spacefaring civilization in such a system with so many easy-to-reach targets for exploration and exploitation.


Image: An artist’s conception of a habitable exomoon (credit: David A. Aguilar, CfA).

After SETIQuest stopped publication and I published a popular-level article on habitable moons in the December 1998 issue of Sky & Telescope, my scientific and writing interests lead me in other directions for the next decade and a half. But in the meantime, scientific work on exploring the issues surrounding habitable bodies in general and habitable moons in particular has continued. The current state of knowledge has been thoroughly reviewed in the recent cover story of the September 2014 issue of the scientific journal Astrobiology, titled “Formation, Habitability, and Detection of Extrasolar Moons” by a dozen scientists active in the field including one of the authors of the first paper on habitable exomoons, Dr. Darren Williams.

Even after 17 years of new theoretical work and observations, the possibility of habitable exomoons still remains strong. The authors show that exomoons with masses between 0.1 and 0.5 times that of the Earth can be habitable. A review of the available literature shows that exomoons of this size could form around EGPs or could be captured much as Triton is believed to have been captured by Neptune in our own solar system. Calculations also show that such exomoons, habitable or otherwise, are detectable using techniques that are available today, especially direct detection by photometric means like that employed by Kepler and by more subtle techniques such as transit timing variations (TTV) and transit duration variations (TDV) of EGPs with exomoons. As the authors state in the closing sentence of their paper:

In view of the unanticipated discoveries of planets around pulsars, Jupiter-mass planets in orbits extremely close to their stars, planets orbiting binary stars, and small-scale planetary systems that resemble the satellite system of Jupiter, the discovery of the first exomoon beckons, and promises yet another revolution in our understanding of the universe.

The fully referenced review paper is René Heller et al., “Formation, Habitability, and Detection of Extrasolar Moons”, Astrobiology, Vol. 14, No. 9, September 2014 (preprint).



New Horizons: Hydra Revealed

by Paul Gilster on September 18, 2014

Since we don’t yet have flight-ready systems for getting to the outer Solar System much faster than New Horizons, we might as well enjoy one of the benefits of long flight times. Look at it this way: For the next ten months, we can look forward to sharper and sharper images and an ever increasing flow of data about Pluto/Charon and associated moons. It’s going to be a fascinating story that unfolds gradually, culminating in the July flyby next year, and then, of course, we can hope for further exploration of a Kuiper Belt object.

So New Horizons, launched in 2006, is going to be with us for a while, and it has already given us a brief look at asteroid 132524 APL and a shakeout of its science instruments during a gravitational assist maneuver at Jupiter. Now we’re getting down to much finer-grained imagery from Pluto. The first image distinguishing Pluto and Charon was returned in July of 2013. The latest imagery using the spacecraft’s Long Range Reconnaissance Imager (LORRI) shows Pluto’s diminutive moon Hydra, taken as part of a long-exposure strategy that controllers are using to search for other moons or debris near Pluto/Charon.

Interestingly, this report on the New Horizons site points out that the science team didn’t expect to detect Hydra until January. The spacecraft took 48 10-second images on July 18 and repeated the process on July 20, combining them to show the evidence of Hydra.

satsearch1_hydra _2

Image (click to enlarge): Hydra revealed in summer data from New Horizons. Credit: Alan Stern/JHU/APL.

To untangle the image, start with the top row. With Pluto overexposed at image center, you can see a dark streak at the right that is an artifact of the imaging process. Both dates show similar images, but at the right is a ‘difference’ image that largely pulls out the background starfield. Here you can see Hydra as an overlapping smudge of bright and dark that appears immediately above Pluto in the image. In the bottom row are the same images, showing Hydra’s expected position on these dates as marked by red and green crosshairs. It’s tough to make Hydra out, but a close look at the enlarged image will identify it.

The JHU/APL article quotes Science Team member John Spencer as saying that at this point, Hydra is several times fainter than the faintest objects New Horizons’ camera is designed to detect. That’s good news overall because it speaks to the quality of the equipment as well as its operational status. It also confirms the efficacy of the plan to look for satellites and possibly hazardous smaller objects as the spacecraft approaches the system. Keeping New Horizons healthy and collision-free is obviously job number one — we only get one shot at this.

I also want to quote New Horizons co-Investigator Randy Gladstone (Southwest Research Institute) on the question of how we’ll study Pluto’s atmosphere during the flyby. We know, of course, that it’s low in pressure, mostly made up of molecular nitrogen with small amounts of methane and carbon monoxide, but Gladstone points out that models of the atmosphere are in disagreement because of the sparse data available. New Horizons will be engaged in survey observations that make few assumptions about what may be found, including a Pluto solar occultation operation:

The Alice ultraviolet spectrograph will watch the Sun set (and then rise again) as New Horizons flies through Pluto’s shadow, about an hour after closest approach. Watching how the different colors of sunlight fade (and then return) as New Horizons enters (and leaves) the shadow will tell us nearly all we could ask for about composition (all gases have unique absorption signatures at the ultraviolet wavelengths covered by Alice) and structure (how those the absorption features vary with altitude will tell us about temperatures, escape rates and possibly about dynamics and clouds).

We’ve only known about this atmosphere since 1988, when Pluto occulted a distant star. The refracted starlight observed then was hard evidence for an atmosphere. Like the atmosphere on Neptune’s large moon Triton, Pluto’s has a surface pressure of 30 to 100 microbars — that’s 3 to 100 millionths of Earth’s surface pressure. Sublimation of ices on the surface is responsible for what little atmosphere Pluto has. As Pluto continues to move away from the Sun following its closest approach in 1989, condensation should take over, but New Horizons should be getting there before the atmosphere has condensed back onto the surface.

And what about Charon? No evidence exists for an atmosphere there, but the moon is small enough that any atmosphere present would have to be considerably less dense than that on Pluto. While sublimation could produce gases that fed the atmosphere for a time, the tiny world would not be able to prevent their rapid dispersion into space.



Crucible for Moon Formation in Saturn’s Rings

by Paul Gilster on September 17, 2014

Hard to believe that it’s been ten years for Cassini, but it was all the way back in January of 2005 that the Huygens probe landed on Titan, an event that will be forever bright in my memory. Although the fourth space probe to visit Saturn, Cassini became in 2004 the first to orbit the ringed planet, and since then, the mission has explored Titan’s hydrocarbon lakes, probed the geyser activity on Enceladus, tracked the mammoth hurricane at Saturn’s north pole, and firmed up the possibility of subsurface oceans on both Titan and Enceladus.

I mentioned the Galileo probe last week, its work at Europa and its fiery plunge into Jupiter’s atmosphere to conclude the mission. Cassini has a similar fate in store after finishing its Northern Solstice Mission, which will explore the region between the rings and the planet. As discussed at the recent European Planetary Science Congress in Cascais, Portugal, the spacecraft’s final orbit will occur in September of 2017, taking Cassini to a mere 3000 kilometers above the planet on closest approach. A final Titan encounter will then provide the gravitational muscle to hurl the craft into Saturn’s atmosphere, where it will be vaporized.

But of course we’re not quite through with Cassini yet. New work out of the SETI Institute delves into the creation and destruction of moons on extremely short time-scales within the Saturnian ring system. Robert French, Mark Showalter and team accomplished their studies by comparing the exquisite photographs Cassini has produced with pictures made during the Voyager mission. The upshot: The F ring has taken on a completely new look.

“The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C,” notes French. “It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps. We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring. These moons are small enough to coalesce and then break apart in short order.”

Short order indeed. It appears that the bright spots can appear and disappear in the course of mere days and even hours. Explaining the phenomenon is the nature of the F ring itself, which is at the critical point known as the Roche limit. This is the range within which the gravitational pull on a moon’s near side can differ enough from that on its far side to actually tear the moon apart. What we seem to be seeing is moon formation — objects no more than 5 kilometers in size — quickly followed by gravitationally induced breakup.


Image: Cassini spied just as many regular, faint clumps in Saturn’s narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did. But it saw hardly any of the long, bright clumps that were common in Voyager images. Credit: NASA/JPL-Caltech/SSI

Mark Showalter compares these small moons to bumper cars that careen through moon-forming material, taking form and then fragmenting as they go. Adding to the chaos is the moon Prometheus, a 100-kilometer object that orbits just within the F ring. Alignments of this moon that occur every 17 years produce a further gravitational influence that helps to launch the formation of the tiny moonlets and propel them through their brief lives. Prometheus, then, should cause a periodic waxing and waning of the clumps of moon activity.

Further work with the Cassini data should help the researchers firm up this theory, for the Prometheus influence should cause an increase in the clumping and breakup activity within the next few years. Cassini has a sufficient lifetime to test that prediction. What adds further interest to the story is the fact that we’re seeing in miniature some of the elementary processes that, 4.6 billion years ago, led to the formation of the Solar System’s planets. Consider Saturn’s F ring, then, a laboratory for processes we’d like to learn much more about as we turn our instruments to young stars and the planets coalescing around them.

The paper is French et al., “Analysis of clumps in Saturn’s F ring from Voyager and Cassini,” published online in Icarus on July 15, 2014 (abstract). This news release from the SETI Institute is also helpful.



‘Hot Jupiters’: Explaining Spin-Orbit Misalignment

by Paul Gilster on September 16, 2014

Bringing some order into the realm of ‘hot Jupiters’ is all to the good. How do these enormous worlds get so close to their star, having presumably formed much further out beyond the ‘snowline’ in their systems, and what effects do they have on the central star itself? And how do ‘hot Jupiter’ orbits evolve so as to create spin-orbit misalignments? A team at Cornell University led by astronomy professor Dong Lai, working with graduate students Natalia Storch and Kassandra Anderson, has produced a paper that tells us much about orbital alignments and ‘hot Jupiter’ formation.

It’s no surprise that large planets — and small ones, for that matter — can make their stars wobble. This is the basis for the Doppler method that so accurately measures the movement of a star as affected by the planets around it. But something else is going on in ‘hot Jupiter’ systems. In our own Solar System the rotational axis of the Sun is more or less aligned with the orbital axis of the planets. But some systems with ‘hot Jupiters’ have shown a misalignment between the orbital axis of the gas giants and the rotational axis of the host star.


Image: ‘Hot Jupiters,’ large, gaseous planets in inner orbits, can make their suns wobble after they wend their way through their solar systems. Credit: Dong Lai/Cornell University.

The Cornell team went to work on simulations of such systems, working with binary star systems separated by as much as hundreds of AU. Their work shows that gas giants can be influenced by partner binary stars that cause them to migrate closer to their star. At play here is the Lidov-Kozai mechanism in celestial mechanics, an effect first described by Soviet scientist Michael Lidov in 1961 and studied by the Japanese astronomer Yoshihide Kozai. The effect of perturbation by an outer object is an important factor in the orbits of planetary moons, trans-Neptunian objects and some extrasolar planets in multiple star systems.

Thus the mechanism for moving a gas giant into the inner system, as described in the paper:

In the ‘Kozai+tide’ scenario, a giant planet initially orbits its host star at a few AU and experiences secular gravitational perturbations from a distant companion (a star or planet). When the companion’s orbit is sufficiently inclined relative to the planetary orbit, the planet’s eccentricity undergoes excursions to large values, while the orbital axis precesses with varying inclination. At periastron, tidal dissipation in the planet reduces the orbital energy, leading to inward migration and circularization of the planet’s orbit.

As the planet approaches the star, interesting things continue to occur. From the paper:

It is a curious fact that the stellar spin axis in a wide binary (~ 100 AU apart) can exhibit such a rich, complex evolution. This is made possible by a tiny planet (~ 10-3 of the stellar mass) that serves as a link between the two stars: the planet is ‘forced’ by the distant companion into a close-in orbit, and it ‘forces’ the spin axis of its host star into wild precession and wandering.

Moreover, “…in the presence of tidal dissipation the memory of chaotic spin evolution can be preserved, leaving an imprint on the final spin-orbit misalignment angles.”

The approach of the ‘hot Jupiter’ to the host star can, in other words, disrupt the previous orientation of the star’s spin axis, causing it to wobble something like a spinning top. The paper speaks of ‘wild precession and wandering,’ a fact that Lai emphasizes, likening the chaotic variation of the precession to chaotic phenomenon such as weather systems. The spin-orbit misalignments we see in ‘hot Jupiter’ systems are thus the result of the evolution of changes to the stellar spin caused by the migration of the planet inward.

The paper goes on to mention that we see examples of chaotic spin-orbit resonances in our own Solar System. Saturn’s satellite Hyperion experiences what the paper calls ‘chaotic spin evolution’ because of resonances between its spin and orbital precession periods. Even the rotation axis of Mars undergoes chaotic variation due to much the same mechanism.

The paper is Storch, Anderson & Lai, “Chaotic dynamics of stellar spin in binaries and the production of misaligned hot Jupiters,” Science Vol. 345, No. 6202 (12 September 2014), pp. 1317-1321 (abstract / preprint)



Emergence of the ‘Venus Zone’

by Paul Gilster on September 15, 2014

In terms of habitability, it’s clear that getting a world too close to its star spells trouble. In the case of Gliese 581c, we had a planet that some thought would allow liquid water at the surface, but subsequent work tells us it’s simply too hot for life as we know it. With the recent dismissal of Gl 581d and g (see Red Dwarf Planets: Weeding Out the False Positives), that leaves no habitable zone worlds that we know about in this otherwise interesting red dwarf system.

I’m glad to see that Stephen Kane (San Francisco State) and his team of researchers are working on the matter of distinguishing an Earth-like world from one that is more like Venus. We’ve made so much of the quest to find something roughly the same size as the Earth that we haven’t always been clear to the general public about what that implies. For Venus is Earth-like in terms of size, but it’s clearly a far cry from Earth in terms of conditions.

Indeed, you would be hard-pressed to find a more hellish place than Venus’ surface. Kane wants to understand where the dividing line is between two planetary outcomes that could not be more different. Says the scientist:

“We believe the Earth and Venus had similar starts in terms of their atmospheric evolution. Something changed at one point, and the obvious difference between the two is proximity to the Sun.”

Kane and company’s paper on this will appear in Astrophysical Journal Letters and is already available on the arXiv server (citation below). At stake here is solar flux, the incoming energy from the planet’s star, which can be used to define an inner and an outer edge to what Kane calls the ‘Venus Zone.’ Venus is 25 percent closer to the Sun than the Earth, but it gets twice the amount of solar flux. Get close enough to the star to trigger runaway greenhouse effects — the results of which make Venus the distinctive nightmare at the surface that it is — and you are at the outer edge of the Venus Zone.

Go further in toward the star and you can pick out the point where a planet’s atmosphere would begin to be eroded by the incoming flux. This is the inner edge of the Venus Zone, and by understanding the boundaries here, we are helping future attempts to characterize Earth-sized worlds in the inner systems of their stars. Find an Earth-sized planet in the Venus Zone and there is reason to suspect that a runaway greenhouse gas effect is in play.


Image: This graphic shows the location of the “Venus Zone,” the area around a star in which a planet is likely to exhibit atmospheric and surface conditions similar to the planet Venus. Credit: Chester Harman, Pennsylvania State University.

The broader picture is an attempt to place our Solar System in context. The Kepler results have consistently demonstrated that any thought of our Solar System being a kind of template for what a system should look like must be abandoned. From the paper:

A critical question that exoplanet searches are attempting to answer is: how common are the various elements that we find within our own Solar System? This includes the determination of Jupiter analogs since the giant planet has undoubtedly played a significant role in the formation and evolution of our Solar System. When considering the terrestrial planets, the attention often turns to atmospheric composition and prospects of habitability. In this context, the size degeneracy of Earth with its sister planet Venus cannot be ignored and the incident flux must be carefully considered.

The study identifies 43 potential Venus analogs from the Kepler data, with occurrence rates similar to those for Earth-class planets, though as the paper notes, with smaller uncertainties. After all, Kepler is more likely to detect shorter-period planets in the Venus Zone than Earth-class planets with longer orbital periods. Overall, the team estimates based on Kepler data that approximately 32% of small low-mass stars have terrestrial planets that are potentially like Venus, while for G-class stars like the Sun, the figure reaches 45%.

Kane notes that future missions will be challenged by the need to distinguish between the Venus and Earth model. We’ll also be looking at the question of carbon in a planet’s atmosphere and its effects on the boundaries of the Venus Zone, the assumption being that more carbon in the atmosphere would push the outer boundary further from the star.

The paper is Kane, Kopparapu and Domagal-Goldman, “On the frequency of potential Venus analogs from Kepler data,” accepted for publication in The Astrophysical Journal Letters and available as a preprint. Be aware as well of the team’s Habitable Zone Gallery, which currently identifies 51 planets as likely being within their star’s habitable zone.



Space Telescopes Beyond Hubble and JWST

by Paul Gilster on September 12, 2014

Ashley Baldwin tracks developments in astronomical imaging with a passion, making him a key source for me in keeping up with the latest developments. In this follow-up to his earlier story on interferometry, Ashley looks at the options beyond the James Webb Space Telescope, particularly those that can help in the exoplanet hunt. Coronagraph and starshade alternatives are out there, but which will be the most effective, and just as much to the point, which are likely to fly? Dr. Baldwin, a consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK) and a former lecturer at Liverpool and Manchester Universities, gives us the overview, one that hints at great things to come if we can get these missions funded.

by Ashley Baldwin


Hubble is getting old.

It is due to be replaced in 2018 by the much larger James Webb Space Telescope. This is very much a compromise of what is needed in a wide range of astronomical and cosmological specialties, one that works predominantly in the infrared. The exoplanetary fraternity will get a portion of its (hoped for) ten year operating period. The JWST has coronagraphs on some of its spectrographs which will allow exoplanetary imaging but as its angular resolution is actually lower than Hubble, its main contribution will be to characterise the atmospheres of discovered exoplanets.

It is for this reason that the designers of TESS (Transiting Exoplanet Survey Satellite) have made sure a lot of its most prolonged viewing will overlap with that of the JWST. Its ability to do this will depend on several factors such as the heat (infrared) the planet is giving out, its size and critically its atmospheric depth (the deeper the better) and the proximity of the planet in question. The longer the telescope has to “stop and stare” at its target planet the better, but we already know lots of other experts want some of the telescope’s precious time, so this will be a big limiting factor.

Planet Hunting in Space

The big question is, where are the dedicated exoplanet telescopes? NASA had a mission called WFIRST planned for the next decade, with the predominant aim of looking at dark matter. There was an add on for “micro-lensing” discovery of exoplanets that happened to pass behind further stars, getting magnified by the stars’ gravity and showing up as “blips” in the star’s spectrum. When the National Reconnaissance Mirrors (NRO) were recently donated to NASA, it was suggested that these could be used for WFIRST instead.

Being 2.4 m in diameter they would be much larger than the circa 1.5 m mirror originally proposed and would therefore make the mission more powerful, especially because by being “wide field” they would view far bigger areas of the sky, further increasing the mission’s potency. It was then suggested that the mission could be improved yet further by adding a “coronagraph” to the satellite’s instrument package. The savings made by using one of the “free” NRO mirrors would cover the coronagraph cost.

Coronagraphs block out starlight and were originally developed to allow astronomers to view the Sun’s atmosphere (safely!). Subsequently they have been placed in front of a telescope’s focal plane to cut out the light of more distant stars, thus allowing the much dimmer light of orbiting exoplanets to be seen. A decade of development at numerous research testbeds such as the Jet Propulsion Laboratory, Princeton and at the Subaru telescope on Mauna Kea has refined the device to a high degree. When starlight of all wavelengths strikes a planet it can be reflected directly into space, or absorbed to be re-emitted as thermal infrared energy. The difference between the amount of light emitted by planets versus stars is many orders of magnitude in the infrared compared to the visible, so for this reason telescopes looking to visualise exoplanets do so in the infrared. The difference is still a billion times or so.

Thus the famous “firefly in the searchlight “metaphor. Any coronagraph must cut out infrared to the tune of a billion times or more for an exoplanet to first be seen and then analysed spectroscopically. The latter is crucial as it tells us about the planet and its atmosphere according to the factors described above. This light reduction technique is called “high contrast imaging” with the reduction described according to negative powers of ten. Typically a billion times reduction is simplified to 10e9. This level of reduction should allow Jupiter size planets, ice giants like Neptune and, at a push, “super earths”. To visualise Earth like, terrestrial planets, an extra order of magnitude, 10e10 or better is necessary.


Image: A coronagraph at work. This infrared image was taken at 1.6 microns with the Keck 2 telescope on Mauna Kea. The star is seen here behind a partly transparent coronagraph mask to help bring out faint companions. The mask attenuates the light from the primary by roughly a factor of 1000. The young brown dwarf companion in this image has a mass of about 32 Jupiter masses. The physical separation here is about 120 AU. Credit: B. Bowler/IFA.

The Emergence of WFIRST AFTA

Telescope aperture is not absolutely critical (with a long enough view), with even small metre-sized scopes able to see exoplanets with the correct coronagraph. The problem is the inner working angle or IWA. This represents how close to the parent star its light is effectively blocked, allowing imaging with minimal interference. Conversely, the outer working angle, OWA , determines how far away from the star a planet can be seen. The IWA is particularly important for seeing and characterising planets in the habitable zone (HBZ) of sun-like stars. By necessity it will need to shrink as the HBZ shrinks, as with M dwarfs, which would obviously make direct imaging of any terrestrial planets discovered in the habitable zones of TESS discoveries very difficult. For bigger stars with wider HBZs obviously the IWA will be less of an issue.

So all of this effectively made a new direct imaging mission, WFIRST AFTA. Unfortunately the NRO mirror was not made for this sort of purpose. It is a Cassegrain design, a so-called “on axis” telescope with the focal plane in line with the primary mirror’s incoming light, with the secondary mirror reflecting its light back through a hole in the primary to whatever science analysis equipment is required. In WFIRST AFTA this would mainly be a spectrograph.

The coronagraph would have to be at the focal plane and along with the secondary mirror, would further obscure the light striking the primary. It would also need squeezing between the “spider’ wires that support the secondary mirror (these give the classic ‘Christmas tree star’ images we are all familiar with in common telescopes).

Two coronagraphs are under consideration that should achieve an image contrast ratio of 10 to the minus 9, which is good enough to view Jupiter-sized planets. Every effort is being made to improve on this and to get down to a level where terrestrial planets can be viewed. Difficult and expensive, but far from impossible. Obviously, WFIRST has quite easily the biggest mirror of the options under consideration by NASA and hence the greatest light intake and imaging range. It could also be possible to put the necessary equipment on board to allow it to use a starshade at a later date. The original WFIRST budget came in at $1.6 billion but that was before NASA came under increasing political pressure on the JWST’s (huge) overspend.

An independent review of cost suggested WFIRST would come in at over $2 billion. Understandably concerned about the potential for “mission creep”, seen with the JWST development, NASA put the WFIRST AFTA design on hold until the budgetary statement of 2017, with no new building commencing until JWST launched. So whatever is eventually picked, 2023 will be the earliest launch date. Same old story, but limited costs sometimes lead to innovation. In the meantime, NASA commissioned two “Probe” class alternative back up concepts to be considered in the “light” of the budgetary statement.

Exoplanet Telescope Alternatives

The first of these is EXO-C. This consists of a 1.5 m “off axis” telescope ( the primary mirror is angled so that the focal plane and secondary mirror are at the side of the telescope and don’t obscure the primary, thus increasing its light gathering ability). There are potential imaging issues with such scopes so they cost more to build. EXO-C has a coronagraph and a spectrograph away from the optical plane. The issue for this concept is which coronagraph to choose. There are many designs, tested over a decade or more with the current “high contrast imaging”( see above) level between 10e9 and 10e10. So EXo-C is relatively low risk and should at a push be able to even see some Earth or Super-Earth planets in the HBZs of some nearby stars, as well as lots of “Jupiters”.

The other Probe mission, even more exciting, is EXO-S. This involves combining a self propelled “on axis” 1.1m telescope with a “starshade”. The starshade is a flower-like (it even has petals) satellite — the choice of a flower shape rather than a round configuration reduces image-spoiling “Fresnel” diffraction from the starshade edges. The shade sits between the telescope and the star to be examined for planets. It casts a shadow in space within which the telescope propels itself to the correct distance for observation (several thousands of kms).

Like a coronagraph, the starshade cuts out the star’s light, but without the difficulty of squeezing an extra device into the telescope. The hard bit is that both telescope and shade need to have radio or laser communication to achieve EXACT positioning throughout the telescopic “stare” to be successful, requiring tight formation flying. The telescope carries propellant for between 3 and 5 years. With several days for moving into position, this is around 100 or so separate stop and stares. The shade concept means two devices instead of one although they can be squeezed into one conventional launch vehicle, to separate at a later point in the mission.


Image: The starshade concept in action. Credit: NASA/JPL.

The good news is that with a starshade the inner working angle is dependent on the telescope starshade distance rather than the telescope. The price of this is that the further apart the two are, the greater the precision placement required. The distances involved depend on the size of the starshade. For EXO-S’ 35 m starshade, this is in excess of thirty seven thousand kilometres. EXO-S, despite its small mirror size, will be able to view and spectrographically characterise terrestrial planets around suitable nearby stars and Jupiter-sized planets considerably further out.

Achieving Space Interferometry

“Formation flying” of telescopes is an entirely new concept that hasn’t been tried before, so potentially more risky, especially as its development is way behind that of coronagraph telescopes. If it works, though, it opens the gate to fantastic discovery in a much wider area than EXO-S. This is just the beginning. If you can get two spacecraft to fly in formation, why not 3 or 30 or even more? In the recent review I wrote for Centauri Dreams on heterodyne interferometers, I described how 30 or so large telescopes could be linked up to deliver the resolution of an telescope with an aperture equivalent to the largest gap between the unit scopes of the interferometer (a diluted aperture). The number of scopes increases light intake ( the brightness of the image) and “baselines” , the gap between constituent scopes in the array, delivering detail across the diluted aperture of the interferometer.

We’re in early days here, but this is heading in the direction of an interferometer in space with resolution orders of magnitude larger than any New Worlds telescope. A terrestrial planet finder yes, but more important, with a good spectrograph, a terrestrial planet characteriser interferometer. TPC-I. To actually “see” detail on an exoplanet would require hundreds of large space telescopes spread over hundreds of kilometers, so that’s one for Star Trek. Detailed atmospheric characterisation, however, is almost as good and not so far in the future if EXO-S gets the go ahead and the Planet Formation Imager evolves on the ground before migrating into space. All roads lead to space.

As an addendum, EXO-S has a yet to be described back-up that could best be seen as WFIRST AFTA-S. Here the starshade has the propulsive system, but the telescope is made from the NRO 2.4 m mirror, thus making the device potentially the most potent of the three designs. Having the drive system on the starshade, along with a radio connector to the telescope, is a concept even newer than the conventional EXO-S . But it is potentially feasible. We await a cost from the final reports the design concept groups need to submit.

In the meantime, various private ventures such as the BoldlyGo Institute run by Jon Morse, formerly of NASA, are hoping to fund and launch a 1.8 m off-axis telescope with BOTH an internal coronagraph AND a starshade. Sadly, the two methods have been found not to work in combination, but obviously a coronagraph telescope can look at stars while its starshade moves into position, increasing critical viewing time over a 3 year mission.

By way of comparison, coronagraphs can and have been used increasingly effectively on ground-based scopes such as Gemini South. It is believed that thanks to atmospheric interference the best contrast image achievable, even with one of the new ELTs being built, will be around 10 to the minus 9, so thanks to their huge light gathering capacity, they too might just discover terrestrial planets around nearby stars but probably not in the HBZ.

The future holds exciting developments. Tantalisingly close. In the meantime, it is important to keep up the momentum of development. The two Probe design groups recognise that their ideas, whilst capable of exciting science as well as just “proof of concept”, are a long way short of what could and should be done. The JWST for all its overspend will hopefully be a resounding success and act as a pathfinder for a large, 16 m plus New Worlds telescope that will start the exoplanet characterisation that will be completed by TPC-I. Collapsible, segmented telescopes will be shown to fit into and work from available launch vehicles, such as the upcoming Space Launch system (SLS), or one of the new Falcon Heavy rockets. New materials such as silicon carbide will reduce telescope costs. The lessons learned from JWST will make such concepts economically viable and deliver ground-shaking findings.

How ironic if would be if we discover other life in another star system before we find it in our own !



Evidence for Plate Tectonics on Europa

by Paul Gilster on September 11, 2014

It was the Galileo mission, which ended in 2003 when the probe descended into the depths of Jupiter’s atmosphere, that brought us the first solid evidence of an ocean beneath the ice of Europa. Galileo made multiple flybys of the Jovian moon, the first spacecraft to do so, with the closest pass being a scant 180 kilometers on October 15, 2001. As you would imagine, the radiation environment near Europa is hazardous, which is why the flybys were reserved for Galileo’s extended mission. We’ve been mining the Galileo data on Europa ever since.

You may remember that Galileo was unable to open its high-gain antenna on the way to Jupiter, so we had to rely on the ingenuity of mission controllers to get the maximum performance out of the low-gain antenna. That 70 percent of the mission’s science goals were still met, and that we are making new discoveries with the Galileo data today, still amazes me. Now we have new work on Europa that flags the evidence for plate tectonics on the distant moon, which would be the first sign of such activity on any world other than our own.

Simon Kattenhorn (University of Idaho) and Louise Prockter (Johns Hopkins University Applied Physics Laboratory) led this work, which offers visual evidence of the expansion of Europa’s icy crust. A look at Europa’s cracked and ridged surface as sent back by Galileo calls into question how the terrain formed, because while new crust is visible, the mechanism for destroying older crust is not apparent. Kattenhorn and Prockter suggest that this ‘missing terrain’ was absorbed into Europa’s ice shell rather than breaking through it into the ocean that lies beneath. But the evidence for plate tectonics is compelling, and the thickness of the ice shell remains controversial.


Image: Scientists have found evidence of plate tectonics on Jupiter’s moon Europa. This false-color image of the trailing northern hemisphere on Jupiter’s moon Europa — the hemisphere that faces away from Jupiter — shows numerous ridges (red) and band (light-colored) features. Subduction zones — regions where two tectonic plates converge and one is forced beneath the other — may also be present in the study area and are identified by arrows. Image credit: NASA/JPL/University of Arizona.

Plate tectonics describes the motion of large plates in the Earth’s outermost shell, causing earthquakes and volcanic activity as well as mountain-building and the formation of trenches in the oceans as the plates meet. Subduction can carry plate material back into the mantle, while new crust can emerge from seafloor spreading. On Europa’s surface, the break up of crustal material and its replacement by bands of fresh ice from below is apparent. The new material fills in broad bands that are kilometers wide. Kattenhorn and Prockter reconstructed what areas of the surface would have looked like before these disruptions occurred.

Just where was the old crust being destroyed so that the new crust could form? When the researchers looked at areas where subduction similar to Earth’s might be occurring on Europa, they found ice volcanoes on the overriding plate. The smoothness of the surface in these areas implied that older material was forced below rather than remaining as crumpled mountainous terrain on the surface. So now we have evidence not only of material moving up through the ice crust but a mechanism for moving surface material back into the shell.

Simon Kattenhorn comments on the significance of the finding:

“Europa may be more Earth-like than we imagined, if it has a global plate tectonic system. Not only does this discovery make it one of the most geologically interesting bodies in the solar system, it also implies two-way communication between the exterior and interior — a way to move material from the surface into the ocean — a process which has significant implications for Europa’s potential as a habitable world.”


Image: Scientists have found evidence of plate tectonics on Jupiter’s moon Europa. This conceptual illustration of the subduction process (where one plate is forced under another) shows how a cold, brittle, outer portion of Europa’s 20–30 kilometer (roughly 10–20 mile) thick ice shell moved into the warmer shell interior and was ultimately subsumed. A low-relief subsumption band was created at the surface in the overriding plate, alongside which cryolavas may have erupted. Image credit: Noah Kroese, I.NK.

Bear in mind the reason for Galileo’s fiery plunge into the Jovian atmosphere. The spacecraft, its systems degrading in the high-radiation environment, its fuel largely spent, was crashed into the giant planet so that there would be no possibility it might contaminate Europa at some point in the future with bacteria from Earth. Europa remains a target of high astrobiological interest, and preventing even the faintest possibility of contamination kept this fascinating moon pristine. We now ponder what kinds of equipment it might take to explore near-Europa space and the surface itself in hopes of finding evidence of life from below.

The paper is Kattenhorn and Prockter, “Evidence for subduction in the ice shell of Europa,” Nature Geoscience, published online 7 September 2014 (abstract). See also this JHU/APL news release.



Binary Stars: The Likelihood of Planets

by Paul Gilster on September 10, 2014

In Greg Bear’s novel Queen of Angels (Gollancz, 1990), a robotic probe called AXIS (Automated eXplorer of Interstellar Space) has used antimatter propulsion to make a fifteen-year crossing to Alpha Centauri. The world’s various networks of the future begin to feast on reports of what it finds, like this one:

“In the past few weeks, AXIS has returned images of three planets circling Alpha Centauri B. As yet these worlds have not been named, and are called only B-1, B-2, and B-3. B-3 was already known to moonbased astronomers; it is a huge gas giant some ten times larger than Jupiter in our own solar system. Like Saturn, it is surrounded by a thin rugged ring of icy moonlets. B-1 is a barren rock hugging close to Alpha Centauri B, similar to Mercury. But the focus of our attention is now on B-2, a justright world slightly smaller than Earth. B-2 possesses an atmosphere closely approximating Earth’s, as well as continents and oceans of liquid water. It is orbited by two moons each about a thousand kilometers in diameter.”

It’s a tale that is only partially devoted to interstellar matters, but those with an interest in artificial intelligence of a high order indeed and its possibilities in future probes will want to become familiar with it. As you can see, Bear’s guess about Centauri Bb is about right, at least based on what little we know about the candidate world located in a scorching inner orbit. We can rule out the gas giant based on subsequent work which has whittled down the possibilities for large worlds, but we do have the region within 2 AU in which to hope for a stable orbit for another planet (outside of that, planetary orbits according to our simulations are quickly disrupted).

Are we likely to find another Alpha Centauri planet, a hypothetical Centauri Bc? We can certainly hope so, but while we await the lengthy period of data acquisition and analysis that may tell us, we can look at recent work from Elliott Horch (Southern Connecticut State) and team, which has shown, using Kepler data, that 40% to 50% of host stars for exoplanets are binary stars. Says Horch: “It’s interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago.”


Image: The Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN telescope in southern Arizona. Credit: NOAO.

Indeed, there was a time not all that long ago when the idea of planets around multiple star systems was considered unlikely because of the gravitational disruptions such systems — at least relatively close binaries — would experience. But a number of studies since the 1990s have demonstrated stable orbits even in systems as close as Alpha Centauri, where the separation between Centauri A and B closes from 40 AU down to a tight 11 AU. That 2 AU of breathing room I mentioned above re Centauri B gives us a planet possibility perhaps as far out as the asteroid belt in our own system if we throw in a fudge factor, but not much further.

As to the work of Horch and company, the researchers used speckle imaging using data from the WIYN telescope located on Kitt Peak in southern Arizona and the Gemini North telescope (Mauna Kea) to look at targets at a rate of 15 to 25 times per second. The resolution achieved through this method, combining the images with suitable algorithms, can detect companion stars that are as much as 125 times as faint as the target star and only 0.05 arcseconds away. The occurrence rate of binaries in this work yields the high percentage of exoplanet host stars that turn out to be binaries, or at least appear to be. From the paper:

After a distance-limited subsample of these objects is constructed, the known statistics concerning binarity among stars near the Sun is added. The simulations predict that the very large majority of sub-arcsecond companions will be physically bound to the Kepler star.

The needed simulations are there to rule out objects that may only be in line of sight with the Kepler Object of Interest star being studied. As this National Optical Astronomy Observatory news release explains, the simulation relies on known statistical properties of binary star systems and line of sight ‘companions.’ Continuing from the paper:

This result suggests that, over the separation range to which we are sensitive, exoplanet host stars have a binary fraction consistent with that of field stars. Our speckle imaging program has identified a sample of candidate binary-star exoplanet systems in which only a modest number of false positives are likely to exist.

Thus the large majority of stellar companions revealed around KOI stars turn out to be actual companion stars rather than line of sight stars not connected with the system. And because we are talking about companion stars with separations between several AU out to no more than 100 AU, we may not always be sure around which star a given planet orbits. Now that binaries are thought to account for about half of known stars, these results suggest that the presence of the companion star does not not adversely affect the formation of planets.

The paper is Horch et al., “Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound,” accepted at The Astrophysical Journal (preprint).