by Paul Gilster | Apr 19, 2016 | Exoplanetary Science |
At Palo Alto’s superb Amber India, I was thinking about Alpha Centauri. There are several Amber India locations in the Bay area, but the Palo Alto restaurant dishes up, among other delights, a spicy scallop appetizer that is searingly hot and brilliantly spiced. Greg and Jim Benford were at the table, Claudio Maccone and my son Miles. It was the night before Breakthrough Discuss convened. And while the topics roamed over many aspects of spaceflight, it was that star system right here in our solar neighborhood that preoccupied me.
How lucky could we be to have not one but two stars this close and so similar to our own? Centauri A is a G-class star, Centauri B a K, and if we hit the jackpot, we could conceivably find planets orbiting both. Then there is Proxima Centauri, an M-dwarf that is the closest star of all to the Solar System. The presence of so many astronomers on the Breakthrough Discuss roster made it clear we’d get the latest on the hunt for planets here, a vital factor as we assessed prospects for the Breakthrough Starshot mission. A nice blue target world would help.
The Binary Star Factor
Growing up, I would haunt the local library for books on astronomy, learning that despite its tantalizing proximity, Alpha Centauri was likely devoid of planets because Centauri A and B were so close to each other. After all, at times they close to within 11 AU — how could planets exist in such a gravitationally unsettled region? The question is still unanswered, as is the question of whether Proxima Centauri is truly part of a triple star system or simply shares a common motion with A and B. But our picture of Alpha Centauri has changed radically since my days in the local library, and the system is under scrutiny as never before.
Image: Gathering outside Stanford’s Arrillaga Alumni Center with coffee in hand as we waited for the first session of Breakthrough Discuss to begin.
Paul Wiegert and Matt Holman discussed stable orbits around Centauri A and B back in 1997, work that led to a generally accepted belief that out to a distance of perhaps 2.5 AU, small planets of Earth-like radius or a bit larger could exist. Giant planets are seemingly ruled out by radial velocity studies, although Jared Males (University of Arizona) would note that we might save James Cameron’s Polyphemus, a gas giant in the film Avatar, by postulating a large radius, low mass planet. But he was quick to add that the prospect was not likely.
The session, titled Exoplanet Detection Programs Focused on Alpha Centauri, was led by Olivier Guyon, but it was Michael Endl (University of Texas at Austin) who presented the overview of Alpha Centauri work so far. The issue of planet formation is far from settled, and as Centauri Dreams readers know, the question is not so much one of stable orbits but whether planet formation can deliver an intact planet in the first place. Key work here has been done by Philippe Thébault and Javiera Guedes, who have reached opposite conclusions, with Guedes arguing for small planet formation, and Thebault arguing against the proposition because of planetesimal encounters and the impact velocities at which they would occur.
Image: One of Michael Endl’s slides, this one discussing planet formation around Alpha Centauri.
The argument will be settled, as Endl pointed out, by our increasingly powerful ability to deploy new technologies. Radial velocity methods are pushing toward the region we’ll need to study, but even now we would need to work at a 10-12 centimeters per second level to find an Earth mass planet, a feat that Endl noted was orders of magnitude below what today’s best instruments can deliver. Remember, too, that the planet we thought we had found around Centauri B probably isn’t there, now considered to be a false positive probably caused by activity on the star itself. In fact, let me quote Xavier Dumusque on this, as he was at the conference and was on the team that performed the original Centauri B work:
We worked with 20,000 spectra on Centauri B taken over four years and found a small 50 cm per second signal that seemed to be a planet in a 3.2 day orbit. Subsequent papers have shown that the signal is in the data, but it is probably not due to a planet. All our techniques are at the limit of their capabilities, which means we should use all the techniques we have, so that if one tells us we have a planet, another can assure us it is real.
The problems Alpha Centauri presents, particularly right now, are manifest. Spectral contamination means that when you’re trying to tease a Doppler signal out of the light from a star like Centauri B, you get light mixing in from Centauri A, for at this point in their orbits, the two stars have closed to their closest point as viewed from Earth. The work Dumusque referred to, drawn from HARPS spectroscopic data at the European Southern Observatory’s La Silla Observatory, may well have been affected by magnetic effects on Centauri B’s surface. But right now we’re in that period when the primary Centauri stars are very hard to analyze.
I’ll remind readers that there has been one possible transit detected. Motivated by the HARPS work on the possible 3.2-day period planet, the search turned up what looked like the transit of an Earth-like planet with a period of less than 12 days, but if it was a transit, it did not recur. As to Proxima Centauri, we still have no planets there, but we can rule out larger worlds, while allowing the possibility of planets of two to three Earth masses in the habitable zone. We’ve followed the work of the Pale Red Dot project that has collected new spectra using the HARPS instrument in these pages and are awaiting the data analysis with great interest.
Image: Natalie Batalha (NASA Ames) raising a point during the panel discussion that followed the Alpha Centauri planet detection talks.
Bringing New Methods to Bear
The point that Michael Endl made, and it was echoed by other speakers, is that we need to throw everything we have at this intractable problem. One way forward is to keep improving radial velocity precision, but we also need to do x-ray monitoring of Centauri A and B to look for activity cycles, and consider the possibilities of astrometry and even direct imaging. Thomas Ayres (University of Colorado) noted the dramatic changes to Alpha Centauri A in x-ray imaging — the star goes dark at x-ray wavelengths in 2005 with an unprecedented darkening by a factor of 50, and is now showing a return to activity levels close to that of our own Sun.
The scientists at Breakthrough Discuss were generally upbeat about the prospects of finding planets in the Alpha Centauri system, though the feeling was not quite unanimous, with Peter Tuthill (University of Sydney) saying he found the likelihood of planets there in the range of 20 percent. Adding “I’ve just put myself out of a job with that comment,” he went on to explain JAM, the JWST Aperture Mask, which would use astrometric methods to look for the tiny stellar motion that a planet tugging either Centauri A or B would induce. A separate mission called TOLIMAN (a medieval name for Alpha Centauri) would use a diffractive pupil aperture mask, with the distortions optical systems produce becoming a ‘ruler’ that detects such motion.
Image: A view across the way from the alumni center during a break. After the intense sessions, it was a pleasure to walk outside for a few minutes to rest my eyes.
And what about observing Alpha Centauri planets from the ground? We’re moving into the era of enormous ground observatories with apertures from 20 meters up to 100 meters across in the works. These Extremely Large Telescopes (ELTs), like the European Extremely Large Telescope (Chile), the Thirty Meter Telescope (Mauna Kea, Hawaii) and the Giant Magellan Telescope (Chile) point toward future instruments as enormous as Colossus, a 100 meter telescope concept that could become the world’s largest optical and infrared instrument.
Needless to say, such instruments can become major tools for studying planetary systems around nearby stars. But as Markus Kasper (European Southern Observatory) explained, we can also perform upgrades on existing instruments — the Very Large Telescope (VLT), Magellan (Chile) and Gemini (sites in Hawaii and Chile) — to perform pathfinder work at thermal infrared wavelengths for future imaging with the giant instruments to come. Thus ground-based instruments become a complement to space telescopes for actual exoplanet imaging.
I was interested in Bruce Macintosh’s presentation on direct imaging from space, because years ago I talked to Webster Cash (University of Colorado) about the prospects of using a starshade, in which the optics for a mission are separated. Rather than using a coronagraph to block out the light of the central star, you create a starshade whose shape is precisely determined to block the same light, with the starshade operating some 25000 kilometers away from the telescope.
Macintosh (Stanford University) said the problem of seeing a planet next to the blazing star that it circles was akin to looking for bioluminescent algae next to a lighthouse, which is why we need a coronagraph or a starshade in the first place. The WFIRST mission (Wide-Field Infrared Survey Telescope), scheduled for launch late in the next decade, will carry an advanced coronagraph, but a starshade would also be compatible with this instrument.
Image: The starshade in position far from the space telescope observing the light, with the central star effectively masked. Credit: University of Colorado.
But maybe we don’t have to wait that long. During a break I had the chance to talk to Cash, who had been a huge help with my original Centauri Dreams book. Cash has been working with starshade concepts for a long time, but even he was surprised when his team began testing small starshades in the atmosphere. In a field of view that included the bright star Sirius, the star would simply disappear. While continuing work on a space telescope/starshade concept called the Aragoscope (after French optical scientist Francois Arago), Cash and team began testing an airborne starshade that could be observed by a telescope on the ground.
All of this could lead to serious results at Alpha Centauri. Cash hopes to use an airborne starshade no more than a meter across that will be observed by a balloon-lofted telescope several hundred kilometers away to probe the habitable zone of Alpha Centauri. “Anything you can do on ground, you should do on ground,” Cash explained. “If we can do it remotely with big telescopes, it’s not a key part of payload that actually goes to Alpha Centauri.”
I’m running out of time today, so I’ll start tomorrow with an Alpha Centauri observing platform called ACEsat, a dedicated space observatory, and move from there into some of the more speculative thoughts of the attendees on what we might find around these stars.
by Paul Gilster | Apr 12, 2016 | Missions |
Here on Centauri Dreams we often discuss interstellar flight in a long-term context. Will humans ever travel to another star? I’ve stated my view that if this happens, it will probably take several hundred years before we develop the necessary energy resources to make such a mission fit within the constraints of the world’s economy. This, of course, assumes the necessary technological development along the way — not only in propulsion but in closed-loop life support — to make such a mission scientifically plausible. I get a lot of pushback on that because nobody wants to wait that long. But overall, I’m an optimist. I think it will happen.
Let’s attack the question from another direction, though, and leave human passengers for a later date, as Yuri Milner’s Breakthrough Initiatives, aided by Stephen Hawking, is doing today in a New York news conference. What if we talk about unmanned missions? What if, in fact, the question is: How soon can we put a scientific payload past another star? Let’s not worry about decelerating — this will be a flyby mission. Let’s build it as soon as possible using every breakthrough technology we have at our disposal. How long would it take for that mission to be developed and flown?
Milner, a philanthropist and investor who was an early backer of Facebook, Twitter, Spotify and numerous Chinese tech companies, tells me his goal is to ‘give back to physics’ in developing just such a mission. Part of that giving back is the $100 million he has already put forward to support SETI, a ten-year project that will produce more telescope time for SETI than any other. Milner is also the founder of the Breakthrough Prize, issuing awards in physics, life sciences and mathematics. But in many respects this third Breakthrough Initiative is the most daring of all.
Time for the Stars
Breakthrough Starshot is an instrumented flyby of Alpha Centauri with an exceedingly short time-frame, assuming research and development proceed apace. Milner is putting $100 million into the mission concept, an amount that dwarfs what any individual, corporation or government has ever put into interstellar research. A discipline that has largely been the domain of specialist conferences — and in the scheme of things, not many of those — now moves into a research enterprise with serious backing.
Could an Alpha Centauri flyby mission be developed and launched within a single generation? I think it’s quite a stretch, but it’s the best-case scenario Milner mentioned in a phone conversation over the weekend. He’s enough of a realist (with a first-rate physics background) to know that the challenges are immense. Even so, he sees no deal-breakers.
Let’s walk through the case and see why he finds reason for optimism. “There are major advances that we can now turn to as we develop this proof of concept,” Milner says. “Twenty years ago, none of these things would have been available to far-thinking scientists like Robert Forward. But now we can put them to use and test their possibilities.”
If you’re thinking of an interstellar mission in the near-term, there is really only one choice of propulsion: The beamed sail. Sails have the advantage of known physics, laboratory experiment and actual deployment in space. We could talk about fusion for some indefinite point in the future, but at present, we don’t know how to do fusion even in massive installations on Earth, much less in the tight confines of a spacecraft engine. Interstellar ramjets are a far-future unknown — they may act more effectively as braking devices than engines, according to recent research. Antimatter is nowhere near readiness for propulsion, either in production methods or storage. Chemical rockets fall victim to the mass/ratio problem and are useless for fast interstellar journeys.
That leaves us with sails carrying very small payloads. To cross the 4.37 light years to the Centauri A and B system, Breakthrough Starshot proposes small spacecraft, taking advantage of advances in nanotechnology to reduce payload size. Think Moore’s Law and the reductions in size and cost that have accompanied the vast increases in micro-chip power. “Moore’s Law,” says Milner, “tells us that now is the time.”
StarChip is the Breakthrough Initiatives’ name for a payload measured not in kilograms but grams, a wafer that carries everything you would expect in a fully functional probe. ‘What was once a 300 gram instrument is is now available at three grams,” Milner continues. “What was 100 grams is now 0.5 grams. This is the trend we are riding.”
The StarChip payload includes cameras, power supply, communications equipment, navigation capabilities and photon thrusters. And it would be thrown across the interstellar gulf at 20 percent of the speed of light by a sail that is itself a miniaturized version of the sails Robert Forward used to discuss. Forget the thousand-kilometer sail (much less the continent-sized sails of the science fiction dreamer Cordwainer Smith). Milner’s team believes we can now talk in terms of a laser-driven lightsail that is no more than 4 meters across. This is actually smaller than the first deployed sail craft, the Japanese IKAROS, which boasts a sail measuring 14 meters to the side.
Advances in metamaterials and additional research should be able to produce, Milner believes, a 4 meter sail whose own weight is tallied in grams, and whose materials allow fabrication at a thickness of a few hundred atoms. A sail that small makes its own statement: Clearly, it’s not going to be under the beam for long, which means we need to focus a great deal of light on it for a very brief time. Lasers are another technology that benefits from rising power and falling cost. The trick here will be to create ‘phased arrays’ of lasers that can scale up to the 100 gigawatt level. A phased array involves not one but a group of emitters whose effective radiation pattern is reinforced in the desired direction by adjusting the phase of the signals feeding the antennae.
This is classic Bob Forward thinking rotated according to the symmetries of our new era. Milner aims for a beamer technology that is modular and scalable. And it fits into a larger infrastructure. Breakthrough Initiatives talks about bringing a ‘Silicon Valley approach’ to the problem of interstellar flight. Build a StarChip that can eventually be mass-produced at no more than the cost of an iPhone. For the Alpha Centauri mission, whenever it flies, is itself a proof of concept that could lead to multiple destinations. And if the cost can be driven as low as Milner believes, then we can think in terms of redundancy, with StarChips sent in large numbers to return a full characterization of any destination system. Assemble the light beamer and, as the technology matures, the cost of each launch falls.
These are ideas that are at once familiar but also exotic, for while Forward talked about enormous power stations in close solar orbit to power up his banks of lasers (and a huge Fresnel lens in the outer system to focus the beam), Milner thinks we can build a ground-based beamer at kilometer scale right here on Earth. I was startled at the idea — surely efficiency favors a space-based installation — but Milner’s point is that he thinks we can begin to launch interstellar craft before we have the technology to build the kind of power station Forward envisioned. If you’re serious about a launch within a few decades (again, it’s a best case scenario, and a dramatic one), then you build an Earth-based beamer and use adaptive optics to cancel out atmospheric effects.
Image: A wide-field view obtained with an Hasselblad 2000 FC camera by Claus Madsen (ESO), of a region around the Southern Cross, seen in the right of the image (Kodak Ektachrome 200, 70 min exposure time). Alpha Centauri is the bright yellowish star seen at the middle left, one of the “Pointers” to the star at the top of the Southern Cross. Although it appears here as a single ‘star,’ it is actually comprised of the G-class Centauri A, K-class Centauri B, and the M-dwarf Proxima Centauri. Credit: ESO/Claus Madsen. Original here.
All this will be subject to tightly focused research, which is what the $100 million is for, but what Milner hopes to see are nano craft delivered to orbit and then boosted on their way with a 30 minute laser ‘burn’ that, reaching 60,000 g’s, drives the sail to 20 percent of the speed of light. That makes for roughly a twenty year crossing to Alpha Centauri. With a craft this small, data return is highly problematic, and in fact I think it’s one of the biggest unanswered questions Breakthrough Starshot will have to face (well, this and the challenge of interstellar dust, and key questions related to sail design and the sail’s ability to stay on thee beam during acceleration). The sail is itself the antenna on a craft of this design, and Jim Benford told me in conversation that it will have to be shaped to one-micron precision. Even so, powering up the system to send imagery and data to Earth is going to be tricky. It will be fascinating to see what kind of solutions emerge as this research gets underway, and what alternative methods may be suggested.
Even so, and granting the cost reductions digital technology makes possible, Breakthrough Starshot embarks upon a multi-year research and engineering phase that will focus on building a mission infrastructure. Creating the actual mission will demand a budget comparable to the largest scientific experiments of our time. These are no small aspirations, but what drives them is something that interstellar studies have never had at their disposal: A dedicated, enthusiastic, well-funded effort with the participation of major scientists.
“We have an advisory board of twenty, including Freeman Dyson and other top scientists,” Milner added. “$100 million will be spent in coming years as we look toward concept verification. Multiple grants should flow from this, research and experiments. We need to complete the initial study and see if building a prototype, perhaps at a scale of 1/100, is then the next step.”
At the very least, we can expect the research behind this project to spin off numerous useful technologies, all of which should be applicable not only to star missions but to in-system exploration, along with, potentially, a kilometer-scale beamer that can double as a large telescope for astronomical observations. And while I doubt we can look at interstellar missions within the next few decades (I am open to being convinced otherwise), I believe that the timing for a fast flyby of Alpha Centauri will be considerably advanced by this work.
There is much to be said about all aspects of the Breakthrough Starshot concept, and as you would imagine, I’ll be covering this closely, beginning with a trip later this week to the Breakthrough Initiatives meeting in California. That meeting will have a large SETI component growing out of Milner’s prior commitment of another $100 million, which is already being translated into active observations at the Green Bank observatory in West Virginia. But as you can imagine, the Alpha Centauri mission will be under discussion as well as the research effort begins to be assembled. What spins out of this will keep us talking for a long time to come.
by Paul Gilster | Dec 4, 2015 | Deep Sky Astronomy & Telescopes |
A dedicated spacecraft just to investigate the Alpha Centauri system? I’ve been fascinated with the nearest stars since boyhood, so the ACEsat concept Ashley Baldwin writes about today would have my endorsement. But budgetary realities and practical mission planning might demand a larger instrument capable of studying more distant targets. Dr. Baldwin, a committed amateur astronomer, is consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK). His deep knowledge of telescope technologies has served us well in the past, and now takes us into the realm of mission planning beyond the James Webb Space Telescope.
by Ashley Baldwin
ACEsat is a revolutionary all silicon carbide, 45 cm telescope concept with a bespoke built in Phase Induced Amplitude Apodisation (PIAA) coronagraph designed to image planets (in five selected visible-wavelength bands from 400-700 nm) in the habitable zones of Alpha Centauri A and B. It was designed to take advantage of studies that show a 10-55% probability of a 0.5-2R Earth planet in these areas. Other objectives included determining exoplanet size, mass and orbit as well as imaging any “debris disk”.
The mission was conceived by a team from NASA Ames led by Ruslan Belikov and Eduardo Bendek, in conjunction with Northrop Grumman, and was recently unsuccessfully submitted for NASA Small Explorer funding (SMEX). Only circa $100 million for a lot of telescope, the design when compared with the ongoing WFIRST (Wide-Field Infrared Survey Telescope) mission highlights the critical issues of directly imaging exoplanets with coronagraph-based telescopes. These are small at present, but the science is central to the potentially large telescopes of the 2030s. As the 2020 Decadal NASA “roadmap” for the next decade approaches, the battle lines are being joined for what comes after JWST. Yes, that far ahead!
Fitting Alpha Centauri into the Roadmap
At present the various astrophysics subcommittees have got together with universities to make recommendations to the NASA Executive for consideration. In terms of telescopes, the main examples are a 10-12m UV, Visible and Near-Infrared “all rounder” telescope that is likely to be segmented like a big JWST and be fitted with high performance coronagraphs for exoplanet imaging (see below ). There is also a large, segmented Far-Infrared telescope like a direct descendent of JWST, an X-Ray telescope and a 4m “Habex” dedicated exoplanet telescope.
Image: Centauri A and B as captured by Cassini over the limb of Saturn. Credit: NASA/JPL.
Science definition groups are in the process of being formed to try and reach a consensus on what will be prioritised in the Decadal roadmap. All of these missions would be classed as “flagship” (multi-billion $), and if WFIRST does fly, as appears likely, the money for the 2020s will be spent before they arrive! This plan is thinking 15 years ahead. There is no guarantee that an exoplanet telescope will be selected, though interest in the field is burgeoning, with one third of papers submitted to journals last year on the topic. The presence of both a Far-Infrared telescope and an X-Ray telescope illustrates the influence some groups historically maintain. The two imaging missions are intertwined and illustrate the process of imaging exoplanets with internal occulters or “coronagraphs”.
The PIAA coronagraph, as usual, stands out in all domains barring average contrast reached. This device has a substantial heritage over the last decade in small exoplanet imaging concepts such as EXCEDE (Exoplanetary Circumstellar Environments and Disk Explorer), PECO (Pupil mapping Exoplanet Coronagraphic Observer) and most recently the Probe concept Exo-C, a smaller version of which ACEsat is based on. Simulations show how important “post processing” of the raw data received from the telescope is.
Image: An off axis telescope without and as ACEsat with a PIAA coronagraph. Credit: Wikimedia Commons.
Conventional methods will improve contrast by up to thirty times. This includes techniques that remove “speckles” or stray starlight within the telescope due to wavefront errors or mirror imperfections. “Angular differential imaging” removes these by rotating the image and subtracting it from the original non-rotated version. “Spectral differential imaging” takes advantage of subtle variations in the spectrum of reflected planet light. This will include the spectra of molecules in any atmosphere, which will be very different from back or foreground targets as well as exo-zodiacal light. Finally, when reflected off a planet, starlight tends to become more polarised, which differentiates it from the original starlight and background sources. Thus a polarimeter can separate out an exoplanetary signal. Ground-based telescopes used for exoplanet imaging, like the VLT, employ just such devices to run as part of an instrument package like SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) that also includes a coronagraph.
All of these methods are conventional and effective and, importantly, applied to the image after it has been taken or during the process of forming it. They are easy to do and don’t require multiple images. Compare this to the newest form of differential imaging, discussed below. These are ways of making the exoplanet stand out from the overall telescope field. That’s enough to take the telescope’s observations from “no image ” to “image”. The PIAA best contrast goes from just under 3×109 to 9×1011. In this case, that translates from seeing a large gas giant to seeing something Earth-sized. The post processing also takes some of the pressure of delivering contrast away from the coronagraph and its designers, too.
Image: Two habitable zones to work with in this close binary. What will we find around Centauri A and B?
How ACEsat Operates
ACEsat is a small, 45cm telescope, yet it can deliver contrasts down to 3×1011. How? Until recently there have been just two post processing techniques. The ACEsat team have cleverly created an additional technique called “Orbital differentiating imaging” (ODI). This uses an exoplanet’s orbit to identify it from the background “noise”. It is so potent that it is even resistant to exozodiacal light (dust from asteroids and comets that reflects light like planets, thus mimicking them — believed to be responsible for the “planet” seen shepherding the Fomalhaut protoplanetary disk) that can contaminate and ruin previous exoplanet imagery.
By way of comparison, ODI can improve contrast by a thousand times. The key is that to deliver ODI, one needs information on a good fraction of the exoplanet’s orbit. That’s why ACEsat had to take 2000 images of Alpha Centauri over a two year period, yet despite its size could deliver images AND spectroscopic characterisation. That approach requires a dedicated telescope, not the one year laid aside for WFIRST, though as can be seen, terrestrial planets could still be viewed — such is the potency of the PIAA versus other coronagraphs. The Hybrid Lyot coronagraph has been chosen for WFIRST so far as its average performance is better, and with limited time to justify inclusion, the coronagraph needs as many planets characterised as possible. Hence the list of known Exo-Jupiters that will dictate its use. Quantity over quality.
Image: From a presentation on ACEsat by Eduardo Bendek at AAS 2015 in Seattle, available online.
The other key feature of the PIAA is its “inner working angle” (IWA). This is the degree to which the parent starlight is compressed inwards as much as possible, concentrated by the PIAA, in order to create a dark disk around the star that will allow any exoplanets to stand out. The PIAA is the only coronagraph considered for WFIRST that has an IWA that would allow a planet in the habitable zone of a Sun-like star to be seen. The smaller the IWA the better. This also allows for smaller orbits, which of course require less time and images to determine, thus allowing ODI to come into play. The price for this precision is that the coronagraph is extremely sensitive to “jitter”, small unwanted movements caused by the steering reaction wheels, and thermal instabilities.
The WFIRST simulation data demonstrate how severe this can be and disruptive to successful exoplanet characterisation. It was because of thermal instabilities that silicon carbide was chosen as a construction material, as it is very stable to changes in temperature, defined by the “coefficient of thermal expansion”. Jitter is actively mitigated by the use of a “fast” tip/tilt mirror incorporated into the optical train that the coronagraph sits in, one that uses starlight rejected by the coronagraph to feed a computer that in turn controls the position of the mirror to correct any wavefront errors. Such is the importance of this part of the telescope that despite the presence of an on-board computer, “real time” modifications will still need to be made from the ground, all of which adds to the “operations costs” described later.
Wavefront errors arising from other sources are corrected in turn by an electromechanical, MEMS-deformable mirror (DM), also in the optical train and also feeding a guiding computer. The equivalent of active optics in a ground-based telescope. Imperfections in the telescope mirror can allow stray starlight in the firm of “speckles ” that can be removed this way.
Working with a Close Binary
With ACEsat, which is in the unusual situation of imaging a close binary system where light from one star will seep into planetary imaging of its neighbour, deformable mirrors managed by a “multi-star wavefront control” computer algorithm counteract this and in doing so open the door to imaging planets around the many binary systems close to the Solar System. All of these will reduce the telescope’s overall throughput, which demonstrates why it is so important not to throw away light in the coronagraph given there is so little to start with. Other conventional glass mirrors also possess such properties, but for a small telescope are prohibitively expensive and heavy. The initial ACEsat bid had the telescope in an “Earth trailing orbit”, drifting away at 0.2 AU per year but in a thermally stable, easy (cheap) orbit in direct contact with Earth. Traditionally such missions utilise the superior but more difficult to reach Earth/Sun Lagrange 2 point. Trade offs are a necessity when funds are low.
The other issue is the amount of light the coronagraph allows through to the all important spectrograph. ACEsat is an off-axis telescope in that its mirror is offset to focus its light to a point outside the telescope, so that there is no obstructive secondary mirror or coronagraph to waste precious incident light (up to 20% is obstructed in on-axis telescopes). The coronagraph thus runs down the side of the telescope and is actually embedded into it, which is possible with a smaller telescope with a less complicated mirror arrangement in its coronagraph, and embedding helps significantly reduce the damaging “jitter” described above.
Image: A simple illustration that illustrates the off-axis concept. Here the red line is the focal plane where spectrograph, imaging camera, polarimeter etc are located. Credit: Wikimedia Commons / Ashley Baldwin.
Inside the PIAA Coronagraph
The PIAA is the only coronagraph with near 100% throughput, thus allowing all light delivered by the telescope for characterisation in the spectrograph at the end of the “optical” train. The importance of this is that spectrographs unavoidably have considerably less than 100% throughput while some coronagraphs have only 20% throughput! A PIAA coronagraph in essence uses two sets of “aspheric” (curved at edges) mirrors to concentrate the Point Spread Function (PSF), or disk of light produced by a telescope when it images a point light source like a star. It removes the sharp diffraction rings that surround the central disk to create a dark area where reflected planetary light can be seen. In essence, the central starlight is slowed down whilst the extreme edge is allowed as normal, leaving a dark area around the edges.
A simple “stop” blocks off the Gaussian-like “peak” of starlight, creating a dark starlight-free zone around the PSF in which, with suitable contrast and light, an exoplanet image can be seen. The PIAA is unique amongst coronagraphs in allowing through most of the planet light, which is vital when one thinks just how faint this is. Up to ten billion times less than the star in the visible spectrum, though less as wavelengths increase into the infrared, which allows for lower imaging contrast but at the price of image resolution, which is derived from wavelength/telescope aperture.
ACEsat takes advantage of the PIAA design to actually embed it around the telescope secondary and tertiary mirrors, which feed the concentrated PSF starlight to an occulter at the first focal plane, where most of it is blocked and some is redirected to a computer that controls the deformable mirror “upstream” that then corrects any wavefront errors. This process helps create the dark area surrounding the central PSF described above, allowing planet light to pass unhindered through the optical train to the final focal point, and imaging via electron magnifying CCD (EMCCD) cooled to minus 85° C. This cooling prevents “dark current”, electrical current arising spontaneously within the pixels of the sensor array and causing them to activate in the absence of genuine photon stimulation.
The EMCCD sensor is now standard for visible imaging, with its individual pixels (1024×1024) releasing additional electrons when stimulated by a light photon in order to boost the final image/signal. The embedding of the PIAA around the telescope mirrors IWA unique design features that are only practical in smaller telescopes but do increase the telescope stability many fold. It is impossible to overstate just how important telescope stability is when imaging faint light sources light years away. Despite a “vibration source isolation system” that separates the telescope from its bus and its 23 fine-pointing reaction wheels, the transfer of some vibration or “jitter” is unavoidable, as is some degree of thermal instability, and this needs to be controlled to within 0.5 milli-arc seconds by tip/tilt mirror and related computer for a successful image to be formed.
Image: ACEsat hardware, from Eduardo Bendek’s presentation at AAS 2015.
The PIAA’s small IWA, though helpful in allowing planets to be imaged as near to the stars as possible, also increases the telescope’s sensitivity to jitter. The larger and more sensitive the scope, the greater this sensitivity. ACEsat is ahead of its time because of its small size, allowing greater stability. This approach is so far only possible up to 1.5m and the main drawback is that, although the planets can be characterised in terms of mass and orbit, detailed spectroscopic analysis, looking for atmospheric constituents for instance, is not possible. The larger versions simply allowing a wider radius of imaging. Even the 45 cm version of ACEsat was intended for an extended mission to image Sirius, Procyon and Altair, with the latter as far out as 16 light years. All of these are either genuine or visual binaries with a bright member, so fitting in well with ACEsat’s imaging strategy.
ACEsat clearly illustrates the incredible precision that would be needed on the kind of multi-metre aperture telescopes that would have the light gathering sensitivity to spectroscopically analyse exoplanets. What it does do, though, is introduce the new and potent orbital differentiating post-processing technique that could work just as well or indeed better with larger telescopes, whilst taking some of the contrast role off of the coronagraph. The need for prolonged imaging to obtain as much of an exoplanetary orbit as possible is evident, which requires dedication. ACEsat proposed observation periods run of up to 100 days at a time.
A Future for ACEsat?
There is clear synergy between ACEsat and WFIRST in that the ongoing research on the latter will improve the PIAA coronagraph further. The complex internal architecture of the National Reconnaissance Office (NRO) array that forms WFIRST is such that it effectively makes the telescope segmented, thus providing a comparison of a monolithic telescope versus the segmented design. If the hybrid Lyot remains the coronagraph of choice for WFIRST, should a revised version of ACEsat become reality then there will be opportunity to compare coronagraphs in the field and not just in the lab. The ACEsat design itself is a smaller version of the Exo-C Probe concept telescope that arose from NASA’s desire to look at exoplanet alternatives to WFIRST. As it is likely that any large space telescope of the future will be equipped with high performance coronagraphs, both WFIRST and ACEsat present perfect test beds.
Image: The monolithic 1.5m off axis Exo-C telescope that is near identical to the biggest version of ACEsat. The only difference is it uses a Hybrid Lyot coronagraph instead of a PIAA and it isn’t embedded in the telescope wall. Otherwise, this is a good representation of a big ACEsat or indeed any unobstructed coronagraph exoplanet imager. Image credit: Exo-C Concept Development page / JPL.
ACEsat was unsuccessful for its SMEX bid. Its design is robust and its construction material, silicon carbide composite, which is used for the entire satellite, has an extensive and successful heritage in Europe. The mission objective was purely Alpha Centauri, so if nothing was discovered for any reason, then the funding is wasted as the telescope at just 45cm was bespoke for our nearest stellar neighbour. It will interesting to see what modifications any funding resubmission has. For one, it is likely to be larger to allow characterisation of more than one star system and not have “all its eggs in one basket”. The design can be extrapolated to at least a 1.5m aperture with eighty times the light gathering of the original ACEsat and more than three times the resolution. This should allow a planets as far out as thirty light years to be imaged over the extended period only a dedicated telescope can provide.
The question is just how much bigger the available funding would allow this telescope to become. The now familiar Dawn mission was cancelled/postponed on several occasions amidst a NASA funding crisis but subsequently reinstated when it’s manufacturers, Orbital Sciences Corporation, agreed to build it at “cost”, so confident were they in the ability of its innovative design to deliver. A decision that more than paid off with great science and publicity.
Image: Potential targets for an expanded ACEsat. Credit: ESO.
It’s worth mentioning that an 80m ground based ELT, based in optical viewing conditions on the Antarctic domes, could be expected to characterise 40 of the nearest Earth-like planets. An enhanced ACEsat of, say, 1.5m stands respectable comparison to this and illustrates the huge advantage of imaging above the atmosphere. And at a fraction of the cost.
The only question that leaves unanswered is, “how big”? The largest funding pool available for such missions is the Explorers programme of TESS fame. It is about $200 million with facility to top up with any savings made on using a smaller than expected launcher – a distinct possibility for a small telescope made from ultra light material. Silicon carbide is 5-10 times less heavy than conventional materials and equivalently cheaper. Capping mission time to, say, just two years is a gamble that releases funds from mission operations and systems engineering (running costs !) into the telescope proper, on the assumption that even in this shorter time enough high quality science will be delivered to justify a significant extension. Consumables (fuel etc) reflect this by being far more than is necessary for the primary mission. If sensitive spectrographs like ESPRESSO come on line in the next two years, their precision work will provide a list of nearby targets to reduce exoplanet search times within previous mission time. The TESS spacecraft bus has a life expectancy of up to seven years or more despite a two year primary mission.
There will be no additional funds for exoplanet imaging next decade. If “ACEsat plus” is approved, that will be two exoplanet imaging missions to supplement PLATO and the ELTs, preparing hopefully for the big telescopes of the next decade. It may not be the USS Enterprise, but an enlarged ACEsat telescope would enable man to boldly go where no man has been before. Unlike a manned spacecraft, its ingenious design and novel construction material, silicon carbide, will cost a small fraction of conventional telescopes and fit within pre-existing funding envelopes. Its coronagraph, sophisticated wavefront control and processing algorithms will allow imaging of our nearest stellar neighbours and open up a whole new investigation area in binary systems.
By far and away the most exciting goal is looking for planets in the habitable zone of the two sun analogues of the Alpha Centauri system. In doing so it will be years ahead of its time, dovetail with the small exoplanet technology demonstrator element of WFIRST and be the forerunner of the large telescopes to come. A tremendous achievement if Ames and Northrop Grumman can pull it off. Dawn of a new age?
We live in interesting times. Parlous financial circumstances have not stopped the eighteen month placement of a lunar UV telescope by a Chinese team, while India is launching an X-ray telescope. Things are moving forward fast. The question is, just how far are this US team prepared to go to achieve their visionary goal and maintain an obvious technological lead? Success could lead to the first habitable exoplanet or at least a terrestrial planet in a habitable zone, so the rewards are potentially huge, while the costs are low.
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Further Reading
“From Cosmic birth to living Earths: A visionary space telescope for UV-Optical-Near IR Astronomy”: AURA, 2015. http://www.hdstvision.org/report/
“How to directly image a habitable planet around Alpha Centauri with a 30-45cm telescope”: AAS Belikov et al., 2015. http://arxiv.org/abs/1510.02479
“Space mission and instrument design to image the habitable zone of Alpha Centauri”: Bendek et al 2015. http://adsabs.harvard.edu/abs/2015AAS…22531102B
“The orbital design of Alpha Centauri Exoplanet Satellite (ACEsat) Heliocentric orbit design: Interplanetary small satellite conference, Beyond LEO”: Weston et al 2015. http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3228&context=smallsat
WFIRST-AFTA, STDT final report: Spergel et al 2013. http://wfirst.gsfc.nasa.gov/science/sdt_public/WFIRST-AFTA_SDT_Final_Report_Rev1_130523.pdf
“The search for habitable worlds 1: The viability of a star shade mission”: Turnbull et al, 2012. http://arxiv.org/abs/1204.6063
“Design study of an 8m monolithic mirror UV/Optical space telescope”: H Philip Stahl 2009. http://www.stsci.edu/institute/atlast/documents/stahl_8mDesign_SPIE2008.pdf
“WFIRST-AFTA Coronagraph instrument low order wavefront sensing and control”: Shi et al, JPL Cal Tech, 2014. http://home.strw.leidenuniv.nl/~kenworthy/_media/nospeckles:shi-afta-c_lowfsc.pdf
“The phase induced amplitude Apodization coronagraph”: Guyon et al, 2006. http://exep.jpl.nasa.gov/TPF/Coronagraph_PDFs/CWP2006_31_Guyon_pp157-161.pdf
“High contrast imaging and wavefront control with a PIAA coronagraph: Laboratory system validation”: Guyon et al 2009. http://arxiv.org/abs/0911.1307
“Low order wavefront sensing and control and Point Spread Function calibration for direct imaging of exoplanets”: JPL presentation : Guyon and Traub 2014. http://exep.jpl.nasa.gov/exopag/exopag10/presentations/Guyon_ExoPAG10_LOWFS.pdf
by Paul Gilster | Nov 9, 2015 | Exoplanetary Science |
Finding a habitable world around any one of the three Alpha Centauri stars would be huge. If the closest of all stellar systems offered a blue and green target with an atmosphere showing biosignatures, interest in finding a way to get there would be intense. Draw in the general public and there is a good chance that funding levels for exoplanet research as well as the myriad issues involving deep space technologies would increase. Alpha Centauri planets are a big deal.
The problem is, we have yet to confirm one. Proxima Centauri continues to be under scrutiny, but the best we can do at this point is rule out certain configurations. It appears unlikely, as per the work of Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), that any planet of Neptune mass or above exists within 1 AU of the star. Moreover, no ‘super-Earths’ have been detected in orbits with a period of less than 100 days. This doesn’t rule out planets around Proxima, but if they are there, so far we don’t see them.
Image: The Alpha Centauri stellar system, consisting of the red dwarf Proxima Centauri and the two bright stars forming a close binary, Centauri A and B. Credit: NASA.
Centauri B, the K-class star in close proximity to G-class Centauri A, was much in the news a few years back with the announcement of Centauri Bb, a candidate world announced by Swiss planet hunters. This is radial velocity work based on data gathered by the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. The signal that Xavier Dumusque and team drew out of the data was 0.5 meters per second, a fine catch if confirmed.
What we thought we had in Centauri Bb was a mass just a little over the Earth’s and an orbit of a scant 3.24 days. As the blistering first planet detected around one of the Centauri stars, it would be a significant find even if it’s a long way from the temperate, life-sustaining world we’d like to find further out. The putative Centauri Bb supported the idea that there might be other planets there, and we’ve known since the work of Paul Wiegert and Matt Holman back in the 1990s that sustainable habitable zone orbits are possible around both the primary Alpha Centauri stars.
But Centauri Bb has remained controversial since Artie Hatzes (Thuringian State Observatory, Germany), using different data processing strategies, looked at the same data and found a signal he considered too noisy, indicating that what might be a planet might also be stellar activity on Centauri B itself. Debra Fischer’s team at Cerro Tololo Inter-American Observatory has also been studying Centauri Bb using the CHIRON spectrometer but has not been able to confirm it. And while a transit search using the Hubble Space Telescope did find a promising lightcurve (about which more in a moment), it couldn’t confirm Centauri Bb.
Image: Of the three stars of Alpha Centauri, the dimmest, Proxima Centauri, is actually the nearest star to the Earth. The two bright stars, Alpha Centauri A and B form a close binary system; they are separated by only 23 times the Earth – Sun distance. This is slightly greater than the distance between Uranus and the Sun. The Alpha Centauri system is not visible from much of the northern hemisphere. The image above shows this star system and other objects near it in the sky. Credit/copyright: Akira Fujii / David Malin Images.
Now we have a new paper from Vinesh Rajpaul (University of Oxford) and colleagues that makes Centauri Bb look more unlikely than ever. Rajpaul praises the thorough work of Xavier Dumusque and the team at the Geneva Observatory, but notes that their attempts to filter stellar activity out of their data evidently boosted other periodic signals that had nothing to do with a planet. The signal grows out of the time sampling, or ‘window function,’ of the data.
What is left behind is what the paper calls ‘the ‘ghost’ of a signal’ that was present all along. The paper argues that when a signal is sampled at discrete times (and the Dumusque team had to use the La Silla instrument only when it was not otherwise booked), periodicities can be imposed on the signal. Rajpaul was able to simulate a star with no planets, generating synthetic data out of which the exact same 3.24-day planetary signal emerged. The problem is particularly acute when working with planetary ‘signals’ as weak as these. From the paper:
D12’s data set [i.e., the data gathered by Dumusque and team] was particularly pathological because the window function happened to contain periodicities that coincided with the stellar rotation period of α Cen B, and its first harmonic; when these signals were filtered out, the significance of the 3.24 d signal was preferentially boosted.
All this is going to be quite useful if it helps us refine our techniques for identifying small planets. Rajpaul proposes that his team will carry out a new study of the spurious but coherent signals that can emerge from noisy datasets that should help us learn how to mitigate the problem:
We alluded to a number of other tests we believe worth carrying out when considering the reliability of planet detections from noisy, discretely-sampled signals. These include using the same model used to detect the planet instead to fit synthetic, planet-free data (with realistic covariance properties, and time sampling identical to the real data), and checking whether the ‘planet’ is still detected; comparing the strength of the planetary signal with similar Keplerian signals injected into the original observations; performing Bayesian model comparisons between planet and no-planet models; and checking how robust the planetary signal is to datapoints being removed from the observations.
Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” We’re going to get a lot out of this investigation even though we lose Centauri Bb.
But back to that HST transit study run by Brice-Olivier Demory (University of Cambridge). I mentioned that it could detect no transit of Centauri Bb, which certainly fits with what we’ve just seen, but there was an interesting lightcurve suggesting a different possible planet, this one in an orbit that might range from 12 to 20 days. If this planet exists, radial velocity confirmation would be even more challenging than for Centauri Bb. Its signal, as Andrew LePage notes in The Discovery of Alpha Centauri Bb: Three Years Later, would be only half that of Centauri Bb.
LePage’s work at Drew ex Machina is definitive, and he has devoted a good deal of attention to Alpha Centauri. Here he explains why that second ‘planet’ is going to be so hard to spot:
Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations. In addition, since the radial velocity signature for this planet would be expected to be maybe half that of α Centauri Bb, this method has little likelihood of providing independent confirmation of this sighting any time soon. Once again, we will have to wait for a few more years for new telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launches in 2017 and may be capable of making the required observations of such a bright target.
Alpha Centauri is frustrating in many ways because you would expect the closest stellar system to have revealed more of its secrets by now. One of the problems, though, and a huge one, is that the angular separation (as viewed from Earth) of the primary Centauri stars has been decreasing as they move through their orbits. It won’t be until December of this year that they’ll reach minimum separation as seen from Earth. We’ll need to give Alpha Centauri a little time, in other words, before we can hope to get data on other possible worlds around Centauri B.
Image (click to enlarge): Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.
The Rajpaul paper is Rajpaul, Aigrain & Roberts, “Ghost in the time series: no planet for Alpha Cen B,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint). The Hatzes paper is “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, (2013) (preprint).
by Paul Gilster | Mar 30, 2015 | Exoplanetary Science |
Alpha Centauri continues to be a maddening and elusive subject for study. Two decades of radial velocity work on Centauri A and B have been able to constrain the possibilities — we’ve learned that there are no gas giants larger than Jupiter in orbits within 2 AU of either of the stars. But lower mass planets remain a possibility, and in 2012 we had the announcement of a planet slightly more massive than Earth in a tight orbit around Centauri B. It was an occasion for celebration (see Lee Billings’ essay Alpha Centauri and the New Astronomy for a glimpse of how that moment felt and how it fit into the larger world of exoplanet research).
But the candidate world, Centauri Bb, remains controversial, and for good reason. The work involved radial velocity methods at a level of precision that pushed our instruments to the limit. Andrew LePage explored the issues in Happy Anniversary α Centauri Bb?, where the question-mark tells the tale. Here he discusses the instrumentation involved in the 2012 work:
The first team to announce any results from their search was the European team using the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. They employed a new data processing technique to extract the 0.5 meter per second signal of α Centauri Bb out of 459 radial velocity measurements they obtained between February 2008 and July 2011. These radial velocity data had a measurement uncertainty of 0.8 meters per second and contained an estimated 1.5 meters per second of natural noise or “jitter” resulting from a range of activity on the surface of α Centauri B modulated by its 38-day period of rotation.
A planet with a 3.24 day orbital period was the result of an extremely low-amplitude signal, and subsequent analysis raised doubts about its validity, with Artie Hatzes (Thuringian State Observatory) finding that additional observations were needed to make sure we weren’t seeing noise in the data instead of a planet. Bear in mind that we also have Debra Fischer (Yale University) and team investigating Alpha Centauri Bb at Cerro Tololo Inter-American Observatory (CTIO) in Chile and a team at Mt. John University Observatory in New Zealand.
Image: An artist’s impression of the still unconfirmed α Centauri Bb, whose discovery was announced on October 16, 2012. The planet is the subject of a new transit search discussed below. (Credit: ESO/L. Calçada/Nick Risinger)
Now comes Brice-Oliver Demory (University of Cambridge), whose team has gone after a different kind of detection, working with the Hubble Space Telescope on a transit search of Centauri B in hopes of finding the signature of the controversial planet. Transits depend upon the alignment of star, planet and observer, so a null result doesn’t demonstrate that the planet doesn’t exist, but using 40 hours of observation, the team was able to rule out a transit of Centauri Bb under the published orbital parameters with a confidence level of 96.6 percent.
The story does have an intriguing coda in the form of a single 2013 event, one that lasted longer than expected for a Centauri Bb transit. The team worked through the possibilities of instrument error and other factors, as the paper explains:
We explore in the following the possibility that the July 2013 transit pattern is due to stellar variability, instrumental systematics or caused by a background eclipsing binary. We do not find any temperature or HST orbital dependent parameter, nor X/Y spectral drifts to correlate with the transit pattern. The transit candidate duration of 3.8 hours is 2.4 times longer than the HST orbital period, making the transit pattern unlikely to be attributable to HST instrumental systematics. As the detector is consistently saturated during all of our observations, we also find it unlikely that saturation is the origin of the transit signal.
Another source of confusion could be activity on the star itself, but the researchers do not see it as a factor:
…the duration of the transit candidate (3.8-hr) is not consistent with the stellar rotational period of 36.2 days…, to enable a spot (or group of spots) to come in and out of view. In such a case, star spots would change the overall observed flux level and produce transit-shape signals, as is the case for stars having fast rotational periods…
We are left with the possibility that this may have been an actual planetary transit with a different orbital period than described in the Centauri Bb discovery paper. Is it a second possible planet around Centauri B, one with an orbital period in the vicinity of 20 days? It will take follow-up photometric observations of an extremely tricky stellar system to tell us more.
In this New Scientist article on the Hubble observations, Demory mentions the possibility of a low-cost, perhaps crowd-funded mission, a small satellite whose sole purpose would be the kind of intensive Alpha Centauri ‘stare’ that busy instruments like Hubble haven’t time for. It’s an interesting idea, and would make for a KickStarter project in the range of $2 million. Says Demory: “Anyone fancy chipping in to find our nearest neighbours?”
The paper is Demory et al., “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri B b,” accepted at Monthly Notices of the Royal Astronomical Society (preprint). For a thorough analysis of the data involved in this work, see Andrew LePage’s essay Has Another Planet Been Found Orbiting Alpha Centauri B?
