by Paul Gilster | Apr 30, 2021 | Exoplanetary Science |
Why the renewed focus on astrometry when it comes to Alpha Centauri (a theme we saw as well in the previous post on ALMA observations from the surface)? One problem we face with other detection methods is simply statistical: We can study planets, as via the Kepler mission, by their transits, but if we want to know about specific stars that are near us, we can’t assume a lucky alignment.
Radial velocity requires no transits, but has yet to be pushed to the level of detecting Earth-mass planets at habitable-zone distances from stars like our own. This is why imaging is now very much in the mix, as is astrometry, and getting the latter into space in a dedicated mission has occupied a team at the University of Sydney led by Peter Tuthill for a number of years — I remember hearing Tuthill describe the technology at Breakthrough Discuss in 2016.
Out of this effort we get a concept called TOLIMAN, a space telescope that draws its title from Alpha Centauri B, whose medieval name in Arabic, so I’m told, was al-Zulm?n. [Addendum: This is mistaken, as reader Joy Sutton notes in the comments. It wouldn’t be until well after the medieval period — in 1689 — that the binary nature of Centauri A and B was discovered by Jesuit missionary and astronomer Jean Richaud. The name al-Zulm?n seems to be associated with an asterism that included Alpha Centauri, though I haven’t been able to track down anything more about it. Will do some further digging.]
The name is now fixed — a list of IAU-approved star names as of January 1st, 2021 shows Alpha Centauri A as Rigel Kentaurus and Centauri B as Toliman, so what goes around comes around. Centauri C’s name, according to the IAU, remains the familiar Proxima Centauri.
Image: This is Figure 5 from an online description of this work. Caption: Habitable zones of Alpha Cen A (left) and B (right) in green, along with dynamical stability boundary (red dashed line), 0.4” and 2.5” inner and outer working angles (IWA and OWA) of a small coronagraphic mission. The inset shows the Solar System to scale. Planetary systems of Alpha Cen A & B are assumed to be in the plane of the binary (the likeliest scenario) and orbits of hypothetical Venus-like, Earth-like, and Mars-like planets are shown. Credit: Tuthill et al.
The TOLIMAN Technology
The TOLIMAN mission is designed to use astrometry by means of a diffractive pupil aperture mask that in effect leverages the distortions of the optical system to produce a ‘ruler’ that can detect the changes in position that flag an Earth-mass planet in the Alpha Centauri system. Tuthill’s colleague Céline Bœhm presented the work at the recent Breakthrough Discuss online.
In TOLIMAN, we actually have an acronym — Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighborhood — but at least it’s one that’s applicable to the system under study. According to Boehm, there are certain advantages to working with nearby binaries. Astrometry normally uses field stars (stars in the same field as the object being studied) as references, making larger apertures an essential given the faintness of many of these reference points and their distance from the target star, and requiring a wide field of view.
But a bright binary companion 4 arcseconds away, as we find at Alpha Centauri, resolves the problem. Now a small aperture telescope can come into play because we have no need for field stars. The TOLIMAN concept puts a diffractive ‘pupil’ in front of the optics that spreads the starlight out over many pixels, a diffraction caused by features embedded in the pupil. Here stability is at play: We’re trying to eliminate minute imperfections and mechanical drifts in the optical surfaces that can create instabilities that compromise the underlying signal. The pupil mask prevents the detector from becoming saturated and reduces noise levels in the signal.
The astrometric ‘ruler’ used to measure the star’s position is thus created by the light of the stars themselves. The diffraction pattern cast by the pupil positions a reference grid onto the detector plane, which registers precise stellar locations. As a 2018 paper on the mission puts it: “Drifts in the optical system therefore cause identical displacements of both the object and the ruler being used to measure it, and so the data are immune to a large class of errors that beset other precision astrometric experiments.”
Narrow-angle astrometry, where the reference star is extremely close, makes for greater precision than wide-angle astrometry, and angular deviations on the sky are larger. Boehm makes the case that TOLIMAN’s diffractive pupil technology should be effective at finding Earth analogs around Centauri A and B, with a more advanced mission (TOLIMAN+) capable of finding rocky worlds around secondary targets 61 Cygni and 70 Ophiuchi.
Image: This is from Figure 3 of the online description of TOLIMAN referenced above. Caption: Left: pupil plane for TOLIMAN diffractive-aperture telescope. Light is only collected in the 10 elliptical patches (the remainder of the pupil is opaque in this conceptual illustration, although our flight design will employ phase steps which do not waste starlight). Middle: The simulated image observing a point-source star with this pupil. The region surrounding the star can be seen to be filled with a complex pattern of interference fringes, comprising our diffractive astrometric grid. Right: A simulated image of the Alpha Cen binary star as observed by TOLIMAN. Credit: Tuthill et al.
Astrometry and Earth-like Planets
Unlike both transit and radial velocity methods, the signal generated by a potential planet increases with the separation of planet and star, which makes astrometry an ideal probe for habitable zones. We should also throw into the mix the fact that radial velocity detections vary depending on the spectral type of the star, which is why we have had pronounced radial velocity success at Proxima Centauri and Barnard’s Star, but face a steeper climb in doing the same around G- and K-class stars. Astrometry becomes more sensitive with rising luminosity of the host star even as we wrestle with the tiny distortions that a planet like this would produce.
Both HIPPARCOS and Gaia have shown what can be accomplished by moving astrometry into space, and we saw yesterday that Gaia is likely to produce gas giants in the thousands, but the idea behind TOLIMAN is that astrometry now needs a dedicated mission to get down to rocky planets in temperate orbits. Tuthill and team have pointed to Alpha Centauri as an ideal target, one that would yield signals potentially 2 to 10 times stronger than the next best systems for study.
The researchers believe their work yields the optical innovations needed to acquire astrometric data on habitable-zone rocky planets with a telescope with an aperture of 10 cm. To explore the concept further, a CubeSat mission called TinyTol is scheduled to fly later this year, serving as a pathfinder to demonstrate astrometric detection. The full-scale TOLIMAN mission is funded and under development, with launch planned for a 3 year mission in 2023. TOLIMAN+ is envisioned as a 0.5-meter aperture telescope capable of sub-Earth class detections at Alpha Centauri and Earth-mass planets at both 61 Cygni and 70 Ophiuchi.
The most recent paper I know of on this work is Tuthill et al. “The TOLIMAN space telescope: discovering exoplanets in the solar neighbourhood,” SPIE Proceedings Vol. 11446 (13 December 2020). Abstract.
by Paul Gilster | Mar 15, 2024 | Exoplanetary Science |
Just a few weeks ago I wrote about stellar interactions, taking note of a concept advanced by scientists including Ben Zuckerman and Greg Matloff that such stars would make for easier interstellar travel. After all, if a star in its rotation around the Milky Way closes to within half a light year of the Sun, it’s a more feasible destination than Alpha Centauri. Of course, you have to wait for the star to come around, and that takes time. Zuckerman (UCLA), working with Bradley Hansen, has written about the possibility that close encounters are when a civilization will attempt such voyages.
I have a further idea along the lines of motion through the galaxy and its advantages to explorers, and it’s one that may not require tens of thousands of years of waiting. We’d like to get to another star system because we’re interested in the planets there, so what if an interstellar planet nudges into nearby space? I’ll ignore Oort Cloud perturbations and the rest to focus on a ‘rogue’ or ‘free-floating’ planet as the target of a probe, and ask whether we may not already have some of these in nearby space.
After all, finding free-floating planets – and I’m now going to start calling them FFPs, because that’s what appears in scientific papers on the matter – are hard to find. There being no reflected starlight to look for, the most productive way is to pick them out by their infrared signature, which means finding them when they’re relatively young. This is what Núria Miret Roig (University of Vienna) and team did a couple of years ago, working with data from the Very Large Telescope and other sources. Lo and behold, over one hundred FFPs turned up, all of them infants and still warm.
Image: The locations of 115 potential FFPs [free-floating planets] in the direction of the Upper Scorpius and Ophiuchus constellations, highlighted with red circles. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. Credit: ESO/N. Risinger (skysurvey.org).
But young FFPs are most likely to be found in star-forming regions, two of which (in Scorpius and Ophiuchus) were subjected to Miret Roig and team’s searches. What’s likely to amble along in our rather more sedate region is an FFP with enough years on it to have cooled down. The WISE survey (Wide-Field Infrared Survey Explorer) showed how difficult it is to pin down red dwarfs in the neighborhood, although it can be done. But even there, when you get down to L- and T-class brown dwarfs, uncertainty persists about whether you can find them. With planets the challenge is even greater.
Sometimes FFPs are found through microlensing toward the galactic core, but I don’t think we can rely on that method for finding a population of such worlds within, say, half a light year. Nonetheless, Miret Roig is not alone in pointing out that “there could be several billions of these free-floating planets roaming freely in the Milky Way without a host star.” Indeed, that number could be on the low side given what we’re learning about how these objects form. Given the excitement over ‘Oumuamua and other interstellar interlopers that may appear, I’m surprised that there hasn’t been more attention paid to how we might detect planet-sized objects near our system.
The ongoing search for Planet 9 demonstrates how difficult finding a planet outside the ecliptic can be right here at home. While pondering the best way to proceed, I’ll divert the discussion to rogue planet formation, which has always been central to the debate. Are the processes rare or common, and if the latter, do most stellar systems including our own, have the potential for ejecting planets? The last two decades of study have been productive, as we have refined our methods for modeling this process.
Recent work on the Trapezium Cluster in the Orion Nebula shows us how the catalog of FFPs is growing. The Trapezium Cluster is helpfully located out of the galactic plane, and there is a molecular cloud behind it that reduces the problems posed by field stars. I was startled to learn about this study (conducted at the European Space Agency’s ESTEC facility in the Netherlands by Samuel Pearson and Mark J McCaughrean) because of the sheer number of FFPs it turned up. Some 540 FFP candidates are identified here, ranging in mass from 0.6 to 13 Jupiter masses, although the range is an estimate based on the age of the cluster and our current models of gas giant evolution.
Image: A total of 712 individual images from the Near Infrared Camera on the James Webb Space Telescope were combined to make this composite view of the Orion Nebula and the Trapezium Cluster. Credit: NASA, ESA, CSA/Science leads and image processing: M. McCaughrean, S. Pearson, CC BY-SA 3.0 IGO.
What stopped me cold about this work is that among the 540 candidate FFPs, 40 are binaries. Two free-floating planets moving together without a star, and enough of them that we have to learn a new term: JuMBOs, for Jupiter-mass binary objects. How does that happen? There are even two triple systems in the data. Digging into the paper:
…we can compare their statistical properties…with higher-mass systems. The JuMBOs span the full mass range of our PMO [planetary-mass object] candidates, from 13 MJup down to 0.7 MJup. They have evenly distributed separations between ∼25–390 au, which is significantly wider than the average separation of brown dwarf-brown dwarf binaries which peaks at ∼ 4 au [42, 43]. However, as our imaging survey is only sensitive to visual binaries with separations > 25 au, we can not rule out an additional population of JuMBOs with closer orbits. For this reason we take 9% as a lower bound for the PMO multiplicity fraction. The average mass ratio of the JuMBOs is q = 0.66. While there are a significant number of roughly equal-mass JuMBOs, only 40% of them have q ≥ 0.8. This is much lower than the typical mass ratios for brown dwarfs, which very strongly favour equal masses.
That last line is interesting. Our FFP binary systems tend to have planets of distinctly different masses, which implies, according to the authors, that if the JuMBOs formed through core collapse and fragmentation – like a star – “then there must be some fundamental extra ingredient involved at these very low masses.” But the binary systems here go well below the mass where this formation method was thought to work. That opens up the ‘ejection’ hypothesis, with the planets forming in a circumstellar disk only to be ejected by gravitational interactions. So note this:
In either case, however, how pairs of young planets can be ejected simultaneously and remain bound, albeit weakly at relatively wide separations, remains quite unclear. The ensemble of PMOs and JuMBOs that we see in the Trapezium Cluster might arise from a mix of both of these “classical” scenarios, even if both have significant caveats, or perhaps a new, quite separate formation mechanism, such as a fragmentation of a star-less disk is required.
Ejection is a rational thing to look at considering that gravitational scattering is a well-studied process and may well have occurred in the early days of our own system. On the other hand, in star-forming regions like Trapezium the nascent systems are so young that this scenario may be less likely than the core-collapse model, in which the process is similar to star formation as a molecular cloud collapses and fragments. The open question is whether a scenario like this, which seems to work for brown dwarfs, is also applicable to considerably smaller FFPs in the Jupiter-mass range.
In any case, it seems unlikely that binary planets could survive ejection from a host system. As co-author Pearson puts it, “Nine percent is massively more than what you’d expect for the planetary-mass regime. You’d really struggle to explain that from a star formation perspective…. That’s really quite puzzling.”
All of which triggered a new paper from Fangyuan Yu (Shanghai Jiao Tong University) and Dong Lai (Cornell University), which takes an entirely different tack when it comes to formation of binary FFPs:
The claimed detection of a large fraction (9 percent) of JuMBOs among FFPs (Pearson & McCaughrean 2023) seems to suggest that core collapse and fragmentation (i.e. scaled-down star formation) channel plays an important role in producing FFPs down to Jupiter masses, since we do not expect the ejection channel to produce binary planets. On the other hand, (Miret-Roig et al. 2022) suggested that the observed abundance of FFPs in young star clusters significantly exceeds the core collapse model predictions, indicating that ejections of giant planets must be frequent within the first 10 Myr of a planetary system’s life.
Yu and Lai look at close stellar flybys as a contributing factor to FFP binary formation. If we’re talking about dense young star clusters, encounters between stars should be frequent, and there has been at least one study advancing the idea that bound binary planets could be the result of such flybys. Yu and Lai model two-planet systems to study the effects of a flyby on single and double-planet systems. Will an FFP result from a close flyby? A binary FFP? Or will the flyby star contribute a planet to the system it encounters?
These numerical experiments yield interesting results: The production rate of binary pairs of FFPs caused by stellar flybys is always less than 1 percent in their modeling, even when parameters are adjusted to make for tightly packed stellar systems. Directly addressing the JWST work in Trapezium and the large number of JuMBOs found there, Yu and Lai deduce that they cannot be caused by flybys, and because ejection scenarios are so unlikely, they see “a scaled-down version of star formation” at work “via fragmentation of molecular cloud cores or weakly-bound disks or pseudo-disks in the early stages of star formation.”
The matter remains unresolved, producing much fodder for future observations and debate. And while we figure out how to detect free-floating planets that may already be far closer than Proxima Centauri, we can create science fictional scenarios of journeys not just to a single rogue planet, but to a binary or even a triple system cohering despite the absence of a central star. I can only imagine how much Robert Forward, the man who gave us Rocheworld, would have enjoyed working with that.
The paper is Pearson & McCaughrean, “Jupiter Mass Binary Objects in the Trapezium Cluster” (preprint). The Miret-Roig paper is “A rich population of free-floating planets in the Upper Scorpius young stellar association,” published online at Nature Astronomy 22 December 2021 (abstract). The Fangyuan Yu & Dong Lai paper is, “Free-Floating Planets, Survivor Planets, Captured Planets and Binary Planets from Stellar Flybys,” submitted to The Astrophysical Journal (preprint).
by Paul Gilster | Nov 24, 2023 | Exoplanetary Science |
Although I think most astronomers have assumed Proxima Centauri was bound to the central binary at Alpha Centauri, the case wasn’t definitively made until fairly recently. Here we turn to Pierre Kervella (Observatoire de Paris), Frédéric Thévenin (Côte d’Azur Observatory) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève). We last saw Dr. Kervella with reference to a paper on aerographite as a sail material, but his work has appeared frequently in these pages, analyzing mission trajectories and studying the Alpha Centauri system. Here he and his colleagues use HARPS spectrographic data to demonstrate that we have at Centauri a single gravitationally bound triple system. This is important stuff; let me quote the paper on this work to explain why (italics mine):
Although statistical considerations are usually invoked to justify that Proxima is probably in a bound state, solid proof from dynamical arguments using astrometric and radial velocity (RV) measurements have never been obtained at a sufficient statistical significance level. As discussed by Worth & Sigurdsson (2016), if Proxima is indeed bound, its presence may have impacted planet formation around the main binary system.
This is a six-year old paper, but I want to return to it now because a new paper from the same team will tighten up its conclusions and slightly alter some of them. We’ve gone from resolving whether Proxima is bound to the A/B binary to pondering the issues involved in the dynamical history of this complex system. That in turn can inform the ongoing search for planets around Centauri A and B at least in terms of explaining what we might find there and how these two systems evolved. The original paper on this work lays out the challenges involved in tracing the orbit of the red dwarf. For HARPS is exquisitely sensitive to the Doppler shifts of starlight, and these data, obtained between 2004 and 2016, contain potential booby traps for analysis.
Image: Pierre Kervella, of the Observatoire de Paris/PSL.
Convective blueshift is one of these. We’re looking at the star’s spectral lines as we calculate its motion, and some of these are displaced toward the blue end of the spectrum because of the structure of its surface convection patterns. The lifting and sinking of hot internal gases has to be factored into the analysis and its effect nulled out. The spectral lines are displaced toward the blue, in effect a negative radial velocity shift, although the effect is stronger for hotter stars. In the case of Proxima, Kervella’s team finds a relatively small convective blueshift, though still one to be accounted for.
A similar though more significant issue is gravitational redshift, which occurs as photons climb out of the star’s gravity well. Here the effect is “an important source of uncertainty on the RV of Proxima” whose value can be established and corrected. How the astronomers went about making these corrections is laid out in a discussion of radial velocities that aspiring exoplanet hunters will want to read.
Image: Orbital plot of Proxima showing its position with respect to Alpha Centauri over the coming millenia (graduations in thousands of years). The large number of background stars is due to the fact that Proxima is located very close to the plane of the Milky Way. Credit: P. Kervella/ESO/Digitized Sky Survey 2/Davide De Martin/Mahdi Zamani.
Out of all this we learn that Proxima’s elliptical orbit around Centauri A and B’s barycenter extends from 800 billion kilometers when closest (periastron) to 1.9 trillion kilometers at apastron, its farthest distance, with an orbital period of approximately 550,000 years. The orbital phase is currently closest to apastron.
The Astronomy & Astrophysics site (this is the journal in which the paper above appeared) is currently down, so I’m quoting from the version of the paper on arXiv, which after noting that the escape velocity of Alpha Centauri at Proxima’s distance (545 +/- 11 m/s) is about twice as large as Proxima’s measured velocity, goes on to speculate in an intriguing way:
Proxima could have played a role in the formation and evolution of its planet (Anglada-Escudé et al. 2016). Conversely, it may also have influenced circumbinary planet formation around αCen (Worth & Sigurdsson 2016). A speculative scenario is that Proxima b formed as a distant circumbinary planet of the αCen pair, and was subsequently captured by Proxima. Proxima b could then be an ocean planet resulting from the meltdown of an icy body (Brugger et al. 2016). This would also mean that Proxima b may not have been located in the habitable zone (Ribas et al. 2016) for as long as the age of the αCen system (5 to 7 Ga; Miglio & Montalbán 2005; Eggenberger et al. 2004; Kervella et al. 2003; Thévenin et al. 2002).
The idea of Proxima b as a captured planet has not to my knowledge appeared anywhere else in the literature. I was fascinated, enough so that I dashed off a quick email to Dr. Kervella asking about this as well as the current status of the orbital calculations. And indeed, his response indicates new work in progress:
… we identified a mistake in our 2017 determination of the orbital parameters of Proxima. In the papier, they are expressed in the Galactic coordinate system, and the orbital inclination is thus not directly comparable to that of the Alpha Cen AB orbit. We are preparing a new publication with revised orbits and parameters for all three stars. The main difference is that now the orbital plane of Proxima is better aligned with that of AB. The gravitationally bound nature of Proxima with Alpha Cen AB is also strengthened, as we include new astrometry and radial velocities.
I’ll cover the new paper as soon as it appears. Dr. Kervella also observes that confirming a scenario of Proxima b as a captured planet would be difficult (Proxima b has ‘forgotten’ the history of its orbital evolution, as he puts it), meaning that working with astrometric data alone will not be sufficient. But the arrival of telescopes like the Extremely Large Telescope, now under construction in Chile’s Atacama Desert with first light planned for 2028, should signal a treasure trove of new information. A spectrum obtained by ELT could show us whether Proxima b is indeed an ocean planet.
The paper on Proxima Centauri’s orbit is Kervella, Thévenin & Lovis, “Proxima’s orbit around α Centauri,” Astronomy & Astrophysics Vol. 598 (February 2017), L7 (abstract/preprint).
by Paul Gilster | Nov 22, 2023 | Exoplanetary Science |
How extraordinary that the nearest star to Earth is actually a triple system, the tight central binary visually merged as one bright object, the third star lost in the background field but still a relatively close 13000 or so AU from the others. Humans couldn’t have a better inducement to achieve interstellar flight on the grounds of these stars alone. We get three stellar types: The G-class Centauri A, the K-class Centauri B, both of which are capable of hosting planets, perhaps habitable, of their own.
And then we have Proxima Centauri, opening up M-class red dwarf stars to close investigation, and we already know of a planet in the habitable zone there, adding to the zest of the venture. If extraterrestrial beings in a system like this would have even more inducement to travel, with another star’s planets perhaps as close to them as our own system’s worlds are to us, we humans are also spurred to undertake a journey, because 4.2 light years is a mere stone’s throw in the overall galactic distribution.
Image: The central binary at Alpha Centauri, with the two stars only resolved in the x-ray image. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zdenek Bardon/ESO.
I like this image, used by Dirk Schulze-Makuch to illustrate a recent popular science article, because it includes the Chandra X-Ray imagery. That’s how we can separate the central stars, which are at times nearly as close as Saturn is to the Sun while they orbit their common barycenter. Centauri Dreams readers will recognize Schulze-Makuch (Technical University Berlin and an adjunct professor at Washington State) not only as a prolific writer but the author of a host of scientific papers including many we’ve looked at in these pages. He’s played a valuable role in presenting astrobiological matters to the general public, part of the flowering of interstellar investigation that continues as we keep finding interesting worlds to explore.
If you’re wondering about Proxima Centauri’s location, the image below flags it. Credit: ESO/B. Tafreshi (twanight.org)/Digitized Sky Survey 2; Acknowledgement: Davide De Martin/Mahdi Zamani).
I like to keep an eye on what appears in the popular press from respected scientists, because they’re bringing credibility to matters that often get distorted by mainstream media attention (not to mention what happens on social media sites). We should always give a nod to scientists willing to explain their work and the broader issues involved given that kind of competition for the public’s attention. It’s interesting in this case to get Schulze-Makuch’s take on habitability at Alpha Centauri. He’s pessimistic about Proxima but is surprisingly bullish on Centauri A and B:
The other two stars in the system are believed to have planets, although they have not been confirmed. (A possible Neptune-size planet was reported in 2021 orbiting Alpha Centauri A at roughly the same distance as Earth orbits the Sun, but this could turn out to be a dust cloud instead.) The apparent lack of any brown dwarfs or gas giants close to Alpha Centauri A and B make the likelihood of terrestrial planets greater than it would be otherwise, at least in theory. The chances of a rocky, potentially habitable planet in our neighboring solar system might therefore be as high as 75 percent.
The Proxima Centauri problem is, of course, the X-ray flux, although Schulze-Makuch also considers tidal lock a distinct negative. The Chandra data (citation below) revealed a relatively benign influx of X-rays for Centauri A and B, making them fine hosts for life if it can develop there. But Proxima is deeply problematic, receiving an average dose of X-rays some 500 times greater than Earth’s, and some 50,000 times as great during periods of flare activity, which M-dwarfs are particularly prone to in their younger days.
Here the word ‘younger’ is a bit deceptive. Recall that this kind of star can live for several trillion years. That’s a bit humbling, considering that the universe itself is thought to be 13.8 billion years old. In that sense all M-dwarfs are ‘young.’
Just as we can zoom in via X-ray to see the central stars, we can also take a look at Proxima Centauri’s movements via spectroscopic data, which we’ll examine next time, along with a fascinating speculation on the origin of Proxima b.
For more on the X-ray environment at Alpha Centauri, see Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (January, 2018), 17 (abstract). The possibility of a ‘warm Neptune’ at Alpha Centauri is discussed in Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12, Article number: 922 (2021). Full text. We’ll be talking about this one a bit more in coming days.
by Paul Gilster | Nov 8, 2023 | Exoplanetary Science |
We’re building some remarkably large telescopes these days. Witness the Giant Magellan Telescope now under construction in Chile’s Atacama desert. It’s to be 200 times more powerful than any research telescope currently in use, with 368 square meters of light collection area. It incorporates seven enormous 8.5 meter mirrors. That makes exoplanet work from the Earth’s surface a viable proposition, but look at the size of the light bucket we need to make it work. Three mirrors like that shown below are now in place, and the University of Arizona’s Mirror Lab is building number 6 now.
Image: University of Arizona Richard F. Caris Mirror Lab staff members Damon Jackson (left) and Conrad Vogel (right) in the foreground looking up at the back of primary mirror segment five, April 2019. Credit: Damien Jemison; Giant Magellan Telescope – GMTO Corporation. CC BY-NC-ND 4.0.
Imaging an exoplanet from the Earth’s surface is complicated by the Rayleigh Limit, which governs the resolution of our optical systems and their ability to separate two point sources. Stephen Fleming showed the equation in his talk on super-resolution imaging at the Interstellar Research Group’s recent meeting in Montreal. I use few equations on this site but I’ll show this one because it’s straightforward and short:
θ = 1.22 * (λ / D)
Here λ is the wavelength and D is the diameter of the mirror. What this says is that there is a minimum angular separation (θ) that allows two point sources to be clearly distinguishable, which in terms of astronomy means we can’t pull useful information out of the image when they are closer than this. I’ve pulled the image below out of Wikipedia (in the public domain, submitted by Spencer Bliven).
Image: Two Airy disks at various spacings: (top) twice the distance to the first minimum, (middle) exactly the distance to the first minimum (the Rayleigh criterion), and (bottom) half the distance. This image uses a nonlinear color scale (specifically, the fourth root) in order to better show the minima and maxima.
Here we have another useful term: An Airy disk is a diffraction pattern that is produced when light moves through the aperture of a telescope system. Light diffracts – it’s in the nature of the physics – and the Airy disk is the best focused spot of light that a perfect lens with a circular aperture can make. We’re looking at light interfering with itself, so in the image, we have a central bright spot with surrounding rings of light and dark. The diffraction pattern depends upon the wavelength being observed and the aperture itself. This diffraction can be described as a point spread function (PSF) for any optical system, and essentially governs how tightly that system can be focused.
Bigger apertures matter as we try to deal with these limitations, and the Giant Magellan Telescope will doubtless make many discoveries, as will all of the coming generation of Extremely Large Telescopes. But when we want to see ever smaller objects at astronomical distances, we run into a practical problem. Nothing in the physics prevents us from building a ground-based telescope that could see an Earth-class planet at Alpha Centauri, but if we want details, Fleming notes, we would need a mirror 1.8 kilometers in diameter to retrieve a 40 X 40 pixel image.
The point of Fleming’s talk, however, was that we can use quantum technologies to nudge into the Rayleigh limitations and extract information about amplitude and phase from the light we do collect. That, in turn, would allow us to distinguish between point sources that are closer than what the limit would imply. The operative term is super-resolution, a topic that is growing in importance in the literature of optics, though to this point not so much in the astronomical community. This may be about to change.
Counter-intuitively (at least insofar as my own intuitions run), a multi-aperture telescope does a better job with this than a large single-aperture. Instead of a 3-meter mirror you use three 1.7 meter mirrors that are spaced out over, perhaps, an acre. This hits at mirror economics as well, because the costs of these enormous mirrors goes up more than exponentially. The more you can break the monolithic mirror into an array of smaller mirrors, you can add to the data gain but also sharply reduce the expense.
In terms of the science, Fleming noted that the point spread function spreads out when multiple smaller mirrors are used, and objects become detectable that would not be with a monolithic single mirror instrument. The technique in play is called Binary Spatial Mode Demultiplexing. Here the idea is to extract quantum modes of light in the imaging system and process them separately. The central mode – aligned with the point spread function of the central star – is the on-axis light. The off-axis photons, sorted into a separate detector, are from what surrounds the star.
So in a way we’re nudging inside the Rayleigh Limit by processing the light, nulling out or dimming the star’s light while intensifying the signal of anything surrounding the star. I’m reminded, of course, of all the work that has gone into coronagraphs and starshades in the attempt to darken the star while revealing the planets around it. In fact, some of the earliest research that convinced me to write my Centauri Dreams book was the work of Webster Cash out at the University of Colorado on starshades for this purpose, with the goal of seeing continents and oceans on an exoplanet. I later learned as well of Sara Seager’s immense contributions to the concept.
Thus far the simulations that have been run at the University of Arizona by Fleming’s colleagues have shown far higher detection rates for an exoplanet around a star using multi-aperture telescopes. In fact, there is a 100x increase in sensitivity for multi-aperture methods. This early work indicates it should be possible to identify the presence of an exoplanet in a given system with this ground-based detection method.
Can we go further? The prospect of direct imaging using off-axis photons is conceivable if futuristic. If we could create an image like this one, we would be able to study this hypothetical world over time, watching the change of seasons and mining data on the land masses and oceans as the world rotates. The possibility of doing this from Earth’s surface is startling. No wonder super-resolution is a growing field of study, and one now being addressed within the astronomical community as well as elsewhere.
