Centauri Dreams

In the ranks of exoplanets we can actually see, we can include the gas giant PDS 70b, a young world orbiting the K-dwarf PDS 70. Bear in mind that of the more than 4,000 exoplanets thus far catalogued, only about 15 have been directly imaged, an indication of how tricky this work is and how far we have to go as we contemplate imaging Earth-size planets and taking spectroscopic measurements of their atmospheres. The most recent PDS 70b work was performed with the Hubble instrument, and is to my knowledge the first direct detection of an exoplanet in the ultraviolet.

About 370 light years from Earth, PDS 70 (also known as V1032 Centauri) is a T Tauri star, a class of variables less than 10 million years old; this one appears to be no more than 5 million years old, and its largest planet is still in the process of building mass. The star is known to host at least two actively forming planets within its circumstellar disk of gas and dust, although only the larger is apparent in these UV observations.

Image: Hubble observations pinpoint planet PDS 70b. A coronagraph on Hubble’s camera blocks out the glare of the central star for the planet to be directly observed. Though over 4,000 exoplanets have been cataloged so far, only about 15 have been directly imaged to date by telescopes. The team’s fresh technique for using Hubble to directly image this planet paves a new route for further exoplanet research, especially during a planet’s formative years. Credit: Joseph DePasquale (STScI).

What we’re seeing here is planet formation as flagged by the radiation produced by hot gas falling onto the planet, allowing estimates of how fast PDS 70b is gaining mass. In its short (in astronomical terms) history, the world has grown to roughly 5 Jupiter masses. The accretion rate is now dwindling, and the team studying the planet, led by Yifan Zhou (University of Texas at Austin), describes it as being at the end of the formation process.

Two disks are actually at play here, the first being the circumstellar disk that is supplying growth materials in the form of gas and dust to the entire infant system; the second is the disk around PDS 70b, which is drawing off material from the larger disk, producing hot patches apparent at ultraviolet wavelengths on the forming planet. We can hypothesize that a system of satellites like Jupiter’s could form here as well.

Image: This illustration of the newly forming exoplanet PDS 70b shows how material may be falling onto the giant world as it builds up mass. By employing Hubble’s ultraviolet light (UV) sensitivity, researchers got a unique look at radiation from extremely hot gas falling onto the planet, allowing them to directly measure the planet’s mass growth rate for the first time. The planet PDS 70b is encircled by its own gas-and-dust disk that’s siphoning material from the vastly larger circumstellar disk in this solar system. The researchers hypothesize that magnetic field lines extend from its circumplanetary disk down to the exoplanet’s atmosphere and are funneling material onto the planet’s surface. The illustration shows one possible magnetospheric accretion configuration, but the magnetic field’s detailed geometry requires future work to probe. The remote world has already bulked up to five times the mass of Jupiter over a period of about five million years, but is anticipated to be in the tail end of its formation process. Credit: NASA, ESA, STScI, Joseph Olmsted (STScI).

According to the paper, the luminosity we see in these data is low compared to the mass accretion rate the researchers had expected through their shock modeling, leading to this note at the end of the paper. The hydrogen alpha (Hα) spectral line referenced below is the brightest hydrogen line in the visible spectral range:

The discrepancy suggests that either Hα production in planetary accretion shocks is more efficient than these models predicted, or we underestimated the accretion luminosity/rate. By combining our observations with planetary accretion shock models that predict both UV and Hα flux, we can improve the accretion rate measurement and advance our understanding of the accretion mechanisms of gas giant planets.

So when it comes to probing the accretion of giant planets, we’re making progress, as witness Hubble’s ability to examine ultraviolet emissions from this growing young world. This is crafty work that extends Hubble’s capabilities by a factor of five when it comes to observing a planet close to its star, a tribute to skillful image processing. Co-author Brendan Bowler, a college of Zhou at the University of Texas at Austin, adds:

“Thirty-one years after launch, we’re still finding new ways to use Hubble, Yifan’s observing strategy and post-processing technique will open new windows into studying similar systems, or even the same system, repeatedly with Hubble. With future observations, we could potentially discover when the majority of the gas and dust falls onto their planets and if it does so at a constant rate.”

Image: The European Southern Observatory’s Very Large Telescope caught the first clear image of a forming planet, PDS 70b, around a dwarf star in 2018. The planet stands out as a bright point to the right of the center of the image, which is blacked out by the coronagraph mask used to block the light of the central star. Credit: ESO, VLT, André B. Müller (ESO).

The paper is Yifan Zhou et al., “Hubble Space Telescope UV and Hα Measurements of the Accretion Excess Emission from the Young Giant Planet PDS 70 b,” Astronomical Journal Vol. 161, No. 5 (29 April 2021), 244. Abstract / Preprint.

Because we’ve just looked at how a carbon cycle like Earth’s may play out to allow habitability on other worlds, today’s paper seems a natural segue. It involves geology and planet formation, though here we’re less concerned with plate tectonics and feedback mechanisms than the composition of a planet’s mantle. At the University of British Columbia – Okanagan, Brendan Dyck argues that the presence of iron is more important than a planet’s location in the habitable zone in predicting habitability.

We learn that planetary mantles become increasingly iron-rich with proximity to the snow-line. In the Solar System, Mercury, Earth and Mars show silicate-mantle iron content that increases with distance from the Sun. Each planet had different proportions of iron entering its core during the planet formation period. The differences between them are the result of how much of their iron is contained in the mantle versus the core, for each should have the same proportion of iron as the star they orbit.

Core mass fraction (CMF) is a key player in this paper, defined as the extent of planetary core formation as a function of total planet mass. We start with similar precursor materials, but variations in the core mass fraction point to the differences in the silicate mantle and the surface crusts that should result on each rocky world. CMF itself “reflects the oxidation gradient present in the proto-planetary disc and the increasing contribution of oxidized, outer solar system material to planetary feedstocks.” We can use CMF as a marker for how a given planet will evolve.

Can we expect a similar growth in iron content in the mantles of planets around other stars? Evidently so. From the paper:

Oxidation gradients have been observed around other main sequence stars… and similar gradients in mantle iron contents are thus expected in other planetary systems possessing rocky differentiated planets… Consequently, even if each rocky body in a multi-planetary system forms from similar precursor material, variations in their core mass fraction will generate silicate mantles and derivative surface crusts that exhibit distinct compositional and petrophysical differences. Hence, variations in CMF may have a disproportionate role in determining a planet’s geological evolution and its future habitability.

Image: Brendan Dyck (University of British Columbia – Okanagan) is using his geology expertise about planet formation to help identify other planets that might support life. Credit: NASA/Goddard Space Flight Center.

To explore how core formation influences both the thickness and composition of a planet’s crust, Dyck and team developed computer models simulating mantle and crust production in planets through a range of core mass fractions. As we saw on Tuesday, Earth’s CMF is 0.32, while Mars’ is 0.24. Dyck’s models investigate core mass fractions between 0.34 and 0.16.

So here we have some interesting observables to juggle. The modeling shows that a larger core points to thinner crusts; smaller cores produce thicker crusts that are more iron-rich, along the model of Mars. And now we circle back to plate tectonics, which is dependent upon the thickness of the planetary crust, remembering that plate tectonics is thought to be critical for a carbon cycle that can support life. The conclusion is apparent: We may well find numerous planets located within the habitable zone whose early formation history makes them unable to support water on the surface.

Adds Dyck:

“Our findings show that if we know the amount of iron present in a planet’s mantle, we can predict how thick its crust will be and, in turn, whether liquid water and an atmosphere may be present. It’s a more precise way of identifying potential new Earth-like worlds than relying on their position in the habitable zone alone.”

These conclusions again point to the critical nature of chemical composition in stellar systems, which is a key area of research made feasible by new instruments like the James Webb Space Telescope. Assuming a (fingers crossed) safe launch and deployment, JWST should be able to measure the amount of iron present in exoplanetary systems, which will offer clues as to whether life is possible there.

The paper is Dyck et al., “The effect of core formation on surface composition and planetary habitability,” in process at Astrophysical Journal Letters (preprint).

Earth’s long-term carbon cycle is significant for life because it keeps carbon in transition, rather than allowing it to accumulate in its entirety in the atmosphere, or become completely absorbed in carbonate rocks. The feedback mechanism works over geological timescales to allow stable temperatures as CO₂ cycles between Earth’s mantle and the surface. As a result, we have carbon everywhere. 65,500 billion metric tons stored in rock complements the carbon found in the atmosphere and the oceans, as well as in surface features including vegetation and soil. It’s a long-term cycle that can vary in the short term but be stabilizing over geological time-frames.

The Sun has increased in luminosity substantially since Earth’s formation, but the long-term carbon cycle is thought to be the key to maintaining temperatures on the surface suitable for life. Does it exist on other planets? It’s an open question, as astronomer Mark Oosterloo (University of Groningen, The Netherlands) points out:

“We don’t know if there are any other planets at all with plate tectonics and a carbon cycle. In our solar system, the Earth is the only planet where we have found a carbon cycle. We hope that our model can contribute to the discovery of an exoplanet with a carbon cycle, and therefore, possibly life.”

Image: This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. Credit: Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.

Oosterloo is lead author of a paper that has just appeared in Astronomy & Astrophysics. Working with researchers at SRON (Netherlands Institute for Space Research) and Vrije Universiteit Amsterdam, the scientist has developed a model designed to analyze whether or not a carbon cycle can emerge on an exoplanet, varying its mass, core size and the amount of radioactive isotopes it contains.

Quite a lot goes into determining how the feedback mechanisms of a carbon cycle work, enough so that our problems in observing exoplanets swim into sharp relief — we’re usually limited to mass and radius measurements along with a degree of atmosphere characterization on those worlds where we can deploy methods like transmission spectroscopy, viewing the light of a star as it is filtered by a planet’s atmosphere.

But we’re learning about the composition of the planets slowly but surely. The authors cite Kepler-452b, a world whose interior has been shown to have a larger fraction of rock than Earth, which would affect the chemical structure of the interior. That in turn affects the amount and type of volatiles outgassed into the atmosphere.

To go beyond this, the authors investigate how planetary interiors of different composition affect long-term carbon cycling as enabled by plate tectonics. This involves not just the relative abundances of radioactive isotopes, the size of the planet’s core and its mass, but also into the evolution of CO₂ in the atmosphere. The team’s two-component model, connecting mantle convection to the emergence of a long-term carbon cycle, has plate tectonics at its center — in fact, the paper refers to mean plate speed as “the key coupling variable between the two models.”

Among the findings thus far is that the cooling of a planet’s mantle (and its effect on the tectonic plate speed) produces gradually declining CO₂ levels over time as outgassing slowly decreases. And note the effects of varying a planet’s internal heating:

A long-term carbon cycle driven by plate tectonics could operate efficiently on planets with amounts of radiogenic heating in their mantles different from Earth. However, planets with their mantles enriched in radioactive isotopes with respect to Earth, may favor the development of warmer climates resulting from a more CO₂ rich atmosphere. This is in particular the case for a planet with a higher thorium abundance. In addition, the carbon cycle operates more efficiently on planets rich in radioactive isotopes, motivating the characterization of planetary systems around stars whose atmospheres are rich in thorium or uranium.

A long-term carbon cycle regulated by plate tectonics, say the authors, may not operate on planets with a core mass fraction greater than 0.8 (for reference, Earth’s core mass fraction is 0.32 of the planetary mass; that of Mars is 0.24). Here the value of mass/radius measurements on Earth-sized planets becomes apparent. And it was surprising to me to learn that the effects of carbon cycling can be felt relatively quickly. In the authors’ models, equilibrium is achieved between 100 and 200 million years. In other words, if plate tectonics is operational, an exoplanet does not have to be old to have its atmosphere affected by a long-term carbon cycle.

Image: An artist’s impression of an Earth-like exoplanet. Can we develop the tools to establish the presence of a carbon cycle on such worlds? Credit: NASA.

The significance of this paper in my estimation is that it shows that carbon cycling can work as a regulator of climate for planets with a wide range of masses, core sizes and radioactive isotope abundances. As noted above, there is more efficient carbon cycling on planets with a high level of thorium or uranium in their mantles. That would imply that mapping these elements as found in their host stars would be a useful observational tool in exploring potential habitability of planets in a given stellar system.

The paper is Oosterloo, et al., “The role of planetary interior in the long-term evolution of atmospheric CO2 on Earth-like exoplanets,” Astronomy & Astrophysics Vol. 649, A15 (3 May, 2021). Abstract / Preprint.

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.

When it comes to finding planets around Centauri A and B, the method that most intrigues me is astrometry. At the recent Breakthrough Discuss sessions, Rachel Akeson (Caltech/IPAC) made the case for using the technique with data from the Atacama Large Millimeter Array (ALMA). My interest is piqued by the fact that so few of the more than 4300 known exoplanets have been discovered using astrometry, although astronomers were able in 2002 to characterize the previously known Gliese 876 using the method. Before that, numerous reported detections of planets around other stars, some going back to the 18th Century, have proven to be incorrect.

But we’re entering a new era. ESA’s Gaia mission, launched in 2013, is likely to return a large horde of planets using astrometry as it creates a three-dimensional map of star movement in the Milky Way. Dr. Akeson’s case for using ALMA to make detections on the ground is robust, despite the challenges the method presents. She points out that if we viewed the Solar System from a distance of 10 parsecs, Jupiter’s impact on the movement of our star would be 500 microarcseconds (µas), which works out to 1.4 X 10^-7 degrees. Keep that figure in mind.

Astrometry is about measuring the movement of a star’s position on the sky, and it has been put to good use for a long time in identifying binary star systems. As the star’s position changes over time, the gravitational pull of an orbiting planet should likewise be revealed. This is something like the well established radial velocity technique for planet detection, but it operates within the plane of the sky instead of along the line of sight (radial velocity measures the Doppler shift in the star’s light as it is pulled alternately toward and then away from Earth along the line of sight). The beauty of astrometry is that it complements radial velocity by being best suited to planets with a wide separation from their star.

Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. This technique can also be used to identify planets around a star by measuring tiny changes in the star’s position as it wobbles around the center of mass of the planetary system. Credit: ESA.

Back to Gaia for a moment. It’s a mission that has given astrometry a dazzling upgrade, offering 10 to 20 µas performance for a large sample of observed stars, making the upcoming release of its exoplanet catalog an event that should vastly enlarge our statistical understanding of planetary systems. Remember that figure from Jupiter as seen from 10 parsecs — the movement of the Sun is 500 µas. Gaia should identify tens of thousands of exoplanets out to 1600 light years from the Sun, all of this by tracking minute changes in the stars’ position. But for the nearest star system, Gaia is far less effective because of Alpha Centauri’s brightness.

Enter ALMA. 5000 meters up on the Chajnantor Plateau in Chile’s Atacama desert, the site offers 66 antennas with baseline lengths of 150 meters to 16 kilometers. At the longest baseline, the resolution is 12 milliarcseconds (one thousandth of an arcsecond, abbreviated mas). We’ve looked often in these pages at ALMA studies of protoplanetary disks (Akeson refers to the famous image of HL Tau, shown below, with the roughly one million year old disk dividing into clearly visible rings and gaps).

With ALMA, the proximity of Centauri A and B becomes an advantage. The relative positions of the two stars can be measured, as opposed to having to compare them to reference stars in other fields, meaning that the precision of the measurement is greatly increased. To be sure, this remains a tough measurement — in 2015 observations, position uncertainties were in the range of 10 milliarcseconds, a figure that needs to be reduced to hundred of microarcseconds.

Can ALMA handle this level of precision? Akeson’s team requested three pairs of observations in 2018-2019, with two achieved and the third incomplete, a dataset that will be complemented with future observations to identify deviations in the orbital motions of these stars. The need for high resolution has meant that only a few of the configurations, using the longest baselines between ALMA antennas, can be used, which puts limits on the observing time available.

Image: ALMA image of the protoplanetary disc around HL Tauri – This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO.

The highest resolution the team has yet achieved is in the August 2019 dataset. Here the absolute position of the stars was limited to 3 milliarcseconds (atmospheric noise is a factor here), and their relative separation showed uncertainties of 300 µas. Out of all this the plan is to use ALMA’s astrometrical data along with archival data (this includes radial velocity) to tighten up constraints on the orbits of both stars. The data allowed the team to improve our calibration of the stars’ masses by 2-3 percent. From a paper (citation below) describing the ALMA work, illustrating how data from numerous sources play into this effort:

The combination of historical measurements of ? Cen A-B position angle and separation, more recent PRV [precision radial velocity] measurements from HARPS, and absolute astrometry from Hipparcos, and ALMA yields improved ? Cen A and B orbital elements, system proper motion and parallax, and component masses.

But I also noticed, from the same paper, this intriguing bit about Proxima Centauri, whose status as bound or unbound to the central binary has remained problematic. The ALMA work seems to resolve the issue:

The significance of the gravitational link between AB and Proxima has therefore increased over the last three years from 4.4? in (Kervella et al. 2017b), to 5.5? in Kervella et al. (2019) to 8.3? in the present work (< 10?15 false alarm probability). Proxima becomes a yet more valuable check on lower main sequence stellar modeling…

Leading to this:

Our accurate determination of the ? Cen AB system parallax, RV [radial velocity], and proper motion, when compared to those values for Proxima Centauri, confirms that the three stars constitute a bound system.

All this is obviously painstaking work, and in the way of astronomical observation, it allowed the scientists to now use the tightened orbital information as a baseline for their continued search for deviations caused by planets. Akeson argues that the highest resolution configuration of the ALMA antennas will allow a resolution 2.5 times higher than the August 2019 data, which would take ALMA into the 100 microarcsecond range. As the paper notes:

Our estimates of ALMA’s relative astrometric precision suggest that we will ultimately be sensitive to planets of a few 10s of Earth mass in orbits from 1-3 AU, where stable orbits are thought to exist.

We may be closing in on a potential planet like Kevin Wagner’s tentatively identified C1. Here we can see the synergy between detection methods, with direct imaging (the subject of multiple proposals) exploring the same space.

But both these methods play off radial velocity studies as well. Remember that astrometric data is best suited to planets in wider orbits than radial velocity. While the latter continue and the ALMA work pushes ahead, we have imaging possibilities through the James Webb Space Telescope to add to the mix. The observational ring around Centauri A and B, then, is tightening as astrometry yields more refined orbital parameters for both stars. We can hope for exoplanet discovery here that can be confirmed in short order through multiple approaches.

For more on the ALMA work on Alpha Centauri, see Akeson et al., “Precision Millimeter Astrometry of the ? Centauri AB System,” in process at the Astrophysical Journal (preprint).