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Imaging Alpha Centauri’s Habitable Zones

We may or may not have imaged a planet around Alpha Centauri A, possibly a ‘warm Neptune’ at an orbital distance of roughly 1 AU, the distance between Earth and the Sun. Let’s quickly move to the caveat: This finding is not a verified planet, and may in fact be an exozodiacal disk detection or even a glitch within the equipment used to see it.

But as the paper notes, the finding called C1 is “is not a known systematic artifact, and is consistent with being either a Neptune-to-Saturn-sized planet or an exozodiacal dust disk.“ So this is interesting.

As it may be some time before we can make the call on C1, I want to emphasize the importance not so much of the possible planet but the method used to investigate it. For what the team behind a new paper in Nature Communications has revealed is a system for imaging in the mid-infrared, coupled with long observing times that can extend the capabilities of ground-based telescopes to capture planets in the habitable zone of other nearby stars.

Lead author Kevin Wagner (University of Arizona Steward Observatory) and colleagues describe a method showing a tenfold improvement over existing direct imaging solutions. Wavelength is important here, for exoplanet imaging usually works at infrared wavelengths below the optimum. Wagner points to the nature of observations from a warm planetary surface to explain why the wavelengths where planets are brightest can be problematic:

“There is a good reason for that because the Earth itself is shining at you at those wavelengths. Infrared emissions from the sky, the camera and the telescope itself are essentially drowning out your signal. But the good reason to focus on these wavelengths is that’s where an Earthlike planet in the habitable zone around a sun-like star is going to shine brightest.”

With exoplanet imaging up to now operating below 5 microns, where background noise is low, the planets we’ve been successful at imaging have been young, hot worlds of Jupiter class in wide orbits. Let me quote from the paper on this as well:

Their high temperatures are a remnant of formation and reflect their youth (~1–100 Myr, compared to the Gyr ages of typical stars). Imaging potentially habitable planets will require imaging colder exoplanets on shorter orbits around mature stars. This leads to an opportunity in the mid-infrared (~10 µm), in which temperate planets are brightest. However, mid-infrared imaging introduces significant challenges. These are primarily related to the much higher thermal background—that saturates even sub-second exposures—and also the ~2–5× coarser spatial resolution due to the diffraction limit scaling with wavelength. With current state-of-the-art telescopes, mid-infrared imaging can resolve the habitable zones of roughly a dozen nearby stars, but it remains to be shown whether sensitivity to detect low-mass planets can be achieved.

Getting around these challenges is part of what Breakthrough Watch is trying to do via its NEAR (New Earths in the Alpha Centauri Region) experiment, which focuses on the technologies needed to directly image low-mass habitable-zone exoplanets. The telescope in question is the European Southern Observatory’s Very Large Telescope in Chile, where Wagner and company are working with an adaptive secondary telescope mirror designed to minimize atmospheric distortion. That effort works in combination with a light-blocking mask optimized for the mid-infrared to block the light of Centauri A and then Centauri B in sequence.

Remember that stable habitable zone orbits have been calculated for both of these stars. Switching between Centauri A and B rapidly — as fast as every 50 milliseconds, in a method called ‘chopping’ — allows both habitable zones to be scrutinized simultaneously. Background light is further reduced by image stacking and specialized software.

“We’re moving one star on and one star off the coronagraph every tenth of a second,” adds Wagner. “That allows us to observe each star for half of the time, and, importantly, it also allows us to subtract one frame from the subsequent frame, which removes everything that is essentially just noise from the camera and the telescope.”

Among possible systematic artifacts, the paper notes the presence of ‘negative arcs’ due to reflections that are introduced within the system and must be eliminated. The image below shows the view before the artifacts have been removed and a second after that process is complete.

Image: This is Figure 2 from the paper. Caption: a high-pass filtered image without PSF subtraction or artifact removal. The α Centauri B on-coronagraph images have been subtracted from the α Centauri A on-coronagraph images, resulting in a central residual and two off-axis PSFs to the SE and NW of α Centauri A and B, respectively. Systematic artifacts labeled 1–3 correspond to detector persistence from α Centauri A, α Centauri B, and an optical ghost of α Centauri A. b Zoom-in on the inner regions following artifact removal and PSF subtraction. Regions impacted by detector persistence are masked for clarity. The approximate inner edge of the habitable zone of α Centauri A13 is indicated by the dashed circle. A candidate detection is labeled as ‘C1’. Credit: Wagner et al.

Over the years, we’ve seen the size of possible planetary companions of Centauri A and B gradually constrained, and as the paper notes, radial velocity work has excluded planets more massive than 53 Earth masses in the habitable zone of Centauri A (by comparison, Jupiter is 318 Earth masses). The constraint at Centauri B is 8.4 Earth masses, meaning that in both cases, lower-mass planets could still be present and in stable orbits. We already know of two worlds orbiting the M-dwarf Proxima Centauri.

You can find the results of the team’s nearly 100 hours of observations (enough to collect more than 5 million images) in the 7 terabytes of data now made available at http://archive.eso.org. Wagner is forthcoming about the likelihood of the Centauri A finding being a planet:

“There is one point source that looks like what we would expect a planet to look like, that we can’t explain with any of the systematic error corrections. We are not at the level of confidence to say we discovered a planet around Alpha Centauri, but there is a signal there that could be that with some subsequent verification.”

A second imaging campaign is planned in several years, which could reveal the same possible exoplanet at a different part of its modeled orbit, with potential confirmation via radial velocity methods. From the paper:

The habitable zones of α Centauri and other nearby stars could host multiple rocky planets–some of which may host suitable conditions for life. With a factor of two improvement in radius sensitivity (or a factor of four in brightness), habitable-zone super-Earths could be directly imaged within α Centauri. An independent experiment (e.g., a second mid-infrared imaging campaign, as well as RV, astrometry, or reflected light observations) could also clarify the nature of C1 as an exoplanet, exozodiacal disk, or instrumental artifact. If confirmed as a planet or disk, C1 would have implications for the presence of other habitable zone planets. Mid-infrared imaging of the habitable zones of other nearby stars, such as ε Eridani, ε Indi, and τ Ceti is also possible.

It’s worth keeping in mind that the coming extremely large telescopes will bring significant new capabilities to ground-based imaging of planets around nearby stars. Whether or not we have a new planet in this nearest of all stellar systems to Earth, we do have significant progress at pushing the limits of ground-based observation, with positive implications for the ELTs.

The paper is Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12: 922 (2021). Abstract / full text.

{ 29 comments… add one }
  • Michael February 11, 2021, 12:09

    Could it be a reflection off a dust ring, maybe orbiting a planet, with the light from Alpha B.

    • Andrei February 11, 2021, 13:38

      IMO the geometry is not quite right to make a dust ring the more likely candidate here. If we had a star with the dust ring edge on, it would brighten quite some on the sides, as there would be more particles / dust in our line of sight. In this case I am more inclined to think it’s a planet.
      It’s prudent that they point out the possibility though.

  • Mike Serfas February 11, 2021, 12:52

    I’m curious what the current thinking is on the habitable zone for moons. Alpha Centauri A is brighter than the Sun, and the candidate planet is at “about” 1 AU. Would the smaller size of a moon and (for inner moons) perhaps some shadowing by the larger planet bring that habitable zone inward a little to make up for the luminosity difference?

    The potential for people, sensu lato, hopping between many small warm moons with varying characteristics invites some interesting fictional scenarios.

    • FrankH February 11, 2021, 18:08

      A “moon” around a large planet in a star’s habitable zone would have to be at least about half the size of the Earth; the combination of an Earth-like temperature and low escape velocity will make it difficult for anything smaller to hold onto an atmosphere over geological time.

      A mini-Neptune or super-Earth in Alpha’s HZ would be bad news, unless the HZ is packed with Earth sized worlds.

      The Alpha Centauri system is also somewhat evolved, so the HZ may be on the move, outwards. So undetected planets that started in the inner HZ may no longer be in it.

      • Mike Serfas February 12, 2021, 16:44

        @FrankH: That certainly is a good objection, but there are some questions in my mind. The solar wind from Alpha Centauri could strip atmosphere from a small body, but not from a Neptune. So I’m thinking that the majority of molecules lost from a moon’s atmosphere should remain in orbit around the planet. Additional hits with solar wind should knock the molecules out of orbit or into the planet with some probability, but how often does that happen before they fall back onto one of the moons? (I have the image of Iapetus in mind, though that is quite a leap) If I’m feeling particularly optimistic I could imagine that a captured ice moon, losing water from an ocean hundreds of kilometers deep, might surround several rocky moons with secondary oxygen atmospheres and water oceans.

    • Edwin February 11, 2021, 19:44

      That’s a very interesting point Mike and something I have thought about too.

  • Brett February 11, 2021, 14:55

    It’s unfortunate that a Neptunian planet might be orbiting inside the HZ of Alpha Centauri A, but it looks like it would be pretty close to the inner edge of that habitable zone. Maybe there are smaller planets orbiting further out inside its HZ.

    • Ronald February 12, 2021, 15:14

      Well, also to FrankH’s comment: we know by now that inner planetary systems can be quite packed with planets in stable orbits, also see my other comment, mentioning Tau Ceti. Which is hopeful.
      Taking TC, which has an estimated age of about 6 gy, as an example, super-earths / mini-Neptunes can have stable orbits of only a few % of an AU apart, and even in the HZ only a few tenths of an AU apart.
      Apparently, orbital spacing is hardly the limiting factor, sufficient planetary building material probably is.

    • Ronald February 12, 2021, 15:33

      Further to my previous comment: that is to say, for small and intermediate planets, spacing and stable orbits isn’t the big issue. For greedy giants, Jupiters, that is a whole different matter of course, they would definitely spoil it for other planets in a *wide* zone.
      If I am not mistaken, the Hill sphere radius of an Earth-sized planet at 0.7 – 1 AU is slightly less than 0.01 AU, and of a Neptune class planet at that orbital distance only 0.02 – 0.03 AU.

  • Gary Wilson February 11, 2021, 15:11

    Do we have an orbiting telescope that is in the IR spectrum that could get a better look? I understand all the background IR is a problem here on Earth. If we don’t have an appropriate orbital platform could it be a future priority for exoplanet work? Please pardon my lack of knowledge in these matters but I’m very interested to know the answer.

    • Robin Datta February 11, 2021, 20:48

      Maybe an upgrade (with appropriate sensors, filters &c.) to Hubble or a telescope still in the works might return the desired results?

    • Michael Losh February 12, 2021, 14:21

      I don’t know offhand, but I think the requirements for a suitable coronagraph to make these observations are likely beyond anything currently in orbit. Having two bright stars in the scene would make it difficult to block the direct starlight and nothing flying has the kind of adaptive optics needed to use the method mentioned in the posting. But these new Alpha Centauri observations may help justify a separately-flying, high-resolution starshade to work with a new or existing orbital observatory. I really hope we can get such a configuration working in space fairly soon!

    • Jens Fridthjof Knudsen February 12, 2021, 16:45

      I am no insider, but believe the James Webb telescope, slated for launch this year will be the best chance we have to get at it withing a reasonable timeframe. It is a very powerful telescope primarily intended for use in the IR part of the spectrum.

      • Michael February 12, 2021, 21:58

        You do know by invoking the name James W..b you have put back the launch day another day ! At least that how it seems and I can’t wait for ‘its’ launch.

  • Martin February 11, 2021, 18:41

    If I had to bet (I don’t like betting–I hate to lose), I would bet it’s an artifact.
    Having said that, I believe the size is an estimate based on the IR brightness. How about an earth-sized planet with rings?

  • Spaceman February 11, 2021, 19:15

    This is very exciting news! In terms of the exo-zodiacal disk, I am surprised that neither ALMA nor any other previous infrared or radio astronomy observatory would not have picked up such a disk by now, 2021. Alpha centauri B’s little world did not stand up to scrutiny, I am hoping that this hint regarding B’s big brother won’t end up going down the same way!

    Also, when it comes to close binaries, and please correct me if I am off base on this one, isn’t the larger star of the two more likely to host a planet if planets exist in such systems?

  • Edwin February 11, 2021, 19:47


    Yes this sure is interesting. I’m sure I sent you the paper a few days ago. The new telescopes and equipment are going to make this something to watch in the years ahead.

    Will it be confirmed as a planet in the near future? We have to wait and see.

    Great to read your update on this as as always.

  • djlactin February 12, 2021, 5:41

    My money is on ‘artifact’.

  • Ashley Baldwin February 12, 2021, 13:29

    Enough is enough. It’s obvious that the current 8-10m state of the art telescopes just don’t have the resolution at the longer mid IR wavelengths necessary to discover terrestrial mass planets in the habitable zones of either Alpha Centauri A or B.

    It’s debatable as to whether even the ELTs will either – with only the E-ELT possessing an IR imager on METIS at anything even approaching the necessary wavelength – with the ground based IR still a massive source of background noise and poor SNR.

    Discovering hab zone planets around Alpha Centauri A/B , and especially terrestrial iterations, is going to have to happen from space. Remember ACESat? Bendek and Belikov’s genius ‘Alpha Centauri satellite’. A. nasa small explorer class bespoke telescope designed for specifically this purpose. Just a humble 0.45m aperture, embedded high contract coronagraph for a sub € 120 million budget . With five observational wavelength bandwidths from 0.4 – 1 microns .Over a two year observation run to deliver the necessary star/planet contrast and inner working angle for such a small ( off axis) aperture.

    Covering the stellar hab zones down to Earth mass for both stars. Hamstrung and rejected due to the immaturity ( at the time ) of the required (PIAA ) coronagraph and the perceived limited science of the subject matter . Hab zone terrestrial planets ( or not) around our closest sun like neighbours (ahem)

    Rephrase – Discovering terrestrial hab zone planets with characterisation -around two sun analogues on our front doorstep !

    For a $100 million dollars -versus $1.5 billion plus for the best and inferior ground based equivalent. Versus €10 billion for JWST. More still by far for the SLS, Shelby launch system .


    Five years on from ACESat – with those years driven by development of the soon to fly Nancy Reagan ( WFIRST) telescope coronographic instrument – and the technological component of ACESat is near if not met.

    Thanks to the contaminating light of the adjacent binary star , neither WFIRST or even the much more capable HabEX concept telescope will include Alpha Centauri as a target . Ridiculous . Forget NEAR, JWST, WFIRST , HabEX , LUVOIR A or B 0r even the E-ELT and let’s focus on ACESat.

    If this article proves anything – there has never been a greater ( or cheaper) priority.

    • Paul Gilster February 12, 2021, 14:32

      Everyone should be sure to see Ashley’s ACESAT: Alpha Centauri and Direct Imaging for more on all this:


      • Ronald February 12, 2021, 15:01

        Great idea, how about crowdfunding for this one? Including some rich persons in this crowd.

    • Alex Tolley February 12, 2021, 15:11

      SLS, Shelby launch system

      I have never seen it described this way before. I love it! I would like to use that in comments elsewhere.

      • Paul Gilster February 12, 2021, 21:34

        Yes, Ashley has nailed it. Shelby Launch System indeed!

    • Michael Fidler February 12, 2021, 23:02

      Thank you Ashley Baldwin for the in depth information on ACESat. Who can we write and contact in Congress and NASA to push this issue and could cooperation with ESA and other space agencies make it easier to fund it. What could make this for broader use of the telescope, would it work for M Dwarfs like Proxima? The larger picture to sell this project for as many benefits as possible to get funding. What about the military Black projects, plenty of money and they are worried about UAP’s…

    • Wojciech J February 14, 2021, 19:53

      Fortunately with the growth of space sector and infrastructure, we probably can count on one of the space tech billionaires to fund such telescope eventually.
      Also don’t forget that with the pollution of night sky by thousands of satellites, there might be an idea to tax companies using them to fund space telescope or two for astronomers…wink wink ;)

  • Afterthought February 12, 2021, 14:49

    Alpha Cen system is so important that it needs it’s own dedicated space telescope.

  • Ronald February 12, 2021, 14:57

    Talking about promising nearby solar type stars with possibly planets in the HZ, there is a recent paper out, that analysed the architecture and orbital dynamics of the planetar system of Tau Ceti, and they predicted at leat 8 planets in stable orbits, of which 1 in the HZ:
    An Integrated Analysis with Predictions on the Architecture of the tau Ceti Planetary System, Including a Habitable Zone Planet, by Dietrich and Apai, Oct. 2020.

  • ljk February 12, 2021, 16:39

    Is It a Planet? Astronomers Spy Promising Potential World around Alpha Centauri

    The candidate could be a “warm Neptune” or a mirage. Either way, it signals the dawn of a revolution in astronomy

    By Lee Billings on February 10, 2021


  • Michael Fidler February 15, 2021, 1:30

    Interesting new algorithm called TRAP which can improve contrast by up to 6x that work on SPHERE and CHARIS, both coronagraph based. The question is will this work on the NEAR coronagraph system with Alpha Centauri A and B so close together and with the 100 hour integration time? Paul could you please see if one of the research group can comment about this?

    TRAP: A temporal systematics model for improved direct detection
    of exoplanets at small angular separations.

    “Context. High-contrast imaging surveys for exoplanet detection have shown that giant planets at large separations are rare. Thus, it
    is of paramount importance to push towards detections at smaller separations, which is the part of the parameter space containing the
    greatest number of planets. The performance of traditional methods for the post-processing of pupil-stabilized observations decreases
    at smaller separations due to the larger field-rotation required to displace a source on the detector in addition to the intrinsic difficulty
    of higher stellar contamination.
    Aims. Our goal is to develop a method of extracting exoplanet signals, which improves performance at small angular separations.
    Methods. A data-driven model of the temporal behavior of the systematics for each pixel can be created using reference pixels at a
    different positions, on the condition that the underlying causes of the systematics are shared across multiple pixels, which is mostly
    true for the speckle pattern in high-contrast imaging. In our causal regression model, we simultaneously fit the model of a planet signal
    “transiting” over detector pixels and non-local reference light curves describing the shared temporal trends of the speckle pattern to
    find the best-fitting temporal model describing the signal.
    Results. With our implementation of a spatially non-local, temporal systematics model, called TRAP, we show that it is possible to
    gain up to a factor of six in contrast at close separations (< 3λ/D), as compared to a model based on spatial correlations between
    images displaced in time. We show that the temporal sampling has a large impact on the achievable contrast, with better temporal
    sampling resulting in significantly better contrasts. At short integration times, (4 seconds) for β Pic data, we increase the signal-to noise ratio (S/N) of the planet by a factor of four compared to the spatial systematics model. Finally, we show that the temporal model
    can be used on unaligned data that has only been dark- and flat-corrected, without the need for further pre-processing."


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