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Imaging an Alpha Centauri Planet

At some point, and probably soon, we’re going to be able to identify planets around Alpha Centauri A and B, assuming they are there and of a size sufficient for our methods. We may even be able to image one. Already we have an extremely tentative candidate around Alpha Centauri A — I hesitate even to call it a candidate, because this work is so preliminary — which could be a ‘warm Neptune’ at about 1 AU. One of the pleasures of the recent Breakthrough Discuss meeting was to hear film director James Cameron on the matter. Cameron, after all, gave us Avatar, where a habitable moon around a gas giant in this system plays the key role.

Despite his frequent protestations that he is not a scientist, Cameron was compelling. He’s obviously well-enough versed in the science to know the terminology and the issues involved in the ongoing deep dive into the Alpha Centauri system, and he’s done wonders in fixing the public’s attention not only on its possibilities but also on presenting a starship concept that, using hybrid propulsion methods, makes a bid for most realistic starship ever in film.

I imagine Kevin Wagner thinks about the Avatar scenario now and again, given that his work on Centauri A has turned up the observation he refers to as C1. Let’s put it in context (and I’ll also send you to Imaging Alpha Centauri’s Habitable Zones, which ran here in February), delighting in the fact that we have more than one habitable zone to talk about.

Wagner (University of Arizona Steward Observatory) and team run NEAR (New Earths in the Alpha Centauri Region), which thus far has been a full-on 100-hour attempt to look into the habitable zones of Centauri A and B. It’s fascinating to realize that these stars are close enough to us that with technology like Hubble, we can actually observe the habitable zones, for these are at separations we can see, at about 1 arcsecond, which is resolvable with large telescopes. Think Sagan’s ‘pale blue dot’ when you imagine the ultimate goal of actually imaging an Earth-like world, although it will take future instrumentation to get us to that level of sensitivity.

Image: This is a familiar image from Hubble showing Centauri A at left and B on the right. Kevin Wagner superimposed the circles showing the size of the habitable zones. The image was captured by the Wide-Field and Planetary Camera 2 (WFPC2), and is drawn from observations in the optical and near-infrared. Credit: Kevin Wagner/ESA/NASA.

NEAR, whose first 100-hour run is complete, used an adaptive secondary telescope mirror working in combination with light-blocking and masking technologies in the mid-infrared to suppress light from each of the binaries in sequence. The NEAR equipment is mounted on the Very Large Telescope’s Unit Telescope 4 in Chile, and the key, according to Wagner, is the deformable secondary mirror, which maximizes adaptive optics without adding warm optics downstream in a tertiary mirror that would degrade the infrared signal. About 1600 magnetic actuators zone out atmospheric distortion even as the coronograph nulls out star light.

Imaging something on the order of a pale blue dot around another star is quite a goal. We’ve only imaged a dozen or so exoplanets thus far, and all of these have been young and massive gas giants that still radiate brightly, no more than tens of millions of years old. Mature planets like those in our own Solar System are much cooler, and if we are after a planet like the Earth, we have to look in areas where the infrared signal is swamped by our own atmosphere. Adds Wagner:

“The earth is a 300 K black body. Here the primary radiation is at 10 microns, which is where we have to look at more mature exoplanets. And the problem is that the atmosphere of our own planet is what we have to look through, and it also radiates at about 10 microns. The sky, the telescope, the camera, everything is glowing at us.”

I ran the figure below in February, but I want to introduce it again, as it shows not only the C1 observation but also, on the left, the systematic artifacts that have to be removed to come up with what the astronomers hope is a clean image. Remember, we are in early days here, and when discussing C1 as a possible planet, we have to keep in mind that other explanations are possible, including distortion in a not yet recognized effect within the equipment itself.

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.

The C1 candidate looks, says Wagner, like what the team’s simulated planetary sources look like, but it could also represent, in addition to a systematic error, dust in the habitable zone, bearing in mind that while the Sun has its own zodiacal light from such dust, the Alpha Centauri system is known to have 50 times more dust. We could be looking, in other words, at dust that is off-center simply because of the orbital perturbations within the binary. “We can’t attribute this to any of the known systematics,” says Wagner, “but we don’t know all the systematics in this new system.”

What’s truly newsworthy in the NEAR work is the sensitivity of the dataset, which demonstrates that a habitable zone planet somewhere between Neptune and Saturn in size is detectable around the Alpha Centauri stars. NEAR is, in other words, sensitive to planets smaller than Jupiter at about 1 AU, and thus we can expect further work to find out whether C1 can be verified as a planet. This could be done through imaging using the James Webb Space Telescope, or through another observing run with NEAR, or via astrometry (about which more in a day or so) or even time-tested radial velocity using the hugely sensitive ESPRESSO.

The current limit on radial velocity detection around Alpha Centauri is on the order of 50 Earth masses in the habitable zone, Wagner added. NEAR itself is not currently in operation but could be reinstalled at UT-4 on the VLT, and of course on top of the other options, we have the next generation of ground-based telescopes coming, extremely large instruments that could accomplish within a single hour what it took the NEAR instrumentation 100 hours to do.

NEAR has demonstrated a technology, then, that is apparently capable of imaging mature Neptune-class planets in this system. Ramp its sensitivity up four times and we get to ‘super-Earth’ detection capability. We’re not yet at Earth-like planet imaging, but within decades, the ELTs should make it possible. We can consider NEAR a pathfinder experiment that has demonstrated the limits of the possible and shown us the way forward as, step by step, Alpha Centauri yields its secrets.

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

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{ 20 comments… add one }
  • ljk April 26, 2021, 14:17
    • Andrei April 26, 2021, 20:19

      When the ship was shown in the movie theater, my friend nudged me asking what I thought since he know I am a space buff.
      I incorrectly thought it was some variety of a ram scoop. But I noted the cooling panels, and said those were way to small, and questioned why they still would be glowing red.
      So he asked what kind of panels an actual start ship would have: And I replied, with the energy levels it would at least need to be built like a butterfly.
      If the ship have had such a design with such truly large radiators, I would have applauded in the cinema.
      I had a look at the linked atimic rocket page, and below is a description of a “Frisbee” ship with 500 kilometer heat radiators – that’s more like it and what it takes for antimatter propulsion!

  • jonW April 26, 2021, 14:52

    > Think Sagan’s ‘pale blue dot’ when you imagine the ultimate goal of actually imaging an Earth-like world, although it will take future instrumentation to get us to that level of sensitivity.

    “Future instrumentation” as in not yet realizable with our technology, or just not yet launched? I’m thinking of JWST. Before hitting “submit” on this question I did a quick search and found this 2019 article https://arxiv.org/abs/1910.09709 , “Searching for Planets Orbiting Alpha Centauri A with the James Webb Space Telescope” which concludes that with enough repeat visits we could hope to detect a planet of just 3 times earth’s mass within a 1- to 3-AU radius. But we’d be very unlikely to be able to image a 1-R_E planet at 1 AU.

  • Geoffrey Hillend April 26, 2021, 15:31

    I like the idea of subtracting the image of one star from the other, but the so called exoplanet image doesn’t look round. It’s hard to rule out some kind of false positive or distortion of light from enlarging the photograph. When we make a poor image larger, the light distortions get larger. It does not seem like it not enough evidence to support an actual exoplanet More photographs over time are necessary.

    • Michael April 27, 2021, 0:08

      Hmmm, I wonder what could make a distant planet look not-round…

      #exomoon

      • Michael M April 27, 2021, 7:02

        It could be a ringed world like Saturn.

        • Ashley Baldwin April 27, 2021, 13:04

          The image is a point source. PSF= point spread function. Any deformation is an artefact . Even for the Alpha Centauri stars a telescope of over a hundred kms aperture ( or an equivalent interferometer ) would be required to provide spatial resolution – and then only across just a few image pixels.

  • Geoffrey Hillend April 26, 2021, 15:42

    Orbital perturbations could be accounted for it there is a mistake in the way the math or the formula is set up. General relativity is tricky and one can’t always use Newtons and Kepler’s laws to explain orbital motion. Two stars follow a center of gravity, so they have an astrometry or wobble as they move through space, so that they perturb each others orbits like a gas giant around a single star. I wonder just how much the so called perturbations which are not explained here deviate from general relativity. It is easy to see what one wants to see in the data, but with science and astrophysics, it is only what is actually there.

  • Fredrik Bjerkeland April 26, 2021, 20:12

    I hope C1 is not a planet. Who wants a Neptune at 1 AU anyway?

    • Michael April 27, 2021, 7:00

      Could be worse, it could be a Uranus

  • Robin Datta April 26, 2021, 23:19

    Speculation serves not only to fill perceived voids in the narrative, contributing by fiction (often satisfyingly), but also through a grasp of reality, fostering the genesis of future projects.

    Both functions are important to society, the former for impetus and the latter for resolve. Too much of either to the neglect of the other would be suboptimal. It would seem that at least for now the thought leaders in this regard are well balanced.

  • Afterthought April 27, 2021, 0:42

    Hoping for exciting findings (even if oversold) to spur what is really needed: a dedicated space based AlphaCen telescope.

  • Victor Nunnally April 27, 2021, 13:08

    This might be irrelevant but does the oort cloud of Alpha Centauri ever interact with our solar systems oort cloud?

  • Geoffrey Hillend April 27, 2021, 16:21

    I stand corrected on the idea of a distortion by photographic enlargement. I didn’t think about that one enough.

    The astrometry of a two star system makes these circular motions as the system moves through space. I was wondering if these values based on mass don’t match what relativity predicts with two bodies, so there must be another third mass there, the Neptune sized exoplanet. There is no mention of the details of the anomalies in this article.

    I am still don’t think there are planets around Alpha Centauri A or B based on Rudolph Kippenhahn’s ring theory of double star formation. If we find a planet in the habitable zone around Centauri A it might invalidate his theory. The idea was that two stars at the right distance apart like from the Sun to Saturn might not have any planets. They have to form in only a ring of gas with the two stars on opposite sides of the ring. There can’t be any gas and dust in the center inside the ring like an accretion disk with a central star because the two stars grab all the angular momentum and gas and dust from the center inside the ring and constrict it to the ring. This is an interesting idea. It will be put to the test with Alpha Centauri A and B. Source; 100 Billion Suns, Kippenhahn, 1983.

  • Michael Fidler April 28, 2021, 9:59

    More nearby star systems observed with NEAR.

    High contrast imaging at 10 microns, a search for exoplanets around: Eps Indi A, Eps Eri, Tau Ceti, Sirius A and Sirius B.

    https://arxiv.org/abs/2104.13032

  • wdk April 29, 2021, 14:19

    The report in Nature Communications was interesting to read, but it was also oddly difficult to do so, considering that the subject was “imaging”.
    Albeit that most of of the light from the Alpha Cen source would be extraneous, clearly delineating the accomplishments of the team was very difficult as displayed. It appears that part of the article described the
    simulation or exercise, but also an unexpected result: perhaps a ghost of a planet or zodiacal dust or simply an unexplained thermal. Still, from years back trying to determine stability of terrestrial planets in this
    system with dynamic simulation, the target area and the ghost were eerie to read of. My results from back in those days indicated that Alpha Cen A would allow a terrestrial planet out to 2 or 3 AUs based on prolonged numerical integration, but the issue of how it would form in the first place remained beyond the scope of such calculations – and still have to wonder how circumstellar disks would behave in such close
    elliptical binary system. It’s like the New York, New York Song: “If you could make it here, you could make it anywhere.” But considering that, would zodiacal dust hang around in a star system depleted of planets?

  • Wojciech Jacyk May 2, 2021, 4:57

    “Think Sagan’s ‘pale blue dot’ when you imagine the ultimate goal of actually imaging an Earth-like world”
    The first image of this kind will truly be life changing for our civilization and iconic. The beauty is that perhaps within a decade or so we will be close for it to be within our grasp. And we can eventually go even further with hypertelescopes.

  • Lawrence May 2, 2021, 9:13

    If C1 is confirmed to be a Neptune-sized planet, it should be aptly named ‘Polyphemus.’

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