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What We Can Rule Out at Alpha Centauri

One of the reasons to pay attention to spectrograph technologies — and we recently talked about ESPRESSO, which has just achieved ‘first light’ — is that we’re reaching the inflection point when it comes to certain key observations. Finding planets around Centauri A and B has been the gold standard for a number of researchers, and as Debra Fischer (Yale University) points out, we’re just now getting to where spectrographic technology is up to the challenge.

Chile is where much of the action is. Here we find ESPRESSO installed on the European Southern Observatory’s Very Large Telescope at Paranal. But Fischer’s team has built CHIRON at Cerro Tololo, and the paper likewise relies on data from the Geneva team’s HARPS and the UVES installation at the Very Large Telescope Array in the United States. Working with Yale’s Lily Zhao, Fischer has re-examined older data with an eye toward turning once again to Centauri A and B with a new round of observations beginning the year after next.

The scientist seems quite optimistic, and not just about the technology. In a Yale University news release she says: “Because Alpha Centauri is so close, it is our first stop outside our solar system. There’s almost certain to be small, rocky planets around Alpha Centauri A and B.”

Let’s dig into that assertion a bit. We’ve proceeded at Centauri A and B just as we did at Proxima Centauri, beginning with observations that allowed us to drill down progressively into the possible planet populations there. For a long time, it has been possible to say that no super-Earths larger than 8.5 Earth masses could be detected around Proxima in orbits with a period of 100 days (this was from work by Michael Endl and Martin Kürster). The same work showed that no super-Earths of 2-3 Earth masses could be found in the Proxima habitable zone.

But by pushing to ever more exacting observations, Guillem Anglada‐Escudé and team eventually discovered Proxima Centauri b, and now we are on the hunt for further planets there. In the close binary Centauri A and B system, Debra Fischer’s team can do something similar. Their new paper, looking at possibilities in the respective stars’ habitable zones, finds no evidence for planets at Centauri A larger than about 50 Earth masses. At Centauri B, we find no planets larger than about eight Earth masses. That leaves a lot of room for planets of that interesting small, rocky description that, in size at least, remind us of our own.

Image: The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Here I always pause to remind people that Beta Centauri is an entirely different star, not part of Alpha Centauri. The stars we know as Centauri A and B are both within the glare of what appears to be the single ‘star’ on the left. The star Beta Centauri is actually in the range of 400 light years from us — don’t confuse it with Centauri B. This image was taken with a Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30 seconds. Credit: Skatebiker at English Wikipedia.

Radial velocity work, looking for the faint back-and-forth motion of a star as it is affected by the gravitational forces of orbiting planets, demands patient and time-consuming analysis. Fischer, Zhao and colleagues used more than a decade of radial velocity measurements for Centauri A and B as well as Proxima, drawing on CHIRON, UVES and HARPS data and using simulated signals to assess the probability that the signal could have been produced by stellar noise alone. The paper shows the lengths to which they went to screen for systematic errors as well.

This gets intriguing, for we know that with radial velocity, a key issue is that we cannot obtain a true planetary mass, but rather a range of masses with a minimum established — this is the result of the fact that in most cases, we cannot know the inclination of the planetary system, so what might appear to be a relatively small planet at minimum could also be much larger.

At Alpha Centauri, though, other factors come into play. Here, the dynamical influences of the binary system mean, according to Zhao and Fischer, that any stable planets are most likely co-planar, or nearly so, with the 79 degree inclination of the stellar binary system. In that case we can derive from radial velocity data a figure that is approximately the actual planet mass for any planets we do find around Centauri A and B. The work demonstrates that terrestrial class worlds could still exist around Centauri A or B and would not have been detected by the past ten years of precision radial velocity searches. The idea that we could find Earth-sized planets using radial velocity methods is still robust, and that includes for searches inside the habitable zone.

Here are the specifics of the result:

At each point in the parameter space of M sin i and orbital period, we sample a Keplerian signal at the actual time of the observations with added white noise scaled to the errors to provide a baseline of planet detection space. These simulations exclude planets within the conservative habitable zone of each planet with a M sin i of greater than 53 M for α Cen A, 8.4 M for α Cen B, and 0.47 M for Proxima Centauri on average. This result for α Cen B comes from the HARPS data set; the CHIRON data set excludes planets in the habitable zone of α Cen B to greater than 23.5 M.

Note that finding on Proxima Centauri, which tells us that there could be planets orbiting there that are less than one-half of Earth’s mass. We have a great deal to learn about planet possibilities around all three stars. Note, too, how this work picks up another theme we’ve looked at recently, the use of existing datasets to draw new conclusions. As to future work, the paper gives us the roadmap: We will need radial velocity precision in the 10 centimeters per second space before we can detect the smaller planets that may lurk around Centauri A and B.

ESPRESSO, anyone?

Bear in mind that work on Centauri A and B is currently tricky thanks to the small separation of the two stars as seen from Earth. That angular separation is now increasing, and the authors believe that the two stars will be ‘ideal targets’ for renewed radial velocity study by 2019. Adds Fischer: “The precision of our instruments hasn’t been good enough, until now.”

The paper is Zhao et al., “Planet Detectability in the Alpha Centauri System,” The Astronomical Journal Vol. 155, No. 1 (18 December 2017). Abstract / preprint.


{ 51 comments… add one }
  • Tom Mazanec December 19, 2017, 13:37

    I think you have to put CHIRON in your EarthExoplanet Project listing.

  • Silver Fang December 19, 2017, 14:36

    I really hope they find a perfect earthlike planet orbiting Centauri A and B, with bright blue skies and clear water for swimming in, 60-80 degrees year round. I’d go there in a heartbeat.

    • Zanstel December 20, 2017, 4:54

      A planet in the goldilocks zone don’t orbit A and B at the same time. Only one of both. The other star would be seen as we could see our star from Saturn.

      I would prefer a sight from a Earth-like planet from B, as B been cooler would appear to be bigger (although it’s the opposite) because the orbit is closer, and A would be shawn brighter than B from A.
      A lot of nights on the year (when the planet it’s between B and A) would be light all the day. The day as we know it, and the “not night, dark day from the far star”. We would need a new word for that.


      • Tom Mazanec December 20, 2017, 13:42

        In the Handbook for Space Pioneers, months with such nights were called “am months” and the others were called “nocht months”.

  • Michael Spencer December 19, 2017, 15:52

    Do we even dare? Do we have anything beyond a faint glimmer that, lying far below the resolving power of current instruments, fine indeed— that, buried deep down in the data lies New Earth?

    There being SF aficionados here about, can we even imagine how such a discovery would play out?

    SF at its finest explores the human reaction to new science. What an experiment we would see! Imagine the news, if you will: Scientists confirm the presence of a rocky but water-covered planet accompanied by a large moon barely 4 LY away!

    Be still my heart!

    • Michael Fidler December 19, 2017, 20:25

      What would be even better if we found planets very similar to earth around both A and B. Then we would see the space programs of the world combine to develop space as Sir Arthur C. Clarke envisioned in 2001 A Space Odyssey written during the peak of the Apollo program.

      • Michael Spencer December 20, 2017, 10:23

        Exactly so! I’ve harped constantly over our directionless space program- the one involved in HSF, that is- a program without direction because we can’t find consensus on a good reason to actually inhabit space.

        But a New Earth? Wow. All sorts of technology would be developed. What a space race! “Could we ever allow those no-good [here, insert your favorite nation] to reach Alpha first? And perhaps discover secrets that would wipe us out?”

        We would see twenty or thirty years of aggressive research into propulsion. We’d see missions to Paul’s favorite spot, the solar focal point, in hopes of better images.

        Stop me…or not, because all of this remains possible.

    • David Herne December 19, 2017, 22:51

      A measly 4 light years… surely we could…

    • Ronald December 20, 2017, 4:48

      The discovery of a terrestrial planet, even remotely habitable or terraformable, around Alpha Centauri A and/or B would probably constitute the greatest incentive for a global renewed and unprecedented interest in space and interstellar travel as never before in human history.
      If we were part of a wide binary star system, with the other stellar component also possessing a terrestrial planet in the HZ, we would already have had an interstellar space program.

  • Harold Shaw December 19, 2017, 17:48

    10 centimeters per second precision from 4+ lys…
    Can anyone put that in context? What can we achieve now and how far out is this precision?

    • Paul Gilster December 19, 2017, 23:01

      Harold, check the recent article on ESPRESSO:


      ESPRESSO is in the 10 centimeters per second range now, with hopes by researchers to push lower.

      • Harold Shaw December 20, 2017, 9:10

        Thank you. I had 3 meters per second as a theoretical limit for controlling for stellar flux stuck in my head. Glad that is wrong.

  • Ashley Baldwin December 19, 2017, 19:14

    What’s slightly disappointing is the comment about the relative immaturity of photospheric noise modelling . This helps remove what is one of the leading impediments to the sort of high precision velocimetry mooted here. Debra Fischer and her team have done more work on this than anyone and propose feeding it into their 100Earths EXPRES spectrometer . This is under construction at the Discovery telescope as we speak and designed around experience gained from CHIRON. The implication appears to be that things are not progressing as hoped. The Alpha Centauri binary constituents are also both relatively stable ( A particularly ) and as well analysed photospectroscopically as just about any stars known outside of the Sun.

  • David Herne December 19, 2017, 22:48

    Paul, I’m interested in your thinking when setting Centauri-Dreams up, about the name and the banner image. There must be such an air of familiarity for you in discussions on the Centauri A/B and Proxima systems.

    A safe and happy Christmas to all.

    • Paul Gilster December 19, 2017, 23:05

      David, I chose the banner image back in 2004 when I set up the site. The name was originally suggested by my editor, Paul Farrell, at Copernicus Books — I can’t remember what I had titled the original manuscript of the book. But the book came out under the Centauri Dreams title, and I just appropriated it, because I wanted the site to be, in a sense, the continuing, upgrading second edition of the book. And yes, talking about these systems is very familiar, because I tend to focus in quickly on any new work re planets there. Alpha Centauri has been an obsession of mine since I was a kid, and remains one!

      • David Herne December 20, 2017, 4:33

        ‘Alpha Centauri has been an obsession of mine since I was a kid, and remains one!’ What a happy obsession to have had. As a kid, I couldn’t understand how the universe could be finite because infinity was impossible.

        • Ashley Baldwin December 20, 2017, 13:28

          Mine too ironically . I remember discussing it frequently as a keen amateur astronomer at high school aged 13. Lamenting that living at latitude 53 degrees North in the ( cloudy ) NW of England I was never going to see it ! Still haven’t as Dallas remains as far South as I’ve ever got where it is visible , and even then only briefly and in the daytime whilst passing though Fort Worth airport.

        • Patient Observer December 20, 2017, 21:55

          As a teenager, I read a DC comic in which the main character, Adam Strange, was periodically teleported by a “Zeta beam” to the planet Rann, which orbited the star Alpha Centauri (I don’t recall if the comic depicted a double star). My main interest, beyond the general sci-fi-ishness, was the gorgeously drawn daughter of the prominent Rannian scientist, Alanna.

      • Ashley Baldwin December 20, 2017, 4:57

        One increasingly near to being realised . ( predictive text offered me “relished ” which would be apt too) . Merry Christmas and an Alpha Centauri Few Years.

  • xcalibur December 20, 2017, 4:26

    As I’ve said before, discovering an earthlike planet orbiting our closest stellar neighbor would be truly encouraging; almost an invitation to venture out into the cosmos. I look forward to the wealth of exoplanet discoveries that still awaits.

  • Alex Tolley December 20, 2017, 10:55

    A decade or so back, I recall lectures given by astronomers that were forecasting time frames for detecting Earth size exoplanets based on improvements in detection.

    Is there any way to do this for Centauri a & b so that if we don’t find anything by n years out, there is likely none there? It think that as technology matures and we gain experience and new data, that we should be moving inexorably, although not monotonically, towards detecting smaller worlds, including Earth-size ones.

  • ljk December 20, 2017, 11:02

    NASA is sending a space probe to Alpha Centauri in 2069!


    Sorry, just got caught up in the media hyperbole/ignorance there for a moment.

  • Thomas Hair December 20, 2017, 11:03

    Dr. Fischer is to be commended for her tenacity, but it is important to realize that the finest mathematical sieve will always gather up far more straw from the haystack than it does those very special needles.

  • Ashley Baldwin December 20, 2017, 13:13

    The Fischer article illustrates how the RV technique is improving all the time. There are numerous impediments to finding an “eta Earth” both in equipment ( which has already been substantially upgraded with ESPRESSO and its soon to be commissioned cousins ) and process. By the time A and B have separated enough to be subjected to high precision RV spectroscopy in 2019 , I’m confident everything will be aligned nicely for success.

    Even if the photospheric noise issue hasn’t been completely ironed out, for a special case like Alpha Centauri the “Proxima b” approach adopted by “Pale Red Dot “can be employed . This being near continuous contemporaneous ground based photometry of the two stars to exclude any spurious signals in the velocimetry data return , arising from stellar jitter instead from a legitimate planet.

  • DJ Kaplan December 20, 2017, 13:28

    “…don’t confuse it with Centauri B.”
    Oh, why might that have happened? Because we are abbreviating Alpha Centauri as “Centauri” which could apply to any of at least 11 stars. :)
    As news percolates to the masses, this will become useful terminology or it will sow confusion and loss of trust in science.
    We are talking about the two components of the Alpha Centauri (AC or a Centauri) system, AC A and AC B. The first confirmed planet around AC A would be called AC Ab. Yikes!
    I prefer Toliman.

    • J. Jason Wentworth December 21, 2017, 6:13

      Thank you for mentioning its name Toliman, which means “grapevine shoot.” Its Greek name, “Rigel Kentaurus” (latinized from ‘Rigil Kentauros,’ I suppose) means “foot of the centaur” (the academician and teacher Chiron, although some sources say that Chiron is represented by Sagittarius). John W. Macvey’s generation starship interstellar pioneers in “Journey to Alpha Centauri” (also republished as “How We Will Reach the Stars”) renamed Alpha Centauri A, the sun of their new home, “Alphauri.” (If we have any relatively nearby neighbors, discussions about what their home stars represent to us would be interesting, and hopefully not offensive to them; ditto for what our Sun may be to them, if they also figured constellations…).

      While this news doesn’t make me want to turn cartwheels (if I could), because these latest studies could possibly have “put some chaff in the wheat bin,” it does make me quietly optimistic nonetheless. Having looked across those 4.3 light-years from Everglades National Park (on the night when Comet Hyakutake was closest to Earth) and wondered if someone might have been looking back at me from the Alpha Centauri system at that moment, I very much want there to be habitable worlds–where we might stand under a strange sun–therein, and:

      Just as the inhabitants of any planets of the two quite Sun-like stars Zeta Reticuli 1 and 2, a wide binary about 39 light-years away (who could each see the planets of the other sun with telescopes, as they are only about 3,750 AU apart), would be spurred to develop interstellar travel by such a nearby other solar system, so too do I hope there are habitable–or even inhabited–Alpha Centauri A and/or B planets, for the same reason. They might be desert worlds due to the mutual influences of the two stars during the accretion of any planets orbiting them, but since our planet’s deserts are hardly bereft of life, this is no fatal objection. Even ~1/2 Earth mass planets that might orbit Proxima Centauri, as Paul pointed out, could possibly be the home of life (either indigenous, or possibly settled from Earth).

    • andy December 21, 2017, 6:15

      And just to add to the possible confusion, there’s also an A Centauri, an a Centauri, a B Centauri and a b Centauri.

      Better to keep the “Alpha” to avoid the problems.

      • andy December 21, 2017, 6:18

        Oh, almost forgot, let’s not leave out the radio galaxies Centaurus A and Centaurus B

        • DJ Kaplan December 22, 2017, 17:50

          And a Gamma Centauri. ;)
          Thanks, Michael; now I wait for Rigil to be confused with Rigel Orionis. :D
          Very good point, Jason. If our sun had a nearby binary neighbor, we certainly would be spurred on to explore more. If we find a parallel civilization on these worlds, they may already be primed for interstellar exploration.

  • andy December 20, 2017, 16:34

    Talking about habitable planets around the Alpha Centauri stars seems wildly optimistic to me. Close binaries have been described as “ruinous” for planets, and there seems to be a fair chance that both Alpha Centauri A and B were accretion-hostile environments. At best, I’m hoping for a system of small rocky planets in close orbits (something similar to Kepler-444A). Habitable-zone terrestrials would be a fantastic surprise, but we have to recognise that this may not be a likely possibility.

    • DJ Kaplan December 20, 2017, 17:33

      Agreed, at this point, in order to be objective, we have to be ready to accept any discovery.

      • ljk December 21, 2017, 10:06

        Remember how relatively dull most astronomers thought the Galilean moons of Jupiter were going to be based on their past experience before Voyager 1 flew past them in March of 1979? “Just” ancient iceballs covered with craters.

        Instead we got a world with active volcanoes spewing molten sulfur hundreds of miles into space and at least two and maybe three other moons with global oceans of liquid water under their icy surfaces.

        • andy December 21, 2017, 12:24

          Not a particularly relevant analogy. In the case of the Galilean moons, no-one had observed ice moons before, and no-one had done the tidal heating calculations that might have predicted the activity (at least not until right before the encounter, so the prediction of large-scale melting on Io got rather overshadowed by the actual observations).

          In the case of planet occurrence in close binaries, there is a body of both observational and theoretical work relevant to the subject, and unfortunately that work points towards reduced planet-occurrence rates in such systems.

          • DJ Kaplan December 22, 2017, 17:52

            Scientifically, we must be ready for either possibility. Or a third surprise.

          • ljk February 12, 2018, 15:17

            Black holes had never been observed but several astronomers considered them as far back as the 18th Century. So 20th Century astronomers should have had an even easier time thinking outside the box when it came to “mere” ice moons, especially since they far outnumber rocky moons in at least our Sol system.

            So, yes, it is relevant.

    • Ronald December 21, 2017, 6:02

      “we find (…) that inside (…) 47 +59 / -23 AU, the planet occurrence rate in binary systems is only 0.34 +0.14 / -0.15 times that of wider binaries or single stars. Our results demonstrate that a fifth of all solar-type stars in the Milky Way are disallowed from hosting planetary systems due to the influence of a binary companion”.

      So, for binaries with an approach of smaller than about 50 AU, the planet occurrence rate is only about 1/3 that of single stars. I wonder how small that chance becomes for a minimum approach of only about 11 AU (as for Alph Cen A and B).
      If I remember well, there are other close binary (< 15 AU) systems known with planets, but not many.
      The very elliptical orbit of Alph Cen A and B ( from 11 to 36 AU) will probably not help much either. Even more so if there is an orbital inclination.

    • DJ Kaplan December 28, 2017, 17:54

      What if by 2069 we still haven’t found evidence of exoplanets around the AC system? Surely the probe could be re-calibrated to aim at a more likely neighbor…

  • kzb December 21, 2017, 8:46

    But if any planets in the HZ of both stars are said to be in stable orbits, would that not also be true for particles in the disk? Also, even if the total mass of the disk is depleted 25-fold, there is plenty left for terrestrial planet formation. In other words, close binaries might well be ruinous for Jupiter formation, but not for Earth formation?

  • James Benford December 23, 2017, 0:32

    Back in the old days, it was referred sometimes as “Alpha C”, which preserves the Alpha, doesn’t focus on the other Centauris. Hence Robert Silverbergs first book ‘Revolt on Alpha C’.

  • ljk December 29, 2017, 13:10

    NASA’s bold mission pegged for 2069


    NASA is reportedly in the very early stages of planning an interstellar mission to search for life outside our solar system, a century after the Apollo 11 mission touched down on the Moon. NASA 2069 New Scientisst

    A small group at NASA’s Jet Propulsion Laboratory (JPL) in California is supposedly drawing up rough plans for the mission despite the fact that the project doesn’t yet have a name and much of the technology that will ultimately be used has yet to be invented. Which is fine because it’s not slated to happen until 2069, a hundred years after Neil Armstrong and Buzz Aldrin’s famous footsteps.

    The mission would be to search for life outside our solar system in the three-star Alpha Centauri system. Astronomers know the star system has at least one planet in orbit and while other planets have been theorised, data to prove their existence has been hard to come by.

    The Alpha Centauri constellation is 4.4 light years away and would require, at a minimum, a craft capable of travelling at 10 per cent of the speed of light.

    But with such a long-term time frame, anything could happen. “It’s very nebulous,” NASA’s Anthony Freeman told New Scientist, who first reported that the project was in the pipeline.

    According to the report, he presented the mission concept at the 2017 American Geophysical Union conference in New Orleans on December 12.

    NASA is said to be considering sending tiny probes powered by lasers which if they can travel at the required speed, could reach their destination in as little as 44 years.

  • Jared Magneson January 2, 2018, 3:59

    One factor I didn’t see anyone else really talking about is the radius involved. For a planet to support an oxygen atmosphere such as ours, it has to be the same radius as the Earth. It can’t be smaller or larger, for the same reasons that Mars can never have a breathable atmosphere, or Venus, or Jupiter. They are too small to float oxygen, nitrogen, and argon. Jupiter is too big. Finding new planets is awesome just the same, more to study and learn, but let’s not get our hopes up about habitable planets. It needs to be the right radius and in a habitable zone, thermally.

  • Stephen January 3, 2018, 15:01

    Isaac Asimov wrote a book called “Alpha Centauri” detailing what we could expect there.

    • Paul Gilster January 3, 2018, 16:39

      Yes, and it’s quite good given its date. I’ve got a copy here in the office.

  • Harry R Ray January 8, 2018, 10:26

    Proxima Centauri planet(s?)and TRAPPIST-1 planets AS A WHOLE(i.e. NOT individually)may be detectable with JVLA and GMRT RIGHT NOW and WILL be detectable INDIVIDUALLY with SKA> “Exoplanet-induced radio emmissions from M-dwarfs.” by S. Turnpennwey, J. D. Nichols, G. A. Wynn, M. R. Burleigh.

  • ljk January 19, 2018, 10:31


    Long-Term Stability of Tightly Packed Multi-Planet Systems in Prograde, Coplanar, Circumstellar Orbits within the α Centauri AB System

    Press Release – Source: astro-ph.EP

    Posted January 18, 2018 10:58 PM

    We perform long-term simulations, up to ten billion years, of closely-spaced configurations of 2 — 6 planets, each as massive as the Earth, traveling on nested orbits about either stellar component in alpha Centauri AB.

    The innermost planet initially orbits at either the inner edge of its star’s empirical habitable zone (HZ) or the inner edge of its star’s conservative HZ. Although individual planets on low inclination, low eccentricity, orbits can survive throughout the habitable zones of both stars, perturbations from the companion star require that the minimum spacing of planets in multi-planet systems within the habitable zones of each star must be significantly larger than the spacing of similar multi-planet systems orbiting single stars in order to be long-lived.

    The binary companion induces a forced eccentricity upon the orbits of planets in orbit around either star. Planets on appropriately-phased circumstellar orbits with initial eccentricities equal to their forced eccentricities can survive on more closely spaced orbits than those with initially circular orbits, although the required spacing remains higher than for planets orbiting single stars.

    A total of up to nine planets on nested prograde orbits can survive for the current age of the system within the empirical HZs of the two stars, with five of these orbiting alpha Centauri B and four orbiting alpha Centauri A.

    Billy Quarles, Jack J. Lissauer

    (Submitted on 18 Jan 2018)

    Comments: 24 pages, 15 figures, 6 tables; accepted for publication in Astronomical Journal

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1801.06131 [astro-ph.EP] (or arXiv:1801.06131v1 [astro-ph.EP] for this version)

    Submission history

    From: Billy Quarles

    [v1] Thu, 18 Jan 2018 17:14:15 GMT (10079kb,D)



  • ljk February 12, 2018, 15:20

    Folks who ponder what planetary systems circling our nearest celestial neighbors may be like should keep the following in mind from this very recent article about interstellar objects captured by our Sol system:


    To quote:

    In the end, they determined that a few thousands captured objects might be found within the Solar system at any time – the largest of which would be tens of km in radius. For the Alpha Centauri system, the results were even more interesting. Based on the likely rate of capture, and the maximum size of a captured object, they determined that even Earth-sized objects could have been captured in the course of the system’s history.

    In other words, Alpha Centauri may have picked up some rogue planets over time, which would have had drastic impact on the evolution of the system. In this vein, the authors also explored how objects like ‘Oumuamua could have played a role in the distribution of life throughout the Universe via rocky bodies. This is a variation on the theory of lithopanspermia, where microbial life is shared between planets thanks to asteroids, comets and meteors.

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