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.
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 ? Centauri B on-coronagraph images have been subtracted from the ? Centauri A on-coronagraph images, resulting in a central residual and two off-axis PSFs to the SE and NW of ? Centauri A and B, respectively. Systematic artifacts labeled 1–3 correspond to detector persistence from ? Centauri A, ? Centauri B, and an optical ghost of ? 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 ? 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 ? 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 ? 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 ? Eridani, ? Indi, and ? 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 ? Centauri,” Nature Communications 12: 922 (2021). Abstract / full text.
It’s always breathtaking to see the band of the Milky Way under good viewing conditions. I remember so well the night I saw it best, about 20 years ago on a cold, absolutely clear night from a boat in the middle of Lake George. This is up in New York’s Adirondacks, and when I glanced up as we crossed the lake heading back to our hotel, I was simply stunned by the vista. When you contemplate what you’re looking at and think of yourself within that ghostly band, you feel somehow a deep connection to all the myriad processes that put us here as observing beings.
Now we have another fine view of the Milky Way, this time from TESS. The scientists working data from the Transiting Exoplanet Survey Satellite have just given us a composite drawn from 208 TESS images taken during the mission’s first year of science operations, which ended July 18. Have a look at the southern sky, and realize what while TESS has found 29 exoplanets thus far, another 1,000 or so are in candidate stage and being investigated.
Image: This mosaic of the southern sky was assembled from 13 images taken by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its first year of science operations, completed in July 2019. The mission divided the southern sky into 13 sectors, each of which was imaged for nearly a month by the spacecraft’s four cameras. Credit: NASA/MIT/TESS.
Lots of good things to see here. TESS has divided the southern sky into 13 sectors, each of which received almost a month’s worth of imaging by the four cameras aboard. The Milky Way’s band is easily recognized, but look in the center to see the Large Magellanic Cloud, and at the top of the image, you should be able to identify the Orion Nebula, a birthing place for stars.
Can you find Alpha Centauri in this image? Here’s a second image, one showing the confirmed TESS planets to date. I’ve inserted an arrow to identify our nearest star(s).
Image: The host stars of the 29 TESS planet discoveries to date are shown on this version of the southern sky mosaic. Credit: NASA/MIT/TESS and Ethan Kruse (USRA).
TESS is doing excellent work, capturing a full sector of the sky every 30 minutes as it hunts for exoplanet transits. In the first year of operations, its CCDs captured 15,347 30-minute science images. These make up a part of the more than 20 terabytes of southern sky data returned thus far. The TESS survey of the northern sky is now underway.
One of the benefits of having Alpha Centauri as our closest stellar neighbor is that this system comprises three different kinds of star. We have the familiar Centauri A, a G-class star much like our Sun, along with the smaller Centauri B, a K-class star with about 90 percent of the Sun’s mass. Proxima Centauri gives us an M-dwarf, along with the (so far) only known planet in the system, Proxima b. Questions of habitability here are numerous. Along with possible tidal locking, another major issue is radiation, since M-dwarfs are known for their flare activity.
As we learn more about the entire Alpha Centauri system, though, we’re learning that the two primary stars are much more clement. They may have issues of their own — in particular, although stable orbits can be found around both Centauri A and B, we still don’t know whether planets are likely to have formed there — but scientists studying data from the Chandra X-ray Observatory have found that levels of X-ray radiation are far lower here than around Proxima Centauri.
This is good news, because high radiation levels could prove fatal for surface life, with the additional effect of possible damage to planetary atmospheres. Chandra has been involved in a multi-year campaign targeting Centauri A and B stretching back to 2005, with observations every six months. No other X-ray observatory is capable of resolving the two primary stars during their current close orbital approach. What we wind up with is a look at radiation activity over time, covering a period analogous to our own Sun’s 11-year sunspot cycle.
Image: A new study involving long-term monitoring of Alpha Centauri by NASA’s Chandra X-ray Observatory indicates that any planets orbiting the two brightest stars are likely not being pummeled by large amounts of X-ray radiation from their host stars. This is important for the viability of life in the nearest star system outside the Solar System. Chandra data from May 2nd, 2017 are seen in the pull-out, which is shown in context of a visible-light image taken from the ground of the Alpha Centauri system and its surroundings. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zden?k Bardon/ESO.
Tom Ayres (University of Colorado Boulder) presented these results at the just concluded meeting of the American Astronomical Society in Denver. Any planets in the habitable zone of Centauri A would actually receive a lower dose of X-rays, on average, than planets around the Sun, while the X-ray dosage for a planetary companion of Centauri B is about 5 times higher than the Sun. This contrasts sharply with Proxima Centauri’s planet, which would receive an average dosage 500 times larger than the Earth, rising to 50,000 times higher during a major flare. If we find planets around either A or B, it may be that Breakthrough Starshot will want to prioritize these at the expense of the more endangered Proxima b.
In the animation below, we can see the proper motion of Centauri A and B.
Image: This movie shows Chandra observations of Alpha Centauri A and B taken about every 6 months between 2005 and 2018. Alpha Cen A is the star to the upper left. The motion of the pair from left to right is their “proper motion”, showing the movement of the pair in our galaxy with respect to the solar system. The change in relative positions of the pair shows the motion in their 80 year long orbit and the wobbles show the small apparent motion (called parallax) caused by the year long orbit of the Earth around the Sun. The Chandra images are shown in black and white. To place these semi-annual images in context, the two colored circles show the expected motion of Alpha Cen A (yellow) and Alpha Cen B (orange) when taking account of proper motion, orbital motion and parallax. The size of the circles is proportional to the X-ray brightness of the source. Credit: Thomas Ayres.
Ayres has also written up some of the results in Research Notes of the American Astronomical Society, where I learned that the central AB pair has actually been under X-ray study for almost four decades, dating back to the late 1970s and the HEAO-2 satellite (also known as the Einstein Observatory), which was the first fully imaging X-ray telescope ever put into space. Subsequent observations were conducted by ROSAT (Röntgen-Satellit), XMM-Newton and now Chandra. Here, Ayres explains why X-ray studies may help us learn about habitability in this system as well as giving us information closer to home:
The modest coronae (106 K) of ? Cen AB are on par with our own Sun’s. X-ray studies of these objects can help us understand how the “Dynamo” in the stellar interior produces the episodic surface magnetic eruptions at the core of solar activity and “Space Weather.” The hard radiation and particle bombardment from flares and coronal mass ejections can affect Planet Earth, so the interest is not solely academic. Exoplanets of other sunlike stars can be exposed to analogous extreme high-energy transients from their hosts, with perhaps serious repercussions for habitability.
Image: Figure 1 from the Ayres note. Caption: X-ray light curves of a Cen AB and the Sun 1995–2018. Credit: T. R. Ayres.
I was fortunate enough to be in the audience when Ayres spoke to Breakthrough Discuss in 2016 in a presentation called “The Ups and Downs of Alpha Centauri.” Here’s Breakthrough’s video of that talk, which I highly recommend.
The research note is Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (22 January 2018). Full text.
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 ? Cen A, 8.4 M? for ? Cen B, and 0.47 M? for Proxima Centauri on average. This result for ? Cen B comes from the HARPS data set; the CHIRON data set excludes planets in the habitable zone of ? 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.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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