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Alpha Centauri Planet Reconsidered

Finding a habitable world around any one of the three Alpha Centauri stars would be huge. If the closest of all stellar systems offered a blue and green target with an atmosphere showing biosignatures, interest in finding a way to get there would be intense. Draw in the general public and there is a good chance that funding levels for exoplanet research as well as the myriad issues involving deep space technologies would increase. Alpha Centauri planets are a big deal.

The problem is, we have yet to confirm one. Proxima Centauri continues to be under scrutiny, but the best we can do at this point is rule out certain configurations. It appears unlikely, as per the work of Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), that any planet of Neptune mass or above exists within 1 AU of the star. Moreover, no ‘super-Earths’ have been detected in orbits with a period of less than 100 days. This doesn’t rule out planets around Proxima, but if they are there, so far we don’t see them.


Image: The Alpha Centauri stellar system, consisting of the red dwarf Proxima Centauri and the two bright stars forming a close binary, Centauri A and B. Credit: NASA.

Centauri B, the K-class star in close proximity to G-class Centauri A, was much in the news a few years back with the announcement of Centauri Bb, a candidate world announced by Swiss planet hunters. This is radial velocity work based on data gathered by the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. The signal that Xavier Dumusque and team drew out of the data was 0.5 meters per second, a fine catch if confirmed.

What we thought we had in Centauri Bb was a mass just a little over the Earth’s and an orbit of a scant 3.24 days. As the blistering first planet detected around one of the Centauri stars, it would be a significant find even if it’s a long way from the temperate, life-sustaining world we’d like to find further out. The putative Centauri Bb supported the idea that there might be other planets there, and we’ve known since the work of Paul Wiegert and Matt Holman back in the 1990s that sustainable habitable zone orbits are possible around both the primary Alpha Centauri stars.

But Centauri Bb has remained controversial since Artie Hatzes (Thuringian State Observatory, Germany), using different data processing strategies, looked at the same data and found a signal he considered too noisy, indicating that what might be a planet might also be stellar activity on Centauri B itself. Debra Fischer’s team at Cerro Tololo Inter-American Observatory has also been studying Centauri Bb using the CHIRON spectrometer but has not been able to confirm it. And while a transit search using the Hubble Space Telescope did find a promising lightcurve (about which more in a moment), it couldn’t confirm Centauri Bb.


Image: Of the three stars of Alpha Centauri, the dimmest, Proxima Centauri, is actually the nearest star to the Earth. The two bright stars, Alpha Centauri A and B form a close binary system; they are separated by only 23 times the Earth – Sun distance. This is slightly greater than the distance between Uranus and the Sun. The Alpha Centauri system is not visible from much of the northern hemisphere. The image above shows this star system and other objects near it in the sky. Credit/copyright: Akira Fujii / David Malin Images.

Now we have a new paper from Vinesh Rajpaul (University of Oxford) and colleagues that makes Centauri Bb look more unlikely than ever. Rajpaul praises the thorough work of Xavier Dumusque and the team at the Geneva Observatory, but notes that their attempts to filter stellar activity out of their data evidently boosted other periodic signals that had nothing to do with a planet. The signal grows out of the time sampling, or ‘window function,’ of the data.

What is left behind is what the paper calls ‘the ‘ghost’ of a signal’ that was present all along. The paper argues that when a signal is sampled at discrete times (and the Dumusque team had to use the La Silla instrument only when it was not otherwise booked), periodicities can be imposed on the signal. Rajpaul was able to simulate a star with no planets, generating synthetic data out of which the exact same 3.24-day planetary signal emerged. The problem is particularly acute when working with planetary ‘signals’ as weak as these. From the paper:

D12’s data set [i.e., the data gathered by Dumusque and team] was particularly pathological because the window function happened to contain periodicities that coincided with the stellar rotation period of α Cen B, and its first harmonic; when these signals were filtered out, the significance of the 3.24 d signal was preferentially boosted.

All this is going to be quite useful if it helps us refine our techniques for identifying small planets. Rajpaul proposes that his team will carry out a new study of the spurious but coherent signals that can emerge from noisy datasets that should help us learn how to mitigate the problem:

We alluded to a number of other tests we believe worth carrying out when considering the reliability of planet detections from noisy, discretely-sampled signals. These include using the same model used to detect the planet instead to fit synthetic, planet-free data (with realistic covariance properties, and time sampling identical to the real data), and checking whether the ‘planet’ is still detected; comparing the strength of the planetary signal with similar Keplerian signals injected into the original observations; performing Bayesian model comparisons between planet and no-planet models; and checking how robust the planetary signal is to datapoints being removed from the observations.

Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” We’re going to get a lot out of this investigation even though we lose Centauri Bb.

But back to that HST transit study run by Brice-Olivier Demory (University of Cambridge). I mentioned that it could detect no transit of Centauri Bb, which certainly fits with what we’ve just seen, but there was an interesting lightcurve suggesting a different possible planet, this one in an orbit that might range from 12 to 20 days. If this planet exists, radial velocity confirmation would be even more challenging than for Centauri Bb. Its signal, as Andrew LePage notes in The Discovery of Alpha Centauri Bb: Three Years Later, would be only half that of Centauri Bb.

LePage’s work at Drew ex Machina is definitive, and he has devoted a good deal of attention to Alpha Centauri. Here he explains why that second ‘planet’ is going to be so hard to spot:

Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations. In addition, since the radial velocity signature for this planet would be expected to be maybe half that of α Centauri Bb, this method has little likelihood of providing independent confirmation of this sighting any time soon. Once again, we will have to wait for a few more years for new telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launches in 2017 and may be capable of making the required observations of such a bright target.

Alpha Centauri is frustrating in many ways because you would expect the closest stellar system to have revealed more of its secrets by now. One of the problems, though, and a huge one, is that the angular separation (as viewed from Earth) of the primary Centauri stars has been decreasing as they move through their orbits. It won’t be until December of this year that they’ll reach minimum separation as seen from Earth. We’ll need to give Alpha Centauri a little time, in other words, before we can hope to get data on other possible worlds around Centauri B.


Image (click to enlarge): Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.

The Rajpaul paper is Rajpaul, Aigrain & Roberts, “Ghost in the time series: no planet for Alpha Cen B,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint). The Hatzes paper is “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, (2013) (preprint).


Comments on this entry are closed.

  • TLDR November 9, 2015, 13:34

    If this is true, then that means that we don’t know what kind of orbit an Alpha Cen exoplanet would have. May we assume that any such planet would be influenced by the gravity of its stars? This would tell whether we could expect transiting data at some point, or fall back to some other method of detection?

  • Steven Torry Rappolee November 9, 2015, 14:39

    Alpha Centauri the movie


    By SETI talks

  • Andrew LePage November 9, 2015, 14:43

    “Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” ”

    WOW! This is quite an admission! Members of the European HARPS team have typically not taken too kindly to criticisms of the existence of some of their recent “habitable zone” finds (e.g. Kapteyn b, GJ 581d, GJ 667Cd). Typically their counterarguments have centered on their staunch belief in the mathematics behind their analytical tools used to derive their results to the exclusion of any other evidence. Of course the results from such analyses are only as valid as the assumptions made about the data being analyzed (e.g. the nature of a bona fide planetary signature, the sources and characteristics of the “noise” in the data). I hope that the issues surrounding the sampling of the RV data can be addressed in future analyses so that smaller planets can be more confidently revealed.

    I also appreciate Paul’s mentioning my recent essay on Alpha Centauri Bb. Links to all of my essays on planet searches in this system can be found here:


  • Charlie November 9, 2015, 15:35

    All very good and well, but I wonder about something which I haven’t seen written anywhere else concerning this star system in particular.

    Consider for the moment that the question of planetary bodies as it appears to be at the present state of affairs does not seem to indicate an outright likelihood that are SIZABLE planetary bodies that might be ‘Earth-Like’. Has anyone considered that perhaps that is not necessarily a game changer with regards to this particular star system ? If there were to exist smaller bodies in the system say equivalent to the sizes of our Moon or a Bit Larger, might that be of sufficiency to permit colonization of that particular star system ? It may not possess an atmosphere, or have any type of life as we know it, but as a solid surface on which to settle a new colony of individuals, it might be just the very best that you can possibly do.

    It would seem unlikely that if there are no Minor Planets in this system, such as we see with the asteroid belt here that that would provide water and other resources that could be exploited. We’re always attempting to find earthlike planets else we refuse to set out to explore such a system. Nature isn’t going to always cooperate with us so we should adopt the attitude that beggars can’t be choosers. If we expect to be able to get anything done at all.

    I’m working on the assumption that a solid , moonlike surface while not ideal, can still serve as a place where underground shelters can be built to protect against the external vacuum and radiation that undoubtedly such a body would be exposed to.

  • TLDR November 9, 2015, 16:35

    @Charlie, if I understand your question correctly, our current instruments would not be capable of finding an Earth-sized planet in the habitable zones of A or B. That does not mean that they don’t exist.

    On the bright side, we don’t see any giant exoplanets in the HZ’s. If such planets were there, they might have swept out any Earthlike planets. No news, sometimes, is good news.

    And we are not even close to preparing to visit those planets. Perhaps not in our lifetimes.

  • Steven Torry Rappolee November 9, 2015, 21:22


    Off course Jupiter and smaller worlds could have exomoons with life.

  • Enzo November 9, 2015, 21:59


    Thanks for finding this video : I have been waiting for months, but the SETI Talks page that normally includes the video was empty, unlike all the other ones.

    BTW, I remember the repeated announcements for Deborah Fisher radial velocity search in 2008-9, with a 5 year time frame for a result.
    See here, but other announcements (or intention to proceed) can be found earlier :

    However, since then 6 or 7 years have passed, depend on how you count, no result.

    I just found this instead that mentions a transit detection for Alpha Centauri B but no specific information :

  • Michael November 9, 2015, 23:13

    I would have thought that if the two stars co-rotate i.e. in the same direction, the two planetary discs during formation would have more likely collided due to the small distance between them pushing planet forming material into the stars or very tight orbits. There may be no large planets at all around Alpha Centauri because of this process just dwarf planets and/or asteroids.

    Now if we did send a probe to AC it may be better to have the main craft as a bus which drops off a probe or leaves one attached to the first stage to go onto proxima with the second stage going onto AC, two birds with one stone.

    The angular distance between AC and PC is quite small.


  • Mike Lorrey November 9, 2015, 23:49

    Until they can say that there’s nothing mars size or larger in the habitable zone, the possibility remains open.

  • Rangel November 10, 2015, 1:47

    As stated the lack of news about planets there is actually very good, it could mean they are there but are too small (Earth or Mars-size) to be detected right now, we just need to wait.

    We have a triple star system as neighbor, i would be shocked if none of them has a terrestrial planet on the habitable zone, i mean for real, every couple of years we search again and restrains the upper mass/size limits of them, at this point we know there are not giant planets there, so we have 3 star system very likely to host terrestrial planets, that’s just amazing.

    It’s a shame we need to wait for new technology, still the new telescope generation promises a lot, i can’t wait to see what we will find, somehow i believe they are gonna be a game changer, specially those giant ones for 2020’s.

  • kamal ali November 10, 2015, 3:47

    they’ve only excluded close (<100d, ie <<1AU) super-earths and Neptunes within 1AU. they have NOT excluded earth-size planets at any distance; including around ~0.8AU which would be where 1 earth-equivalent insolation would be received. for all we know there could be 4 earth-size planets around B.

  • RobFlores November 10, 2015, 12:08

    Well, I hope the question of planets in centaurus system, is
    resolved relatively quickly. Even a full negative result would be eminently useful, We could discard twin suns, between 5AU-20 AU distant from planetary search, for suns approximately this size.

    Interestingly, if Beta Centauri did not exist and Proxima where orbiting on a elongated orbit that brought it between 25 AU – 50 AU from Alpha Centauri, it would be much more probable that the larger star would have habitable planets.

  • Andrew LePage November 10, 2015, 15:44

    @RobFlores November 10, 2015 at 12:08

    Beta Centauri is a triple star system consisting of three huge B-type stars about 390 light years away. Aside from being in the same constellation, this system has no connection to the triple star system of Alpha Centauri (which consists of a pair of Sun-like stars in an 80-year orbit, designated A and B, and a distant M-dwarf, Proxima Centauri). And while searches to date have eliminated the possibility of planets the size of Jupiter or Saturn orbiting inside the HZ of either Alpha Centauri A or B, the question of the presence of smaller planets including potentially habitable Earth-size worlds is still unresolved. Unfortunately, it is beginning to look more and more likely that precision RV measurements will be unable to resolve the issue for a variety of reasons and we will have to wait for space-based instruments capable of performing direct imaging searches that will (hopefully) be launched within the next decade or so. It is doubtful the issue will be resolved anytime soon.

  • Charlie November 10, 2015, 21:25

    In reference to my earlier comment a day or so ago concerning the presence or absence of some type of extrasolar body around the trio of stars in the alpha Centauri system. It occurred to me that there are very well might be a reasonable expectation that there could exist asteroidal or cometary bodies which would be capable of acting as a solid platform on which to, if the explorers wished, permit the establishment of colonies or what have you.

    I’m further struck by the fact that we have found a new addition in our own solar system, which I relate below as a link and as the announcement itself. Given the fact that it would be reasonable to expect that extrasolar star systems would be born in the same manner as our sun it would be than expected that there would be left over residual materials from the condemned station cloud that gave birth to that star. It would seem almost INCONCEIVABLE that there would exist a system which would be COMPLETELY swept clean of all possible debris of all types, as well as volatiles. I would imagine that even in the absence of a direct detection of such bodies, there could be a reasonable expectation that they would exist. In lieu of the fact that there would be a desire to explore this system a robotic probe would be the first choice to make a reconnaissance and to determine if there exists anything in the system, such as I hypothesized. I leave you with the promised announcement below :


    Most distant solar system object yet could hint at hidden planet.

    The inky black of the outer solar system just got a little brighter. A speck of light spotted in October 2015 is a rocky world more than 3 times more distant than Pluto – the farthest body in our solar system ever seen.

    “We don’t know anything about its orbit,” says Scott Sheppard of the Carnegie Institute of Washington, whose team discovered the new addition. “We just know it’s the most distant object known.”

    Sheppard announced the new object, called V774104, on 10 November at a meeting of the American Astronomical Society’s Division for Planetary Sciences, held in National Harbor, Maryland.

    From how it shifted in the sky as the Earth moved over a few hours, Sheppard’s team calculates that V774104 is about 103 astronomical units (AU) away from the sun, where one AU is the distance from Earth to the sun. That’s about as far away as the twin Pioneer probes, which have been traveling since 1972 and 1973.

    To be as bright as it is at that distance, the object needs to be between 500 and 1000 kilometres in diameter, less than half the size of Pluto.

    And it’s not alone. The same deep sky survey, conducted with the Subaru telescope in Hawaii and the Dark Energy Survey Camera in Chile, has also turned up about a dozen other objects around 80 to 90 AU from the sun. Since these distant bodies move around the sky slowly, it will take about a year of follow up observations to understand their orbits – and their origins.

    If it turns out their paths will take them inward near Neptune’s orbit, they were probably kicked out of the inner solar system after a brush with Neptune. But Sheppard hopes that some will turn out to belong to a class of true weirdos: the inner Oort Cloud.

    Dark planet

    Only two other known objects are thought to be members of this exclusive club: Sedna, discovered in 2003, and 2012 VP113, found in 2012. Neither of them ever comes closer to the sun than 50 AU. The rest of the Oort Cloud, which is thought to be a storage lot for long-period comets, extends out a hundred or even a thousand times farther than these objects.

    “Sedna and VP113 are the only object in the known solar system whose orbits cannot be explained by things in the known solar system,” says Michael Brown of the California Institute of Technology. They are far enough away from the giant planets to avoid gravitational tweaks to their orbits, and close enough to the sun that they don’t respond to other passing stars.

    One explanation for the strange orbits is the pull of a massive but very dark rocky planet. “Something might be shepherding the objects,” Sheppard says.

    But the more likely explanation, according to Sheppard and Brown, is that the inner Oort Cloud preserves signs of an era at the very start of the solar system, when the sun and planets were born in an interstellar nursery packed tightly with nearby stars.

    Over the next year, monitoring the new candidate objects may uncover a rare few that never venture into the inner solar system. If so, their orbits, added to Sedna and VP 113, should help us understand what influences such far-out worlds. Meanwhile, we have to wait to see what becomes of V774104.

    “You don’t know whether it’s just a gee-whiz record holder or something super cool,” Brown says. “I’ve got my fingers crossed for super cool.”

  • P November 10, 2015, 22:10

    The latest SETI Institute talk by Michael Endl is relevant here:


    His high cadence work with a modest instrument (1m) at Mt John in New Zealand very interesting, although the close approach of A and B has basically ended that work for several years until the stars start to seperate.

    In the meantime why not a high cadence RV campaign on Proxima? Also, surely we can expect some astrometric news from Gaia sometime before TESS launches?


  • TLDR November 11, 2015, 13:40

    @Drew, I suspect Rob was referring to the hypothetical exoplanet in the Alpha Centauri system.

  • TLDR November 11, 2015, 15:44

    Thank you, that was very informative.
    So, five years or so before this system is ready for its close-up.
    Meanwhile, we can put more birds in the sky and eyes on the ground to prepare for the stars’ separation.
    Could the inevitable signal contamination between the two stellar elements be used to help screen out the signals from exoplanets? In other words, we can determine how much gravitational influence the stars have on each other, and this should diminish as they become separated. so certain features in their signals should diminish – but others wouldn’t.

  • TLDR November 11, 2015, 19:07

    If AC’s exoplanets are not transiting (edge-on), we needn’t despair; for face-on (non-transiting) exoplanets, the Gaia satellite, launched in 2013, should be able to use astrometry to find AC’s planets.
    And the radial velocity method, which helps characterize an exoplanet’s mass, will be helped by ESPRESSO, due in 2017

  • RobFlores November 12, 2015, 12:09

    Yes, sorry for the miscue,

    Let me rephrase my prior message. A G type star with a single companion
    M type star with a long period (200 Yrs, no closer than 30 AU) is more likely to have habitable planets, than the A-B Centauri orbital/mass dynamics. Is this a fair assumption ?

  • Mark Zambelli November 12, 2015, 12:32

    Do we have any data on the spin axis alignment of either Alpha Cent A or B so we can get a feel for whether to even expect transits? I’m not assuming that the planes of planetary systems have to be aligned to the star’s rotation but angular momentum leads me to guesstimate most should. As TLDR pointed out, maybe Gaia could provide us with some data what with radial velocity measurements being a pain and transit measurements coming up empty (so far).

  • andy November 12, 2015, 16:13

    the Gaia satellite, launched in 2013, should be able to use astrometry to find AC’s planets.

    Maybe, maybe not. Alpha Centauri is brighter than the initially planned bright magnitude threshold for the Gaia mission. According to this page, there are planned observations in a special mode for stars with Gaia magnitudes less than 3 (which would include Alpha Centauri), but it remains to be seen what kind of accuracy can be achieved.

  • JAMES STILWELL November 13, 2015, 16:48


  • Robert Eachus April 3, 2016, 9:02

    There is an “easy” way to settle all this. There should be occasions where a satellite in geosynchronous orbit (or other orbit) passes between the Hubble and Alpha Centauri A . Hoping for two occulting events (one for A one for B) at the same time would be asking for a bit much. But even if the remote satellite did not provide a 100% occultation picking out objects otherwise lost in the glare should be possible. (Using the dark edge of the moon as an occulter is also possible. It might not work for Alpha Centauri, but should be doable for stars nearer the celestial equator.)

    Better is to try this with the JWST once launched. There are other satellites at L1 which might be used. The best would be to build an occulter for the JWST that could be positioned a thousand miles or so from the JWST, and moved around to occult other nearby stars–or even not so nearby. If the disk were purpose designed, it should be possible to see planets at 1 AU around any non-giant star within a few hundred parsecs.

    Note that multiple observations are going to be necessary. A pointlike object could be a planet, or a more distant star. Enough observations would allow an orbit to be calculated, but even two observations should make a planet more likely than a distant star if (relative) motion is detected.