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Happy Anniversary α Centauri Bb?

A physicist and writer well-versed in the intricacies of the exoplanet hunt, Andrew LePage now turns his attention to the question of planets around Centauri B, and in particular the controversy over whether the highly publicized Centauri Bb does in fact exist. Today is the second anniversary of the discovery announcement, and we still have work to do to resolve whether ‘noise’ in the data — explained below — may account for what seems to be a planet. The good news is that multiple teams continue to work on Alpha Centauri, and we should expect answers within several years, or just possibly, as LePage explains, a bit sooner than that.

by Andrew LePage


Time certainly seems to fly at times. It has already been two years since the October 16, 2012 announcement by a Geneva-based team of astronomers of the discovery of a planet orbiting our Sun-like neighbor, α Centauri B, using precision radial velocity measurements. While this planet, designated α Centauri Bb, was hardly the Earth-like planet for which interstellar travel enthusiasts had been waiting so long, its presence demonstrated that the closest star system to us harbored at least one planet and held the promise of more to be discovered. But two years after this momentous announcement, many questions still remain and this important discovery has yet to be independently confirmed.

First some background: at the heart of the α Centauri system 4.37 light years away are a pair of Sun-like stars, designated α Centauri A and B, locked in an eccentric 79.9-year orbit. The probable third member of this star system, located about 15,000 AU from the main pair of stars, is a dim red dwarf better known as Proxima Centauri. With a distance of 4.24 light years, it is the closest known star to our solar system. Despite the distance between α Centauri A and B varying from 11 to 35 AU during the course of one revolution, various dynamical studies performed over the decades have confirmed that regions with stable planetary orbits do exist in this system. These studies have shown that orbits out to about 3 AU, give or take, would be stable depending on their inclination to the plane of the orbit of α Centauri A and B about each other. What has not been so clear is if planets could form around this pair of stars.

A number of studies performed over the past couple of decades have been more or less evenly split on the question of whether or not planets could form around α Centauri A and B. Some studies have shown that the building blocks for planets, called planetesimals, would be able to collect themselves together into planets out to some reasonable distance. Still other studies have suggested that the presence of the two stars would have stirred up the orbits of the planetesimals too much. Instead of collecting into larger bodies, the planetesimals would tend to smash themselves apart upon contact so that planets could not form. As in the story about the ancient Greek philosophers arguing about how many teeth a horse has, it made sense to open the horse’s mouth and simply count them – it was time to look for planets orbiting α Centauri A and B.

Given the difficulty of detecting extrasolar planets even in a nearby star system like α Centauri, the first technology that offered reasonable chance of success was the precision measurement of changes in the stars’ radial velocity resulting from the small reflex motion of an orbiting planet. But after almost two decades of measurements with increasingly better instruments, the results of searches for planets orbiting α Centauri A and B published up to 2011 had found nothing. This null result combined with dynamical arguments only demonstrated that planets larger than Saturn or Jupiter did not orbit within about 2 AU of either α Centauri A or B. This still left a lot of possibilities including Earth-size planets orbiting comfortably inside the habitable zones of these stars but much more precise radial velocity measurements would be required to detect them.

Beginning around seven years ago, several teams employing various observing approaches are known to have started looking for lower-mass planets orbiting α Centauri A and B with instruments capable of making radial velocity measurements with uncertainties on the order of one meter per second – a factor of up to four more precise than in previously published results for the system. The first team to announce any results from their search was the European team using the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. They employed a new data processing technique to extract the 0.5 meter per second signal of α Centauri Bb out of 459 radial velocity measurements they obtained between February 2008 and July 2011. These radial velocity data had a measurement uncertainty of 0.8 meters per second and contained an estimated 1.5 meters per second of natural noise or “jitter” resulting from a range of activity on the surface of α Centauri B modulated by its 38-day period of rotation.


Image: An artist’s impression of the still unconfirmed α Centauri Bb whose discovery was announced on October 16, 2012. (Credit: ESO/L. Calçada/Nick Risinger)

The HARPS team’s analysis indicated the presence of a planet with a minimum mass or Mpsini (where i is the unknown inclination of the planet’s orbit to our line of sight) of just 1.1 times that of Earth, locked in a tight orbit with a radius of just 0.04 AU and a period of 3.24 days. This was well below the upper limits set by earlier searches. Given sufficient observation time, the team estimated that they could detect a planet with a Mpsini of about four times that of the Earth in a 200-day orbit inside the habitable zone of α Centauri B. While the result generated much excitement, it was also met with a healthy amount of skepticism in the astronomical community because of the previously untried technique used to process the data to extract such a low-amplitude signal.

One of the first critics, American astronomer Artie Hatzes (Thuringian State Observatory, Germany), performed his own analysis of the publicly available HARPS data set using two different data processing techniques to look for the radial velocity signal of α Centauri Bb. Formally published in June 2013, Dr. Hatzes’ analysis did indeed find a signal buried in the radial velocity data with a period of 3.24 days but it had a false alarm probability of a few percent – far too high to be considered a reliable detection. Furthermore, his analysis of the “random” noise in the data showed that it had periodicities in the 2.8 to 3.3 day range and amplitudes on the order of half that of the alleged planetary signal. Given the recent situation of planetary false alarms with GJ 581 and GJ 667C, this finding suggested that noise in the data, whether from the instrument or activity on α Centauri B, might have been mistaken for a planet. Dr. Hatzes concluded that additional data were needed to better understand the nature of the noise in the radial velocity measurements and confirm the planetary nature of the radial velocity signal.

Other teams have already been taking data in order to confirm the existence of α Centauri Bb as part of their ongoing observing programs although no results have been formally published to date. A team of astronomers working with the 1.5-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile are using CHIRON (CTIO Higher Resolution Spectrometer) to search for planets orbiting α Centauri A and B in part with the support of The Planetary Society. The project’s principal investigator, Debra Fischer (Yale University), quoted in a blog on The Planetary Society’s web site posted earlier this year that they had insufficient data to detect α Centauri Bb when its discovery was announced in October 2012. They launched a renewed effort to gather much more data at a higher cadence starting in 2013 aimed specifically at detecting the purported planet’s 3.24-day signal. To date they have not detected α Centauri Bb in their data but their simulations indicate that any such detection would have been marginal at best so far.

One of the issues complicating continuing efforts to gather more data needed to resolve the situation with α Centauri Bb is the increasing amount of stray light from α Centauri A that is degrading the quality of radial velocity measurements. As viewed from the Earth, the apparent separation of α Centauri A and B has been decreasing at an accelerating rate for about a third of a century as they move in their inclined elliptical paths around each other. The two stars will reach a near-term minimum separation of just four arc seconds at the end of 2015. The conventional wisdom has been that it will be several more years before the separation of α Centauri A and B increases enough to acquire new data of sufficient quality to confirm α Centauri Bb. But we might not have to wait this long after all.

One of the other groups known to be searching the α Centauri system for planets is a team of astronomers using the HERCULES (High Efficiency and Resolution Canterbury University Large Echelle Spectrograph) spectrograph on the one-meter McLellan Telescope at the Mt. John University Observatory in New Zealand. In July 2014 they submitted a paper for publication where they described a new technique to reduce significantly the effects of stray light contamination in precision radial velocity measurements.

In order to test the effectiveness of their new technique, they observed four double-line spectroscopic binaries (i.e. pairs of unresolved stars that can only be differentiated by periodic Doppler shifts in their spectral lines) whose blended images represent the worse-case scenario of “contamination”. With the new technique, they were able to recover accurate radial velocities of both components of the observed spectroscopic binaries. The New Zealand-based team now plan to use their new analysis method to reduce the data they are continuing to gather as part of their observing campaign of α Centauri that started in 2007. Their calculations show that they should be able to detect α Centauri Bb if it exists. If they are successful, the situation with α Centauri Bb might be resolved much sooner than later.

A more detailed account with general references to the discovery of α Centauri Bb along with more background information on the system can be found on my web site in the post titled The Search for Planets Around Alpha Centauri. A second post in this series, The Search for Planets Around Alpha Centauri – II provides details of the results of past searches for planets orbiting α Centauri A and B as well as what current and soon-to-be-started search programs hope to find.


Comments on this entry are closed.

  • James M Essig October 16, 2014, 9:29

    Awesome! Pandora, here we come! Perhaps. I hope additional observations reveal more planets in the Centauri System.

  • Marshall Eubanks October 16, 2014, 9:53

    Here is the Hatez paper http://arxiv.org/abs/1305.4960 , which I found fairly compelling. I would expect that, if this is a real planet, the close approach of Alpha Centauri A would perturb its orbit (probably by a rapid precession of nodes, maybe also of periapse). If that could be detected in Doppler data it would be pretty unassailable IMO. (This planet would be 30 milliarcseconds from Alpha Centauri B at maximum elongation – that’s far enough you have to wonder if the Webb telescope could separate them in the Infrared.

    By the way, have you heard any results from the Proxima Centauri microlensing opportunity http://arxiv.org/abs/1401.0239 ?

  • ljk October 16, 2014, 11:21

    What an interesting coincidence. October 16 is the day we sent our first manned colonizing mission to Alpha Centauri. In 1997.

    Don’t you folks remember? It got lost – in space – on the way due to sabotage….


  • Daniel October 16, 2014, 14:19

    Talk about Apha Centuari system,is Mesolensing event for this October for the Proxima Centauri that could eventually detect planets around it:


    Anyone remember it?

  • Alex Tolley October 16, 2014, 14:42

    OT but relevant when discussing life on exo planets.
    New Genes = New Archaea?

    If the results hold up to further inquiry, however, the implications for scientists’ understanding of the early evolution of microbes could be huge. Martin compares the large-scale gene acquisitions in archaea to the origin of eukaryotes, which may have occurred when an archaeal cell incorporated a bacterial cell (along with its genes), which became the mitochondrion. “If this is widely accepted, it makes the origin of eukaryotes not as special,” said Doolittle. “That would be quite a paradigm shift in the way we think about things, because most people regard the origin of eukaryotes as a uniquely almost miraculous event.”

    Back on topic, is HERCULES suitable for spectrographic analysis of exoplanet atmospheres?

  • Andrew LePage October 16, 2014, 17:51

    @Marshall Eubanks October 16, 2014 at 9:53

    I’d have to see some detailed dynamical simulations to convince me that the orbit of Alpha Centauri Bb with a semimajor axis of just 0.04 AU would have its orbit perturbed in any measurable way during the periapsis pass of Alpha Centauri A in 2035. And, of course, how the orbit changes would depend critically on the mutual inclination of the orbit of Alpha Centauri Bb with the orbit of Alpha Centauri A and B which can not be determined directly by radial velocity measurements. I am also rather dubious about measuring any precession in the line of nodes, etc. of the orbit of Alpha Centauri Bb. The discovery paper (Xavier Dumusque et al., “An Earth-mass planet orbiting α Centauri B”, Nature, Vol. 491, pp. 207-211, November 8, 2012) states that the eccentricity of the orbit is poorly constrained by the current data and is consistent with a circular orbit which would be expected with such a small semimajor axis due to tidal dampening. Given how difficult it is to even detect this planet through the natural noise noise or jitter of Alpha Centauri B, I seriously doubt that the radial velocity measurements can ever be precise enough to detect the subtle change of the alignment of such a nearly-circular orbit.

    As for the JWST ability to detect Alpha Centauri Bb through direct imaging, again, I do not think that this will be possible. First of all, JWST will have a resolution on the order of only many tens of milliarcseconds at best (its design is optimized for longer IR wavelengths). Even if somehow its short wavelength performance was much better than its specs AND there was an imager available with a suitable pixel scale (the NIRCam, for example, has a pixel scale of ~32 mas which is insufficient to resolve such an object), JWST will be unable to spot an object 30 mas from a star with a contrast ratio on the order of 10^-5, give or take. JWST simply is not designed for that kind of work.

    Finally, yes, I was aware of the microlensing opportunity with Proxima Centauri but I am unaware of any attempts to take advantage of it – especially searching for microlensing events from any planets (which would require a lot of dedicated time on very large telescopes to pull off with a mag 20 background star).

  • Andrew LePage October 16, 2014, 18:06

    @Alex Tolley October 16, 2014 at 14:42

    I seriously doubt that HERCULES will be able to examine the atmosphere of Alpha Centauri Bb assuming the planet exists and it has an atmosphere (its stellar flux is high enough that it might not, assuming it has an actual mass within a couple times that of Earth’s). No Earth-based telescope can detect the planet directly even with adaptive optics because of its 30 milliarcsecond seperation so that direct spectroscopy is impossible. And I don’t believe HERCULES has the sensitivity needed to do transmission spectroscopy during a transit of Alpha Centauri Bb (which, to the best of my knowledge, have not been observed and would likely be undetectable from the ground anyway).

  • Harry R Ray October 17, 2014, 9:58

    LATE BREAKING NEWS! Dark matter has been DIRECTLY for the firt time! AND: Not by a particle accelerator, but; by an X-ray telescope! A solar SEASONAL X-ray flux can ONLY be explained by the disintigration of solar axions in the presence of the solar magnetic field! Thus the SEASONAL effect is produced due to the most precice alignment of that field with the telescope! OTHER AXION NEWS: It appears hat the mysterious short period radio bursts can be BEST explained EITHER by a white hole coming into esistance OR by an Axion star COLLIDING with a neutron star. If the LATTER is true, the Axion star would be a strange beast indeed, having the mass of a large asteroide and the radius of a few centimeters!

  • ljk October 17, 2014, 13:01

    An article on the possible dark matter find:


    Inexplicable signal from unseen universe provides tantalizing clue about one of astronomy’s greatest secrets — dark matter

    Date: October 16, 2014

    Source: University of Leicester

    Summary: The first potential indication of direct detection of dark matter — something that has been a mystery in physics for over 30 years — has been attained. Astronomers found what appears to be a signature of ‘axions’, predicted ‘dark matter’ particle candidates.

  • Joëlle B. October 17, 2014, 14:46

    @Harry R Ray

    Arthur C. Clarke’s novel The Songs of a Distant Earth took place in a universe where humanity tries to evade the (now debunked) Solar neutrino problem by travelling to Alpha Centauri. Of course, now we know that the “problem” was with our lack of knowledge and equipment; interesting, in such regards, to see how our observation of axions will pan out in the near future and possibly how they will affect our understanding of dark matter and (maybe) the coronal heating problem.

    On a side note, I randomly discovered a video of a young and spicy Crystal Gayle on the Muppet Show singing her song ‘We Must Believe in Magic’, and I thought the lyrics befitting, considering the current angst in planet hunting in the system. I knew about her from my father having a large poster of her in the garage of my childhood home–very intriguing to wonder how much of our lives evolve through unintentional, oftentimes subliminal influences. :)


  • Joëlle B. October 17, 2014, 15:26

    The correct title of the Clarke novel is ‘The Songs of Distant Earth’, minus the ‘a’.

    Another interesting read is Isaac Asimov’s Homo Sol, also including a planet in the system. I, too, believe this is an example of one of the first ‘first contact’ sci-fi stories whereby alien civilizations officially recognize humanity after the discovery of FTL travel, contemporaneously with Star Trek? I wonder how the concept first came up to both Asimov and Roddenbury? It’s really a nice piece of speculative science fiction, also touching on psychology and the role it may play in a future interstellar society.


  • Andrew LePage October 18, 2014, 10:29

    I find this discussion about Isaac Asimov to be rather interesting given how his NONfiction influenced me over three decades ago. While I was hugely interested in space exploration at an early age, I did not read much about science in general until I stumbled upon and read a paperback collection of Isaac Asimov science essays when I was 13 years old. Afterwards I was hooked on Asimov’s nonfiction and bought every book of his I encountered all through high school and college.

    I still recall one of Asimov’s essays, “The Planet of the Double Sun”, that I read in “Asimov on Astronomy” (a copy of which I bought in July 1979 and still have in my personal library) which speculated about how Earth’s mythology might have been affected if, instead of orbiting a single star, Earth orbited Alpha Centauri A with a bright Alpha Centauri B orbiting far away. These types of essays and the way Asimov approached calculating various things out of curiosity had a profound influence on my thinking as a teenager (contributing to my wider interest in science and getting me interested in physics) and my writing as an adult.

  • Joëlle B. October 18, 2014, 20:08

    I’m aware that Asimov was a very prolific writer, although people tend to hate on his earlier SF. As for Earth’s mythology, it would seem that it wouldn’t exist in such a system from the above data we’re working with. :) If we believe in magic, then I guess we could postulate more gods and two Japanese emperors.

  • Tom Baty October 18, 2014, 22:52

    Unless it turns out to be yet another “cold fusion” type of fiasco , perhaps, just maybe, Skunk Works surprise effort at a fusion reactor might change our outlook for a possible unmanned probe to the Centauri system. If a practical fusion power system becomes a reality in a decade or so, as they are talking about, it will change space programs considerably. The first probe to Alpha Centauri could be only decades away instead of a century plus. Much could be wrong or off base…we will see.

  • andy October 19, 2014, 15:58

    Interesting to note that the nearest sunlike stars (Alpha Centauri, Tau Ceti, Epsilon Eridani) all have significant questions about the reality of their planetary systems.

    One recent development with regards to Alpha Centauri is the inclination determination for Alpha Centauri B, as detailed in two arXiv papers (here and here). It appears to be substantially misaligned with the binary plane, which is not particularly surprising given the statistics of binary stars of comparable separation, but perhaps not good news for the prospects of habitable planets existing around the star if the system formed with a large mutual inclination.

  • Eniac October 19, 2014, 22:01

    Tom Baty is correct. If a relatively compact hot fusion reactor can be built, propulsion is likely to be its most straightforward application. Instead of messing with power conversion, you just introduce a leak at one point where a fraction of the plasma can escape, and you have a fairly efficient rocket.

    As the Skunkworks project indicates, and the graph here shows: http://flowoftheprocess.wordpress.com/2011/07/03/fusion-research-ahead-of-the-moores-law-curve/ , fusion is in many ways more attainable technology in the (relatively) short term than most of the alternatives so often discussed here.

  • william October 20, 2014, 4:12

    @Andrew LePage

    I was quite intrigued by your comments regarding Alpha Centauri possible planetary body. A question or two on what you stated; you said that you would like to see a detailed dynamical simulation on the possible planet. When you speak of detailed dynamical simulation in what form would your analysis consist of ? What I mean here is would you have a detailed computer printout or some type of plot generated to scale regarding the possible orbit ? I’m extremely interested in just how one makes a determination of ANY possible orbit analysis.

    Secondly, you mentioned that there would be some natural noise or jitter associated with Alpha Centauri B; of what would this expected noise or jitter from a signal standpoint consist of ?

  • Ronald October 20, 2014, 5:54

    So many fascinating things to read, so little time…
    I have been very busy lately, so little or no time to react substantively, but I do want to commend and compliment Andrew LePage on his very interesting website! I have been reading the mentioned articles (and a few others) with great interest, even fascination. What an interesting, original and accessible site, almost as awesome as Centauri Dreams ;-)

  • Andrew LePage October 20, 2014, 10:30

    @william October 20, 2014 at 4:12

    Re: dynamical simulations, I would want to see documented results (tabular or graphic with a description of how the simulation was performed) of how the key orbit parameters of Alpha Centauri Bb change over time and how these changes correlates with the orbit of Alpha Centauri AB to prove Marshall’s speculation that “the close approach of Alpha Centauri A would perturb its orbit (probably by a rapid precession of nodes, maybe also of periapse)”. With that information in hand, one could then figure out if the determination of the orbit parameters of Alpha Centauri Bb by means of RV measurements would ever be accurate enough to detect these changes and provide dynamical proof of the existence of Alpha Centauri Bb. While certainly interesting, my gut feeling is that the changes in the orbit would be too small to ever detect using RV data.

    As for the the sources of natural noise or jitter in the RV data from Alpha Centauri B (or indeed any star), I discussed this in the first of my detailed essays on my web site cited above (http://www.drewexmachina.com/2014/08/11/the-search-for-planets-around-alpha-centauri/). But in brief, such noise can be caused by changes in granulation or the star’s convection pattern, starspots, phlages and other types of magnetic surface activity that changes over time and varies with the rotation of the star. Mature stars like Alpha Centauri B typically have low levels of jitter but it still amounts to about 1.5 m/s – about three times larger than the amplitude of the radial velocity signature of Alpha Centuari Bb. Astronomers’ ability to accurately model this jitter and remove it from their data is what will eventually limit the ability of the RV method of planet detection and is at the heart of the doubts about the existence of Alpha Centauri Bb.

  • Mark Zambelli October 23, 2014, 11:19

    Thanks Andrew, and I’m sure I’m not alone in finding the possibility of a planet(s) residing in our nearest neighbouring system a lifelong tantalizing. It’s good to know of the new techniques being employed to tease something of the data rather than waiting for more favourable seperations.

    Here is the most recent Planetary Society update which Andrew mentioned…


  • Andrew LePage February 23, 2015, 9:44

    To complete the discussion of the search for planets in the Alpha Centauri system, here is an essay on the results of various searches for extrasolar planets orbiting our closest neighbor, Proxima Centauri:


  • Andrew LePage March 28, 2015, 9:19

    The results of attempts to spot the photometric signature of transits of Alpha Centauri Bb using the Hubble Space Telescope has been recently submitted for publication. While no transits of Alpha Centauri Bb were spotted, a clear transit-like event was observed suggesting the presence of another as yet unconfirmed extrasolar planet orbiting Alpha Centauri B perhaps once every 10 to 20 days.