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Reflections on Centauri B b

When planet-hunter Greg Laughlin (UC-Santa Cruz) took his turn at the recent press conference announcing the Alpha Centauri B findings, he used the occasion to make a unique visual comparison. One image showed the planet Saturn over the limb of the Moon, as shown immediately below in a 1997 photo from Krzysztof Z. Stanek. Think of this as the Galilean baseline, for when Galileo went to work on the heavens with his first telescope, the Moon was visually close at hand and Saturn a mysterious, blurry object with apparent side-lobes.

Laughlin contrasted that with the image I ran yesterday, showing the Alpha Centauri stars as viewed from Saturn, a spectacular vista including the planet and the tantalizing stellar neighbors beyond. Four hundred years after Galileo, we thus define what we can do — a probe of Saturn — and we have the image of a much more distant destination we’d like to know a lot more about. The findings of the Geneva team take us a giant step in that direction, revealing a small world of roughly Earth mass in a tight three-day orbit around a star a little smaller and a little more orange than the Sun. What comes next is truly interesting, both for what is implied and for what we are capable of doing.

Image: Have another look: Alpha Centauri as seen from Saturn space. These days we can see Saturn’s features in sharp detail thanks to Cassini, but Centauri A and B remain distant and enigmatic. The discovery of Centauri B b is the first step in sharpening our focus. Credit: JPL/NASA.

Be sure to check Alpha Centauri B b on Greg’s systemic blog for his latest thoughts.

Closing on Alpha Centauri

Surprisingly, there is a 10 percent chance that Centauri B b is a transiting world, and a slightly higher chance still if the new planet is in the orbital plane of the binary stars. Remember, the primary Centauri stars are just eleven degrees from being seen edge-on to us. We’re talking about a very challenging detection scenario, but one that’s not out of the question for an instrument like the Hubble Space Telescope. Clearly, a transit would be a major boost, allowing us to determine the radius and the density of the planet (and, obviously, confirming its existence). Stéphane Udry (Observatoire de Genève) told the news conference that a proposal to examine Centauri B for transits has already been sent to the Hubble team.

Radial velocity work on the Centauri stars has proven tricky business, requiring 500 nights of observing time spread over a nine year period. But things are going to get a bit more complicated still, as Laughlin explained in an email on Tuesday. For Centauri A and B are not exactly static, and stray light from the brighter Centauri A can contaminate the studies of B:

I really like the particular way that the narrative is unfolding. The presence of the 3.2-day planet, taken in conjunction with the myriad Kepler candidates and the other results from the HARPS survey, quite clearly points to the possibility, and I would even say the likelihood, of finding additional planets at substantially more clement distances from the star. Alpha Cen A and B, however, are drawing closer together over the next several years, severely metering the rate at which high-precision measurements can be obtained. This builds suspense! It reminds me a bit of a mission like New Horizons, where the long coast to the destination serves to build a groundswell of excitement and momentum for the dramatic close encounter.

At the news conference, Laughlin likened our current state to halftime at a football game. We’ve pulled out a major detection but even as we start to speculate about rocky worlds further out in the system, we’re faced with increasingly difficult observations. We can expect the Alpha Centauri story to unfold slowly, but Xavier Dumusque (Centro de Astrofísica da Universidade do Porto) pointed out how much more difficult it becomes to find planets as we move further out in the Centauri B system, adding that it would take at least twice as many measurements as the Geneva team has now made. Right now the researchers are saying the HARPS spectrograph might be limited to a planet with a lower mass limit of about four Earth masses here, but Stéphane Udry added that new ESO instrumentation was in the works that offered, in the not so distant future, good prospects for finding an Earth-mass planet in the habitable zone.

A 230-day orbit around Centauri B should put us right in the middle of the habitable zone, the place we’d most like to find a terrestrial world. Fortunately, it’s a region of orbital stability — the effects of Centauri A only become problematic as we move as much as 3 AU out from the star. Before we can find a habitable zone planet, we’ll need to confirm Centauri B b and begin to study it, which is where that useful transit could come in. The probable picture is stark — a rocky, lava-world with a surface temperature somewhere around 1500 Kelvin, surely in a tidally locked orbit. Not exactly a clement place, but the implication of other worlds in this system will urge us forward.

Putting the new planet in perspective involves seeing the overall picture of exoplanet research, which could be changed by the discovery. In his email, Laughlin noted the possibilities:

I think that this is important for a society that is increasingly expectant of immediate interactivity and instant gratification… I hope that this detection of Alpha Cen B b provides an impetus for the funding of additional radial velocity infrastructure, and also for space-based missions such as TESS, which can find and study the very best planets orbiting the very nearest stars.

Will the focus now shift from the statistical wonders being revealed by Kepler to the many nearby stars about which we have little planetary information? We saw the other day that the ESA mission concept called NEAT offers new ways to study Alpha Centauri and other nearby stars, and TESS (Transiting Exoplanet Survey Satellite) is likewise out there as a concept that would allow us to survey 2.5 million of the brightest stars in the sky. Data from a mission like this could be handed off to the James Webb Space Telescope for more intense investigation. All that depends on funding and the continued awakening of interest in planets around neighboring stars.

The Choice of Centauri B

We have three stars in the Alpha Centauri system, but Centauri B has been at the center of the current effort, not only by the Geneva team but by Debra Fischer’s team and a third group in New Zealand. Why Centauri B and not its brighter partner, Centauri A? Both could have planets, and in the case of A we can’t rule out planets as large as ten Earth masses (gas giants would probably have been detected by now). What a fascinating scenario that is: Two planetary systems, each with the possibility of a planet in the habitable zone. Imagine the spur to space travel that would give any civilization there, to find a habitable world within easy striking distance!

The focus is on Centauri B because it is a more promising study for the radial velocity methods used in this investigation. Its level of stellar activity is low, which means there are fewer perturbations that can distort radial velocity measurements. It’s also a cooler star than our Sun, which means the habitable zone will be closer to the star than around the brighter Centauri A. Remember that with radial velocity methods we’re looking at incredibly tiny distortions in the movement of the star (in the case of Centauri B b, 51 centimeters per second, or 1.8 kilometers per hour, the highest precision ever achieved using this method). A smaller mass means stronger radial-velocity variation for a planet of similar mass, hence an easier detection.

Image: An artist’s impression of the planet orbiting Alpha Centauri B, a member of the triple star system that is the closest to Earth. Alpha Centauri B is the most brilliant object in the sky and the other dazzling object is Alpha Centauri A. Our own Sun is visible to the upper right. The tiny signal of the planet was found with the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. Credit: ESO/L. Calçada/N. Risinger (skysurvey.org).

Not that Centauri B is an easy target. There are plenty of factors intrinsic to the star that can introduce jitter in these observations and thus render planet detection difficult. A huge part of the Geneva team’s work has been to examine HARPS spectrograph observations between February 2008 and July 2011 to model and remove all non-planetary sources of perturbation. From the paper:

The raw radial velocities of α Centauri B… exhibit several contributing signals that we could identify. Their origin is associated with instrumental noise, stellar oscillation modes, granulation at the surface of the star, rotational activity, long-term activity induced by a magnetic cycle, the orbital motion of the binary composed of a Centauri A and B, light contamination from a Centauri A, and imprecise stellar coordinates.

Each of these factors had to be modeled and subtracted from the data. The team performed Monte Carlo simulations to check against the signal at 3.236 days being an artifact from the elimination of the stellar signals and was able to conclude that the signal is real. Xavier Dumusque, lead author of the discovery paper, showed the press conference graphs of Centauri B’s magnetic activity, noting that as the latter went up, radial velocity jitter increased. Notice how subtle these effects are and remember that they must be understood to get the real picture:

For the Sun, as for other stars similar to α Centauri B in spectral type, convection induces a blueshift of the stellar spectra. Therefore, no convection means no convective blueshift inside these regions, and so the spectrum of the integrated stellar surface will appear redshifted. Because a redshift means a measured positive radial velocity, a positive correlation between the magnetic cycle variation and the long-term radial velocity variation is then expected.

Get the noise out of the data and that 51 centimeter per second signal persists as Centauri B b.

Significance of the Find

There was a sense of exhilaration in the air on Tuesday as the buzz around an Alpha Centauri planet built, and when the embargo was lifted, reports of the find filled the social media as the early articles began to appear online. Just how big a deal is Centauri B b? A skeptic could point out that while finding an Earth-mass planet is significant, it must still be confirmed, and in any case, this is an Earth-mass planet that is nothing like a clement, habitable world. Then too, the level of investigation involved here was so intense that it may be years before we learn about other planets in this system, not to mention planets around Centauri A or Proxima.

NASA’s John Grunsfeld, Science Mission Directorate Associate Administrator for the agency, had this to say about NASA’s plans in the days after the discovery:

“NASA’s James Webb Space Telescope (JWST) will provide a unique facility that will serve through the next decade as the mainstay for characterization of transiting exoplanets. The main transit studies JWST will be able to undertake are: discovery of unseen planets, determining exoplanet properties like mass, radius, and physical structure, and characterizing exoplanet atmospheres to determine things like their temperature and weather. If there are other planets in the Alpha Centauri system farther from the star, JWST may be able to detect them as well through imaging.

“NASA is also studying two medium-class exoplanet missions in our Explorer program, and in the spring of 2013 will select one of them to enter development for flight later in the decade.”

Clearly the game is afoot. A confirmed Centauri B b would tell us that planet formation is indeed possible in the nearest stellar system to our own — this had by no means been obvious, and the debate over planet formation mechanisms in close binaries has been brisk. The presence of a rocky planet here obviously implies the presence of other worlds, and the Geneva team holds out strong hope that we’re up to the task of finding them. From the paper:

The optimized observational strategy used to monitor α Centauri B is capable of reaching the precision needed to search for habitable super-Earths around solar-type stars using the radial-velocity technique. However, it requires an important investment in observation time, and thus only few targets can be observed over several years. Recent statistical analyses and theoretical models of planetary formation suggest that low-mass rocky planets and especially Earth twins should be common. We are therefore confident that we are on the right path to the discovery of Earth analogues.

Alpha Centauri is obviously a prime target for any future interstellar probe because it is so much closer than other stars. Space-based instrumentation will one day be able to tell us something about the larger Centauri B system, assuming other planets are present. The discovery of a terrestrial world in the habitable zone here would be a spur to exploration that could drive public interest and funding for increasingly sophisticated technologies. Maybe a distant but theoretically reachable green and blue world is out there around our nearest neighbor, but we won’t know until we commit the resources to continue the investigation. Centauri B b is an exciting start to characterizing this fascinating system, a process that will demand time, patience, and effort just as rigorous as the Geneva team put in here. Well done to all involved!


Comments on this entry are closed.

  • henk October 18, 2012, 9:19

    some people already say the chance is 100% we find 1 habitable planet there, maybe a and b have both habitable worlds. I hope after all this hype about this star system that we will not be disappointed.

  • David October 18, 2012, 10:03


    It is stimulating some talk . His liberal base on daily kos would love the spending on such a mission…….

  • ljk October 18, 2012, 10:29

    Extreme Solar Systems: Why Aren’t We Finding Other Planetary Systems Like Our Own?

    October 16, 2012

    Artist concept of a previous multi-planet solar system found by the Kepler spacecraft. Credit: NASA/Tim Pyle

    Most planetary systems found by astronomers so far are quite different than our own. Many have giant planets whizzing around in a compact configuration, very close to their star. An extreme case in point is a newly found solar […]

    Read more: http://www.universetoday.com/#ixzz29f2dPNgZ

  • Dwight Williams October 18, 2012, 10:52

    Like I was saying to friends: it’s real. It’s there. It’s a baked rock, but it’s there.

    Looking forward to seeing whatever else is also there.

  • Joe October 18, 2012, 11:25

    I’d like to see some atmospheric modelling done for ALF Cen B b. 1500 K on the dayside is pretty nasty, but if the atmosphere is thin enough (or absent) the nightside could be quite cold. Also the nightside would not be in perpetual darkness, because it would receive the light from ALF Cen A. Finally, an Earth-mass rocky world with a 3.5 day rotation should have a healthy magnetic field. Healthy enough to shield the surface from a star 0.04 AU away? I don’t know, but this could present effectively two worlds. One incredibly hot with constant bright daylight, the other cool with a dim 3.5 day/night cycle. Could a thin superrotating atmosphere moderate portions of the latter?

  • Kappy October 18, 2012, 11:27

    From the article: “Get the noise out of the data and that 51 centimeter per second signal persists as Centauri B b.”

    My question is, while this is an incredible result, are we really so sure we already know all the possible causes of radial velocity noise? Isn’t it possible that this 51 centimeter per second change is caused by some as yet unobserved or unknown phenomenon?


  • Tulse October 18, 2012, 11:47

    I’m curious how the close but somewhat eccentric orbit of A might affect the habitable zone of B. As I understand it, A is 1.5 times as bright as the Sun, and its distance from B varies from the equivalent of Saturn at its closest to Pluto at its farthest. Does anyone know how much that affects the energy received by a planet in B’s potential habitable zone?

  • lurscher October 18, 2012, 12:56

    the first interstellar mission will be unmanned, but my hope is that this unmanned probe will be able to do a lot that we wouldn’t expect from a first mission. Hopefully by then, we can develop probes able to mine and build other probes in an autonomous way, so that their presence will be permanent on the system

  • Joe October 18, 2012, 15:07

    Found a handy atmosphere retention plotter. You have to extrapolate to get up to 1500 K, but for a radius of 6500 km and density of 6.0 g/cm^3 it looks like water and lighter molecules are likely to escape from the dayside atmosphere. Even N2 and O2 may be at risk, leaving a CO2-dominant atmosphere. So the most likely result would be an equilibrium between CO2 driving a runaway greenhouse effect and temperatures getting so high that CO2 itself escapes. Wasn’t there a paper on Venus possibly being hotter and cloudless in the past? Could be relevant here.


  • Joe October 18, 2012, 15:10

    Tulse, A doesn’t really affect B’s HZ except gravitationally and vice versa. From Bb, A would appear about as bright as our Sun would from Jupiter or Saturn. Brighter than our full moon, bright enough to read by at periastron, but little heat.

  • Wojciech October 18, 2012, 15:14

    “There is now great impetus to send a probe with a camera to Alpha Cen to study the three stars there (including Proxima Centauri) and to study the planets and moons there,” Marcy said. “What a rich opportunity for NASA and ESA, working with all nations on Earth, to send a probe to Alpha Centauri, galvanizing interest from people of all ages around the world.”

    That’s just impractical, it will be probably cheaper, quicker, and more efficient to study those planets with telescopes(including ones that can image continents) than sending a probe.

  • Ron S October 18, 2012, 15:18

    Kappy, have you read the paper? What are the concerns you have with it?

  • Tulse October 18, 2012, 16:13

    In a prior post on the proposed FOCAL telescope, it was mentioned that it could potentially images cars on the streets of an inhabited Centauri planet. One hopes that this new finding might give further impetus to that project.

  • Paul October 18, 2012, 18:09

    Since we have nothing remotely capable of sending a probe to Centauri, we might as well work the telescope angle.
    Someone mentioned that a proposal has already been sent to the Hubble people, and supplemental equipment could also be strapped on to the Webb for an additional $700 million (they pointed out that the film”Avatar” which was about a planet in the Centauri system grossed 3 billion).
    Hopefully, the dreams of humans that look and think “up” will be energized.
    I’m sure I won’t live to see a probe, but I will pass on hoping for that technology, and I might be here long enough for telescopes to make the big find.

  • Frank Smith October 18, 2012, 18:52

    That image of Alpha Centauri by Cassini is outstanding! Anyone have a link to the full-rez version? I’d like it as wallpaper.

  • Brasidas October 18, 2012, 21:42

    Very creative idea. Counterintuitively, we learn more about the Centauri system traveling directly away from it. We get results quicker, as the focal point of the sun’s gravitational lense is much closer tha even Proxima. In addition, if we do decide to send a probe to the Centauri system, we can watch it most of the way, as long as we can keep Focal functioning, or send up rotine replacements.

  • Brian Koester October 18, 2012, 22:32


  • David October 18, 2012, 23:17


    Here is some entertainment from Hollwood in the 1960s vision of a trip to Alpah Centauri. Some one in post one mentioned it was more Jupiter 2 than Enterprise. Enjoy the confusion of intergalactic with interstellar.
    You will want to look at maybe the first 10 minutes of teh first 2 episodes. The first is the unaired pilot and second is the aired pilot .
    There is a fun reference to deep space telescopes finding the planet.
    I am also amaued that a society that can build enough atomic powered spaceships with suspened animation that can get to centauri in wither 5 years or 98 depending on pilot cant figure out birth control……..oh well have some fun but avoid the great vegetable rebellion to prevent brain damage

  • Dave Moore October 19, 2012, 0:19

    Some thoughts:
    For those of us who have had a particular interest in the Alpha Centauri system, this discovery is wonderful news, and a start for learning a lot more about the system. It is also a vindication for those who persisted in modeling planetary systems around Alpha Centauri despite the supposition that it didn’t have one.

    I suspect that ACBb will have a similar effect on the study of the Alpha Centauri system as the the discovery of 51 Peg. b did on exosolar planetary studies. The study of Alpha Centauri will go from been considered something marginal to something legitimate.

    Paul made a comment about the orbital inclinations of any planets around both stars. I would expect that anything under about 0.5 au will orbit coplanar with their host star’s equator due to tidal effects, and that anything further out from that will orbit on the plane of A and B’s orbits. An analysis of the Kepler data indicates most planetary systems are flat.
    See http://www.astrobio.net/pressrelease/5088/most-planetary-systems-are-flatter-than-pancakes
    This is also the situation with our solar system where Mercury orbits around the Sun’s equator and the rest of the planets orbit within a degree or two of the Sun-Jupiter plane.

    ACBb’s close orbit is an indication that it was perturbed there, which does not bode well for other planets around ACB or other planets with circular orbits. Perturbation does however reenforce the case that there might asteroid belts around each star (they turn up outside of the outermost planet in most models). And if there are asteroid belts, then there will be a zodiacal dust cloud. As the system is oriented almost edge on to us, we will be looking through it at maximum density. If we could through occultation or interference nulling find the edge of these clouds, then we would know the outer limits of the systems; it would also be a good calibration for the planetary models of the system.

  • Adam Crowl October 19, 2012, 7:20

    No doubt we’ll eventually build a hyper-telescope to map Alpha Cen’s planets in 1 pixel per 100 km detail, but gathering sufficient photons per second to image any interesting detail would require a herculean effort that would make a probe cheap by comparison.

    Using gravity lensing to examine distant galaxies via radio will be straight-forward, communicating via gravity lensing will no doubt work with some effort, but keeping a steady line to image exoplanets in 1 metre detail is highly unlikely.

  • andy October 19, 2012, 13:32

    Tulse, A doesn’t really affect B’s HZ except gravitationally and vice versa

    Actually there should be temperature variations of a few degrees for HZ planets due to the presence of Alpha Cen A, in addition to the orbital perturbations. Planets would probably have substantial Milankovitch cycles.

    Forgan (2012) “Oscillations in the Habitable Zone around Alpha Centauri B”

  • spaceman October 19, 2012, 14:06

    By no means is it my intention to throw a wet blanket on an announcement as momentous as this one; however, I cannot help but remember the same degree of excitement being exhibited when a habitable planet was thought to exist around the star Gleise 581 only to have this planet’s very existence be throw into doubt on multiple occasions between now and then. I really hope history does not repeat itself with this stellar find, as already astronomer Paul Hatzes has referred to this detection as still up for “debate.” Hence, the importance of confirmation of this planet and as well as the importance of finding other planets further out in this system.

    Apparently, there is another high precision search for planets around Alpha Centauri B. How long before an independent confirmation of this small rocky world?

  • Wojciech October 19, 2012, 16:14

    I posted this under another topic, but I might as well repeat it here, ESA has decided to go through with Cheops

    I am no specialist, so I don’t know how it will affect possibilites of other space telescopes being constructed like TESS or EcHo, and if this telescope can help study Alpha Centauri system.

    Adam Crowl October 19, 2012 at 7:20

    “”No doubt we’ll eventually build a hyper-telescope to map Alpha Cen’s planets in 1 pixel per 100 km detail, but gathering sufficient photons per second to image any interesting detail would require a herculean effort that would make a probe cheap by comparison.'”

    Well, that is an interesting question. I am not sure studies regarding the cost of such telescopes have been made(interstellar probes have been more analyzed in terms of economic costs). It would be interesting to see a study on this.
    One thing that a telescope has as an advantage are immediate results from the work, a probe that would take 100 years to reach other system(with uncertainty if it will work at all, and us being unable to repair it) seems much too risky.
    Anyway-just reasonably guessing, we have more chances to build telescopes first than probes, which like mention need enormous infrastructure to create(at least ones that seem most workable), and having such infrastructure will allow to have hypertelescopes in the first place

  • andy October 19, 2012, 18:51

    I am no specialist, so I don’t know how it will affect possibilites of other space telescopes being constructed like TESS or EcHo, and if this telescope can help study Alpha Centauri system.

    If the orbits of the planets are aligned with the orbit of the binary then a transit mission isn’t going to be of much help for Alpha Centauri, except maybe for extremely hot planets (even closer-in than Alpha Cen Bb).

  • C. O. October 19, 2012, 22:02

    Excellent article thanks!

    “Maybe a distant but theoretically reachable green and blue world is out there around our nearest neighbor, but we won’t know until we commit the resources to continue the investigation. ” — What are we waiting for? The wars in the middle east cost way over 3,000,000,000,000 (3 trillion) dollars. I think I read in the previous article that a sunshade for the JWST would cost 700,000,000$. One of these things kills people and the other uplifts humanity.

    Thanks for the excellent science writing, centauri-dreams.org. This is easily the best space blog I’ve found on the internet.

  • Raimo Kangasniemi October 20, 2012, 0:49

    Cheops should be able to study planets down to one Earth mass. If Alpha Centauri Bb transits Alpha Centauri B, then my understanding is that Cheops could and even should be able to observe it.

  • torque_xtr October 20, 2012, 7:58

    A possibility of tidally-locked super-earths with their day- and nightsides being essentially two different worlds is a fascinating one, since they are very possibly not suitable for the starting and evolution of life in the first place (but who knows, maybe Alpha Cen Bb has liquid water seas on the nightside with ecosystems powered by geothermal heat from non-zero excentricity) but much more fit for human colonisation, with some abiogenic oxygen atmospheres from photolysis of water on the dayside…
    And Alpha Cen Bb has a bonus of Alpha Cen A’s light which could keep N2/O2 from freezing on the nightside with only a little help from circulation or/and geothermal heat, thus expanding the range of atmospheric properties which can allow for temperate zones on the nightside far into the range of thinner atmospheres which would otherwise freeze.
    A hot super-earth in a double system with primary in the HZ of the secondary, one half a balmy Earth with seas and day-night cycles and other an ocean of lava, looks like real possibility somewhere not too far away…
    Of course it’s also possible that Alpha Cen Bb is the core of evaporated mini-neptune like Kepler-11e, with five-mile deep molten salt oceans, wild atmospheric composition (Cl2? phosphorus-Sulphur oxohalydes? N2/O2 or complete vacuum if the salts are basic?) and halosiloxane/transition-metal-polymer-oxide fish swimming in it. (just imagine, that could be a very common lifeform in the Galaxy, based on all that exoplanetary statistics data, only much less suitable for crawling up from their ocean and become spacefaring than cool-temperature low pressure gas-breathing carbon-based us … %))

    Time to think again for sparse-mirror hypertelescope at L4 or L5 and look at the surface features of Gliese 581 g & d (and if they exist at all), measure the nightside conditions of alpha Cen Bb, determine if Gl 581 e is a super-Venus or a Janus hot-superearth!
    (if there are just free-flying thin-film mirrors, controlled by laser range meters and moved around by ion drives, a total area of about 10000 m^2 possibly could be achieved in a single launch of a heavy lifter with mirrors stacked inside, and there’ll be no need to trouble aboud long expositions…)

  • Eniac October 20, 2012, 14:47


    (if there are just free-flying thin-film mirrors, controlled by laser range meters and moved around by ion drives, a total area of about 10000 m^2 possibly could be achieved in a single launch of a heavy lifter with mirrors stacked inside, and there’ll be no need to trouble aboud long expositions…)

    I love this idea. A telescope could be composed of thousands of identical units, each mass-produced and collectively able to align precisely into an optically perfect adaptive super-mirror the size of which is only limited by the ever growing number of units available.

    Later, flocks of these could have nuclear propulsion units attached to them to move them into interstellar space, to eventually approach nearby stars to get a closer look, while still available for general astronomy at any time.