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A Possible Proxima Centauri c

While we continue to labor over the question of planets around Alpha Centauri A and B, Proxima Centauri — that tiny red dwarf with an unusually interesting planet in the habitable zone — remains a robust source of new work. It’s surely going to be an early target for whatever interstellar probes we eventually send, and is the presumptive first destination of Breakthrough Starshot. Now we have news of a possible second planet here, though well outside the habitable zone. Nonetheless, Proxima Centauri c, if it is there, commands the attention.

A new paper offers the results of continuing analysis of the radial velocity dataset that led to the discovery of Proxima b, work that reflects the labors of Mario Damasso and Fabio Del Sordo, who re-analyzed these data using an alternative treatment of stellar noise in 2017. Damasso and Del Sordo now present new evidence, working with, among others, Proxima Centauri b discoverer Guillem Anglada-Escudé, and incorporating astrometric data from the Gaia mission’s Data Release 2 (DR2). The result of the new analysis is a possible planet with an orbital period of 5.2 years and a minimum mass of 5.8 ± 1.9 times the mass of the Earth.

Image: This is Figure 5 from the paper. Caption: Outcomes of the combined analysis of the astrometric and RV datasets. Left: True mass of Proxima c versus the sine of the orbital inclination, as obtained from the astrometric simulations. The black line is the simulated exact solution, the blue dots represent the values derived from the Gaia astrometry alone, while the red dots are the values derived by combining the Gaia astrometry with the radial velocities. Right: Fractional deviation of the true mass (defined as the difference between the simulated and retrieved masses for Proxima c divided by the simulated value) versus sine of the orbital inclination. Credit: Damasso et al.

Remember that when dealing with radial velocity results, we can only draw conclusions on the minimum mass in question, as we don’t know how the system is inclined around the star. The researchers find that by analyzing the photometric data and spectroscopic results, they cannot explain the planetary signal through stellar activity, but they also argue that a good deal of follow-up work is needed through a variety of means. The paper notes, for example, that Proxima was observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in 2017, with an unknown source detected at 1.6 AU. Is this evidence for Proxima c?

It’s quite an interesting question, and one that involves more than a new planet:

ALMA imaging could corroborate the existence of Proxima c if the secondary 1.3-mm source is confirmed: In this sense, ALMA follow-up observations will be essential. In (28), the possible existence of a cold dust belt at ∼30 AU, with inclination of 45°, is also mentioned. If Proxima c orbits on the same plane, its real mass would be mc = 8.2 M

Image: Artist’s impression of dust belts around Proxima Centauri. Discovered in data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the cold dust appears to be in a region between one to four times as far from Proxima Centauri as the Earth is from the Sun. The data also hint at the presence of an even cooler outer dust belt and may indicate the presence of an elaborate planetary system. These structures are similar to the much larger belts in the Solar System and are also expected to be made from particles of rock and ice that failed to form planets. Such belts may also prove useful in helping us investigate the presence of a possible second planet around this star. Credit: ESO / M. Kornmesser.

But Gaia astrometry is also crucial, for there is some evidence of an anomaly in Proxima’s tangential velocity that, if confirmed, would be compatible with the existence of a planet with a mass in the 10 to 20 Earth range, and a distance between 1 and 2 AU. Further work with Gaia data is clearly in the cards:

Given the target brightness and the expected minimum size of the astrometric signature…, Gaia alone should clearly detect the astrometric signal of the candidate planet at the end of the 5-year nominal mission, all the more so in case of a true inclination angle significantly less than 90°. Proxima is one of the very few stars in the Sun’s backyard for which Gaia alone might be sensitive to an intermediate separation planetary companion in the super-Earth mass regime.

A final consideration is that while the flux contrast between the hypothetical Proxima c and the parent star (depending on albedo, among other things) is beyond the capabilities of our current direct imaging technologies, the apparent separation of planet and star should be accessible to future high-contrast imaging instruments, perhaps the European Extremely Large Telescope, which the paper mentions along with other ground- and space-based instruments. So we have what the authors describe as ‘a very challenging target,’ but one with huge interest for astronomers continuing to characterize this closest of all stellar systems.

It seems premature to get too far into a discussion of how Proxima c formed, since we have yet to confirm it. However, the authors make the case that if it is there, this planet would challenge us to explain how it formed so far beyond the snowline, where super-Earths could take advantage of the accumulation of ices. Perhaps the protoplanetary disk here was warmer than we’ve assumed. In any case, the apparent circularity of the orbit and the absence of more massive planets closer in makes migration from the inner system unlikely. And I think we should leave formation issues there while we await new work, especially the authors note, from ALMA.

The Damasso paper reanalyzing the Proxima Centauri radial velocity data in 2017 is “Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities,” Astronomy & Astrophysics 599, A126 (2017) (abstract/ preprint). The new Damasso et al. paper is “A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU,’ Science Advances Vol. 6, No. 3 (15 January 2020). Full text.



{ 20 comments… add one }
  • Wojciech J January 16, 2020, 15:06

    Any planets in the closest system to ours is good news for future interstellar travel.While detectibg biosphere would be amazing, access to rich and diverse resources already enables potential colonization using artificial habitats build in space.
    From there our descendants(perhaps political or cultural fringe groups fleeing global culture on Earth) can travel onwards towards other stars.

  • Enzo January 16, 2020, 17:19

    BTW, has anyone heard what’s happening for Alpha Centauri A & B ?
    Back in June there were the news of NEAR, an ESO infrared coronograph purposely designed for this :

    Observations were supposed to be completed by June 11, 2019.

    • Harry R Ray January 17, 2020, 10:49

      No planets were imaged, indicating that no planets Neptune sized or larger orbit in the habitable zones of either Alpha Centauri A or Alpha Centauri B. Speaking of imaging planets in the system: If it does turn out that GAIA is able to confirm Proxima Centauri c, and the planet’s TRUE MASS turns out to be 20 Earth mass instead of the MsinI value, Proxima Centauri b’s true mass would be ~5 Earth mass, making it EASILY(instead of only a 50-50 chance)imageable RIGHT NOW WITH CURRENT TECHNOLOGY(i.e. mounting an EXISTING high amplitude pupil mask on one of the VLT’s telescopes in concert with the SPHERE/ZIMPOL polarized light imaging instrument package), providing that Proxima Centauri b also orbits in the same orbital plane as the putative Proxima Centauri c does. This makes these observations EVEN MORE IMPORTANT, and they should be scheduled IMMEDIATELY!

      • Ashley Baldwin January 17, 2020, 17:49

        Though nearby Proxima certainly isn’t too bright for Gaia’s sensor array – which is good. The original aspiration was that Neptune mass planets would be constrained within 15 light years or so … So still possible. For astrometry the baseline semi major axis detectable will be determined by the mission length. So to detect and constrain a 5 years plus orbit Gaia will have to operate into its extension mission – which is still looking good as of now with consumables for 9.5 years + and the optics holding up .

        Processing all that data will take time though so it will be five years or more before Gaia can confirm this planet – under the most optimistic estimates , though it’s likely that one or all of the next gen precision RV observations will “msini” it before then. If it’s there and certainly the “signal” seems to be building…..

        • Michael Fidler January 18, 2020, 10:28

          Ashley and Harry, one way that a positive image can be observed without taking up the time of the large observatories is thru violet to UV flares. This should be possible with smaller scopes that can be available when a monitoring telescope picks up the start of a bright flare. This paper has a in depth report on the observation flaring activity of Proxima Centauri from TESS:
          Flaring activity of Proxima Centauri from TESS observations: quasi-periodic oscillations during flare decay and inferences on the habitability of Proxima b.

          Take a look at the table 2 and 3 on page 12 and 13, this is the 53 days that TESS observed it with some 72 flares total. What is interesting is that the brighter flares where all above 10 seconds in duration with two long flares of much higher intensity, #18 and 30.
          Day Seconds Energy
          18. 1613.7400 87.8975 8.715e+31 triple peaked
          30 1623.1664 175.6592 1.742e+32

          The planet Proxima c should light up some 12 minutes after Proxima flares, being at 1.5 AU. This depends on how Proxima c is placed in it orbit around Proxima and if it is fully illuminated by the flare.

          • Ashley Baldwin January 18, 2020, 16:27

            The problem with Proxima Centauri is that it is so dim despite being so close there are not many photons reaching Earth from it – and far, far less either reflected or absorbed and re-emitted from even just 38 microarc seconds away Proxima b. Let alone more than an arc second away “c” . If it exists. Even allowing for brighter flares these are far from continuous though unpleasantly regular in terms of habitability. Had the NEAR apparatus been set up and applied to Proxima c it’s unlikely it could not be imaged other than with prohibitively long observation times ( which ultimately so hampered NEAR for the much brighter Centauri A & B systems) .

            I’m afraid imaging any Proxima planets will have to wait to the advent of the much larger ELTs and especially via polarimetric imager on the E-ELT. Though whether even this can image the Proxima planets and especially b , will require many ( indeed 100s, yes 100s ) hours of observation in order to gain enough photons . This will also depend upon the efficiency of its coronagraph, both in terms of is contrast reduction ( an absolute theoretical 1e8 maximum from the ground ) and throughput ( how much exoplanetary light gets through the instrument – bearing in mind there isn’t much to start ! And that’s before it goes for analysis in a spectrograph afterwards ) . So called high contrast imaging, HCI.

            A better technique involves using “high dispersion” (or ultra high resolution) spectroscopy (circa 150000 – or 1e5) HDS, to tease out exoplanetary spectra from those of the Earth’s atmosphere and that of the its parent exostar . This firstly requires a lot of photons. These are available from a 40m aperture telescope . The well known spectrum of the Earth is relatively easy to remove as is that of the exostar. The hard bit is picking out the spectrum of an exoplanet orbiting it. A princess of “cross – correlation spectroscopy’, Doppler de -convolution is employed. The Doppler or “RV” ( radial velocity) spectroscopic signature of Proxima b is now well described as it was central to the discovery of the planet in 2016. So the high resolution spectrum of Proxima Centauri can be screened for Proxima b’s tell tale radial movement. Easy in theory but systematically exquisitely difficult in practice. But this has already been done successfully for 51 pegasi b ( the first hot Jupiter) and several other stars – and then used to pick out absorption signature of molecules such as methane , ammonia and most recently – water ( though not for 51 Pegasi b) .

            The HDI technique can be helped if used in conjunction with HCI . The coronagraphic imager (HCI) can thus work with the high dispersion spectrograph ( HDS) in the process of high dispersion imaging (HDI) to deliver precision atmospheric characterisation , picking out the individual spectral signatures of gases including potentially biosignatures on terrestrial hab zone planets. However this will first require extensive atmospheric modelling – how can you spot a specific molecular spectral signature in a huge spectrum if you don’t know what you are looking for ? In a hypothetical atmosphere . And that’s assuming all the relevant theory and hardware works ! At present the ELT will have a high res NIR spectrograph and hopefully will gain approval for the versatile UVOIR HIRES spectrograph . This particularly would work ideally for HDI.

            For further info see Snellen , de Kok et al Arxiv 2015 and “Prospects for ground-based characterisation of Proxima b” by the Brogi and the same authors .

            • Michael Fidler January 19, 2020, 2:04

              Yes, I see what you mean, but I’m looking at it from a more practical aspect. No matter which way we are observing the planet, it is the number of photons that will be available. There are two ways this may work in our favor, first the super flares. In the spring of 2018 Proxima Centauri flared to the magnitude of 6.8 and visible to the unaided eye, it’s normal magnitude is 11.0. The TESS observations in the paper above that I mentioned came to this conclusion:

              “Superflares (events with energy output over 10 33 ergs) are expected ≈ 3 times per year, flares a magnitude larger (with 10 34 ergs) every second year”

              Now there is a 12 minutes delay before they reach Proxima c and the flare should of subsided on Proxima Centauri by that time. This 12 minutes would be plenty of time for telescopes in the 2 to 5 meter class in the southern hemisphere to swing over and start taking measurements.

              The second is what type of object could this actual be, since we have little data on what a super earth or sub-neptune planet may look like. The possibilities are anywhere from a ice covered very reflective planet to a bright Venus cloud shrouded thermally heated planet, a neptune blue planet, or a very non-reflective organic residue goo. So a 50/50 chance it may be bright in the blue/UV light from the flare. The brightness is also going to depend on the what the phase angle of the planet is from our viewing angle on earth, in it’s 5.5 year orbit it could be anywhere from a thin crescent to a full hemisphere.

              One problem is the area that is covered when they take the photometry measurements, if the field is less than two to three arc seconds it may miss the secondary reflection from Proxima c. The best way to observe it is with a camera that captures the majority of photons in a movie and stack the images as done with imaging of solar system planets. I would be very interested to see a picture or movie of a red dwarf flaring since there seems to no such monster available.

              The archives of Proxima Centauri flares is another place to look for signs of Proxima c and the TESS and GALEX data may be the best place to look.

              Ultraviolet Insights into Red Dwarf Flares.

              https://www.centauri- dreams.org/2017/06/08/ultraviolet-insights-into-red-dwarf-flares/

  • Alex Tolley January 16, 2020, 22:20

    It really doesn’t matter what type of planets are discovered around Proxima. The more we find, the more tempting it is to send a probe to explore and bring back images and data. While Breakthrough Starshot, if it happens, might give us a glimpse, a Voyager type probe able to send back data of at least a few planets in the system would be a revelation, and make the idea of interstellar exploration a reality, at least for the first voyages. The great prize will be a close flyby, even orbit, of a living world around one of the nearer stars. The costs will be high, and the timeframe long, but the opportunity to open the eyes of the global population to make our place in a galaxy with other star systems with visitable planets, is immense. Just as the other planets in our system became bodies, rather than just wandering points of light, inspired our exploration to know more, these systems with exoplanets will widen our horizons further.

    I can only hope that we can develop propulsion systems capable of fractional c velocities that are cheap enough and powerful enough to propel capable probes to other systems and send the information back. While I will be long gone, my children and their children will perhaps live to see such marvels, inspiring their generations to eventually send people (post-humans?) to such worlds with technologies we can only imagine.

    • Ron S. January 17, 2020, 10:18

      Unfortunately with high speed flybys, which is all we can currently aspire to, achieving a trajectory that gets the spacecraft close enough to one or more planets requires lots of onboard propellant for course adjustment and quite some distance must be maintained to take even low resolution photographs. The project may be inspirational but of minimal science value. In-system deceleration is almost mandatory for doing science.

      • Alex Tolley January 17, 2020, 17:11

        If Starshot Breakthrough managed to send back even very low-resolution images of planets in the system, it would be inspirational, even if of low scientific value. It would make the planets seem far more tangible than inferred from transits, radial velocity, and even points of light by direct imaging.

        A larget probe would likely have some sort of magsail to decelerate, even if it couldn’t completely stay within the system. That should allow more science than a fractional c flyby.

        Speaking for myself, the recent New Horizons flyby of Pluto made that planet far more real that the almost star-like images of my youth, and even the very low Hubble images. Pluto is now much more fixed in my mind a world to visit again, one where exploration can truly begin, with orbital probes and landers. The Jovian and Saturnian moons similarly became more tangible with the Pioneer and Voyager probes before this. If we can image a rocky world in the Proxima system, that means we can eventually map its geography and label its features, making this a world a place to drive further exploration, and with it a drive to create better propulsion systems, robust instruments, and probably AIs to command the probe’s actions. I see this as being like the first telescopic images of Mars that sparked the imagination (however fevered) and eventually led to exploration program of that planet that we now have.

    • Antonio January 17, 2020, 13:30

      “It really doesn’t matter what type of planets are discovered around Proxima.”

      I never understood that obsession with liquid water on the surface (wrongly called habitability). After all, if/when humans travel to other stars, it will only be after having colonized the Solar System, and here only Earth has liquid water on its surface!

      • Andrei January 19, 2020, 13:04

        Habitability is defined as if any life can survive on a world, not merely human. If that definition is wrong, then it’s a matter of semantics.
        I fully agree that humanity eventually colonise the Solar system.
        The question is if it will be this current civ that do so.
        Probably not when most nations, regions and countries do not address the mountain of problems that face us, and the few who do way little.

        Anyway, it was the ‘it really doesn’t matter’ quote that landed my reply here. Because I agree, but for a different reason.

        Finding such planets is important to show how common planets are found at red dwarf stars. While Proxima not only is a flare star but even got super flares – described above.
        It might very well get a visit from one unmanned probe, but with the investment and the fact that any space probe could be sent to any target further away with only a longer flight time.
        I have a feeling that other targets will get selected.
        In interstellar flight it really doesn’t matter if you go 4 light years or 12.
        It’s the same for interplanetary flights also, as the extended missions for some spacecraft have shown.

        • Antonio January 20, 2020, 11:11

          “Habitability is defined as if any life can survive on a world, not merely human”.

          That’s the only sensible definition of habitability, but it’s not the definition that the astronomers and almost all the public uses, hence my complaint.

          “Probably not when most nations, regions and countries do not address the mountain of problems that face us”

          That mountain is a myth that generates recurring discussions here and elsewhere, so I’ll not reopen them again.

          “but with the investment and the fact that any space probe could be sent to any target further away with only a longer flight time”

          I think that, as Solar System colonization progresses, the water-on-the-surface blinders will fade away.

  • Gary Wilson January 17, 2020, 14:48

    How much deceleration can we get from a light sail brought close to Alpha Proxima? Could we stay in system long enough to get images of Proxima b and possibly c? I’m thinking of something like Starshot but with more maneuvering capability. A second generation Starshot possibly. I obviously don’t know the technical details but surely people are thinking about what would be required technically?

    • Paul Gilster January 18, 2020, 8:38

      Good thinking. Rene Heller and Michael Hippke have looked extensively into this idea. See, for example, By ‘Photogravitational Assists’ to Proxima b:


      • Gary Wilson January 20, 2020, 14:42

        Thanks Paul. I just read the article describing the Heller and Hippke work. It seems a very smart way to go and allows for some investigation of all three star systems. It takes a while to get to Proxima Centauri (146 years) but the probe(s) end up in orbit travelling at 1280 km/sec. It also sounds more cost effective as there is no need to build the massive laser launch system. I didn’t see a cost estimate but it must be at least an order of magnitude lower than the StarShot method surely? None of us will see data from the first probes to the Centauri system but possibly our children’s children will?

  • Michael January 18, 2020, 7:07

    With the mass of the planet it does raise the possibility of moons with possible oceans beneath.

  • Mike Serfas January 18, 2020, 18:11

    I think some people have been a little too fast to reject the idea of a “Neptune”-like planet as a world for life. Bear in mind our own Neptune has a surface gravity just 14% more than that of Earth and is composed mostly of water, spiked with ammonia and methane! Alas, our Neptune has the drawback that it is much too hot for life: it has some internal heat source, radiates much more energy than it receives from the Sun, and the top of its mantle is already over 3000 K.

    Do you think a planet three times smaller around a dim star might be just the right size to have accessible liquid oceans? If we suppose the planet has too much hydrogen in circulation to become an oxidizing environment, perhaps it could have floating continents made of hydrocarbon derivatives, where “trees” use geothermal energy to store scarce oxygen atoms in fibers of cellulose, under which “fish” adapted to aqueous and viscous hydrophobic conditions vie for the bounty of their delicate roots…

    • Michael Fidler January 25, 2020, 10:23

      Mike, very good point, super earths and sub neptunes may be the most common planet type and probably the ocean worlds is were most civilizations developed. I’m wondering what type of communication would develop in such environments. We see the dolphins and whales use sound waves and something along that line like Scalar waves or longitudinal may be the common form of communication.
      Good article on human biases in searching for ET:

      To find intelligent alien life, humans may need to start thinking like an extraterrestrial.

  • Harry R Ray January 30, 2020, 11:20

    Tod Lauer twetted this 14 hours ago: “Hey! New Horizins is so far away that Proxima Cen shifts by32 arcsec and Wolf 359 by 16, and we’re going to show that by taking images of their fields on April22-23 and comparing them to simultaneous Earth-based images.” Obviously an absolutely ZERO percent chance of imaging either of Proxima Centauri’s planets, BUT: What a great idea! After New Horizons’ principal mission is done, KEEP THE CAMERA ON(unlike the Voyagers) and KEEP TAKING FIELDS of the closest stars IN CONCERT WITH SIMULTANEOUS EARTH-BASED AND GAIA IMAGES to get TRIANGULATED data on these stars.

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