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Proxima Centauri c?

A possible second planet around Proxima Centauri raises all kind of questions. I wasn’t able to make it to Breakthrough Discuss this year, but I’ve gone over the presentation made by Mario Damasso of Turin Observatory and Fabio Del Sordo of the University of Crete, recounting their excellent radial velocity analysis of the star. Proxima c is a fascinating world, if it’s there, because it would be a super-Earth in a distant (and cold) 1.5 AU orbit of a dim red star. Exactly how it formed and whether it migrated to its current position could occupy us for a long time.

But is it there? The first difficulty has to do with stellar activity, which Damasso and Del Sordo were careful to screen out; it’s one of the major problem areas for radial velocity work in this kind of environment, for red dwarf stars are often quite active. During the question and answer session, another key question emerged: We know from Kepler that many stars are orbited by multiple planets, and there is no reason to assume that Proxima Centauri has but one.

The question: If there are other, smaller worlds in play here, could the effect of their combined masses produce a ‘phantom’ Proxima c in the orbit Damasso and Del Sordo have discussed?

The two astronomers are completely open to this possibility, and point to the need for follow-up observations with ESPRESSO, not to mention the useful Gaia measurements that could give us even more detail. Flare activity is always an issue in any case — it may have affected the results of Anglada et al. in 2018 (citation below), when researchers found possibly two inner dust belts and one outer belt around the star (see Proxima Centauri Dust Indicates a Complicated System). The Damasso and Del Sordo work is comprehensive as far as it can go, but both were careful to note that we are dealing solely with a candidate, not a confirmed world. And it could well be the result of other, unseen planets affecting the star as well as stellar noise.

This work draws on the earlier Proxima Centauri radial velocity dataset compiled by Guillem Anglada-Escudé (University of London) and team, but folds in an additional 61 RV observations, with considerable attention to the question of filtering out the 85 day rotation period of the parent star and the associated noise of stellar surface perturbations. The instrument in play is the European Southern Observatory’s High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at La Silla.

I suspect we’re going to find a number of small worlds around Proxima Centauri, so we’ll see how their gravitational interactions might affect the spectroscopic data and hence the confirmation of the current candidate. But if this detection is confirmed, this is what we’ve found: The planet would mass about six Earths — remember that because this is radial velocity, we can only measure a minimum mass, because we don’t know planetary inclination — and would orbit Proxima Centauri with a period of 1900 days at 1.5 AU. Not exactly a habitable place for the likes of our species. Del Sordo estimates temperatures there would be about 40 K.

We may know, via Gaia, whether Proxima Centauri c is an actual world by the end of this year. A key follow up question is, can we snag a direct image in visible light? If so, it would mark the first such detection of a planet outside our Solar System, the imaged worlds found thus far having been discovered via infrared. There is plenty, in other words, to like about the hypothetical Proxima Centauri c, provided it’s really there. Waiting a few more months could give us a firm answer.

On another matter, as a great admirer of Thoreau, I was pleased that Damasso and Del Sordo quoted him at the beginning of their presentation, and to good effect: “If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.” That’s a good metaphor for RV studies as exceedingly delicate as these. I’ll add a favorite bit from one of Thoreau’s poems:

For lore that’s deep must deeply studied be,
As from deep wells men read star-poetry…

There’s poetry indeed in the spectroscopic data of our nearest star, if we can just tease out its meaning. And here’s an image that might evoke a bit of poetry to close today’s entry.

Image: Rigil Kentaurus is the bright star near the top of this broad southern skyscape. Of course it’s probably better known as Alpha Centauri, nearest star system to the Sun. Below it sprawls a dark nebula complex. The obscuring interstellar dust clouds include Sandqvist catalog clouds 169 and 172 in silhouette against the rich starfields along the southern Milky Way. Rigil Kent is a mere 4.37 light-years away, but the dusty dark nebulae lie at the edge of the starforming Circinus-West molecular cloud about 2,500 light-years distant. The wide-field of view spans over 12 degrees (24 full moons) across southern skies. Credit & Copyright: Roberto Colombari.

The paper on dust belts around Proxima Centauri is from Guillem Anglada, “ALMA Discovery of Dust Belts Around Proxima Centauri,” Astrophysical Journal Letters Vol. 850 No. 1 (15 November 2017) (abstract). (Note: This is not Guillem Anglada-Escudé, despite the similarity in names!) The Damasso and Del Sordo paper is as yet unpublished, though undergoing peer review. Video of their presentation is available at https://www.youtube.com/watch?v=DLzzg9p0-AI&t=15648s (go to about 4:16:45 on the video).

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  • Daniel April 15, 2019, 10:20

    Worth repeating is the tentative layout of the system from the ALMA paper in 2017:
    Planet at 0.05 AU
    Warm Dust? ~0.4 AU
    Unknown Source 1.6 AU
    Cold Belt ~1-4 AU
    Outer Belt? 30 AU

    As of 2017 they were unsure if the Unknown Source at 1.6 AU was a background galaxy or a ringed planet within the system… but it slots in very nicely with the proposed planet indicated via Radial Velocity. They estimated the planet had an orbital period of at least 5.8 years – just a bit above the 5.4 years proposed here.

  • Michael C. Fidler April 15, 2019, 11:00

    Thanks Paul, has been a busy week in astronomy and space! Looking at the dust rings, they show the Unknown Source at 1.6 AU; could this be the planet?
    https://iopscience.iop.org/2041-8205/850/1/L6/downloadHRFigure/figure/apjlaa978bf4
    Have they been able to at least get a idea how Elliptical the orbit might be and if it matches the position for the Unknown Source?

    Of course my favorite subject, the possibility that the planet can be captured with in UV light…
    April 12, 2019, 23:57
    Just did some simple calculations on the distance Proxima Centauri c is from Proxima Centauri in arcsecond. It turns out to be 1.147 arcsecond, calculated from Proxima Centauri b distance of 0.0485 AU to Proxima Centauri c distance of 1.5 AU gives 31 times. Proxima b is 37 milliarcsec from Proxima Centauri time 31 equals 1147 (milliarcsec) which is 1.147 arcsecond, this could easily be resolved by a twenty inch (.5 meters) telescope. So any amateur astronomer could image this planet when an UV flare is emitted from Proxima Centauri. All they would have to do is wait 12 minutes after the flare for Proxima c to light up in the UV…

    THE FIRST NAKED-EYE SUPERFLARE DETECTED FROM PROXIMA CENTAURI.

    “Proxima b is a terrestrial-mass planet in the habitable-zone of Proxima Centauri. Proxima Centauri’s high stellar activity however casts doubt on the habitability of Proxima b: sufficiently bright and frequent flares and any associated proton events may destroy the planet’s ozone layer, allowing lethal levels of UV flux to reach its surface. In March 2016, the Evryscope observed the first naked-eye- brightness superflare detected from Proxima Centauri. Proxima increased in optical flux by a factor
    of ∼68 during the superflare and released a bolometric energy of 10 33.5 erg, ∼10× larger than any previously-detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in bolometric energy from 10 30.6 erg to 10 32.4 erg; coupling those rates with the single superflare detection, we predict at least five superflares occur each year. Simultaneous high-resolution HARPS spectroscopy during the Evryscope superflare constrains the superflare’s UV spectrum and any associated coronal mass ejections. We use these results and the Evryscope flare rates to model the photochemical effects of NO x atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring may be sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years; complete depletion may occur within several hundred kyr. The UV light produced by the Evryscope superflare would therefore have reached the surface with ∼100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would have to undergo extreme adaptations to survive in the surface areas of Proxima b exposed to these flares.”

    arxiv.org/pdf/1804.02001.pdf

    Plenty of UV light to see this planet Proxima c, 12 minutes after the flare on Proxima Centauri, even with my 12.5″ telescope…
    All that is needed is someone looking and recording it when it happens.

  • gdp April 15, 2019, 12:58

    With a minimum mass of ~6 earth-masses and a distance of ~1.48 AU, IMO Proxima c (if it exists) is far more likely to be a “Mini-Neptune” than a “Super-Earth”.

    Moreover, if Chen and Kipping are correct, https://arxiv.org/abs/1603.08614, there are no “Super-Earths” _anywhere_ — there are only “terrestrial” planets massing less than ~2 Earths, “Neptunes” massing more than ~2 Earths but less than ~0.4 Jupiters, and “Jovians and Brown Dwarfs” massing between ~0.4 Jupiters and ~0.08 Sols. Everything more massive than ~0.8 Sols is a star.

    See also: https://medium.com/starts-with-a-bang/sorry-super-earth-fans-there-are-only-three-classes-of-planet-44f3da47eb64

    • Nicky April 16, 2019, 1:10

      For the lack of super-Earth and direct transition from terrestrial to mini-Neptunes in Chen & Kipping is more likely the result of selection bias.

      “Chen & Kipping (2017) include the minor Solar System bodies. However, including this data without inflating the uncertainties to match those we obtain for extrasolar systems leads to misleadingly tight constraints for ∼1 M⊕ planets. Indeed, there is only one planet between 1 and 3 M⊕ in their data set, so most of the information about the “Terran” transition point comes from extrapolating up from Solar System minor bodies and extrapolating down from Neptunes, rather from measurements at the transition point itself. ”
      Bo Ning et al 2018 ApJ 869 5

      Most rocky planet radius threshold studies put it right at 1.5 Re.

      But regardless of the true mass of the candidate planet, forming at a large distance beyond the snowline almost surely makes the planet a volatile- and/or gas-rich mini-Neptune.

    • Ronald April 16, 2019, 8:30

      “more massive than ~0.8 Sols is a star”. Of course you meant 0.08 Sols.
      Other than that, thank you for e very interesting and relevant paper, which makes a strong and rather logical case that there are only 3 types of planets, delimited by type of hydrogen/helium envelope;
      – No hydrogen/helium envelope: Terran (terrestrial) planets.
      – Hydrogen/helium envelope: Neptune class gas planets. This is the largest/most common class, including the gas dwarfs/mini-Neptunes and most so-called super-Earths.
      – Compressed hydrogen/helium envelope: Jovian class gas planets, or gas giants.

      I liked the linked article very much, very clearly written, particularly the conclusion: our solar system is not exceptional after all, we are not missing any planet type (super-Earth), because those don’t exist, they are really just smaller Neptunes.

      I just wonder: would it be possible to get a real ‘secondary’ super-Earth, if an original small Neptune orbited close to its star and the gas envelope was boiled off?

      • Michael C. Fidler April 18, 2019, 4:38

        The fact of the matter is that we will most likely be shocked by the diversity of planets and life forms that exist in the universe. The line between rocky and Neptunes will probably be the most varied of all with life, geology, atmospheres and oceans of such strangeness that no two will be similar. Just imagine the amount of time it will take to study and explore these worlds, whole universities and fleets of starships just to study each individual planet. ;-}

  • Nicky April 15, 2019, 23:20

    If we ignore the fact that the belts and unknown source are possibly due to flare activities and background source and assume the unknown source at 1.6 AU is indeed the emission of the super-Earth candidate, there will be a large gap in terms of the understanding of the system inclination, and the gap is needed to be filled up by current planet formation theories.

    In Guillem-Anglada et al paper, the detected emission requires a giant planet with the mass of Saturn to explain. If this emission does come from the super-Earth candidate, its orbit viewed by us must be almost perfectly face-on (inclination <<10°) otherwise the discrepancy between the masses estimated by radial velocity and emission intensity would be unexplainable. Given that most planets are nearly coplanar or at least the inclination dispersion of them is not too large, the true mass of Proxima b would highly likely to be above 4-5 Me, making it a potentially volatile- and/or gas-rich planet.

    The possible outer belt (which is not rule out by flare activities) at 30 AU detected by ALMA shows an inclination of 45°. This would make the true masses of planet b and c to be 1.8 Me and 8.5 Me, but in this case planet c would not be the source of the detected unknown emission at 1.6 AU.

    If the outer belt is confirmed, this would indicate that either the 1.6 AU unknown emission is not a planet, or there is a HUGE (I mean HUGE) inclination dispersion in this system. The later is yet to be explained by planet formation models.

  • Enzo April 16, 2019, 3:06

    Flares could allow us to “see” the planets with this NIAC concept :
    https://www.nasa.gov/sites/default/files/atoms/files/niac_2016_phasei_mann_stellarechoimaging_tagged.pdf

    • Michael C. Fidler April 17, 2019, 20:47

      A Framework for Planet Detection with Faint Light-curve Echoes
      Chris Mann, Christopher A. Tellesbo, Benjamin C. Bromley, Scott J. Kenyon
      (Submitted on 21 Aug 2018 (v1), last revised 15 Oct 2018 (this version, v3))

      Here is a new paper that is a little more down to earth and more oriented to results. The only problem is that this only works with close in planets and not at the distance of Proxima c. The use of direct UV flare imaging from the echo off Proxima c has still not been addressed. This would be the first real image off an exoplanet from reflected light of the star since previous imaging has been IR from the thermal emission of giant planets. As I have said this should be easy and may also pick up other planets, rings, moons and possible dust/asteroid/comet belts around Proxima Centauri.

      https://arxiv.org/abs/1808.07029

  • Michael C. Fidler April 16, 2019, 9:00

    Thanks Enzo, this looks to be a long range spacecraft project but I would say that the ChileScope which has http://www.chilescope.com a 1 meter and two half meter telescopes that could detect Proxima c right now. Their high altitude should be able to view the UV A and B from 400nm to 280nm when Proxima Centauri flares. This would give the capability to confirm this candidate and possiable others.

  • Harry R Ray April 16, 2019, 10:07

    “A key follow up question is, can we snag a direct image in visible light?” Last year astronomers stated that they have ALREADY DESIGNED AND SUCCESSFULLY TESTED a high amplitude pupil mask that working in concert with the SPHERE/ZIMPOL(which operates in the optical spectrum, as opposed to the SPHERE/VISR package, which operates in the infra-red spectrum)package on one of the four telescopes comprising the VLT. It still remains a mystery whether this pupil mask has indeed been fully integrated and calibrated on any of these telescopes yet, but if it has, these astronomers claim that that telescope could possibly be able to image Proxima b! Proxima c, should it exist, and ESPECIALLY if it has a ring system, should be a slam-dunk!

    • Michael C. Fidler April 17, 2019, 22:29

      One point that should make this much easier to do is a small group of UV monitoring telescopes set up around the world. These could be 10 inch commercial (Celestron’s) telescopes in the southern hemisphere that are pointed at Proxima Centauri (Right ascension 14h 29m 42.94853s Declination −62° 40′ 46.1631″) to set off the alarm of a large UV flare. Then any telescopes could image the UV echo coming from Proxima c 12 minutes later. By that time Proxima Centauri will have fallen back to its previous levels of just red to infrared radiation. The UV reflections from Proxima c will be easy to image because of the 1.15 arc second distance from Proxima Centauri. The idea of having time critical observations in the UV with large scopes seems to be a losing proposition, but with the many smaller 1/2 meter to 1 meter observatories around the southern world this would still be easy to do!!!
      As with most grandiose ideas, the time and money makes it hard to achieve, but using smaller instruments, even professional amatuer telescopes to do this would be much quicker. The method used in Lucky imaging would work for such targets. https://en.wikipedia.org/wiki/Lucky_imaging
      https://www.ast.cam.ac.uk/research/instrumentation.surveys.and.projects/lucky.imaging/latest.results/amateur.lucky.imaging
      http://www.ajax.ehu.es/Juno_amateur_workshop/talks/02_03_ImagingWorkflow_Christopher_Go.pdf
      http://planetaryimagingtutorials.com/getting-started/

      Any of you that have a telescope, try it, it not that hard to do!