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ALMA: Interesting Objects in the Outer System

Two papers have appeared on the arXiv server suggesting hitherto undiscovered objects in the outer Solar System (thanks to Centauri Dreams reader Stevo Darkly for the pointer). Both papers use data harvested by the Atacama Large millimeter/submillimeter array (ALMA), an interferometer of radio telescopes in Chile’s northern high desert. Here some 66 12-meter and 7-meter radio telescopes work the sky at millimeter and submillimeter wavelengths, with targets that have ranged from galactic dust in the early universe to magnetic fields near a supermassive black hole.

ALMA’s uses closer to home are made clear in the new papers, which demonstrate that this array can be a major tool in helping us probe the outer system well into the Oort Cloud. In the first paper, the researchers draw on three periods of observation with ALMA to detect point-like emissions at different positions in two of the periods. The two emissions are thought to be the same source, considering what the authors call “the negligible probability of having identified two independent highly variable background sources.” Assuming a single source, the team dubs the object Gna, noting that it was not visible in the third observing period 42 days later.

So what exactly do we have here? If gravitationally bound, the object would be at a distance between 12 and 25 AU, most likely a Centaur with a semi-major axis between Jupiter and Neptune, though a large one, in the range of 220-800 kilometers depending on distance and albedo. The authors note that the location of the field of view (close to the galactic plane toward the star W Aquilae) may explain why the object was not seen sooner despite its size.

But there is another possibility, that Gna is unbound. “In that case,” write the authors, “the most exciting possibility is that we have observed a planetary body or brown dwarf in the outer reaches of the Oort cloud.” Which leads to recent WISE studies that found no evidence of a Saturn-class object out as far as 28000 AU, or of a Jupiter out to 82000 AU, while a Jupiter-sized brown dwarf could be excluded out to 26000 AU at the locations previously suggested by John Matese and Daniel Whitmire. The latter have argued that a large planetary body in the Oort would explain the seemingly clustered orbits of many comets.

Could Gna be that object? The paper notes the WISE data in reference to the new findings, stating that a submillimeter flux of 3 mJy at a distance of 5000-50000 AU would point to an object that has retained residual heat or generates emission of its own. It would not, in other words, absorb enough sunlight to display the measured flux. As the paper notes:

This would imply a large planet or brown dwarf and could be reconciled with the Jupiter sized brown dwarf or Saturn/Neptune size planetary limits provided by WISE (Luhman 2014), if Gna is located around or beyond ∼ 20000 AU.

But the high proper motion seen here seems to rule out anything beyond about 4000 AU. While unable to exclude a large gravitationally unbound body within 4000 AU, the authors think it most likely that Gna is a bound Centaur in retrograde orbit, currently between 12 and 25 AU out and with a size of 220-880 kilometers.

But we’re not through with ALMA just yet. Another set of ALMA observations ten months apart (2014-2015) by the same research team has revealed a point source that appears to share the proper motion of Alpha Centauri A and B. Is there an Alpha Centauri D? Apparently not, for at the distance of the Centauri stars, the measurements would correspond to an M2-class dwarf, a star bright enough to be clearly visible. If there were an Alpha Centauri D, we would have seen it by now.

Instead, the authors make the case that the object is a Trans Neptunian Object (TNO). From the paper:

For reasons of sensitivity (or rather, lack thereof), TNOs on highly eccentric orbits have traditionally been firstly detected when close to their perihelia. Further away, there would have remained unseen… However, a sizable population of such bodies is expected to exist at large distances from the Sun. It is clear, therefore, that ALMA with its high submm sensitivity provides presently the only existing means to detect TNOs far from their perihelia, where temperatures are merely some tens of Kelvin. There must be a vast reservoir of objects between, say, roughly 100 and 1000 AU, of which we hitherto have seen only a tiny fraction.


Image: An illustration of the relative sizes, albedos and colors of some of the largest known TNOs. Credit: Wikimedia Commons. Licensed under CC BY-SA 3.0.

Just what kind of an object would this be? The paper continues:

We argue that the object is most likely part of the solar system, in prograde motion, albeit at a distance too far to be detectable at other wavelengths, viz. an ETNO [Extreme Trans Neptunian Object] (≫ 100 AU), a hypothesized Super-Earth (∼ 300 AU) or a super-cool brown dwarf (∼ 20 000 AU).

From an ETNO to a brown dwarf is quite a range, highlighting how much we still don’t know. So we can’t say we’ve found the large perturber discussed by Matese and Whitmire, but we do have interesting work on new objects, one of which could conceivably fit the description. It will take a lot of work to learn more about the new ALMA objects, but we’re finding out how the array can fine-tune our capabilities in probing out into the Oort Cloud. If such perturbers exist, we’re going to turn them up sooner or later as we continue to map the system’s farthest depths.

The paper on Gna is Vlemmings et al., “The serendipitous discovery of a possible new solar system object with ALMA,” submitted to Astronomy & Astrophysics (preprint). The paper on the point source near Alpha Centauri in the sky is Liseau et al., “A new submm source within a few arcseconds of α Centauri: ALMA discovers the most distant object of the solar system,” submitted to Astronomy & Astrophysics (preprint). Eric Berger offers a highly skeptical take on this work here.


Comments on this entry are closed.

  • Harry R Ray December 10, 2015, 11:25

    I would like Jason Wright’s input on this. In my wildest dreams, I invision an M2 Dwarf FAR ENOUGH AWAY from A and B to NOT cause ANY astrometric perturbations on them that is COMPLETELY ENSHROUDED by a Dyson swarm, blocking ALL of the VISABLE LIGHT, absorbing ALL of the infra-red light, and re-immiting the heat at a wavelength that ALMA can detect. The ods against this are ASTRONOMICAL ON STEROIDS, but, to me, so are the odds of finding an, as yet, undiscovered MEGACENTAUR right at Alpha Centauri AND CO-MOVING with the system! CAVAET: Phil Platt’s(www.slate/badastronomy.com) take on this is that is just probably an artifact, which makes a lot more sense to me than the extremely co-incedental scenarios presented in the paper, whigh I truely believe are LESS LIKELY than my “Dyson swarm” scenario.

  • Andrew LePage December 10, 2015, 11:47

    I feel we have only begun to scratch the surface of what orbits beyond Neptune in our solar system. Unfortunately, I fear that the popular press is going to butcher the reporting of this discovery by conflating it with planets orbiting Alpha Centauri. More details on what past searches for planets orbiting Alpha Centauri can be found here:


  • Alex Tolley December 10, 2015, 13:36

    So more likely a small object within the solar system, rather than a large object in the Oort or beyond, assuming these are truly robust observations and not artifacts?

  • TLDR December 10, 2015, 13:54

    They could have chosen a goddess with an easier pronunciation than Gná.
    BTW, by “W Aquila”, do you mean “Omega1 Aquila”?

  • ljk December 10, 2015, 14:15
  • Paul Gilster December 10, 2015, 15:51

    Alex Tolley writes:

    So more likely a small object within the solar system, rather than a large object in the Oort or beyond, assuming these are truly robust observations and not artifacts?

    Yes, that’s how I see it. Assuming we’re not dealing with some kind of systematic error.

  • Paul Gilster December 10, 2015, 16:39

    TLDR writes:

    BTW, by “W Aquila”, do you mean “Omega1 Aquila”?

    I see that Omega1 Aquilae and Omega2 Aquilae share the ω Aquilae designation, and that they’re separated by 0.51° on the sky. This is evidently an asymptotic giant branch (AGB) star with a faint companion.

  • andy December 10, 2015, 18:19

    W Aquilae is a variable star designation, it is not the same as ω Aquilae (Omega Aquilae) which is a Bayer designation.

    W Aquilae is an S-type star with a V-magnitude of 10.14. SIMBAD does not list a parallax for this star. This is the AGB star mentioned in the paper. According to this paper it is a binary star at a distance of about 400 parsecs: the secondary is a dwarf star of spectral type F8–G0.

    The Bayer designation ω Aquilae is shared by two stars: ω¹ Aquilae and ω² Aquilae.

    ω¹ Aquilae is an F0 subgiant with a V-magnitude of 5.3 at a distance of 127 parsecs.

    ω² Aquilae is an A2 main sequence star with a V-magnitude of 6.0, at a distance of 81 parsecs.

    Given the different distances of ω¹ and ω² Aquilae, they are clearly not a physical binary. Neither is associated with W Aquilae which is located about 18.6° away from ω¹ and ω².

  • Paul Gilster December 10, 2015, 21:02

    andy, thanks for straightening that out — I didn’t understand the Bayer designation. And yes, going back to the paper, I see the AGB reference vs. ω¹ and ω² Aquilae. Mea culpa!

  • Infinite123Lifer December 11, 2015, 0:24

    Just wanted to say … I love Centauri Dreams. You have always brought an engaging, systematic but tranquil retreat to my evenings. Thank you Paul and all who contribute.

  • Mike Fidler December 11, 2015, 4:36

    To explain source U in terms of a contaminating background source could hardly be considered compelling1 , given that that object had to share
    the high proper motion of αCen. On the other hand, if a passing
    object, it had to be in the far outskirts of the solar system, belonging
    to the Edgeworth-Kuiper Belt or the Oort cloud (Fig. 5).
    The probability could be low though, since we would need to
    catch it very close to the footprints of the loop of the parallax
    motion (cf. Fig. 2). In addition, bound motion at a distance of,
    say, 1000 AU (the aphelion of Sedna) would result in an orbital
    motion of less than 40″ yr−1, but the apparent motion in the sky
    would be dominated by the annual parallax (∼ 200″).

    Sounds pretty bodacious!

  • ljk December 11, 2015, 9:00
  • Paul Gilster December 11, 2015, 9:00

    Infinite123Lifer writes:

    Thank you Paul and all who contribute.

    And thank you for being a part of the conversation! This site thrives on discussion, and I’m always appreciative of the quality and tone of the ongoing debates.

  • Harry R Ray December 11, 2015, 12:17

    Andrew LePage: Should the Alpha Centauri object candidate be imaged AGAIN by ALMA in the Alpha Centauri system(which I SERIOUSLY doubt), how should we follow up? You mentioned in a previous comment that JWST would probably not be able to look at Alpha Centauri B out of fear of overexposure. Is this object candidate far enough offset to allow JWST to image it?

  • Mark Zambelli December 11, 2015, 14:16

    Andrew LePage said…

    “I feel we have only begun to scratch the surface of what orbits beyond Neptune in our solar system….”

    I heartily agree. With the recent addition of an Inner Oort Cloud to the suburbs and knowing we’ve found all these members near their closest, the mind boggles at how many objects of substantial size must litter those sheer volumes. Now that ALMA has opened up the way for detection at these wavelengths the population should blossom in number. The other thing is their diversity in surfaces, as so beautifully seen at Pluto. We’re in for a treat.

  • TLDR December 11, 2015, 14:46

    JWST will not be able to image AC? That’s news to me.

    “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.”

  • ljk December 11, 2015, 15:01

    Maybe it is something that just recently appeared on the edge of our Sol system – and it is headed this way.

  • Alex Tolley December 11, 2015, 16:23

    Is there any data/charts showing total mass for each shell volume around the sun? Using n AU increments, I assume each shell of space contains so much mass. Is there a pattern to its distribution that we can say declines after some peak, and if so, what is the extent? While easy to determine out to Pluto, what happens as one goes further out into the Kuiper belt and then the Oort?

  • Stevo Darkly December 11, 2015, 21:05

    Aha! Thank you for writing about this, Paul Gilster. I ran across the references to the papers on Twitter, but as a layman I found them a bit bewildering. Now I am about to read this entry and learn more of what all the hubbub’s about.

    PS: I was curious why they named the alleged object “Gna” and did a bit of Googling. As some of you already know but maybe others don’t, Gna comes from Norse mythology and is the name of a handmaiden to the goddess Frigg (or Frigga). Just mentioning that on the off chance that it saves someone else a bit of headscratching. At first I thought it was some kind of acronym or something.

  • Jim Franklin December 14, 2015, 16:44

    Whilst it is more probable that what is being recorded is likely artifacts or other related anomalies in the data sets, perhaps a fault in the electronics being used by the team, it is disappointing to read some of the comments by otherwise prominent members of the Astronomy community.

    Mike Brown’s comment about 200,000 Earth sized planets based on the observation is ill-informed, immature and frankly unprofessional. If he has actual repeatable data or can otherwise demonstrate that the suppositions of the team is inaccurate or plain wrong then so be it, but school yard comments are not required in science.

    Could there be an object between 250 and 800 miles in diameter in the outer solar system, highly probable, in fact I think most would be surprised if we discovered we had found them all, so the data may be correct but calculations may be off for unknown reasons. Could it be as close as suggested, probably not, but probably is not definitely so comments dismissing the premise, when the commentator has not themselves studied the data sets, are foolhardy.

    Time will tell, more data sets are required, testing of the equipment and data collection methodology need to be rigorously tested to repeat the results and then we will know more clearly if this is an error by the hardware, software or the human interface – – Or if they have truly made an astounding discovery.

  • andy December 18, 2015, 12:15

    The Alpha Centauri paper (Liseau et al.) has been withdrawn until further data is available.

  • Mike Fidler December 31, 2015, 22:10

    Tentative planetary orbital constraints of some scenarios for the possible new Solar System object recently discovered with ALMA.
    Lorenzo Iorio – 17 December 2015

    A Neptune-type body at some 2500 au.

    Same date that the Alpha Centauri paper (Liseau et al.) was withdrawn, but under General Relativity and Quantum Cosmology.


  • andy January 21, 2016, 7:07

    The Gna paper (Vlemmings et al.) has also been withdrawn.