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A Prehistoric Close Pass

Given the vast distances of interstellar space, you wouldn’t think there would be much chance of stars colliding. But it’s conceivable that so-called ‘blue straggler’ stars are the remnants of just such an event. A large blue straggler contains far more hydrogen than smaller stars around it, and burns at higher temperatures, with a correspondingly shorter life. When you find a blue straggler inside an ancient globular cluster, it’s natural to ask: How did this star emerge?

Packing stars as tightly as globular clusters must produce the occasional collision, and in fact astrophysicist Michael Shara (then at the American Museum of Natural History) has estimated there may be as many as several hundred collisions per hour somewhere in the universe. We would never be aware of most of these, but we could expect a collision every 10,000 years or so within one of the Milky Way’s globular clusters. In fact, the globular cluster NGC 6397 shows evidence for what may have been a three-star collision, the result of an outside star moving into a binary system and eventually coalescing (see Two Stars Collide: A New Star Is Born).

In any case, we can think of our own Solar System’s history to reflect on relatively close passes between other stars and the Sun. WISE J072003.20−084651.2 is the designation for a star more mercifully known as Scholz’s Star, discovered in 2013 in the southern constellation Monoceros. It took a scant two years for Eric Mamajek and co-researchers to report that Scholz’s Star passed through the Oort Cloud some 70,000 years ago.

It was evident that Scholz’s Star showed little tangential velocity. But which way was it moving? Mamajek discussed the matter in a 2015 news release:

“Most stars this nearby show much larger tangential motion. The small tangential motion and proximity initially indicated that the star was most likely either moving towards a future close encounter with the solar system, or it had ‘recently’ come close to the solar system and was moving away. Sure enough, the radial velocity measurements were consistent with it running away from the Sun’s vicinity – and we realized it must have had a close flyby in the past.”

Now 20 light years away, the star is the subject of new work on Solar System orbits. For a close stellar pass can leave traces that linger. A team led by Carlos and Raúl de la Fuente Marcos (Complutense University of Madrid), working with Sverre J. Aarseth of the University of Cambridge, has created numerical simulations to analyze the positions of some 340 objects on hyperbolic orbits. The idea is to work out the radiants, positions in the sky from which these objects appear to come. You can see why this paper caught my eye given our recent discussion of ‘Oumuamua and how we might calculate future such arrivals.

Assuming the objects on hyperbolic orbits are moving toward us from the Oort Cloud, which seems a reasonable assumption, we would figure that they would be more or less evenly distributed in the sky. Instead, the paper identifies what the authors call “a statistically significant accumulation of radiants,” an over-density that projects in the direction of Gemini. This, in turn, fits with the system’s encounter with Scholz’s Star 70,000 years ago. From the paper:

It is difficult to attribute to mere chance the near coincidence in terms of timing and position in the sky between the most recent known stellar fly-by and the statistically significant overdensity visible in Figs 3 and 4. It is unclear whether other clusterings present may have the same origin or be the result of other, not yet-documented, stellar fly-bys or perhaps interactions with one or more unseen perturbers orbiting the Sun well beyond Neptune…

And later:

The overdensity of high-speed radiants appears to be consistent in terms of location and time constraints with the latest known stellar fly-by, that of Scholz’s star.

I’ll send you to the paper for the actual figures — they won’t reproduce well here.

Scholz’s Star is a binary system, a red dwarf orbited by a brown dwarf, and it is likely that there was a time when our ancestors could see it in the sky. But only barely — Eric Mamajek has pointed out that even at its closest approach, the apparent magnitude would have been in the range of 11.4, which is five magnitudes fainter than what the naked eye can see, even in the pristine skies of paleolithic Earth. What might have been visible would have been flares from the M-dwarf, which could have created short-lived transient events, fleeting but noticeable.

Image: At a time when modern humans were beginning to leave Africa and the Neanderthals were living on our planet, Scholz’s star approached to within less than a light-year. It may have been briefly visible during flare events on the M9.5 primary. Credit: José A. Peñas/SINC

Here’s a graph that Mamajek published on Twitter in 2015.

So we have the evidence of disrupted trajectories to back up the finding that Scholz’s Star made a close pass. ‘Oumuamua, incidentally, is not implicated in any of this. Its radiant is in the constellation Lyra, meaning it is not part of the over-density observed by the De la Fuente Marcos team. On the matter of deep space interlopers, though, it’s interesting that the paper names eight hyperbolic comets as being good candidates to have an interstellar origin.

The paper is C. de la Fuente Marcos, R. de la Fuente Marcos, S. J. Aarseth. “Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies,” MNRAS Letters, 2018 (preprint). The Mamajek paper is Mamajek et al., “The Closest Known Flyby of a Star to the Solar System,” Astrophysical Journal Letters 800 (2015), L17 (preprint).


{ 28 comments… add one }
  • Michael March 22, 2018, 10:24

    Here is a short clip of how BH’s move within a GC (the light bending effect is enhanced).If Techaliens get established in a GC they would be very powerful indeed able to see great distances and gain great escape velocities. If Starshot gets up and running I would not mind seeing some gavilen probes dedicated to observing them.



  • Antonio March 22, 2018, 14:46

    I wonder when these hyperbolic objects from Gemini will fly-by the Sun. It’s a pity not having any distance information.

    • Antonio March 22, 2018, 16:49

      Oh, I found the data in the Mamajek et al. paper. They will come in ~2Myr.

      • Harry R Ray March 25, 2018, 13:46

        Eight candidates have already been detected now and require follow-up observations, according to Dr Mamajek’s LATEST tweet.

  • Tom Mazanec March 22, 2018, 16:10

    Gliese 710 will pass the sun at only 13,000 AU around 1,352,000 AD

  • andy March 22, 2018, 18:26

    Does this result for the close encounter with Scholz’s star have any effect on the case for/against Planet Nine, I wonder?

    Talking of stellar close encounters and merges, there’s a recent paper (currently on the A&A “forthcoming” list) that suggests that the Cepheid variable primary of the Polaris system may be a merger remnant. A preprint is available on the arXiv: Anderson (arXiv:1803.07413 [astro-ph.SR]), “Homing in on Polaris: A 7 M⊙ first-overtone Cepheid entering the instability strip for the first time

    • Andrei March 24, 2018, 10:46

      I Think this hyphotesis suggest there is no large planet in a very wide orbit. But the hypthesis of a Mars side KBO might still be viable.

  • Coacervate March 22, 2018, 20:10

    Again I am moved to say thank you for following up on this interesting topic. To sample and study material from another star system would be very exciting. I hope I am still here when we achieve that one!

    BTW my email has changed. I’ll forward my new address

  • Robin Datta March 22, 2018, 21:33

    The approach of another star or for that matter even a rogue planet could threaten disruption of the solar system well before the projected demise of tne sun, being yet another reminder of tenuousness of life (and intelligence) in the universe and a reason for celebration of life and wise stewardship of the planet.

  • J. Jason Wentworth March 23, 2018, 1:30

    The following is admittedly very speculative, but the transients (flare events) of Scholz’s star that ancient humans might have seen during its periastron (with respect to our Sun) reminds me of something, and it may possibly suggest other relatively close star-Sun encounters (perhaps involving stars that orbit the Milky Way in the retrograde direction [Kapteyn’s Star, an M1 red subdwarf star just 12.76 light-years away, is a retrograde-orbiting star]):

    One or more ancient (but within the historical past) accounts insist that Sirius once–for a time–glowed red rather than white. (Unfortunately, I can’t remember the ancient works in question, or which book I read about this in, because it was many years ago, but I do recall that it was an astronomy book.) One passage (ancient Greek, if memory serves) said something very similar to, “The Dog star’s red heat split the stone statues.” Regardless of what might have split the statues (it could hardly have been star-caused), astronomers strongly doubt (as the book also said) that Sirius’ light emission was visibly red at any time after it began burning “at full intensity” on the Main Sequence, especially as recently as the time of Classical Greece, but:

    If an otherwise-invisible (*as a point of light*, like Scholz’s star at its periastron) red dwarf with a high proper motion (like Barnard’s star) passed between Sirius and our Sun, or lined up closely enough with Sirius that the human eye couldn’t distinguish the angular separation, people might have perceived Sirius as being red. An M1 red subdwarf (a very dim, very small, but hydrogen-burning star [it’s not a brown dwarf]) could pass quite close to the Sun, relatively speaking, without (like Scholz’s star at periastron) being visible as a luminous point, and:

    A retrograde-orbiting star, regardless of its mass and luminosity, would have a very high proper motion. While such “against the flow of galactic traffic” stars aren’t common, they could–if they were dim, low-mass, and passed by us closely, or were high-mass and passed by distantly (they might be dim, too, if their intrinsic brightness was relatively low [maybe due to a “carbon-sooty” atmosphere?])–disrupt the Oort Cloud, form clustered radiants for “infalling” objects, and “vanish” relatively quickly (in astronomical time). In addition:

    Depending on the particulars of such stellar encounters (involving either retrograde or direct [prograde] orbiting stars), distantly-orbiting objects around the star might possibly be “pulled away” by our Sun, and either “immediately,” or over time (after other encounters, with stars or perhaps dense interstellar gas and/or dust clouds), these unbound (or weakly bound, to the Sun) interstellar objects might be perturbed to fall inward toward the Sun, perhaps on hyperbolic paths. (If their inbound paths passed even generally close to an ice giant or a gas giant outer planet, their movements could become hyperbolic.)

  • Andrew Palfreyman March 23, 2018, 1:30

    It’s a nice relaxing calendar schedule between these encounters, as befits a mellow species such as ours will become. (We can dream).

    • J. Jason Wentworth March 24, 2018, 0:44

      “All of everyone is nothing.” Don’t judge all human beings by your own culture or sub-culture. In addition to people being individuals (many if not most of whom *are* mellow–they’re the very people one is least likely to be aware of, because such people, by definition, don’t attract attention), there are cultures and sub-cultures which are quiet, in which “exciting” individuals are exceptions. Two examples that immediately come to mind are the Amish and the Mennonites, but there are many others, including modern technological nations (Switzerland is but one example). But your frustration (which I share) raises an interesting, starflight-related possibility:

      A common assumption about interstellar travel is that it’s so complex and expensive–in terms of energy as well as money–that it can only occur when a united humanity will have the necessary wealth and knowledge base to undertake. This may well be the case where crewed missions are concerned. But as technology advances, this may not be the case for starprobes. Especially if miniaturized payloads and “Sun-diver” solar sails are used (the immense power of the Sun could dispatch 1% – 5% [and maybe 10%] of c solar sail probes), individual nations–and perhaps even wealthy space companies and/or individuals–may eventually be able to afford them.

    • Michael March 24, 2018, 17:07

      We are mostly harmless…

  • DCM March 23, 2018, 3:37

    Maybe we’re fortunate not to be closer to the galactic center. Life may have a better chance the further out it is because of the greater distances between stars.

  • Antonio March 23, 2018, 4:13

    Good news, ARIEL is approved: http://sci.esa.int/cosmic-vision/59796-esa-s-next-science-mission-to-focus-on-nature-of-exoplanets/

    I suppose this is the end for FINESSE.

  • ericSECT March 23, 2018, 7:16

    If this star passed so close and disrupted our Oort cloud as seems likely…. where is the shower of comets, or are they right now on their way?

    • Kevin in Wi March 27, 2018, 23:33

      Ok. That’s just…. disconcerting.

  • Harry R Ray March 23, 2018, 9:34

    After many decades of RV observations, the putative Jupiter analog orbiting Epsilon Indi A is NO LONGER putative! It has been CONFIRMED!!! Minimum mass: 2.71+2.19-0.44Mj. Semi-major axis: 12.82+4.18-0.71AU. Orbital period: 52.62+27.70-4.12 years. “Detection of the closest Jovian exoplanet in the Epsilon Indi triple system”. by Fabo Feng, Mikko Toumi, Hugh R. A. Jones.

    • J. Jason Wentworth March 24, 2018, 1:13

      I’ll drink pomegranate/moonbeam nectar to that! Epsilon Indi (see: http://en.wikipedia.org/wiki/Epsilon_Indi ) has long been one of the “most favored” stars for eventual colonization (if no one lives there; John W. Macvey’s cheerful 1983 book “Where Will We Go When the Sun Dies?” lists Epsilon Indi as a possible new home), and for the possible presence of indigenous intelligent life, and:

      While no Earth-like planet(s)–either inside or outside the star’s habitable zone–have been detected there, such worlds could exist; improved instruments will facilitate searches for them. The system’s two brown dwarfs orbit very far out (thousands of AUs), and the Jovian planets orbit well clear of Epsilon Indi A’s habitable zone, so the orbits of any habitable planets in the HZ shouldn’t be perturbed by the giant planets. The two brown dwarfs might also have interesting (although probably not habitable) planets; this system, like Alpha Centauri, would be an interesting “multi-stop destination” to explore.

      • Andrei March 25, 2018, 5:27

        When the Sun dies. Epsilon Indi will have run away.
        Interesting system, but one bad news for rock planets, very little dust can be seen.

    • Michael C. Fidler March 24, 2018, 22:24

      Maybe will will have some interesting results for Barnard’s star in the not too distant future.

      Optical and Near-Infrared Radial Velocity Content of M Dwarfs:
      Testing Models with Barnard’s Star.

      “High precision radial velocity (RV) measurements have been central in the study of exoplanets during the last two decades, from the early discovery of hot Jupiters, to the recent mass measurements
      of Earth-sized planets uncovered by transit surveys. While optical radial-velocity is now a mature field, there is currently a strong effort to push the technique into the near-infrared (nIR) domain (chiefly Y , J, H and K band passes) to probe planetary systems around late-type stars.
      The combined lower mass and luminosity of M dwarfs leads to an increased reflex RV signal for planets in the habitable zone compared to Sun-like stars. The estimates on the detectability of planets rely on various instrumental characteristics, but also on a prior knowledge of the stellar spectrum. While the overall properties of M dwarf spectra have been extensively tested against observations, the same is not true for their detailed line profiles, which leads to signifi-cant uncertainties when converting a given signal-to-noise ratio to a corresponding RV precision as attainable on a given spectrograph. By combining archival CRIRES and HARPS data with ESPaDOnS data of Barnard’s star, we show that state-of-the-art atmosphere models over-predict the Y and J-band RV content by more than a factor of ∼2, while under-predicting the H and K-band content by half.’


  • Steve Muise March 23, 2018, 13:06

    Shades of John W. Campbell’s “The Black Star Passes”.

  • Tom Mazanec March 24, 2018, 15:34

    Another future possible close pass is HIP 85605 (not yet confirmed)

    • Jeff Wright March 28, 2018, 15:52

      That even beats Gliese 710

      Other designations
      BD–01° 3474, HIP 89825, HD 168442, NSV 10635[2]

      I wonder if it is this star that Mallove and Matloff called DM+ 61 366 in THE STARFLIGHT HANDBOOK…

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