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Scholz’s Star: A Close Flyby

The star HIP 85605 until recently seemed more interesting than it may now turn out to be. In a recent paper, Coryn Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg) noted that the star in the constellation Hercules had a high probability of coming close enough to our Solar System in the far future (240,000 to 470,000 years from now) that it would pass through the Oort Cloud, potentially disrupting comets there. The possibility of a pass as close as .13 light years (8200 AU) was there, but Bailer-Jones cautioned that distance measurements of this star could be incorrect. His paper on nearby stellar passes thus leaves the HIP 85605 issue unresolved.

Enter Eric Mamajek (University of Rochester) and company. Working with data from the Southern African Large Telescope (SALT) and the Magellan telescope at Las Campanas Observatory in Chile, Mamajek showed that the distance to HIP 85605 has been underestimated by a factor of ten. As Bailer-Jones seems to have suspected, the new measurement takes the star on a trajectory that does not bring it within the Oort Cloud. But in the same paper, the team names an interesting system called Scholz’s Star as a candidate for a close pass in the past.

Studying the star’s tangential velocity (motion across the sky) as well as radial velocity data, the team found that despite being relatively close at 20 light years, Scholz’s Star shows little tangential velocity. That would imply an interesting encounter ahead, or one that had already happened. Mamajek explains:

“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.”

Red and Brown dwarf binary system

Image: Artist’s conception of Scholz’s star and its brown dwarf companion (foreground) during its flyby of the solar system 70,000 years ago. The Sun (left, background) would have appeared as a brilliant star. The pair is now about 20 light years away. Credit: Michael Osadciw/University of Rochester.

The paper on this work, recently published in the Astrophysical Journal, determines the star’s trajectory, one that shows that about 70,000 years ago, it would have passed some 52,000 AU from the Sun. This works out to about 0.82 light years, or 7.8 trillion kilometers, quite a bit closer than Proxima Centauri, and probably close enough to pass through the outer Oort Cloud. The star was within 100,000 AU of the Sun for a period of roughly 10,000 years.

Scholz’s star (W0720) is a low-mass object in the constellation Monoceros also tagged WISE J072003.20-084651.2 and only recently discovered (by Ralf-Dieter Scholz in 2014) thanks to its dimness in optical wavelengths, its proximity to the galactic plane and its low proper motion. Adaptive optics imaging and high resolution spectroscopy has demonstrated that the star is actually a binary, an M-dwarf with a companion at 0.8 AU that is probably a brown dwarf.

The question that immediately comes to mind is what kind of object the Scholz’s star system would have presented in the night sky some 70,000 years ago. The answer is not dramatic, for at its closest approach the binary would have had an apparent magnitude in the range of 11.4 (note: there is a typo in the paper, as noted here, which had specified an apparent magnitude of 10.3). This is five magnitudes, or a factor of 100 times, fainter than the faintest naked eye stars. But the paper notes that M-dwarfs like this one are often given to flare activity that might have made Scholz’s star a brighter object. From the paper:

If W0720 experienced occasional flares similar to those of the active M8 star SDSS J022116.84+194020.4 (Schmidt et al. 2014), then the star may have been rarely visible with the naked eye from Earth (V < 6; ∆V < −4) for minutes or hours during the flare events. Hence, while the binary system was too dim to see with the naked eye in its quiescent state during its flyby of the solar system ∼70 kya, flares by the M9.5 primary may have provided visible short-lived transients visible to our ancestors.

And take a look at this graph, which Eric Mamajek published on Twitter yesterday.


As you can see, Scholz’s Star was moving out. If it had been visible, what would ancient skywatchers have made of it? We also have to wonder what other close encounters our Solar System may have had with other stars. Note this point from the paper about M-dwarfs:

Past systematic searches for stars with close flybys to the solar system have been understandably focused on the Hipparcos astrometric catalog (García Sánchez et al. 1999; Bailer-Jones 2014), however it contains relatively few M dwarfs relative to their cosmic abundance. Searches in the Gaia astrometric catalog for nearby M dwarfs with small proper motions and large parallaxes (i.e. with small tangential velocities) will likely yield addition candidates.

So much still to learn about M-dwarfs!

The paper is Mamajek et al., “The Closest Known Flyby of a Star to the Solar System,” Astrophysical Journal Letters 800 (2015), L17 (preprint). The Bailer-Jones paper discussed above is “Close Encounters of the Stellar Kind,” in press at Astronomy & Astrophysics (preprint). For more on Bailer-Jones, see Stars Passing Close to the Sun.


Comments on this entry are closed.

  • RobFlores February 19, 2015, 12:05

    Very Interesting, more interesting, I wonder what frequency do stars come
    close enough to disrupt each other’s Ort Cloud.
    I am not sure of the errors margin of errors in terms of Years, but
    70,000 is in the Time period of the Toba Genetic Bottle Neck. Now that
    event is named after super volcano which is suspected as a trigger. Dark
    times for the ancestors of modern man, as they nearly went extinct.
    What if that is not the total story. What if the effects of Scholtz’s star
    near approach resulted in increased frequency of cometary strikes on
    the Earth. It’s probably another factor supporting the Fermi Paradox

    As side note, the plot Jack McDevitt’s novel SEEKER posits a colossal blunder on the part of colonists settling on a new Earth, one of his best novels.

  • Liem February 19, 2015, 13:26

    If you scale 1AU to 1 inch, Scholz’s star would be a pinhead 4/5ths of a mile from our pinhead sized Sun at 50,000AU.

  • Bob Steinke February 19, 2015, 13:32

    I doubt our ancestors would have noticed anything unusual about this star even if flares did occasionally make it visible. From the chart it looks like it moved from 20 degrees to 60 degrees declination in about 10,000 years. That’s about 0.00001 degrees per day. In comparison Saturn moves about 0.03 degrees per day against the background stars.

    It would be a very faint star that sometimes you can see and sometimes you can’t among a whole bunch of other very faint stars, and it wouldn’t move a noticeable amount within a human lifetime.

  • Ashley Baldwin February 19, 2015, 15:52

    Thanks Paul. Great post as ever. Makes me realise what a dynamic place the Universe is all the way from clashing galaxies all the way down to dust inducing cometary collisions. As astrometry finally matures as a science we will be able to calculate a lot more of these “near misses” both forwards and backwards in time. Gaia should really bring things on and its potential for synergy with WFIRST , which is also exploring astrometry closely ( on top of direct imaging and microlensing too, phew !) is mouth watering in what it might discover planetary wise closer to home.

  • Paul Gilster February 19, 2015, 16:09

    Rob Flores writes:

    As side note, the plot Jack McDevitt’s novel SEEKER posits a colossal blunder on the part of colonists settling on a new Earth, one of his best novels.

    Thanks for the tip! I’ve read most of Jack’s work but not this one. I’ve just ordered it.

  • Al Jackson February 19, 2015, 19:00

    Been known for a long time there is an anisotropy in the inclinations of long period comets, even accounting for the sun grazer group.
    I am guessing someone somewhere is doing the calculation to see what scattering Scholz’s Star did to the outer Oort cloud. I am thinking 70 thousand years may be still to short a time to see an enhancement of the long period comet flux.
    Though I have the recollection from reading the comet literature over the last 25 years that some comet experts think there is slight increase in the long period flux into the solar system over what one might expect the steady state to be. Not sure what the thinking on that is today.

  • Lionel Ward February 19, 2015, 20:13

    Does anyone know whether models of solar system formation and dynamic evolution factor close encounters with passing stars?

    I’ve read that in order to maintain consistency with the solar system’s current state, the Nice Model often factors in a fifth gas giant originally being in our solar system and getting flung out by Jupiter. (without this, the Model shows that Mars would likely have been flung out or something like that) Perhaps factoring in close passing stars will require very high probabilities that our solar system consisted yet more planets. These would likely that now drift as rogues around the milky, or perhaps if they were lucky got captured into another system

    Also, Paul not long ago wrote about the proposals that one or more larger-than-earth planet(s) could might exist out at several hundreds of AU, and the schematics I’ve seen present these as being in circular orbits. But are there really high probabilities that such a super-earth or ice giant far beyond the Kuiper belt could stay 4billion years unmolested at that distance? …I do hope somebody out there is researching/simulating these questions because I find the implications fascinating

  • Paul Gilster February 19, 2015, 22:01

    Apropos of both of the above is this section of Eric Mamajek’s website dealing with Scholz’s star, addressing the question “How close does a star have to come into the solar system to perturb enough to trigger comets coming into the inner solar system?”:

    The “danger zone” so to speak is apparently within 20,000 AU of the Sun – the so-called “inner Oort cloud” or “Hills cloud”. In 99.999% of the simulations of the trajectory of Scholz’s star passed well beyond 20,000 AU (only 1 out of 10,000 simulations brought it within 20,000 AU). The likelihood that Scholz’s star passed through the “outer”, low-density Oort Cloud (based on the current velocity and distance data) is about 98%.

    The answer to “How close does a star have to come into the solar system to perturb enough to trigger comets coming into the inner solar system?” A star within a parsec or so could perturb “some” comets towards coming into the inner solar system, but there are fewer comets in the Oort Cloud that far out to perturb. One way of quantifying this was proposed by Feng & Bailer-Jones (2014) — they scale their results by defining a proxy indicator of the encounter-induced flux of Oort Cloud comets as \gamma = (mass of star)/(velocity of star X flyby distance). Their simulations are suggestive that gamma < 10^-5.3 (Msun * s / km / AU) are unlikely to generate an enhancement in the flux of long-period comets. For all of the simulations, none of them resulted in an encounter-induced flux of Oort Cloud comets that came close to generating a significant enhancement in the flux of long-period comets.

  • P February 19, 2015, 22:43

    I know I’m being a pedant, but I do wish astronomical illustrators (or their editors) would get the colour of red dwarfs closer to right. Yes I know this is an M9 dwarf, but this is still too red! I see similar illustrations being used for M2-3 stars which would appear pretty white to the naked eye. Which brings us back to the M-star classification thing someone mentioned a few days back. An M0 and an M9 don’t have much in common with each other.

    Its going to be hard to shake this meme now alas…


  • Mike lorrey February 19, 2015, 23:12

    Any comets disturbed by Scholz will take 2 million years to get here.

  • Sarah Newton February 20, 2015, 5:55

    I must admit the Toba bottleneck Rob Flores mentions upthread was the first thing that occurred to me, too. Also, with my science-fiction writery head on, the prospect of a red dwarf + brown dwarf binary system plus any attendant planets and moons in the vicinity of Earth for 10,000 years is packed with possibilities, especially as red dwarfs seem very promising candidates for advanced life to evolve. Imagine if the evolutionary bottleneck was due to our ancestors packing up and leaving with the Scholz’s Star inhabitants, with just a few remaining behind. ;-)

    Does anyone know of any exoplanet detections around Scholz’s Star other than the brown dwarf?

  • kzb February 20, 2015, 8:52

    Any comets from this encounter are still to get here. The infall time is said to be 2 million years, so there is 1.93 million years to wait. So the encounter can’t have influenced evolution yet.
    That’s not to say previous encounters millions of years ago could not. In fact if these things happen every 100,000 years on average, we should see that signature in the evolutionary record, if it is indeed a significant effect.

  • Michael February 20, 2015, 11:24

    @P February 19, 2015 at 22:43

    ‘I know I’m being a pedant, but I do wish astronomical illustrators (or their editors) would get the colour of red dwarfs closer to right. Yes I know this is an M9 dwarf, but this is still too red! ‘

    Here is an interactive that may help people with the colour issue, an M9 dwarf should be more orange, just play with the options.


  • ljk February 20, 2015, 11:33

    Does anyone who has a copy of Vistas of Many Worlds by Erik Anderson know if the author mentioned HIP 85605 and/or Scholz’s Star in his book?

    Here is the Centauri Dreams article on Anderson’s Vistas from November of 2012:


  • Paul Gilster February 20, 2015, 12:51

    Larry, I believe Scholz’s Star is a recent discovery, too recent to be in Erik’s book. I seem to remember HIP 85605 being in there but I can’t find it if so — the book needs an index! Erik reads this site, so perhaps he’ll comment when he sees this.

  • FrankH February 20, 2015, 16:48

    @p, @Michael – It bugs me to no end, too. A 2300K star is still pretty much yellow in color.

  • kamal ali February 21, 2015, 5:06

    Either we’re very “lucky” to have seen a star within the last 100K years that came within 0.8LY of Earth, or this kind of close encounter happens more frequently than is commonly understood. Does anyone have results on simulations that can tell us things like
    * what’s the mean wait time for another system to get within 10K AU ? or 1K AU??
    Imagine, if within each 100M year period, we get 10 or so “close” visits?!
    Besides perturbing comets, what mass would a 10KAU visitor have to have to perturb planetary motion for, say Pluto? what mass would a 1KAU visitor need to perturb Pluto (I realize this depends on the amount of perturbation).

  • Anthony Mugan February 21, 2015, 8:51

    I wonder if this, or to be more precise similar events over billions of years may be relevant for recent discussion on the origin of earth’s water?
    Could these events produce a mixing of comets formed in different star systems and could this lead to a change in the typical chemical composition, or perhaps create create greater variability?

  • Lionel Ward February 22, 2015, 8:31

    @kamal ali

    A 1K AU visitor, at the right place and right time, would be able to interfere with, capture or fling away Sedna if it was around Aphelion (936 AU). We are almost certainly going to find further dwarf planets in our solar system than Sedna, and the further away they are, the more chances for interaction they will have had with close passing stars; as we push out further we will find objects that we will either suspect or expect to have had origins in entire different star systems, perhaps having been passed around between multiple stars over billions of years

  • Alex Tolley February 22, 2015, 17:02

    See this topic discussed at CD here:


    I particularly refer you to Al Jackson’s comment and reference on historic encounters.

  • Al Jackson February 22, 2015, 19:32

    @Anthonly Mugan
    Stellar encounters with the solar system are important in the study of the stability of solar system. I have mentioned the French planetary physicist Jacques Laskar has made extensive studies of this for more than 30 years now.
    From stellar statistics there are about 5 stars per million years that pass within 1 pc of the solar system. 5 Gyr integrations of the solar system , with stellar encounter statistics, show no major disturbance of the planetary arrangement. So the solar system has been free of “when worlds collide” since it settled down from the final stages of formation.*
    Perturbations of the Oort cloud by stars and giant molecular clouds is still a subject of study.

    * I should point out that the problem here is the solar system as a celestial mechanics many body problem, external forces are perturbations. All these years after Newton ….still a tough problem.

  • Lionel Ward February 23, 2015, 15:11

    @Alex Tolley

    Many thanks! I’ve read Al Jackson’s comment and am looking at the research paper now. Something that must be qualified:

    Al Jackson quotes the article by saying “No object .1 Solar Mass has passed through the solar system in [the approximate age of the solar system], and no object ~ 3 Jovian masses has passed within the Earth’s orbit.”.

    Looking at the figures in the research paper, as we might from a paper written in 1988, *the solar system* in the above quote means only up to the aphelion of Pluto (48 AU). If we look at Figure 3, we see this – the maximum possible mass of a passing star around 50AU is 0.1 mass sol.

    Very interestingly, figure 3 also shows that around passing stars of up to 1 solar mass have not been ruled out from distances further than 120AU. (at the time of writing)

    Figure 4 is really interesting and very pertinent to what kamal ali and I have asked above. It shows the probability of visiting star passages as a function of distance from the sun. The probability of a star having passed us at 100AU is about one percent (this is negating the effect of the planets current orbits, however I expect that at these distances >3x the distances of Neptune, the current orbits cannot tell us so much).

    Figure 4 only goes up to 300AU from the sun, and at this point there is a 10% chance of a star having passed through and if I’m reading the graph right it says there’s a 100% chance of a 0.07 sol mass brown dwarf having passed through. (however beliefs about the abundances of brown dwarfs has changed much since this paper was written)

    It certainly looks like the “All Stars” line will continue going and reach or be close to 100% by the distances of 1000AU, or around the aphelion of Sedna, i.e. a star has almost definitely passed within Sedna’s furthest distance from the sun during the age of our solar system.

  • Lionel Ward February 23, 2015, 15:36

    @Al Jackson:
    “From stellar statistics there are about 5 stars per million years that pass within 1 pc of the solar system.”

    In that case, our civilisation has come about at a truly fortuitous time for interstellar travel, because we are scheduled for almost a million-years’ worth of visitors over the 50,000 years :)


  • Lionel Ward February 23, 2015, 15:39

    @ Al Jackson:
    “From stellar statistics there are about 5 stars per million years that pass within 1 pc of the solar system.”

    In that case, our civilization has arisen at a really fortuitous time for interstellar travel, because we are scheduled for almost a million-years’ worth of passing stars over the next 50,000 years :)


  • Al Jackson February 25, 2015, 10:58

    @Linoel Ward
    One thing I have not been able to verify is the stability of the solar system mean motion resonances. These take a long time to establish and are sensitive to external perturbations.
    Many are important , Laskar discovered that the inner planets are chaotic on times scales of 10 million years. That mostly applied to where they are in their orbits , their phases. However Laskar showed that Mercury had a 1% chance of getting an eccentricity of .6 in 5 Gyr , which is kind of weird.
    The Pluto-Neptune 2 to 1 mean motion resonance keeps Neptune from ejecting Pluto.
    I don’t know how well sensitivity of solar system resonances to stellar passage have been studied.
    I do know a surprising result. It has been found that one must include General Relativity in the long time many body integrations of the Solar System. Excluding GR make the Solar System 60% more unstable over 5 Gyrs!

  • Daniel de França April 9, 2015, 16:33

    What is the statistic distribution in terms of distance and time of the closest stars to ever pass near the sun since its formation?

  • Robert Walker August 17, 2015, 8:44

    @ Al Jackson – do you happen to know which paper he did this in?

    “5 Gyr integrations of the solar system , with stellar encounter statistics, show no major disturbance of the planetary arrangement. So the solar system has been free of “when worlds collide” since it settled down from the final stages of formation.*”

    Lots of papers by Jacques Laskar so not sure where to look. I haven’t yet found a paper on this mentioning stellar encounters. Thanks!

  • ljk August 27, 2015, 13:10


    Radio Emission and Orbital Motion from the Close-Encounter Star-Brown Dwarf Binary WISE J072003.20-084651.2

    Adam J. Burgasser (UCSD), Carl Melis (UCSD), Jacob Todd (UCLA), Christopher R. Gelino (NASA Exoplanet Science Center/IPAC), Gregg Hallinan (Caltech), Daniella Bardalez Gagliuffi (UCSD)

    (Submitted on 26 Aug 2015)

    We report the detection of radio emission and orbital motion from the nearby star-brown dwarf binary WISE J072003.20-084651.2AB. Radio observations across the 4.5-6.5 GHz band with the Very Large Array identify at the position of the system quiescent emission with a flux density of 15±3 μJy, and a highly-polarized radio source that underwent a 2-3 min burst with peak flux density 300±90 μJy. The latter emission is likely a low-level magnetic flare similar to optical flares previously observed for this source. No outbursts were detected in separate narrow-band Hα monitoring observations.

    We report new high-resolution imaging and spectroscopic observations that confirm the presence of a co-moving T5.5 secondary and provide the first indications of three-dimensional orbital motion. We used these data to revise our estimates for the orbital period (4.1+2.7−1.3 yr) and tightly constrain the orbital inclination to be nearly edge-on (93.6\deg+1.6deg−1.4deg), although robust measures of the component and system masses will require further monitoring. The inferred orbital motion does not change the high likelihood that this radio-emitting very low-mass binary made a close pass to the Sun in the past 100 kyr.

    Comments: 12 pages, 22 figures, accepted for publication to AJ

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:1508.06332 [astro-ph.SR]
    (or arXiv:1508.06332v1 [astro-ph.SR] for this version)

    Submission history

    From: Adam J. Burgasser [view email]

    [v1] Wed, 26 Aug 2015 00:02:54 GMT (329kb)