A Visiting Star Jostled Our Solar System 70,000 Years Ago

Around the same time our ancestors left Africa, a dim red dwarf star came to within 0.8 light-years of our Sun, marking the closest known flyby of a star to our Solar System. New research suggests Scholz's Star, as it's known, left traces of this interstellar encounter by perturbing some comets in the outer Oort Cloud.

Artists depiction of Scholz's Star and its brown dwarf companion, with our Sun twinkling in the background. Illustration: University of Rochester

Though Scholz's Star visited the outer reaches of our Solar System some 70,000 years ago, awareness of this celestial meet-and-greet first emerged three years ago in a study published in the Astrophysical Journal Letters.

According to this research, a dim red dwarf star, along with its even dimmer brown dwarf companion - a kind of bloated gas giant that failed to ignite into a full-blown star - came to within 0.8 light-years of our Sun (7.2 trillion km, or 50,600 AU, where 1 AU is the average distance of the Earth to the Sun), and possibly as close as 0.6 light-years (5.7 trillion km, or 37,900 AU). That's a close shave, at least by cosmological standards. By comparison, MU69, a Kuiper Belt object that will be visited by the New Horizons spacecraft on New Year's Day 2019, is about 43 AU from the Sun.

Scholz's Star, this research suggested, just grazed the outer reaches of the Oort Cloud, that remote bubble of debris that marks the outermost limits of the Sun's dominant gravitational influence. The red dwarf has been drifting ever since, and it's now about 20 light-years away. The University of Rochester researchers who conducted the 2015 study said it was unlikely that Scholz's Star, with a mass around nine per cent of the Sun, and it's brown dwarf, at about six per cent the Sun's mass, were able to disturb or jostle any of the objects in the Oort Cloud to a significant degree.

But new research published this week in Monthly Notices of the Royal Astronomical Society suggests this interpretation was wrong, and that Scholz's Star did in fact influence the trajectories of some Oort Cloud objects. In the study, astronomer brothers Carlos de la Fuente Marcos and Raúl de la Fuente Marcos from Complutense University of Madrid, along with Sverre J. Aarseth from the University of Cambridge, identified the movements of dozens of known Oort Cloud objects as having been influenced by this ancient encounter.

To do it, the researchers analysed 339 Solar System objects known to be in hyperbolic orbits, that is, orbits that are very exaggerated, and with a characteristic V-shape, unlike the usual elliptical orbits occupied by most planets and asteroids. Using computer models, the researchers calculated the "radiants", or positions in space from which these hyperbolic objects were coming in from. Statistical analysis showed that some of these distant comets featured trajectories that were very likely to have been influenced by Scholz's Star.

Normally, the positions of these objects should be evenly distributed in the sky, but the researchers found a "statistically significant accumulation" of calculated positions - a pronounced over-density that happens to be in the direction of the Gemini constellation, which fits in rather nicely with the location of Scholz's Star. The red dwarf did not disturb all hyperbolic objects, just those that were close when the star made its flyby. Of the 339 objects studied, 36 had positions in the region located towards Gemini.

"Keep in mind that the sample detected is made of objects that passed relatively close to our planet," Carlos de la Fuente Marcos told Gizmodo. "The number of objects that were possibly perturbed by this stellar flyby could have been significantly higher." What's more, the data also meshes nicely with the timing of the encounter, which happened 70,000 years ago.

Interestingly, the researchers discovered eight possible interstellar comets that now warrant further investigation. Most of these objects have largely unknown orbits, but the best candidates include C/2008 J4 (McNaught) and C/2012 S1 (ISON). Interstellar comets are objects that originated outside of our Solar System, and are either passing by or have been captured by our Sun's gravity. These objects were known before, but their speed and trajectories suggest interstellar origin.

Currently, the only known interstellar asteroid is 'Oumuamua, which was detected in October 2017. As the new study points out, 'Oumuamua is not one of the 36 objects jostled by Scholz's Star, as it was too far away when the red dwarf came for a visit.

Eric Mamajek, the lead author of the 2015 study, said the new paper is solid, and that it was written by "some of the world's experts on dynamics in the Solar System". He said stars pass through the Sun's Oort Cloud fairly regularly on geologic timescales involving millions of years.

"Scholz's Star is probably only the most recent example," Mamajek told Gizmodo, adding that "the effects of the pass were utterly negligible on Earth directly". As for these jostled comets representing a threat to Earth, he said there aren't very many of them, especially when compared to other small bodies in the Solar System that might present a danger. "I'm not losing sleep over comets perturbed by Scholz's Star," he said. "There are many, many more immediate concerns on Earth, and most are fixable."

Wesley Fraser, a physicist and mathematician at Queen's University-Belfast, has some serious concerns about the new paper, telling Gizmodo he's "not convinced". His primary issue is that the researchers can't actually trust the statistics in the way they're used in the paper.

"First I will say that it is intriguing that the radiant [that is, the position in the sky from which an object comes from] of Scholz's Star and those of some hyperbolic comets are similar," Fraser told Gizmodo. "But I am not at all convinced by the statistics. That's because the statistics implicitly assume that the distribution of comets is not observationally biased."

The researchers chose all known objects on hyperbolic orbits, but Fraser says the dataset used appears to be observationally biased, evidenced by the fact that there are too few objects with radiants in the galactic plane - the band that comprises the Milky Way in the night sky. "Just counting from that figure, only 14 of 339 objects fall on the galactic plane, but from considerations of area - how much of the night sky is occupied by the galactic plane - we would expect at least twice that number," said Fraser.

"In my opinion, the data are clearly biased," he said, in that all known objects in hyperbolic orbits may be subject to an unknown selectional effect and that the dataset is not truly a random or an indicative sample of what's out there in the Oort Cloud. "Which is not surprising, as it is very difficult to make an unbiased sample from a set of comet data which is essentially a mishmash of reported comet observations over the last half century."

Dan Brown, a researcher from Nottingham Trent University's Physics and Mathematical Sciences department, also pointed out that the precision of the data used in the study isn't sufficient to make clear statements about the origin of specific objects - something the authors themselves admit. "Which makes sense given the briefness of the objects' observability and therefore our knowledge of its trajectory," Brown told Gizmodo. That concern aside, Brown likes the new study, saying, the new research "engages with the existing and challenging sample of objects on hyperbolic orbits" that "illustrates how our Sun and the solar system can be influenced by other stars - not only hypothetically - but illustrated with some statistical significance".

On the topic of the eight new interstellar comet candidates, Fraser said the researchers were "disingenuous" by using "arbitrary" inbound velocities to separate hyperbolic objects originating from the Oort cloud from those with interstellar origins, and that the authors failed to perform "a physical analysis to see what the distribution of velocities might be from infalling Oort cloud comets". Fraser would have liked to have seen evidence from physics or computer models showing that the velocities described in the study are actually viable.

Alan Jackson, a postdoctoral researcher at the Centre for Planetary Sciences at the University of Toronto-Scarborough, expressed a similar concern. "I think their choice of a velocity of 1.5km/s to identify objects as interstellar might be a bit too low, I suspect some of the eight bodies they identify as interstellar candidates are likely to actually be Solar System objects," he told Gizmodo.

Overall, however, Jackson said the methods used in the study were "sound", and the clustering of inbound comet paths near the location of the close approach of Scholz's Star "certainly seems reasonable". A star "passing within 52,000 AU of the Sun should perturb the Oort Cloud which we would expect to result in sending some comets into the inner Solar system from that direction," Jackson told Gizmodo.

Artist's depiction of a spear-wielding human gazing up at Scholz's Star - a scene that probably never happened, given the red dwarf's low luminosity. Illustration: José A. Peñas/SINC

As a final note, Scholz's Star passed through our neighbourhood when early humans were already romping around Africa and Eurasia. It's tempting to think our ancestors saw a tiny red dot in the night sky during this brief astronomical era, but that probably didn't happen (and why we refrained from using the above artwork, provided by the authors of the new study, as the main image for this post). As I wrote back in 2015:

When Scholz's Star was in the neighbourhood, it would have been a 10th magnitude star (red dwarfs are very dim). That's about 50 times fainter than what can be seen with the naked eye at night. Under normal circumstances, it would be invisible. But because red dwarfs are magnetically active, it could have briefly "flared-up" (i.e., V-band flares) to become thousands of times brighter. The astronomers say it's possible that the star was visible to our paleolithic ancestors for a few minutes or hours if this rare flaring event transpired at the time.

Which seems bloody unlikely. Sadly, this star's passing likely went unnoticed by our ancestors.

[Monthly Notices of the Royal Astronomical Society]

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