- Another star buzzed the solar system 70,000 years ago and may have sent comets hurtling our way.
- It is possible our prehistoric ancestors saw the star in the sky.
- A new study also ID'ed eight objects that could have come from beyond our solar system.
About 70,000 years ago, around the time our ancestors were just beginning to leave Africa, a small red dwarf star passed remarkably close to the solar system. Scholz's star came within a light-year of the sun, the closest such encounter we know of. Today, Scholz's star sits about 20 light-years away, its stellar flyby a distant memory. But we are still seeing the influence of its visit.
Back in 2015, researchers led by Eric Mamajek, professor of astronomy and physics at the University of Rochester, studied the galactic orbits of Scholz's star and our sun. They discovered that Scholz's star and its brown dwarf binary companion probably . At the time, the scientists concluded that Scholz's star did not disturb the orbits of any objects in the solar system in any notable way when it passed by.
However, a new in the Monthly Notices of the Royal Astronomical Society suggests this stellar flyby did in fact nudge comets from the Oort cloud, the ring of icy objects far beyond the planets that marks the edge of the solar system. These comets are now close enough to study in more detail, and while looking for them, the scientists also found eight objects that could be from outside the solar system entirely.
Comets From a 70,000-Year-Old Star Flyby
The Oort cloud is a swarm of cometary bodies believed to be between 5,000 and 100,000 AU from the sun. Occasionally, these objects fall toward the sun, and some gain enough velocity to reach so-called hyperbolic orbits. These trajectories will eventually take them out of the solar system—unless they are influenced by another gravitational encounter with a giant planet, for example.
The new study, conducted by researchers at the Complutense University of Madrid and the University of Cambridge, analyzed the positions and velocities of 339 hyperbolic objects in the solar system. Normally, you'd expect the distribution of these objects across the sky to be more or less even. However, a group of them seem to be coming from the same radiant, or point of origin in the sky, and that area aligns with the encounter with Scholz's star. Any objects that might have been pushed by Scholz's star 70,000 years ago would only now be approaching the inner solar system.
"Out of those 339 [objects], 36 appear projected towards the relevant region in the constellation of Gemini (about 11 percent)," lead author Carlos de la Fuente Marcos of Complutense University of Madrid told Seniorhelpline in an email. "These are the ones that we suggest may have had an origin in the flyby with Scholz's star."
The researchers also found the suspect comets are traveling at a velocity around 1 km/s, or 2,200 mph, which could point to an "impulsive interaction" such as a stellar encounter. "Some of them were sent outwards and away from the solar system... some others were sent inwards and reached the inner solar system," Marcos says. "Out of those sent inwards, a fraction passed close enough to the Earth to be detected by ongoing surveys. Most of the 36 objects pointed out above may fall into this category, the others may be chance alignments."
The study identifies an exciting connection between these comets and the estimated location of the Scholz's star flyby, but to truly confirm that the hyperbolic objects were pushed by that star in prehistoric times, additional modelling of both the comets and the star will be needed. By analyzing the 339 hyperbolic objects, however, the team turned up another tantalizing find—possible interstellar objects.
New Interstellar Suspects
In addition to the comets that could have been hurled in by Scholz's star, the study found eight objects that may be new interstellar candidates. The only confirmed object from outside the solar system to visit our neighborhood of planets is 'Oumuamua, an elongated asteroid that made a close pass to Earth in October 2017. These additional eight objects are icy hyperbolic bodies with a velocity of 1.5 km/s or more, suggesting they could be interstellar comets.
"Statistically speaking, their paths are probably interstellar as their inbound velocities were significantly higher than those of the bulk of the other hyperbolic comets [2 to 30 times higher]," says Marcos. "As far as we can tell, the eight candidates to being interstellar interlopers have been singled out in our work for the first time."
It's unlikely that all eight of these objects have interstellar origins, as some may have formed in the solar system and accelerated during encounters with large objects. "I do think that the 1.5 km/s they used as a cut-off is probably a bit low, so I suspect that some of the eight candidates they identify are not true interstellar objects," says Alan Jackson, a postdoc researcher at the Centre for Planetary Sciences at the University of Toronto who recently published a study that suggests 'Oumuamua likely came from a binary star system. "The two that they identify as their best candidates, C/2008 J4 (McNaught) and C/2012 S1 (ISON), are I think fairly robust."
If any of these objects are confirmed as interstellar interlopers, it would provide an icy comet from beyond the solar system to study alongside the rocky asteroid 'Oumuamua, which turned out to be an object unlike anything astronomers had ever seen. Most astronomers believe interstellar objects fly through the solar system regularly, and our telescope survey projects are advancing toward the point of finding them much more frequently.
Red Star in the Ancient Sky
When the red dwarf star and its brown dwarf companion flew by the outer reaches of the solar system, it is possible that humanity's ancestors saw the star in the sky.
Scholz's star was almost certainly not visible generally, but red dwarfs can periodically flare up due to magnetic activity and grow many times brighter than their standard magnitude. The dimmest stars humans can see with the naked eye have an apparent magnitude (intrinsic magnitude modified by distance) of about 6 (lower numbers are brighter). If Scholz's star was on the farther end of estimates, it would have had an apparent magnitude of about 11, and closer in an apparent magnitude of perhaps 7.9.
Even at the greater distances, "those flares could be visible to human eyes without optical aids," says Marcos. (However, Marcos stresses that whether or not the star was visible to human ancestors is not discussed in the paper.)
It is tempting to imagine that prehistoric humans looked up one night to catch a glimpse of Scholz's star flaring up in the sky. Perhaps they considered it a sign of a bountiful hunt, or an omen of doom. Seventy thousand years later, equipped with all the advantages of modern science, we may have found new evidence of Scholz's visit to the solar system, sending the most distant comets of the Oort cloud in for a closer look.