Tag Archives: rogue star

Some 70,000 years ago, a rogue star duo whooshed by our solar system — and early humans probably saw it

If you happened to be alive 70,000 years ago, you might have witnessed an extremely rare event: a duo of rogue stars passing relatively close to the Sun — close enough to gravitationally disrupt the comets and asteroids in the outskirts of the solar system.

Artistic depiction of a brown dwarf. Image credits: NASA / JPL.

In a time when humans were just leaving Africa and Neanderthals were still around, two small reddish stars were approaching our solar system. We now call them Scholz’s star, after the astronomer who discovered them (Ralf-Dieter Scholz). The system is currently some 20 light-years away from Earth, but 70,000 years ago, it was much closer. A team of astronomers now analyzed the orbit of nearly 340 objects in the solar system with hyperbolic orbits (very open, not quite typical elliptical), finding that the trajectory of some of them was influenced by Scholtz’s star. They report that some comets and asteroids in the Oort Cloud — the outer limit of our solar system — were disrupted by the passing of the star.

“Using numerical simulations we have calculated the radiants or positions in the sky from which all these hyperbolic objects seem to come,” explains Carlos de la Fuente Marcos, who together with the other coauthors publishes the results in the MNRAS Letters journal.

“In principle,” he adds, “one would expect those positions to be evenly distributed in the sky, particularly if these objects come from the Oort cloud; however, what we find is very different: a statistically significant accumulation of radiants. The pronounced over-density appears projected in the direction of the constellation of Gemini, which fits the close encounter with Scholz´s star.”

Despite the name, Scholz’s star is actually a binary system consisting of a red dwarf and brown dwarf, with the entire system having just 0.15 solar masses. Because it was so small, it didn’t disturb all the hyperbolic objects in the solar system. Authors explain that the disturbance might also just be a coincidence, completely unrelated to Schultz’s star — but since both the timing and the location of the disturbance fit so well, it’s highly unlikely.

Scholtz’s star passed approximately 52,000 astronomical units away from the Sun (1 astronomical unit is equivalent to the distance between the Earth and the Sun) — the equivalent of 0.82 lightyears. It’s estimated that a star passes through the Oort Cloud every 100,000 years. However, an approach closer than 50,000 astronomical units only happens every 9 million years, according to current models.

The results have been published in two papers:

  • Carlos de la Fuente Marcos, Raúl de la Fuente Marcos, Sverre J Aarseth. Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies. Monthly Notices of the Royal Astronomical Society: Letters, 2018; 476 (1): L1 DOI: 10.1093/mnrasl/sly019
  • Eric E. Mamajek, Scott A. Barenfeld, Valentin D. Ivanov, Alexei Y. Kniazev, Petri Väisänen, Yuri Beletsky, Henri M. J. Boffin. The closest known flyby of a star to the solar system. The Astrophysical Journal, 2015; 800 (1): L17 DOI: 10.1088/2041-8205/800/1/L17
Credit: Lucasfilm

More than a million years from now, our solar system will briefly house two stars

A double sunset such as the fictitious one on Tatooine from Star Wars has always been on many people’s minds. About 1.3 million years from now, if anyone is still around, he could actually experience a similar scenery here on Earth.

Credit: Lucasfilm

Credit: Lucasfilm

Uninvited stellar guests

Nothing is static in the Universe. Though our senses tell us we’re standing still, since the days of Copernicus we’ve known that the planet we call home revolves around the Sun, which in turn moves around the center of Milky Way, which yet again revolves around the center of mass lying in between Andromeda and Milky Way. In other words, everything is in perpetual motion relative to a reference point. This means that sometimes cosmic objects and systems can behave in very surprising ways.

A massive survey of 300,000 stars and their motion relative to the Sun was performed by astronomers working with the ESA’s Gaia satellite. The scientists plotted the stars’ closest approach to the Sun determined for up to five million years in the past and future. They found 97 stars which should pass within 150 trillion kilometers, with 16 coming within about 60 trillion km.

A particular encounter stands out, that with Gliese 710, which will pass within just 2.3 trillion km or about 16 000 Earth–Sun distances, some about 1.3 million years from now. For a sense of measure, the outermost planet of the solar system, Neptune, orbits around the sun at 30 Sun–Earth distances.

That puts Gliese 710 well within the Oort Cloud, a humongous shell of icy objects that exist in the outermost reaches of the solar system, extending out to 15 trillion kilometers from the Sun or 100 000 times the Sun–Earth distance.

Understanding alien stars that approach our solar system is important work that might one day even avert a planetary catastrophe. It’s thought that most comets in our solar system come from the Oort Clouds. These are mainly perturbed by the gravitational influence of stars which jolts them into orbits that bring the comets closer to the inner solar system. Some of these comets could enter a collision course with Earth or other planets.

Coming this close to the sun, Gliese 710 will certainly stir the Oort Cloud bee-hive, though in what way remains unclear at this point.

We do know that this alien star has a mass of 60% that of our Sun and that it travels much slower than most stars: nearly 50 000 km/h at closest approach, compared with the average 100 000 km/h. This relatively slow motion will likely amplify the Oort Cloud perturbation effect than otherwise in the case of faster stars.

What’s also certain is that Gliese 710 will shine brightly in the night’s sky as seen from Earth’s surface. Astronomers estimate its brightness will be three times that of Mars.

Meeting a rogue

The following animation put together by ESA focuses on Gliese 710 wandering through the galaxy, ultimately performing a close encounter with our Sun in 1.3 million years by passing within the Oort Cloud reservoir of comets in the outskirts of our Solar System. The ESA’s press release explains further that:

“The motion can be likened to what an observer standing beside a road would see looking at an approaching car, and then swinging around to continue to follow it as it moves away. As a result, the objects in the background – in this case distant stars – become blurred as you move quickly to maintain a visual on the passing object.”

Of course, Gliese 710 wouldn’t be the first star to cause a racket in our solar system’s backyard. About 70,000 years ago, during a time when our ancestors were busy staying alive in the aftermath of the Toba super-eruption, a low-mass star system nicknamed “Scholz’s star” passed roughly 52,000 astronomical units away from the sun.

Scientific reference: “The completeness-corrected rate of stellar encounters with the Sun from the first Gaia data release,” by C.A.L. Bailer-Jones, is published in Astronomy & Astrophysics.

star flyby oort cloud

70,000 Years Ago, a Rogue Star Passed Through Our Solar System

Too close for comfort – a team of astronomers from the US, Europe, Chile and South Africa concluded that a dim star passed through the Oort cloud, our solar system’s distant cloud of comets. The star missed the Earth by less than one light year, and passed five times closer than the current closest star, Proxima Centauri.

star flyby oort cloud

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.

In a paper published in The Astrophysical Journal Letters, lead author Eric Mamajek from the University of Rochester and his team studied the velocity and trajectory of a low-mass star system – WISE 0720-0846 (nicknamed “Scholz’s star”). Due to its low luminosity, the star was discovered only a year ago by astronomer Ralf Dieter-Scholz in Potsdam, Germany, through the use of NASA’s WISE (Wide Field Infrared Survey Explorer), which mapped the entire sky in infrared during the years 2010 and 2011.

Since it was discovered, it had some interesting characteristics. despite being fairly close (“only” 20 light years away), it showed very slow tangential motion, that is, motion across the sky. By studying its trajectory and velocity, astronomers found that the star was either moving towards, or away from our solar system. They reconstructed its past movement and quickly realized it was moving away from our solar system, which means that it passed through it (or very close to it) sometime in the past.

“Most stars this nearby show much larger tangential motion,” says Mamajek, associate professor of physics and astronomy at the University of Rochester. “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.”

Scholz’s star moved much faster than expected, and missed Earth by “a whisker” – in astronomical terms, that is. It passed roughly 0.8 light years away from Earth, at 8 trillion kilometers; this happened 70,000 years ago. It may seem like a lot, but it’s really too close for comfort. This fits with an earlier theory, which proposed that such close flybys take place every 100,000 years or so. These encounters could hit the Oort cloud and trigger “comet showers” in the solar system.

“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,” Mamajek said in a news release.


The star is a rogue star – a star that has escaped the gravitational pull of its home galaxy and is moving independently in or towards the intergalactic void; the movement of rogue stars is often hard to predict. A 2012 study claimed that rogue planets riddle the Milky Way, and while rarer, there are also rogue stars in our galaxy.

Currently, Scholz’s star is a small, inconspicuous red dwarf in the constellation of Monoceros, about 20 light years away. The star is part of a binary star system, composed of a low-mass red dwarf star (with mass about 8% that of the Sun) and a “brown dwarf” companion (with mass about 6% that of the Sun). Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but due to their low luminosity, they are difficult to observe and study. Brown stars are “failed stars” – substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike … well, stars.

Journal Reference:

  1. Eric E. Mamajek, Scott A. Barenfeld, Valentin D. Ivanov, Alexei Y. Kniazev, Petri Väisänen, Yuri Beletsky, Henri M. J. Boffin. THE CLOSEST KNOWN FLYBY OF A STAR TO THE SOLAR SYSTEM. The Astrophysical Journal, 2015; 800 (1): L17 DOI: 10.1088/2041-8205/800/1/L17