How A Passing Star Lit Up The Ancient Sky

By Judith E Braffman-Miller

For millions of years all human beings, both early and modern, had to find their own food, and were forced to spend most of each day gathering plants and hunting animals in order to survive. Then, within only the past 12,000 years, our species made the revolutionary transition from being hunters and gatherers, to being able to produce our own food. Nevertheless, about 74,000 years ago, modern humans almost became extinct as a result of dramatic climate changes, and the human population may have been reduced to only about 10,000 adults of reproductive age. It was around this time, approximately 70,000 years ago, that a small reddish star floated close to our Solar System and gravitationally shook up comets and asteroids--sending them screaming inward towards our young Sun. In March 2018, a team of astronomers from the Complutense University of Madrid in Spain and the University of Cambridge in England announced that they have verified that the movement of some of these comets and asteroids was effected by that close stellar encounter.

At a time when modern humans were first beginning to migrate from Africa, and Neanderthals were dwelling with them on Earth, Scholz's star--named after the German astronomer who discovered it--floated less than one light-year from our Sun. Currently, this little red star is almost 20 light-years away, but 70,000 years ago it created a disaster when it wandered into our Solar System's Oort Cloud, a remote reservoir of trans-Neptunian objects (TNOs) located at the outer limits of our Solar System. TNOs are icy and dusty comet nuclei that dwell in the distant dark deep freeze of our Sun's region of gravitational influence.

This discovery was first made public in 2015 by a team of astronomers led by Dr. Eric Mamajek of the University of Rochester in New York (USA). The details of that catastrophic stellar flyby, the closest documented so far, were published in the February 10, 2015 issue of The Astrophysical Journal Letters.

Stellar Ships That Passed In The Night

Our Sun is a solitary star, but even though it lives alone, it sometimes has visitors. Such a visitor was the dim and alien Scholz's star when it paid our Solar System a visit. This faint, small stellar invader is thought to have skimmed through the Oort Cloud--the remote shell of comet nuclei that surrounds our entire Solar System.

Scholz's star is a low-mass red dwarf star that is a member of a binary system, and it sports the puny mass of merely 8% that of our Sun. The other member of the duo is a brown dwarf, a failed star, that is even smaller than Scholz's star with a mass of only 6% solar masses. Red dwarf stars are the smallest true stars in the Cosmos, as well as the most numerous and longest-lived. In contrast, puny little brown dwarfs are fascinating little stellar failures. This is because, even though brown dwarfs are likely born the same way as true stars--from the collapse of an especially dense blob of material embedded within one of the many giant, dark, and frigid molecular clouds that haunt our Milky Way Galaxy--they never manage to gain sufficient weight to ignite their nuclear-fusing star-fire. Even though puny little brown dwarfs never acquire sufficient mass to begin the process of nuclear fusion, they are still more massive than gas giant planets, such as our own Solar System's spotted and banded behemoth, Jupiter. Red dwarf stars, in contrast, did manage to acquire enough mass to begin the process of nuclear fusion--which produces sufficient pressure to battle against the force of gravity, thus keeping the star bouncy against its own fatal collapse. Radiation pressure pushes the stellar material out and away from the star, while gravity tries to squeeze everything in. The two warring forces help a star maintain stellar equilibrium--but the end must come, sooner or later. As soon as the star finishes burning its necessary supply of nuclear-fusing fuel--whereby it fuses heavier atomic elements out of lighter ones--gravity wins the war against its rival, and the star collapses. However, it is likely that there are no dead red dwarf stars in the Cosmos--yet. Small stars take their stellar "lives" easy and burn their fuel--very, very slowly. Indeed, it probably takes trillions of years for a red dwarf to perish, and our Universe isn't even 14 billion years old yet. In contrast, massive stars live fast and die young, and some may only "live" for millions, as opposed to billions--let alone trillions--of years. Our Sun is a small star, but it is much more massive than a red dwarf. Our Star is approximately 4.56 billion years old, and it has about another 5 billion years to go before it blows off its outer gaseous layers, leaving its relic core behind in the form of a tiny dense object called a white dwarf.

Scholz's star is an inhabitant of the Monoceros constellation, which is located about 20 light-years from Earth. However, when the tiny faint red dwarf closely brushed our young Solar System in Earth's prehistory thousands of years ago, it would have appeared as a 20th magnitude star. This is about 50 times more faint than can usually be seen with the naked human eye at night. However, Scholz's star is very magnetically active, and this can make it "flare". For one brief shining moment on a cosmological time scale, Scholz's star can potentially become thousands of times brighter. This means that it is entirely possible that Scholz's star was visible to our prehistoric ancestors 70,000 years ago--for minutes or hours at time during its rare episodes of flaring.

Scholz's star is more formally designated WISE J072003.20-084651.2. It derived its less technical nickname to honor the astronomer Dr. Ralf-Dieter Scholz of the Leibniz-Institut fur Astrophysik Potsdam (AIP) in Germany. Dr. Scholz is the first to have announced the discovery of the dim little red dwarf star late in 2013. The WISE component of Scholz's star's formal name refers to NASA's Wide-field Infrared Survey Explorer (WISE) mission, responsible for mapping the entire sky in infrared light in 2010 and 2011. The J part of the formal designation refers to the red dwarf's coordinates.

The little star's trajectory suggests that 70,000 years ago it floated about 52,000 astronomical units (AU) from Earth (0.8 light-years)--which is equal to 5 trillion miles. One AU is equivalent to the average distance between the Sun and Earth, which is about 93,000,000 miles. The authors of the 2015 paper noted that they are 98% certain that Scholz's star skimmed the Oort Cloud, a mysterious and unexplored domain situated at the edge of our Solar System. The Oort Cloud is generally thought to be the home of trillions of frozen, glittering, icy comet nuclei that are about a mile--or more--across. This distant cloud is also thought to be the origin of long-period comets that swing into orbit around our Sun after their orbits have been gravitationally disrupted.

The Oort Cloud is named for its two discoverers, the Dutch astronomer Jan Oort (1900-1992) and the Estonian astronomer Ernst Opik (1893-1985). This spherical shell is the habitat of icy planetesimals, left over from our Solar System's formation more than 4.5 billion years ago. Icy planetesimals were the building blocks of the quartet of giant gaseous planets inhabiting the outer Solar System--Jupiter, Saturn, Uranus, and Neptune. In contrast, the asteroids--mostly found inhabiting the Main Asteroid Belt between Mars and Jupiter--are the relic population of rocky and metallic planetesimals that built up the quartet of solid inner Solar System planets--Mercury, Venus, Earth, and Mars. In the primeval Solar System planetesimals--both icy and rocky--collided with one another and merged to create ever larger and larger bodies, thus forming the familiar planets of our Sun's family. The Oort Cloud is thought to surround our Solar System at a distance of as much as 100,000 AU, which situates it half-way to the nearest star to our Sun, which is Proxima Centauri.

The Kuiper Belt and Scattered Disk--which also house frozen comet-like objects--are less than one thousandth as far from our Sun as the Oort Cloud. The outermost edge of the Oort Cloud marks the boundary of our Star's region of influence. It is the boundary of our Sun's gravitational dominance.

The Oort Cloud is generally believed to be composed of two regions: a disk-shaped inner cloud called the Hills cloud, and a spherical outer cloud. Most of the remote, frozen inhabitants of the Oort Cloud are made up of ices, such as water ice, methane ice, and ammonia ice.

Our Sun was probably born as a member of a dense open stellar cluster containing thousands of sibling stars. Many astronomers believe that our newborn Sun was either unceremoniously evicted from its natal cluster as the result of gravitational perturbations caused by other stars, or that it simply floated away of its own free will about 4.5 billion years ago. Our Star's stellar siblings have long since wandered off to more distant regions of our Milky Way Galaxy, and there very well may have been as many as 3,500 of these nomadic solar-siblings.

Today, our Sun is in active mid-life. It is a main-sequence (hydrogen-burning) star on the Hertzsprung-Russell Diagram of Stellar Evolution. As stars go, our Sun is not particularly special. Our Solar System is located in the far suburbs of our majestic--though typical--barred-spiral Galaxy, the Milky Way.

Shining In The Prehistoric Sky

Two astronomers from the Complutense University of Madrid (Spain), the brothers Dr. Carlos and Dr. Raul de la Fuente Marcos, along with their colleague Dr. Sverre J. Aarseth of the University of Cambridge (UK), have now analyzed, for the first time, the nearly 340 objects objects dwelling in our Solar System with hyperbolic orbits (very open V-shaped, as opposed to the typical elliptical). In the process, the three astronomers discovered that the trajectory of some of these objects are influenced by the passage of Scholz's star.

"Using numerical simulations we have calculated the radiants or positions in the sky from which all these hyperbolic objects seem to come," explained Dr. Carlos de la Fuente Marcos in a March 10, 2018 La Ciencia es Noticia (SiNC) Press Release.

"In principle, 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," he continued to note.

The exact time in which Scholz's star passed close to Earth, as well as its position during prehistory, coincide with the date determined in the new investigation--and also with those calculated by Dr. Mamajek and his team. "It could be a coincidence, but it is unlikely that both location and time are compatible," Dr. De la Fuente Marcos continued to explain in the March 10, 2018 SiNC Press Release. He further pointed out that their simulations indicate that Scholz's star approached even closer than the 0.6 light-years proposed in the earlier 2015 study as the lower limit.

This close brush with the little red star 70,000 years ago did not disrupt all of the hyperbolic objects in our Solar System, only those that were closest to it at that time. "For example, the radiant of the famous interstellar asteroid Oumuamua is in the constellation of Lyra (the Harp), very far from Gemini. Therefore, it is not part of the detected over-density," Dr. De la Fuente Marcos added. He further said that he is confident that new studies and observations will confirm the idea that Scholz's star passed close to us in relatively recent times. Indeed, it is likely that are ancestors, gazing up at the sky, saw its dim reddish light in the dark nights of prehistory.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.