Dwarf Stars Did a Drive-By 70,000 Years Ago
70,000 years ago our solar system experienced a close encounter with a binary star system which passed through the outer edge of our Oort cloud of comets.
This interesting bit of astronomical sleuthing started when it was noticed that a newly discovered, 20 light year distant star (Scholz star) had a very small proper (tangential) motion. This is the motion across the sky, either left-right, up-down or something in between. This caused the star to stand out because most stars near us have a high degree of tangential movement. This could only mean that this star was moving directly towards us or directly away from us (this is called its radial velocity). I suppose it’s possible that the star could have been moving in the same direction and speed as the sun, giving it a radial velocity of zero, but that pretty much never happens.
To determine what its radial velocity is, astronomers used the spectrum of light emitted by the star to determine what its doppler shift is. Like the pitch of a train whistle rising then lowering as it approaches and passes us, doppler shift records the stretching or compressing of light waves caused by an object receding from or approaching us.
It was shown that Scholz was in fact receding from us. This star isn’t however just a star. It’s actually a binary system consisting of a star that’s just barely a star, in mutual orbit around another object that’s almost a star but not quite. This is because these are dwarf stars (or are they called Little Stars now?)
One of the binaries is a red dwarf star with 85 jupiter masses. These stars have a low mass and a luminosity so faint they cannot be seen with the naked eye. Their extraordinary dimness however is matched by their extraordinary ubiquity. It is believed that fully 75% of the stars in the Milky Way are red dwarfs, including 20 of the nearest 30 to us, the most prominent being Proxima Centauri, the closest of all.
Its binary partner is a brown dwarf with 65 jupiter masses. As I said, this isn’t really a star since its mass is too low to sustain hydrogen fusion in its core. Still though, its density and mass, dwarfs (so to speak) even gargantuan planets such as Jupiter. These sub-stellar objects then exist in a somewhat fuzzy area between gas giants like Jupiter and the lowest mass stars.
The researcher then used both the minimal tangential motion and its radial motion and plugged them into a simulation with other variables such as our galaxy’s gravitational field. They ran the simulation 10,000 times and discovered that 98% of the time, the binary system went through the outer portion of our Oort cloud.
You might be wondering if this bugger was visible to the ancient humans on earth at that time. The answer is no but with a caveat. Remember, we’re talking about 2 very dim objects here. It’s true that at a distance of 0.8 light years, they were 5 times closer than the closest star is now, but that still is not close enough to see this binary without instrumentation. Red dwarf stars however can be magnetically active. They can experience surface eruptions that are like solar flares but much much more powerful. This means that there were brief periods when this close encounter could have been naked-eye visible. So it is possible that Atouk and his girlfriend Lana, after some good zug-zug, were pondering the stars when they saw a fleeting and mysterious point of light in the sky.
This close encounter with the outer portion of the Oort cloud is actually a good finding. The outer portion of our cloud of comets is not what is called, dynamically active. They don’t really contribute to the long-period comets that occasionally pelt the inner solar system signifying a threat to life on earth. If you add that to the fact that Scholz is also a low mass system that traveled fairly quickly through our system, then there is high confidence that nothing untoward resulted from this binary fly-by.
Still though, one of the simulations did show an interaction with the inner dynamically active Oort cloud. This means there’s a small chance that the dwarfs could have perturbed some comets that are at this very moment hurtling towards the inner solar system, perhaps they even have the earth in their sites.
Since this event took place 70,000 years ago and it would take 2 million years for any comets to reach us, we do have a bit of a buffer for a worst-case scenario.
Still, it wouldn’t hurt to get your affairs in order.
Image Credit: NASA/JPL-Caltech/UCB