It’s the stuff of science fiction – companion planets to our Earth, or a companion star to our sun. Wouldn’t it be cool (or maybe hot) to see two suns in the sky? Maybe even of different colors? Could our sun actually have one and we just don’t know it?
No, I’m not talking about the one bad even-numbered Star Trek movie. Three decades ago, a group of scientists examined the geologic record and found that, seemingly like clockwork, there was a major extinction event on Earth roughly every 27 million years. Our planet doesn’t go through a cycle like that, though, by itself (that we know of), and cycles of that length of time are usually an indication of an astronomical source. After all, we are talking about astronomical timescales here (I love words with double meanings).
The paper was published in 1984, by two paleontologists: Raup, D.M., and Sepkoski, J.J. (1984). “Periodicity of Extinctions in the Geologic Past.” Proceedings of the National Academy of Science, 81, pp. 801-805. Kindly, the paper is available online for free (Click Me!). I skimmed through the paper and did a text search, and I mention this because no where do the authors actually state that there may be a companion star. Rather, they are more vague and simply state:
“[We may be] seeing the effects of a purely biological phenomenon or whether periodic extinction results from recurrent events or cycles in the physical environment. If the forcing agent is in the physical environment, does this reflect an earthbound process or something in space? If the latter, are the extraterrestrial influences solar, solar system, or galactic? … [W]e favor extraterrestrial causes …”
Two papers followed that one in 1984, this time by astronomers, suggesting that the unnamed extraterrestrial forcing agent was a possible companion star to the sun that orbited very far away, but would sometimes come close in and disturb the Oort Cloud (a vast region of cometary nuclei that extends far from the sun). This disturbance would send some comets to the inner solar system, periodically at that fixed orbital interval, and an impact event on Earth could then cause a mass extinction.
That’s the basic idea behind Nemesis.
A Binary Companion?
This is not so odd as it may seem. It is estimated that fully 60% or more of stars in our galaxy are in a binary system, where two stars are gravitationally linked together and orbit each other around a common center of mass. Most that we have identified have orbital periods that are reasonably short, like less than a few thousand years.
It’s possible for a binary system to have an orbital period significantly longer. The problem is that we can’t really tell. By far, the easiest way to detect a binary system should be to see the two stars orbiting each other. The problem is that stars don’t do this very quickly, and for two stars that are far enough apart to actually see as separate objects from Earth, they won’t orbit each other in a human lifetime.
Obviously, since we know of many binary star systems, there are other ways to detect them. That’s not the focus of this blog post, though. I just wanted to talk about it for background information.
But, it sets the stage for the sun’s potential companion: Another star, a small, faint one (a brown dwarf or red dwarf), that takes roughly 26 million years to complete one orbit. This means that its average distance from the sun is roughly 90,000 times the Earth-sun distance. That’s really far away in terms of trying to see something that’s really small and faint.
Why This Post?
This is a fairly random topic to talk about, and one may be wondering why I’m doing it. It’s because there was a recent Wired Science article about it, with the typical poor media headline that makes something much more sensational than it actually is: “Death Star Off the Hook for Mass Extinctions.”
That’s not the science, it is the conclusions of a new study that extends the original database of extinctions from 250 million years to 500 million years. The authors of this latest study say that the extinctions occur at almost “exactly” 27 million years. Because the proposed Nemesis star is relatively far away from the sun, it would be perturbed in its orbit by passing stars, and this would cause some fluctuation in the 27 million-year cycle. The new authors claim that the observed pattern is too predictable for a star that can be perturbed.
The rebuttal by many is that the geologic record is not precise enough to say that it’s “exactly” 27 million years, and that the margin of error in the dates more than allows there to be a changing frequency that a companion star can be responsible for.
Personally, I agree.
Absence of Evidence Is Not Evidence of Absence
The obvious problem for the Nemesis hypothesis is that we haven’t found the star. Many have looked for it, but a rather convenient happenstance of timing is that right now it would be about as far as it gets from the sun, so it is the most difficult to detect from Earth. Nemesis has not been found.
However, it would be a fallacy to claim that this absence of evidence is evidence of absence, at least for the time being. There are now several telescopes that have the sensitivity and are looking over the entire sky that are fully capable of finding a Nemesis-type star. We’ll see what happens. If these surveys come up empty-handed, then it becomes much less likely that the star is out there, and a different mechanism will need to be proposed.
I’ve always thought the Nemesis hypothesis is an interesting one. I like the idea that something as basic as a companion star to the sun is still out there, undiscovered. I also like to think astronomy is important, and an extraterrestrial cause for mass extinction events would definitely be a notch on the “YES, Astronomy is Important” side of the scoreboard.
However, I would be remiss if I didn’t at least mention that some people have fallaciously tied Nemesis into 2012 doomsday scenarios, saying that it’s Nemesis that is Planet X that will cause everything to go boom in 2012, or saying that it’s the Christian biblical “Wormwood” from the book of Revelation that it itself will destroy Earth in an apocalypse. Hopefully needless to day, those ideas for Nemesis are baseless.