Does the Solar System Have Another Star?
While we may be used to the eight solar system planets orbiting just one sun, having a single star isn’t the norm for the rest of the universe. Binary star systems, where two stars orbit a common point – or barycenter – are actually incredibly common… leading some scientists to wonder why it is that our sun is off on its own, or whether we might actually have miscounted?
This is Unveiled, and today we’re answering the extraordinary question; could the solar system have another star?
It may sound dangerous and unstable to have two or more stars in such close proximity to each other… but many systems do boast numerous stars and are appropriately called “multiple star systems”. Similarly, we also have binary black hole systems in the universe, with two (sometimes supermassive) black holes also orbiting a common point, thought to be as a result of galactic mergers long ago. With something like our sun, though, there’s some research to suggest that perhaps all stars form in pairs. A 2017 study into the Perseus molecular cloud, which is home to lots of new stars less than four million years old, appears to show just that - that every single star (or, at least, most of them) once had a partner. Naturally, then, if it exists, there’s some debate over where, exactly, our sun’s twin is. There are a few potential answers, too… but none are stranger than what’s been dubbed the “Nemesis Hypothesis”.
In 1984, two teams of physicists independently published academic papers about the perceived periodic mass extinctions on Earth. At the time, there was an assumption that those extinctions were all caused by comets arriving at apparent intervals of 26 million years… and that there must have been something that was responsible for this. That something, according to the theory in ‘84, was the Nemesis star; a second star orbiting the sun at a distance of 1.5 lightyears, which every twenty-six million years or so disturbed distant objects in the Oort Cloud and sent them hurtling towards Earth. In 1988, Richard Muller (one of the physicists working on the theory) even wrote a book on the subject, called “Nemesis: The Death Star”. The idea was that the Nemesis star would have to be a red or brown dwarf, both of which are only visible using infrared technology and spectrographs which didn’t exist in the 1980s. Muller conceded that improving technology would either prove – or disprove – the theory but, when the first papers were written, there was no way to assess it. For those who believe in it, it is the gravitational influence of the Nemesis Star which alters the paths of asteroids and causes mass extinctions.
Today, the Nemesis Theory doesn’t quite capture attention as it once did, and quickly begins to fall apart once it’s examined. First, the claim toward a twenty-six-million-year cycle of mass extinctions is controversial enough in its own right, and has never been conclusively proven. After all, what’s generally considered the last great extinction, which heralded the end of the dinosaurs, happened sixty-six million years ago. Second, though the extinction of the dinosaurs has been widely connected to a comet strike, some of the others before it haven’t. And then there’s the fact that researchers disagree on exactly how many mass extinctions there have even been. The so-called “big five”, however, the extinctions that are almost universally accepted, definitely didn’t occur at twenty-six-million-year intervals and weren’t all necessarily triggered by an asteroid strike.
When we look at the Oort Cloud itself, the science behind the Nemesis Star appears to get even shakier. The Oort Cloud is the most distant region of the solar system; anything beyond is too far away to be affected by the sun’s gravity. It’s currently a theoretical construct because it hasn’t been directly observed, but scientists and astronomers are generally confident that it does exist, stretching from between 2,000 Astronomical Units away from the sun to 200,000 - which is 3.2 lightyears away at its furthest. The Nemesis Star is speculated to lie 1.5 lightyears away, so it would be in the perfect spot to interfere with the icy comets and planetoids orbiting in the Oort Cloud. But is the Oort Cloud really so easily influenced? Perhaps not.
70,000 years ago, a binary star system called Scholz’s Star, made up of a red dwarf and a brown dwarf, came gliding through the solar system just 0.82 lightyears away from the sun, much closer than Nemesis is said to be. Scholz’s Star is also definitely, verifiably real, and is currently twenty-two lightyears away from us. But the effect it had on comets when it visited our star system is thought to be negligible. While it could take millions of years for a displaced Oort Cloud comet to reach us (giving us lots of time), we haven’t actually spotted any comets that appear to be making this journey so far, with any apparent “peaks” in comet distribution usually explained away as the normal effects of what’s called the “Galactic tide”. If the Scholz’s Star flyby, then, hasn’t doomed us all, it seems unlikely that the Nemesis Star could ever be influential enough to regularly, every twenty-six million years, send Oort Cloud comets on a collision course with Earth through so much empty space. Space is rarely as reliable or predictable as all that!
Of course, it isn’t Muller’s fault that the theory doesn’t hold much water all these years later – lots of scientific theories get disproven. We now have the benefit of infrared surveys, which use specialist telescopes capable of seeing beyond the visible light spectrum - they’re effectively purpose-built to find things like red and brown dwarfs. Between 1997 and 2001, the Two Micron All-Sky Survey, otherwise known as 2MASS, discovered all sorts of new objects that up until then we’d had no idea about – but it didn’t find the Nemesis Star. NASA’s Wide-field Infrared Survey Explorer, or WISE, launched in 2009 and has been collecting data ever since. Again though, the proposed red dwarf star about 1.5 lightyears away hasn’t been spotted. What’s more, WISE did discover Luhman 16, comprised of the closet known brown dwarfs to us, a whole 6.5 lightyears away. It makes sense, then, that if we can observe Luhman 16 with our current telescopes, we should also be able to observe the ever-elusive Nemesis… but, no luck.
For most scientists, this is all the proof needed to definitively state that the Nemesis Star doesn’t exist… but we are still left with its unexplained absence. Say the idea that stars form in pairs holds true (which it may or may not), then even if Nemesis isn’t real, there has to be something, somewhere. But the simplest explanation is often the best, and now there are increasing scientists and cosmologists of the opinion that the sun did form with a companion star a long time ago, but that that companion has long since left. The same stars in the Perseus molecular cloud that show us how stars form in binaries have also shown that it’s possible for these stellar pairs to break apart and drift away from one another - which could have easily happened at any point in the last 4.5 billion years since the sun formed. With that in mind, the sun’s twin (if it exists) also need not be the malign, ominous force that the Nemesis Hypothesis suggests it would be, leading some to argue that it deserves a friendlier name.
Finally, though, the Nemesis star is rarely talked about on its own. It’s usually connected to another mysterious, celestial, solar system body that supposedly influences distant comets: Planet Nine. Those who believe in one often believe in both… with one of the most far-out theories being that Planet Nine, which also has very little by way of evidence for its existence, is actually orbiting Nemesis in an evil copy of the entire solar system.
Another popular topic where both of these hypothetical objects are concerned is Sedna, a distant planetoid with an extremely unusual, elongated orbit. For some Nemesis and Planet Nine advocates, the general strangeness of Sedna could be explained if it’s only being pulled so far from the sun because there’s another star out there, with at least one other planet under its control. There are, of course, other theories as to why Sedna’s orbit is so unusual - including that it was actually redirected long ago by an ancient, rogue planet. But for as long as there’s an element of the unknown, there’s at least a sniff of possibility for second star theorists.
For now, though, the science does stack against the Nemesis Star existing. If it were real, it should’ve been picked up by numerous infrared surveys over the last few decades. Still, it is an exciting thought that our sun could well have once had a twin… and that that twin could now be anywhere else in the Milky Way galaxy, or perhaps even further afield. The universe never fails to astound.