Five Astronomical Mysteries

People like to talk confidently about what we do know. And the way some people talk, you’d think we know everything.

Not so.

So, I’m going to do a series of posts on astronomical mysteries over the next few days. Here is the first…

1: Is the Sun binary?
The short answer is probably not. However, perhaps surprisingly, we don’t know this for an absolute fact.

What we do know is that the Sun is certainly not a close binary; two Suns would be hard to miss in the sky! However, the possibility does exist that there might be a wide-separation companion. (‘Wide’ in this context means thousands of AU*.) Wide-separation binaries do exist; take our next nearest stellar system, Alpha Centauri. The inner two stars, A and B, also have a companion in the form of the red dwarf Proxima, which is currently more than a tenth of a light-year closer to us. That’s a huge separation, something like 6000 AU. Wider-separation binaries are also known to exist. (There is a K-giant, L-dwarf binary in the literature with a 15,000 AU separation.)

It is possible that a small and faint companion, like Proxima, might exist at an extreme separation from the Sun. Such an object would certainly be in existing star catalogues – it would have a magnitude between 7 and 12 – but there are lots of red dwarfs in star catalogues. Not every one has a parallax measurement. Also, the object in question would move with the Sun, so it might not appear to have a high proper motion, which would select it out of important catalogues like the Luyten Half-Second.

(Also, a brown dwarf companion would be even harder to spot…)

Surprisingly, there may be a shred or two of indirect evidence to support the Sun-as-binary idea. In the 1980s, some researchers found a few hints of a 26-million year periodicity in mass extinctions. Such a cycle would be too long to be produced by anything terrestrial. They suggested a possible cause – a companion to the Sun, orbiting at around 1 LY out. This object would periodically disturb the Oort Cloud, resulting in a ‘pulse’ of comets falling Sunwards. This would in turn raise the likelihood of the Earth taking a direct hit, leading to mass extinctions…

…Of course, all of that said, there are problems with the hypothesis. One is that no-one’s yet spotted Nemesis. Another is that the periodicity is controversial with geologists.

If the Sun does have a companion, the current generation of wide-field surveys (Pan-STARSS, Wise, LSST and so on) should be able to find it. Also, one would expect it to be in 2MASS somewhere as well. So, this one at least should be soluble sometime soon.

* Terminology note: 1 AU = the Earth’s mean orbital radius, or about 149,600,000 Km.


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