Interesting fact of the day: examining the fossil record suggests that mass extinctions on Earth occur approximately once every 26 million years (Myr). One possible explanation for this is a companion dwarf star to the Sun on a 26 Myr orbit. Every time is passes by, the theory goes, it messes up the Oort cloud and throws comets our way, greatly increasing the possibility of a collision (and all the nasty effects that could go with it). This potential star was christened Nemesis for obvious reasons.
If Nemesis exists then its orbital period (the time it takes to complete one orbit) is predicted to vary by a few Myr every orbit, because of the influence of other stars in the Galaxy. Remember this – it will be important later!
The theory that the mass extinction periodicity is caused by the Nemesis companion star was first suggested in 1984. Since then the quality of the fossil record datasets has improved significantly which has led two scientists, Adrian Melott and Richard Bambach to re-examine the hypothesis using these new modern data. They found strong evidence that mass extinctions have been occurring every 26.8 Myr, stretching back over 500 Myr (interesting aside: that’s approximately as far back as they could go in the fossil record apparently, as that’s how long things with hard body parts, which make good fossils, have existed).
Their results seem at first glance to support the existence of Nemesis. However, the mass extinctions they see are too regular – if they were being triggered by the visits of Nemesis the gap between them should vary more, given the predicted variation in the length of the star’s orbit.
If Nemesis isn’t responsible for these apparently regular extinctions then what is? The alternative explanations range from oscillations in the galactic plane to geological instabilities in the Earth itself, and if someone publishes new research on any of them I’ll maybe try and explain them too. One final thing to note: if Nemesis is out there after all, the new all-sky surveys to be carried out by Pan-Starrs, WISE or the planned LSST should see it.
Adrian L. Melott, & Richard K. Bambach (2010). Nemesis Reconsidered MNRAS arXiv: 1007.0437v1