The idea of ‘Nemesis,’ a hypothetical dark companion to the Sun, won’t quite go away, and it’s possible that the WISE mission may help us either identify such an object or else demonstrate that it’s not there. The idea is simple enough: Sol’s companion would perturb the Oort Cloud in its orbit, causing comets to enter the inner Solar System, thus increasing the likelihood of an impact with the Earth. Throw in an apparent periodicity in extinction events first described back in 1984 and you have an intriguing case.

But Adrian Melott (University of Kansas) and Richard Bambach (Smithsonian Institution) have reconsidered Nemesis in terms of extinction events in a new paper, one that looks at the timing of these incidents in light of the movements of Nemesis over time. They extend the original 26 million year extinction periodicity slightly, to 27 million years, and are careful to note that there is no consensus on the matter among paleontologists. But the real question they tackle is whether the apparent cycle really can be explained by the actions of a distant, massive object.

This is a useful contribution to the discussion particularly because the authors stretch the span over which periodic extinction events are studied to 500 million years. What they find is that the events seem to occur too regularly over that period to be tied to Nemesis. If that seems counterintuitive, realize that the effects of such an object depend upon the stability of its orbit. Two major causes of perturbation to that orbit have been considered, one the galactic tidal gravitational field, the other the effect of passing stars. Both carry a punch. From the paper:

Hut (1984) was specific that irregularity of the period of revolution of such an object over the past 250 My should be about 20% due to perturbation from the Galaxy tidal gravitational field and by passing stars, and sharp peaks should not be expected in Fourier analysis. Torbett & Smoluchowski (1984) reached the same conclusion, but with a somewhat larger estimate of the fluctuations from the Galactic tide alone, dependent on the inclination of the Nemesis orbit with respect to the Galactic disk. Hills (1984) estimated a period change of 4% per Nemesis orbital period from the effects of passing stars. Using a t1/2 amplitude scaling expected from a random walk, the orbital period should drift by 15 to 30% over the last 500 My. This change in the period will broaden or split any spectral peak in a time series frequency spectrum, so Nemesis as an extinction driver is inconsistent with a sharp peak.

So how sharp is the peak? Sharp enough for the authors to conclude that there is 99 percent confidence in rejecting the hypothesis that there is no association of mass extinctions with the 27 million year cycle. The periodicity is demonstrated for a much longer period of time, its timing revised to roughly 27 million years (over the previous 26 million) and the confidence level in the results has gone from 95% to 99%, surely a confirmation of a cycle, but what is causing the extinctions?

It’s not likely to be Nemesis, argue the researchers:

Fossil data, which motivated the idea of Nemesis, now militate against it and suggest another mechanism is needed to explain extinction periodicity. An attempt to associate the periodicity with passage through the Galactic mid-plane …has its own set of problems: 54 My is rather too short for most estimates of the period of the Sun normal to the plane, and our passage within the last My or so of the mid-plane… is inconsistent with the phase of the 27 My signal we have detected, with its recent maximum at 11 My ago.

This is a fascinating finding. On the one hand, we do see the expected timing for extinction events, but the very regularity of that timing argues against its being the result of Oort Cloud perturbations caused by a Nemesis-like object. Nemesis’ orbit couldn’t be that stable. We are left to ponder the cause of these events, now measured over a span of some 500 million years and found to meet the confidence levels of three different statistical tests.

The paper is Melott and Bambach, “Nemesis Reconsidered,” accepted by Monthly Notices of the Royal Astronomical Society and available as a preprint.