What are the current observational constraints on the existence of Nemesis?

Question

Nemesis is a hypothetical companion to the Sun on a very eccentric, long-period orbit. The star supposedly returns every few tens of millions years, driving comets into the inner solar system and causing extinction events. Given our very stringent observational limits from infrared surveys (such as WISE), is its existence definitively ruled out?

Answer 1

Few important points about WISE:

• it was able to detect anything with a temperature above 70-100 K, whereas the coolest brown dwarfs are in the 500-600 K range (the coolest was discovered by WISE itself, see Mainzner et al., 2011);
• it was able to detect objects larger than 1km up to 3 AU from the Sun, or objects of 2-3 Jupiter masses in a distance up to 7-10 light-years (see here and here);
• the closest brown dwarf detected by WISE is a brown dwarfs binary system, located at 6.6 light-years from the Sun.

So I would say we can be pretty confident that Nemesis existence has been ruled out.

Answer 2

Partial answer: the existence of Nemesis on theoretical grounds is shaky. The argument is that an object so far away form the Sun would be easily perturbed by other stars, so it would also be unstable, but the lifetime is predicted to be about 5.5 billion years. The Solar System is not that old, so if Nemesis exists we are approaching its "end of life", but this does not rule out Nemesis entirely.

However, the same perturbations is expected to change Nemesis' orbital period. So the 26 million year periodicity of extinction events should not be a strict 26 year periodicity. It should vary over time. The expected change is roughly a few million years each orbit. From Adrian Melott & Richard Bambach,

Using a $t^{1/2}$ amplitude scaling expected from a random walk, the orbital period should drift by 15 to 30 per cent over the last 500 Myr. 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.

Analyzing the fossil record, they find that:

The peak we have found is measured over ∼500 Myr (possible with modern paleontological data) and appears with a confidence level of p= 0.01 with two different statistical tests based on extinction intensity, and p= 0.02 on one based on ‘lesser peaks’ of this function. It shows less than 10 per cent variation in period by a spectral test, and less than 2 per cent by an extinction timing test, over the entire time period. In fact, our cross-spectral peak has the narrowest bandwidth possible, consistent with the level of probable random error in the fossil record.

The variation is too small.