Nemesis is a myth

Max Planck researchers refutes the claim that Earth is periodically hit by asteroids or comets

August 01, 2011

Danger looms from out of space: asteroids and comets are a threat to our planet. The history of Earth has always been punctuated by cosmic catastrophes. Several studies have claimed to have found periodic variations, with the probability of giant impacts increasing and decreasing in a regular pattern. Now a new analysis by Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA) shows those simple periodic patterns to be statistical artifacts. His results indicate either that the Earth is as likely to suffer a major impact now as it was in the past, or that there has been a slight increase impact rate events over the past 250 million years

An elevation map of the map, reconstructed from satellite data. On this type of maps, the crater can be seen much more clearly than on an ordinary satellite image.

For MPIA's Coryn-Bailer-Jones, these results are evidence not of undiscovered cosmic phenomena, but of subtle pitfalls of traditional (“frequentist”) statistical reasoning. Bailer-Jone: “There is a tendency for people to find patterns in nature that do not exist. Unfortunately, in certain situations traditional statistics plays to that particular weakness.”

That is why, for his analysis, Bailer-Jones chose an alternative way of evaluating probabilities (“Bayesian statistics”), which avoids many of the pitfalls that hamper the traditional analysis of impact crater data. He found that simple periodic variations can be confidently ruled out. Instead, there is a general trend: From about 250 million years ago to the present, the impact rate, as judged by the number of craters of different ages, increases steadily.

There are two possible explanations for this trend. Smaller craters erode more easily, and older craters have had more time to erode away. The trend could simply reflect the fact that larger, younger craters are easier for us to find than smaller, older ones. “If we look only at craters larger than 35 km  and younger than 400 million years, which are less affected by erosion and infilling, we find no such trend,” Bailer-Jones explains.

On the other hand, at least part of the increasing impact rate could be real. In fact, there are analyses of impact craters on the Moon, where there are no natural geological processes leading to infilling and erosion of craters, that point towards just such a trend.

Whatever the reason for the trend, simple periodic variations such as those caused by Nemesis are laid to rest by Bailer-Jones' results. “From the crater record there is no evidence for Nemesis. What remains is the intriguing question of whether or not impacts have become ever more frequent over the past 250 million years,” he concludes.


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