No 'Nemesis' Boosting Comet Impacts

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Trying to predict how much time we have before the next asteroid or comet impact event may sound like a fool's errand. After all, how can we forecast when a rare, yet devastating, space rock will careen through the inner solar system?

For starters, we could use statistics. Looking for patterns in a number of previous impact events is a valuable tool when trying to understand how often Earth was pummeled in the past. Once we know this, projections can be made for the risk of getting hit again.

However, according to a study by scientists of the Max Planck Institute for Astronomy (MPIA), the statistics we use to make these projections may be fundamentally flawed.

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Over the past few decades, scientists have used a valuable "marker" for understanding the Earth's impact rate and used that data to assess the risk of when the next strike will be. Impact craters scarring our planet's surface contain a lot of data — through erosion processes and other factors, these craters are dated.

Once many craters are analyzed, various statistical techniques can be applied to find any periodicity in the impacts. As a consequence, claims are made about when we can expect the next impact. Also, out come the hypotheses about what could be causing this perceived cycle of impacts.

One hypothesis is that, as the solar system "wobbles" through the galactic disk during its orbit around the galactic core, gravitational effects cause comets from the Oort Cloud (the hypothetical "shell" of large frozen chunks of cometary embryos) to periodically dive into the inner solar system, creating a swarm of cometary impacts.

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Another fun hypothesis that gets doomsday theorists all flustered is the idea that there may be a second stellar body with a very eccentric orbit around our sun. In this case, as this "second sun" — known as Nemesis — sweeps through the Oort cloud, it scatters comets, again causing a swarm of impacts on Earth with regular cycles that ebb and flow with the orbit of Nemesis.

However, according to the MPIA researchers, the statistical techniques employed may be producing false indications of a pattern. If there's no cyclical pattern, the basis for believing in Nemesis — or any other interstellar behemoth for that matter — is flawed.

"There is a tendency for people to find patterns in nature that do not exist," said MPIA researcher Coryn Bailer-Jones. "Unfortunately, in certain situations traditional statistics plays to that particular weakness."

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So, Bailer-Jones approached the statistical problem with an alternative way of evaluating probabilities — a method known as "Bayesian statistics." According to the MPIA press release, this method avoids many of the pitfalls that hamper traditional analyses of impact crater data.

Immediately, Bailer-Jones found simple periodic variations in impacts could be ruled out. If there's no obvious cycle of impacts through Earth's history, the perceived "swarms" of impacts through the ages are nothing more than statistical anomalies — therefore any suggestion of a "Nemesis," "Tyche" or "Planet X" playing cometary pinball is nothing more than a fairytale.

But that's not to say he hasn't discovered something else in the newly analyzed data. There appears to be a general trend that demonstrates a steady increase in impact rates over the past 250 million years. However, there may be a good (logical) reason for this.

As per the MPIA release:

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.

There is, however, some supporting evidence to suggest the impact rate is increasing. By looking at craters on the moon that aren't worn down by atmospheric erosion processes, there does appear to be an increasing rate of impact events.

Whether or not the rate is increasing, one thing is for certain, according to Bailer-Jones: the strongest piece of cited evidence for the existence of Nemesis has been ruled out by this new research.

"From the crater record there is no evidence for Nemesis," concluded Bailer-Jones.

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Image: Tenoumer Crater, Mauritania (NASA/Earth Observatory)

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