Nemesis is a myth

August 1, 2011
Barringer Crater, also known as Meteor Crater, in Arizona. This crater was formed around 50,000 years ago by the impact of a nickel-iron meteorite. Near the top of the image, the visitors center, complete with tour buses on the parking lot, provides a sense of scale. Credit: National Map Seamless Viewer/US Geological Service

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.

Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.

Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.

Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.

One proposed mechanism for these variations is the periodic motion of our Solar System relative to the main plane of the . This could lead to differences in the way that the minute gravitational influence of nearby stars tugs on the objects in the Oort cloud, a giant repository of comets that forms a shell around the , nearly a light-year away from the Sun, leading to episodes in which more comets than usual leave the Oort cloud to make their way into the inner Solar System – and, potentially, towards a collision with the Earth. A more spectacular proposal posits the existence of an as-yet undetected companion star to the Sun, dubbed “Nemesis”. Its highly elongated orbit, the reasoning goes, would periodically bring Nemesis closer to the Oort cloud, again triggering an increase in the number of comets setting course for Earth.

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.

Explore further: Getting WISE About Nemesis

More information: C. A. L. Bailer-Jones, Bayesian time series analysis of terrestrial impact cratering, Monthly Notices of the Royal Astronomical Society

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1.9 / 5 (12) Aug 01, 2011
the Earth is as likely to suffer a major impact now as it was in the past,

Probably not.

Earth and other rocky, terrestrial planets started accreting iron meteorites that formed directly from the Fe-rich material surrounding the supernova core almost immediately after the supernova explosion -even before isotopes of molybdenum (made by different nuclear reactions) were homogenized!

See the results of Qi Lu's 1991 PhD thesis:

Today most of the stellar debris has been swept away from the inner part of the solar system, but the break-up of planets further out from the Sun could scatter new debris in this area.

With kind regards,
Oliver K. Manuel
1 / 5 (3) Aug 01, 2011
I believe that nemesis is a crackpot theory, however I don't believe the evidence here does anything to change any minds. When it comes to larger craters, the trend goes away.

Since 'Nemesis' would affect asteroids gravitationally at certain periods of time, it would be expected that there would be some variation between the asteroid is affected and when it's altered trajectory coincides with earth's.

Also, it would be presumptive to assume that 'nemesis' would affect an equal density of asteroids every time it came in. Saturn's rings have variation in density.

Thanks a lot, coryn bailer-jones, you just fed the crackpots. How do you feel about yourself now?? scientists should not make as many assumptions as you do...
5 / 5 (1) Aug 01, 2011
There are galaxy-scale motions that occur with periods on the same scale as that attributed to Nemesis. If Nemesis' orbit is so large and weakly bound to the sun that it takes as much time to orbit the sun as the sun takes to orbit the galaxy, or to 'bob' in the galactic plane, it cannot be truly said that Nemesis 'orbits' the sun.
2.3 / 5 (3) Aug 01, 2011
There are galaxy-scale motions that occur with periods on the same scale as that attributed to Nemesis. If Nemesis' orbit is so large and weakly bound to the sun that it takes as much time to orbit the sun as the sun takes to orbit the galaxy, or to 'bob' in the galactic plane, it cannot be truly said that Nemesis 'orbits' the sun.

Certainly true enough. I was just stating that the researcher here does not add any good arguments to this equation.
not rated yet Aug 01, 2011
Thank god it's a myth, we're safe now.
4 / 5 (4) Aug 02, 2011
What good is the information about a trend if a singular event is all it takes? Risk assessment is only useful if you can start a cost/benefit analysis.

Once the cost comes down to "all you've got" then a cost/benefit analysis becomes meaningless.

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