arnab gupta

It’s all in the Statistics!

August 01, 2011

As is well known, Earth plays host to numerous meteors, some of which are big enough to reach the Earth’s surface as meteorites. What scares us, the human species as a whole, of course, are the ones large enough to cause significant damage—especially the ones that can cause mass extinctions.

Are the rates at which meteorites arrive at Earth cyclic? Can we predict when the next mass extinction (meaning almost certainly the end of the human race as we know it) will be upon us? It’s all in the statistics!

Of course, there’s no other way make predictions, other than to watch for patterns and extrapolate the timescales involved into the future. Based on past records, there are certain hypotheses about when and why some big rocks hurl towards the Earth—and the most interesting of them involves a phantom solar companion. Christened “Nemesis”, this hypothetical star is supposed to be lurking at the outer edges of the solar system, and like a classical villain hurling chunks of rock from the [Oort Cloud][linkoort] into the inner solar system. Essentially, he’s made out to be playing Duck Hunt, with Earth as the Duck, of course.

But all of this story (and other cyclical hypotheses) depends on a robust analysis of data, which involves robust statistical methods. Everything is a question of probability, of course—you can’t get absolute numbers in situations like these—and it’s no easy matter to set limits to what the error tolerances are. To understand how difficult the whole process is, consider the data in this case: cataloguing previous impact craters on Earth against how old they are. There are numerous craters, of course, but there are also erosion processes that make life difficult.

Well, according to this article by Ian O’Neill, there isn’t much statistical evidence after all for Mr. Nemesis. Researchers at the Max Plank Institute of Astronomy turned to Bayesian probabilities to look at the data, and concluded that there isn’t periodicity in the data as previously thought. Bayesian probabilities are a little more complex implementation of probability theory, where the question asked is: “What is the probability of event A, given that event B precedes it?” Event B, of course, has its own probability of occurring. (There are other complexities, of course, but that’s the simplest case.)

We’re not doomed by Nemesis after all! Apparently robust statistics has saved the day once again. :)

Or has it? The scientists did not find cyclical evidence that would point to Nemesis, but they think they’ve found evidence for an increasing rate of meteorites over the past 250 million years. And before you say “erosion!”–which would decrease the number of older craters—apparently the same trend is observed on the Moon, where there is, of course, no erosion going on.

So—we’re not out the woods yet! ;)

(In case you’re seriously wondering—no, there isn’t a serious chance that we’d be hit by a big one soon. There isn’t even a less serious chance. Doesn’t mean it’s impossible, though!)