The Nemesis Theory is one of the great ideas in the history of the world. It really is.
It's like this...
A paleontologist sifting through layers of fossil beds noticed that every so often the layers of fossils were topped-off by a bizarre layer of a substance called Iridium.
Iridium is one of the rarest elements on Earth. Yet here it was, in layers, topping off strata, or layers, in fossil beds all over the world.
Careful measurement of the age of the amount of time passing between the Iridium layers yield a modal average of about 62,000,000 years.
So, there was an appearance that once every 62,000,000 years, something was causing a layer of one of the world's rarest element to be spread like icing on a cake all over the world.
While it is very rare on Earth (except between fossil layers), Iridium is a common component in asteroids. So, the researchers thought, "Assuming asteroids pulverizing themselves into dust by smashing into the Earth once every 62,000,000 years is the reason for the Iridium and the regular mass extinctions generating so many fossils, what mechanism would cause asteroids to target Earth once every 62,000,000 years?"
And then somebody had a wonderful idea, again maybe the best idea anyone in science has ever had.
If we look skyward through our telescopes, we see that, surprisingly, most stars in our galaxy are binary or triplex star systems -- 1 of 2 or 3 stars orbiting one another.
Frequently, orbits of anything around something else in space are extremely elongated -- much longer than wider. If the Earth is part of a binary star system, and if the Earth's partner in that binary star system is a relatively small, almost invisible brown star companion, then that brown star companion might have one of those elongated orbits, 62,000,000 years in length, so that, once every 62,000,000 years, the brown star companion punches through the cloud of ice and rock around the Sun, beyond Pluto's orbit, called the Oort Cloud, and drags millions of asteroids and comets in with it by its own gravity.
These asteroids and comets become bullets in a shooting gallery, smashing into the planets for tens of thousands of years, until the supply of bullets is exhausted, and the planets thereafter enjoy millions of years of relative peace until the brown star companion punches its next hole through the Oort Cloud. Before that period of peace, Earth endures a mass extinction as it is bombarded with the brown star's asteroid and comet “artillery,” and is once again powdered with Iridium dust.
Scientists named the theoretical brown star companion of Earth "Nemesis" -- enemy.
Currently, some astronomers are engaged in a search for Nemesis, through the Oort Cloud, a difficult endeavor. Nemesis, if it exists, is believer to be about 1 to 1.5 light years away right now, racing for the highpoint of its orbit.
No, we don't expect another visit soon.