For over 30 years scientists have been collecting evidence of the chilling regularity of mass extinctions on Earth. Two researchers now think the most likely culprit is the hypothesized ninth planet of the Solar System, which they dub "Planet X," although more commonly referred to now as "Planet Nine".
According to retired Professor Daniel Whitmire, the yet undiscovered planet could trigger periodical comet showers. The planet’s orbit would slowly rotate, making the planet pass through the Kuiper Belt every 27 million years and unleashing a swarm of comets towards the inner Solar System. The research was published in the Monthly Notices of the Royal Astronomical Society.
An apparent periodicity of mass extinctions every 27 million years was first discovered in 1984, and it has been shown to date back 500 million years. Geological records also show that the phenomenon is highly regular with a variation no larger than 10 percent.
To explain these recurring extinctions, several scientists, including Whitmire, have suggested an extraterrestrial origin. The most famous suggestion was the existence of Nemesis, a red (or brown) dwarf companion to the Sun. This idea was dismissed as no such star has ever been found.
Another possible explanation required interactions between the Solar System and the Milky Way. The Sun moves vertically across the galactic plane, and this movement could cause oscillations in the Oort cloud, sending material speeding towards the Sun. But this was only thought to occur every 33 million years, inconsistent with the calculated extinction period every 27 million.
Planet X is the third suggested astronomical model. It was proposed by Whitmire and his colleague John Matese in 1985, and to produce the desired comet shower, the model required the existence of the Kuiper Belt and a gap in material about 7.5 billion kilometers (4.7 billion miles) from the Sun. The belt and the gap were discovered a few years later, but Planet X wasn’t, so the hypothesis remained untested.
However, the more recent discoveries of several large objects in both the Kuiper Belt and the Oort cloud has allowed the scientists to better refine their theory. These objects have a perihelion (the closest point to the Sun) very near to each other, suggesting the presence of a large object more than 10 times the mass of Earth, which is consistent with the hypothesized existence of Planet Nine, suggesting that the objects could be one and the same.
“I’ve been part of this story for 30 years,” Whitmire said in a statement. “If there is ever a final answer I’d love to write a book about it.”