'Planet X' caused mass extinctions on Earth?:

"Planet X, a suspected ninth planet in our solar system, may have caused
periodic mass extinctions on Earth, according to a new study. The yet
undiscovered "Planet X" triggers comet showers linked to mass extinctions on
our planet at intervals of approximately 27 million years, researchers said.
Scientists have been looking for Planet X for 100 years.

In previous work, researchers inferred its existence based on orbital anomalies
seen in objects in the Kuiper Belt, a disc-shaped region of comets and other
larger bodies beyond Neptune. According to Daniel Whitmire, from University of
Arkansas in US, and his colleagues, as Planet X orbits the Sun, its tilted
orbit slowly rotates and the planet passes through the Kuiper belt of comets
every 27 million years, knocking comets into the inner solar system.

The dislodged comets not only smash into the Earth, they also disintegrate in
the inner solar system as they get nearer to the Sun, reducing the amount of
sunlight reaching Earth. In 1985, a look at the paleontological record
supported the idea of regular comet showers dating back 250 million years.
Newer research shows evidence of such events dating as far back as 500 million
years.

The researchers believe that Planet X would be between one and five times the
mass of Earth, and about 100 times more distant from the Sun. "What's really
exciting is the possibility that a distant planet may have had a significant
influence on the evolution of life on Earth," said Whitmire. The study was
published in the journal Monthly Notices of the Royal Astronomical Society."

See:

http://www.deccanchronicle.com/scien...-on-earth.html