As the Solar System passes through the spiral arms of the Milky Way, dragging the Earth and its other planets along for the ride, it can swing by exploding supernovae - a prime source of cosmic rays, high energy particles made up mostly of protons. These rays are the biggest ionizer of air molecules in the Earth's lower atmosphere, and thought to play a role in cloud formation, which in turn affects the climate of the planet.

Shaviv studied the Earth's historical exposure to cosmic rays over eons by looking at the exposure of 42 iron meteorites from different times in the planet's history. Examination of the meteorites indicated that the Earth's comic ray exposure changes on a period of about 143 million years, which matches up well with the historical incidence of the planet's ice ages and the Solar System's path through the galaxy's spiral arms.

There have been four main periods known as ice ages over the last 800 million years, the first in the Proterozoic period lasting about 200 million. The most recent and less extreme, of the events occurred some 430 million years ago in across the epochs known as the Ordovician and Silurian periods.

The Solar System is currently located in the Orion spiral arm, a minor appendage that Shaviv states exposes Earth to about half the amount of cosmic rays it would in a major arm. The galactic position, he added, places Earth in the aftermath of a major ice age and is consistent with global temperatures seen today.

Shaviv stresses that his cosmic ray model does have a weak link. The timing of glacial periods, when vast sheets of ice spread across the world and led to an ice age, remains uncertain and could affect his cosmic ray climate model. Further geological studies are needed.