Our Cruel Sun: Exposed Earth to Cosmic Rays

The sun protects the earth from cosmic rays and dust from the solar system but squeezing of various stars could leave us unprotected (Image: NASA/HST collection)

THE sun provides ideal conditions for life to thrive, right? In fact, it periodically leaves Earth open to assaults from interstellar nasties in a way that most stars do not.

The sun protects us from cosmic rays and dust from beyond the solar system by enveloping us in the heliosphere - a bubble of solar wind that extends past Pluto. These cosmic rays would damage the ozone layer, and interstellar dust could dim sunlight and trigger an ice age. However, when the solar system passes through very dense gas and dust clouds, the heliosphere can shrink until its edge is inside Earth's orbit.

In a paper to appear in Astrobiology, David Smith at the University of Arizona in Tucson and John Scalo at the University of Texas, Austin, calculated the squeezing of various stars' protective "astrospheres". They found Earth is exposed to between one and 10 interstellar assaults every billion years. Habitable planets around a red dwarf, which account for three of every four stars, are never exposed. That's because they need to be close to these dim stars to be warm enough to be habitable. "The bottom line is that habitable planets around red dwarfs are better protected from climate catastrophes than Earth is," says Smith.