It's possible because "empty" is a relative term and the solar system is big. Hell, a whole 'nother star system passed through the solar system (if we include the Oort cloud) just 1,000 lifetimes ago. https://en.wikipedia.org/wiki/Scholz%27s_Star

> If space outside the solar system is empty,

Empty? You've seen stars in the night sky before surely?

The space outside the solar system is mostly empty, but also really big. Maybe these things are more common than we think.

Rare events also happen all the time; something being extremely improbable doesn't mean it will never happen.

Why should it be empty?