Earth’s ultimate fate may be written in the stars, but scientists say the planet could become uninhabitable long before the Sun reaches its dramatic final stage. Research into the Sun’s evolution suggests that its gradual brightening could eventually disrupt Earth’s climate and reduce the oxygen available to support complex life. The timeline is astonishingly distant—but the implications are enormous.

Researchers from Tōhō University in Japan and NASA used computer models to examine how Earth’s atmosphere might change as the Sun grows brighter. Their findings, published in Nature Geoscience, suggest that oxygen levels could eventually collapse, making it impossible for humans and most other oxygen-dependent organisms to survive. The study estimated that the atmosphere could maintain oxygen levels above 1% of today’s level for roughly 1.08 billion years, with considerable uncertainty.

The Sun’s later transformation will bring an even more dramatic change. In approximately five billion years, it is expected to exhaust the hydrogen fuel in its core and expand into a red giant. Mercury and Venus are expected to be engulfed, while Earth’s ultimate fate remains uncertain. Long before then, however, rising solar brightness is expected to make our planet increasingly hostile to life as we know it.

That distant future helps explain why Elon Musk argues that humanity should become a multiplanetary species. He has described Mars as a form of “life insurance” for life itself, arguing that a settlement would need to become self-sufficient rather than depend indefinitely on supplies shipped from Earth. In his vision, a truly independent Mars colony could preserve human civilization if a catastrophic event ever made our home planet uninhabitable.

Yet building such a settlement presents enormous challenges, from radiation and limited resources to producing food, water and breathable oxygen. NASA’s budgets and mission priorities also influence how quickly space exploration can progress. The biggest question may not be whether humanity can reach Mars, but whether it can build a second home capable of surviving on its own. The Sun’s distant evolution gives scientists a reason to think about the future—but any successful backup plan would need to solve problems much closer to home first.