Ep. 789: What Happens When a Planet's Star Dies

The Sun will exhaust hydrogen in about five billion years, expand into a red giant, and shed roughly half its mass. This swelling will certainly engulf Mercury and Venus, while Earth’s fate hinges on the amount of mass loss; reduced gravity can push the orbit outward enough to avoid engulfment, though surface conditions become extreme. The outer giants—Jupiter, Saturn, Uranus, and Neptune—will migrate outward as the Sun’s gravity weakens, and icy moons such as Europa may briefly enter a temporary habitable zone before the star sheds its outer layers into a planetary nebula.

After the red‑giant phase the Sun’s core collapses into a white dwarf, a Earth‑sized object supported by electron degeneracy pressure and radiating intense ultraviolet light. As the white dwarf cools, it initially vaporizes nearby rock, but within a few hundred million years a dust disk reforms from the debris. Recent observations show that medium‑aged white dwarfs host such disks, where planetesimals can coalesce and even create a second‑generation habitable zone a few million kilometers from the star. A crystallization pause in the cooling curve can extend this habitable phase for billions of years.

In the far future the white dwarf will radiate away its remaining heat, eventually evaporating over trillions of years if proton decay or Hawking‑like radiation occurs. Meanwhile, galactic tides and stellar encounters will gradually strip away remaining planets, a process that can take up to a hundred billion years. The final remnant will be a cold, Earth‑sized degenerate core orbiting a lone, tidally‑locked planet or none at all, drifting through a dark universe where only gravitational lensing marks its existence.