What if sun was a red giant
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The changing sun may provide hope to other planets, however. When stars morph into red giants, they change the habitable zones of their system. The habitable zone is the region where liquid water can exist, considered by most scientists to be the area ripe for life to evolve. Because a star remains a red giant for approximately a billion years, it may be possible for life to arise on bodies in the outer solar system, which will be closer to the sun.
"When a star ages and brightens, the habitable zone moves outward and you're basically giving a second wind to a planetary system," exoplanet scientist Ramses M. Ramirez, a researcher at Cornell's Carl Sagan Institute, said in a statement. "Currently objects in these outer regions are frozen in our own solar system, like Europa and Enceladus — moons orbiting Jupiter and Saturn."
The window of opportunity will only be open briefly, however. When the sun and other smaller stars shrinks back down to a white dwarf, the life-giving light will dissipate. And supernovae from larger stars could present other habitability issues.