The ability to host life might not be an exclusive of the young-ish star. Even after stars grow old and swell up they are able to create the right conditions for life, although with some caveat.

This is the finding of a new study by astronomers Ramses Ramirez and Lisa Kaltenegger from Cornell’s Carl Sagan Institute, and published in the Astrophysical Journal. Red Giant stars can provide a stable environment for at least 200 million years and up to 9 billion years depending on their mass.

Most stars, like our Sun, are fusing hydrogen in their core and they are part of the so-called "main sequence". When the hydrogen runs out, after millions or billions of years, they begin to fuse helium but it comes at a price: the stars get larger, brighter and hotter.  

“When a star ages and brightens, the habitable zone moves outward and you’re basically giving a second wind to a planetary system,” said Ramirez, the lead author of the study, in a statement.

The "habitable zone" is the area of a star system where water could be found as both solid or liquid and gas on the surface of planets, based exclusively on the luminosity of the star. The region is not too hot and not too cold, hence the alternative nickname, the "goldilocks zone".

^{The Solar System's habitable zone now (top) and how it will move in the future as the Sun expands (bottom). Wendy Kenigsburg}

In our Solar System, it extends from halfway between Venus and Earth to beyond Mars, but it will reach Jupiter and Saturn when the Sun turns into a red giant in 5 billion years. Moons like Europa and Enceladus might partially thaw, creating the right conditions for life to thrive.

“Long after our own plain yellow Sun expands to become a red giant star and turns Earth into a sizzling hot wasteland, there are still regions in our Solar System – and other solar systems as well – where life might thrive,” Kaltenegger said.

“For stars that are like our sun, but older, such thawed planets could stay warm up to half a billion years. That’s no small amount of time,” said Ramirez.

This research suggests a possible shift in target in our hunt for potentially habitable worlds. Old stars could be just as good hosts as the "middle age" stars that are the most observed objects in our search for exoplanets.

There is also another reason to hunt for planets around close-by red giants: Our telescopes are good enough to directly image objects in their habitable zones. The first habitable exoplanet could be just one observation away.