Astronomers have modelled the locations of habitable zones surrounding several aging stars for the first time, highlighting a move in the habitable zone as a star ages and providing unprecedented insight into how long planets can occupy said zones.
The habitable zone – also dubbed the ‘Goldilocks Zone’ – is the region around a star in which water on a planet’s surface remains liquid, making it a potential host for life.
The model, developed by Cornell University’s Carl Sagan Institute research associate Ramses M Ramirez and Carl Sagan Institute director and associate professor of astronomy Lisa Kaltenegger, is detailed in a paper released today in The Astrophysical Journal.
In the search for habitable planets, astronomers tend to observe middle-aged stars, such as our own Sun. However, the diversity of Kepler planets (exoplanets found using NASA’s Kepler telescope), suggests that around other stars, initially frozen worlds located further from their star could become habitable as that star ages.
“When a star ages and brightens, the habitable zone moves outward and you’re basically giving a second wind to a planetary system,” explained Ramirez. “Currently objects in these outer regions are frozen in our own solar system, and Europa and Enceladus – moons orbiting Jupiter and Saturn – are icy for now.”
Depending on the mass of the star, orbiting objects can stay in a habitable zone for up to 9bn years. Our own planet has been in our sun’s habitable zone for about 4.5bn years and has, as a result, seen a mass of varied life.
Ultimately though, our sun will become a red giant and those planets currently in its habitable zone will become far too hot to support life. Mercury and Venus will be engulfed by the red giant, while Earth and Mars will become rocky and scorched. The habitable zone itself will move outwards, warming outer planets such as Jupiter, Saturn and Neptune.
“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, ” said Kaltenegger.
“In the far future, such worlds could become habitable around small red suns for billions of years, maybe even starting life, just like Earth. That makes me very optimistic for the chances for life in the long run.”
As the graph above shows, the planets that occupy the expanded habitable zone of a red giant, once thawed, could stay warm for up to half a billion years, and provide the potential for life.
The study of these older stars could contribute to the possibility of finding life in other regions of the galaxy, and our own understanding of Earth’s future.
Indeed, though it may be billions of years from now, it is not inconceivable that the future may see humans looking to planets such as Jupiter or Neptune as a new base for our species.