Once frozen worlds could provide habitable conditions once a star ages.
All throughout the universe, there are many different types of stars in many different phases and stages of life. Astronomers usually look at middle-aged stars like our sun to search for habitable worlds, but new research suggests that to find life, scientists need to look around stars of all ages.
Ramses M. Ramirez, research associate at Cornell’s Carl Sagan Institute, and Lisa Kaltenegger, associate professor of astronomy and director of the Carl Sagan Institute, modeled the locations of the habitable zones for aging stars and how long planets can stay in them. The “habitable zone” is the region around a star in which water on a planet’s surface is liquid and signs of life could potentially be remotely detected by telescope.
"When a star ages and brightens, the habitable zone moves outward and you're basically giving a second wind to a planetary system," said lead author Ramirez in a Cornell University press release. "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."
Although it depends on the mass, or weight, of the host star, planets and their moons can hang out in the habitable zone up to 9 billion years.
For example, Earth has been in our sun’s habitable zone for about 4.5 billion years, but in a few more billion years, our sun will become a red giant, engulfing Mercury and Venus, and turning Earth and Mars into scorched and lifeless rocky planets. On the other hand, planets like Jupiter, Saturn, and Neptune — and their moons — will be in the newly established red giant habitable zone by that time.
Current habitable zone of our middle-aged yellow sun. Photo credit: Cornell University
Future habitable zone of our old red giant sun. Photo credit: Cornell University
"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," explained Kaltenegger in the release.
In other words, once-frozen Earth-sized worlds could provide habitable conditions for life as stars age.
"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," said Kaltenegger.
Their research, "Habitable Zones of Post-Main Sequence Stars," is published in the Astrophysical Journal.
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