All throughout the universe, there are stars in varying phases and ages. Planetary diversity suggests that around other stars, initially frozen worlds could be the size of Earth and provide habitable conditions once the star becomes older. The oldest detected Kepler planets (exoplanets found using NASA's Kepler telescope) are about 11 billion years old. Our Sun is currently 4.6 billion years old. Astronomers usually looked at middle-aged stars like our Sun, but to find habitable worlds, one needs to look around to stars of all ages, including red giants.
In their work, 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, have modeled the locations of the habitable zones for aging stars and how long planets can stay in them. Their research, "Habitable Zones of Post-Main Sequence Stars," is published in the Astrophysical Journal May 16.
The "habitable zone" is the region around a star in which water on a planet's surface is liquid and signs of life can be remotely detected by telescopes.
"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. "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."
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