All throughout the universe, there are stars in varying phases and ages. The oldest detected Kepler planets (exoplanets found using NASA's Kepler telescope) are about 11 billion years old, and the planetary diversity suggests that around other stars, such initially frozen worlds could be the size of Earth and could even provide habitable conditions once the star becomes older. Astronomers usually looked at middle-aged stars like our sun, but to find habitable worlds, one needs to look around stars of all ages.
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 it. 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.