A new study led by Thea Kozakis of the Carl Sagan Institute explores the surface habitability potential of planets orbiting white dwarfs. White dwarfs, are dead starts, the final state of Sun-like stars, are objects that have ceased internal nuclear fusion and gradually cool over time. Although still as massive as a star, these dense objects are only about the size of Earth, making them exciting targets for planet detections employing the transit method, such as Kepler and TESS spacecrafts.
This new publication studies the potential for surface life around a white dwarf, which is a dead star. The habitable zone changes around such a White dwarf moves inwards as the white dwarf cools. However the temperatures of White dwarfs becomes stable enough to allow a planet to remain within the habitable zone for over 6 billion years, which is good news for a second time of habitability!
The changing UV radiation from the White dwarf changes while the white dwarf ages - but the surface life UV environment does not change much on an orbiting Earth-like planet. When the White dwarf emits a large UV flux, ozone in the planet’s atmosphere protects the surface from harmful UV. When the cooling white dwarf becomes less active as it ages, the UV levels decrease as well.
Unlike on Earth where we worry about the increasing greenhouse effect, on a planet around a cooling white dwarf such greenhouse gases can even extend the amount of time that the planet can maintain a surface water and support surface life, what could even increase the time for surface habitability beyond 6 billion years.
Links to the paper: UV Surface Environments and Atmospheres of Earth-like Planets Orbiting White Dwarfs,
Thea Kozakis, Lisa Kaltenegger, and D. W. Hoard, ,
Astrophysics Journal: http://iopscience.iop.org/article/10.3847/1538-4357/aacbc7