A universe full of white dwarfs
A white dwarf has about half the mass of the Sun, but that mass is compressed into a volume roughly the size of Earth, with its electrons pressed as close together as the laws of physics will allow. The sun has a radius 109 times the size of Earth's – this size difference means that an Earth-like planet orbiting a white dwarf could be about the same size as the star itself.
White dwarfs are extremely common: An estimated 10 billion of them exist in our galaxy. And since every low-mass star is destined to eventually become a white dwarf, countless more have yet to form. If it turns out that life can exist on planets orbiting white dwarfs, these stellar remnants could become promising and plentiful targets in the search for life beyond Earth.
But can life even exist on a planet orbiting a white dwarf? Astronomers have known since 2011 that the habitable zone is extremely close to the white dwarf. This zone is the location in a planetary system where liquid water could exist on a planet’s surface. It can’t be too close to the star that the water would boil, nor so far away that it would freeze.
The habitable zone around a white dwarf would be 10 to 100 times closer to the white dwarf than our own habitable zone is to our Sun, since white dwarfs are so much fainter.