White dwarf planets are planets which orbit white dwarfs - the glowing remains of dead stars which used to be roughly the size and brightness of Earth's own Sun. In recent years astronomers have grown interested in white dwarf planets because it is easier to find exoplanets orbiting particularly small stars, like red dwarfs and white dwarfs.
When a star runs out of hydrogen in its core, the fusion reactions which have sustained the star during its lifetime are no longer able to do so. What occurs next depends upon the size of the star. Stars that are approximately the size of the Sun, according to the Chandra X-Ray Observatory, begin to swell up, becoming red giants several times their normal size. As their fuel depletes further, the outer layers of the star are blown off, forming a massive cloud of gas and dust called a planetary nebula. The core of the star, glowing white-hot but no longer undergoing sustained nuclear fusion, remains behind. This remnant is what astronomers refer to as a "white dwarf."
Because any white dwarf observable today used to be a main-sequence star, astronomers expected that at least some white dwarfs would actually have exoplanets orbiting them. These exoplanets would have formed around the star early in its life and been far enough away to survive the red giant phase as well as the creation of the planetary nebula. Bathed in the dying light of a white dwarf, they might not be warm enough to support liquid water or carbon-based life anymore. However, they would still be further evidence that exoplanets are actually extremely common, and could reveal some surprising details to scientists about how planets survive the deaths of their host stars.
For the most part, the search for white dwarf planets so far has turned up mainly debris instead of actual planets. In 2012, for instance, National Geographic reported that a new study had found debris fields around four white dwarf stars, indicating that these stars had once hosted planets but that the planets had been torn apart. The destruction probably occurred during the stars' death throes, and might be a disturbing indication of what is likely to happen to Earth and other planets in the solar system in several billion years. In 2013, the Hubble Space Telescope turned up further evidence of planetary debris around two additional white dwarf stars in the Hyades cluster, about 150 light-years from Earth.
For several years, however, several astronomers, like Matt Burleigh of the University of Leicester, have been arguing that some planets probably survived the collapse of their host stars into white dwarfs. Burleigh calls his research program the "Degenerate Objects around Degenerate Objects Project," or "DODO," for short. His team has turned up some tantalizing evidence. For instance, system Gliese 3483 appears to contain a white dwarf and either an orbiting planet or a brown dwarf - a hydrogen-rich mass much larger than Jupiter, but too small to become a star. Currently, astronomers are trying to clarify how far from its star a planet has to be in order to survive the red giant and planetary nebula phases, and what conditions might occur on the surface of a planet orbiting a white dwarf.