Astronomers have made the first direct discovery of a white dwarf star being orbited by a disintegrating minor planet that will ultimately collide into it. The observation, made by the Kepler space telescope, offers a glimpse into what could happen to the Earth in a few billion years as the Sun, like most stars, becomes a white dwarf.
The study, published in Nature, also adds to a growing number of studies reporting that dwarf stars can have atmospheres polluted with heavy elements – in some cases the constituents of water. Knowing that planets can be the source of such contamination gives weight to a hypothesis that says water on Earth was deposited by rocky bodies from distant regions in our solar system.
Strange Pollution
Most stars, including our Sun, will become white dwarfs as they die – before going out as a black dwarf or supernova – when they have exhausted their nuclear fuel. Our standard model for white dwarfs would predict white dwarfs to have no elements heavier than helium in the atmosphere, but a growing number of measurements of white dwarf atmospheres show the presence of heavier elements such as oxygen, magnesium, silicon and iron. This is surprising because the star’s strong surface gravity is expected to cause heavy elements to sink quickly to the centre – leaving simple atmospheres of hydrogen and helium.
One explanation for this atmospheric pollution is that rocky bodies with similar properties to those in our solar system collide with a white dwarf and replenish its atmosphere with new heavy elements, including water. However, until now, there was no direct evidence that such rocky bodies exist or could fall onto a white dwarf.
The authors of the new study discovered the planet by noticing a dip in the brightness – a transit-like signal – from the white dwarf WD 1145+017 (also known as EPIC 201563164). They also detected similar but weaker signals – all with periods ranging between 4.5 and five hours. Using additional data from a range of Earth-based telescopes, they interpreted these dips as times when low-mass objects orbit in front of the white dwarf, blocking out some of its light.
