A white dwarf may be consuming part of a planet that formed from the material expelled by its own predecessor star, according to a study led by Jamie Williams of the University of Warwick. The research, published in Nature Astronomy, suggests that HS 0209+0832 could host a second-generation planet created from the remnants of the star’s earlier evolution.
The hypothesis is based partly on the white dwarf’s unusual chemical composition. Researchers found very little silicon and almost no iron, alongside unusually high amounts of copper, zinc and niobium. The niobium abundance is about three times the solar level, and the element had not previously been identified in a white dwarf of this type.
Niobium can be produced inside certain red giants through a process known as slow neutron capture. Williams’s team proposes that a planet formed from material expelled during that phase could now be orbiting close to the resulting white dwarf. If the star is stripping material from the planet’s atmosphere, the niobium could be returning to the object from which it originated.
The chemical evidence alone cannot establish that a planet is present, so the researchers also examined changes in the star’s brightness. Observations from NASA’s TESS space telescope show a periodic variation of roughly 4.4 days. The cycle appears too slow to be explained by the white dwarf’s rotation, but it could be consistent with a nearby gas giant completing an orbit at a distance of around six million kilometres. Changes in the visible portion of the planet’s illuminated side could produce the observed fluctuations.
HS 0209+0832 has about half the Sun’s mass compressed into a body roughly comparable in size to Earth, with a temperature of approximately 35,000 kelvin. The star had already attracted attention because of its unusual physical and spectral characteristics. Hubble observations in 1999 revealed an unexpectedly high proportion of helium, while later analyses and additional observations helped identify more of the elements in its atmosphere.
Heavy elements normally sink inside white dwarfs because of their intense gravity, leaving atmospheres dominated by hydrogen and helium. However, between a quarter and a half of white dwarfs show chemical pollution, generally attributed to the remains of former planetary bodies. In HS 0209+0832, the combination of elements points to a different and still unconfirmed history.
The proposed process remains difficult to demonstrate around a white dwarf. Zifan Lin of Washington University in St. Louis noted that observational evidence for a white dwarf actively absorbing material from such a planet is still lacking. Juliette Becker of the University of Wisconsin-Madison said that other systems of this kind could plausibly exist.
Williams plans to use the James Webb Space Telescope to search for the planet through its infrared emission and determine whether the suspected second-generation world is really present.
Sources: elDiario.es.